A Thesis Entitled Geophysical Mapping of Concealed Karst And

Home , Delaware Limestone, Greenfield Dolomite, Rochester Shale, University of Toledo

A Thesis

entitled

Geophysical Mapping of Concealed Karst and Conduits North of Bellevue, OH

Biniam H. Estifanos

Submitted to the Graduate Faculty as partial fulfillment of the requirements for the

Masters of Science Degree in Geology

______

Dr. Donald Stierman, Committee Chair

______

Dr. Richard Becker, Committee Member

______

Dr. James Martin-Hayden, Committee Member

______

Dr. Patricia R. Komuniecki, Dean

College of Graduate Studies

The University of Toledo

May 2014

This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author.

An Abstract of

Geophysical Mapping of Concealed Karst and Conduits North of Bellevue, OH

Biniam H. Estifanos

Submitted to the Graduate Faculty as partial fulfillment of the requirements for the

Master of Science Degree in Geology

The University of Toledo

May 2014

The Bellevue region consists of thin glacio-lacustrine sediments underlain by a succession of Devonian and Silurian evaporites, carbonates and shales. These formations developed intrastratal karst due to dissolution of gypsum which was followed by subsequent collapse of the voids. This is manifested in surface expressions such as sinkholes, dolines, depressions and springs. However, there are also concealed karsts that lack surface expression. The objective of this study is to test the hypothesis that gravity can delineate subsurface mass deficit and to detect water table variations. The study also tests whether electrical resistivity can detect water table variations and delineate iii

underground rivers that flow toward Sandusky bay. Two topographic depressions about 4 km2 and 2 km2 in area were the target of this study. A microgravity survey was carried out at State Route 269, Strecker and Southwest roads while, dipole-dipole electrical resistivity surveys were done at Strecker road, a field north of Strecker road west and

Hale road. A total of 346 gravity measurements and 2 km of dipole-dipole electrical resistivity profiles were conducted. A total of nine gravity lows: four on State Route 269, another four at Strecker road and one at Southwest road were delineated. Their maximum amplitude ranges between -0.075 and -0.26 mGal. The gravity lows are associated with topographic lows. The volume of the void space was calculated from the negative

3 residual gravity and it ranges between 0.12 and 0.69 km depending on the density of the infill material.

At Strecker road, the dipole-dipole electrical resistivity delineated three 10-20 m wide throats of sediment filled-sinkholes and a low resistivity zone underlies the Columbus limestone. The microgravity survey also suggests these sinkholes are filled with a lower density material. Repeat measurements using both methods detected changes in water table elevation. The study showed that it is possible to map areas of mass deficit within the concealed karst and both methods can detect changes in water table elevation.

However, outlining underground rivers from the dipole-dipole data was not successful.

Acknowledgements

I would like to extend my deepest gratitude to my advisor, Dr. Donald Stierman, for his supervision, helping with electrical resistivity survey, reviewing the thesis manuscript, and advising during my study. Drs. Richard Becker and Jamie Martin-Hayden who are my thesis committee members provided with helpful suggestions throughout the semesters. Dr. Becker also played a great role in securing field help. Without the help of the following colleagues the data collection would not have been possible. I thank Jon

Sanders, Joseph Blockland, Chris Maike, Joseph Fugate, Kyle Siemer and Kirk

Zmijewski for their help. Jon Sanders was crucial in the field work. Doug Aden of

ODNR shared his sinkhole data which was very helpful in this study.

I thank the department of Environmental sciences for funding field transportation. I would like to thank all the faculty and staff at the department of Environmental sciences,

University of Toledo for all the help I received during my study. Finally, special thanks go to my family and friends for their encouragement and support.

Table of Contents

Abstract iii

Acknowledgements v

Table of Contents vi

List of Tables ix

List of Figures x

List of Abbreviations xii

1 Introduction 1

1.1 Background…. ……………………………………………………………....1

1.1.1 Study Area ……………………………………………………………. .2

1.1.2 Climate………………………………………………………………….3

1.1.3 Significance……………………………………………………………..4

1.1.4 Hypothesis……………………………………………………………....4

1.1.5 Previous Study ……………………………………………………….…5

2 Geology, Hydrogeology and Karst 8

2.1 Geology……………………………………………………………………....8

2.1.1 Bedrock Geology………………………………………………………..9

2.1.2 Surficial Geology……………………………………………………....12 vi

2.1.3 Joint Orientation………………………………………………………..14

2.2 Hydrogeology……………………………………………………………….14

2.3 Karst………………………………………………………………………….19

3 Methodology 21

3.1. Gravity …………………………………………………………………...21

3.1.1 Data Reduction………………………………………………………..22

3.1.1.1 Latitude Correction…………………………………………….23

3.1.1.2 Free Air Correction…………………………………………….23

3.1.1.3 Bouguer Density Correction…………………………………..24

3.1.1.4 Simple Bouguer Anomaly…………………………………….24

3.1.2 Regional-Residual Gravity Separation………………………………..24

3.1.3 Gravity Modeling ……………………………………………………25

3.1.4 Void Volume Estimation……………………………………………...27

3.1.5 Repeat Gravity Measurement……………………………………….....28

3.2 Electrical Resistivity…………………………………………………………28

3.2.1 Repeat Electrical Resistivity Measurement…………………………...33

3.3 Electrical Resistivity Signature of Seneca Caverns…………………………35

3.4 Well Logs… ………………………………………………..……………….36

4 Results 39

4.1 Gravity ….. …………………………………………………………………..39

4.1.1 Simple Bouguer Gravity ……………………………………………….39

4.1.2 Residual Gravity...……………………………………………………...40

4.1.3 Microgravity …………………………………………………………...43 vii

4.1.4 Volume of Voids……….………………………………………………47

4.1.5 Repeat Gravity Measurement …………………………………………….49

4.2 Electrical Resistivity………… ……………………………………………….53

5 Discussion 57

5.1 Gravity………………………………………………………………………...57

5.2 Electrical Resistivity ………………………………………………………..63

6 Conclusion and Recommendations 65

6.1 Conclusion …… ………………………………………………………………65

6.2 Recommendations……………………………………………………………..67

References 68

A. Observed Gravity 73

viii

List of Tables

2.1 Stratigraphy of the Bellevue area………………………………………………..10

2.2 Bedrock water yield of the study area……………………………………….…..15

3.1 Resistivity of some earth materials ...... 32

3.2 Well log that contains mud seam within the carbonate bedrock………………..36

3.3 Well logs that contain broken limestone within the carbonate bedrock………...37

3.4 Well logs that contain porous Limestone within the carbonate bedrock………..37

4.1 Calculated range of void volume …………………………………………….....49

4.2 Summary of the gravity, mass and volume changes…………………………….53

List of Figures

1- 1 Location map of the study area ...... 3

2- 1 Geologic map of the Bellevue region……………………………………………….11

2- 2 Bedrock elevation map...... 12

2- 3 Drift thickness of the Bellevue area ...... 13

2- 4 Potentiometric surface map...... 16

2- 5 Drainage pattern of the Bellevue region...... 17

2- 6 Water level at USGS monitoring well north of Bellevue...... 19

3- 1 Gravitational field over a buried horizontal cylinder…………………………….... 26

3- 2 Geometry of a resistive cylinder…………………………………………………….29

3- 3 A simple diagram illustrating electrical resistivity measurement setup...... 30

3- 4 A general set-up of a dipole-dipole resistivity array...... 31

3-5 AGI Supersting resistivity meter and its accessories...... 32

3- 6 Inversion of a dipole-dipole electrical profile...... 33

3- 7 Photo of Strecker road during first resistivity measurement ...... 34

3- 8 Photo of ponded surface water in a field adjacent to Strecker ...... 34

3- 9 An empty sinkhole next to ponded surface water in Figure 3- 8...... 35

3- 10 Dipole-dipole resistivity profile at the Seneca Caverns...... 36

3- 11 Map of wells with logs with broken, porous and mud seam ...... 38

4- 1 Location map of new gravity stations……………………………………………….40

4- 2 Bouguer gravity contour map ………………………………………………………41

4- 3 Regional gravity separation with graphical smoothing...... 42

4- 4 Residual gravity of the study area...... 43

4- 5 Microgravity profiles along State Route 269 ...... 45

4- 6 East-west microgravity profile along Strecker road...... 46

4- 7 Spring adjacent to Strecker road near gravity low IV...... 46

4- 8 Microgravity profile along Southwest road...... 47

4- 9 Map showing distribution of mass calculated from the residual gravity...... 48

4- 10 Map showing the volume of missing carbonate bedrock...... 49

4- 11 Repeat Gravity measurement at Strecker road...... 50

4- 12 Changes in gravity, mass and volume changes at field base station...... 52

4- 13 Dipole-dipole electrical resistivity profiles locations...... 54

4- 14 Dipole-dipole inversion results at Strecker road...... 55

4- 15 Dipole-dipole resistivity at Hale road...... 55

4- 16 Resistivity inversion profile north of Strecker road west...... 56

4- 17 A north-south resistivity inversion profile north of Strecker road west...... 56

5-1 Simple Bouguer gravity and sinkholes overlain over overburden thickness…..…...59

5- 2 Map showing location of gravity lows II, V-IX and sinkholes...... 60

5- 3 Location of gravity lows I, III and IV and sinkholes...... 61

5- 4 Sinkholes overlain over residual gravity...... 62

5- 5 Overburden thickness from well logs overlain over the residual gravity...... 63

List of Abbreviations

mGal ...... milligals (1mGal = 10-3 cm/s2)

USGS……………….United States Geological Survey

ODNR………………Ohio Department of Natural Resources

xii

Chapter 1

Introduction

1.1 Background

The Bellevue area of Ohio lies on a karst landscape that is expressed on the

Columbus Limestone subcrop and features sinkholes, sinking streams, springs and depressions. The sinkholes serve as recharge points to the carbonate aquifer and cause the absence of well-developed surface drainage. Both natural and manmade sinkholes were used to dump sewage into the carbonate aquifer leading to the pollution of the groundwater (Daugherty, 1941) and contamination of the groundwater with e-coli

(VerSteeg & Yunck, 1932). The carbonate aquifer was polluted enough that during heavy precipitation, waste ridden water flooded the surface (Dean et al., 1991). At present some sinkholes are being used to drain storm water.

Direct infiltration of surface water into the carbonate aquifer triggers a rapid rise in water table level during episodes of heavy precipitation. For example, heavy precipitation caused the sinkholes to discharge water and flood the surface to a depth of 6-12 ft (2-3.5 m) in the spring of 1913 and 1936 (Daugherty, 1941). The flood also accelerated the

formation of sinkholes by widening and deepening depressions. During the spring of

1930, the groundwater discharge caused 3-4 ft (1m) of flooding in areas without any visible sinkholes (VerSteeg & Yunck, 1932). There was also another flooding event in

1969 (ODNR, 2009). Five inches of rain in three days raised the water table by 50 ft (15 m) in 1992 (ODNR, 1994). In the spring of 2008, flooding affected more than 200 families for weeks (ODNR, 2009).

The main purpose of this study is to map the concealed karst using gravity and electrical resistivity. Both gravity and electrical resistivity work well where there is a significant contrast in density and electrical conductivity respectively between the limestone and the infill materials in the karst [e.g., Roth et al., 2000; Zhou et al., 2000; Ahmed & Carpenter,

2003; Kruse et al., 2006]. Prior studies which included geophysical and hydrogeological surveys were conducted to the north and south of the study area in 1990’s (Drane, 1993;

Chaffee 1995) and were successful in mapping karst.

1.1.1 Study Area

The study area is located north of the town of Bellevue and adjoins Erie, Sandusky,

Huron, and Seneca Counties in North Central Ohio (Figure 1-1). This area lies within the

Bellevue-Castalia karst plain. It is a flat, low topography covered by glacial tills and lake sediments deposited by glaciation and post-glacial lakes, on the Columbus Limestone.

The area is characterized by beach ridges, numerous sinkholes and depressions, and lacks surface drainage. The main land use is agricultural. A large tract of the area is underlain by hydric soil.

Figure 1-1: Location map of the study area (C) modified from Carlson (1991).

1.1.2 Climate

Climate had a direct impact on this study as the water table fluctuation monitoring in the karst aquifer was influenced by the recharge from precipitation. As a result the drought of summer 2012 (NOAA, 2012) has delayed this study. The study area has a temperate climate with Lake Erie having a moderating influence (USDA, 2006). The average high temperature in July for the 30 years (1980-2010) was approximately 82°F

(27.8°C) while the average January low was 32 °F (0°C) (NOAA, 2013). The 30 years

(1980-2010) mean annual precipitation for the study area was 31.5 in. (80 cm) of precipitation per year.

1.1.3 Significance

The absence of well-developed drainage system indicates that most surface water drains into the karst. However, when recharge exceeds the storage capacity of the karst, low areas are flooded (VerSteeg & Yunck, 1932). While some sinkholes are visible, others are buried under sediment cover. Daugherty (1941) describes “Depressions not more than a foot across and a few inches deep developed into holes 20 ft (6 m) or more across and three or four feet deep during these floods”.

Many sinkholes are located in agricultural fields where fertilizers and pesticides are utilized, so the direct recharge of surface water into sinkholes is a source of groundwater pollution. Having knowledge of the location, size and extent of the concealed karst will help in zoning hazard prone areas.

1.1.4 Hypothesis

The following hypotheses were set forth at the beginning of the study:

(1). Gravity measurements can constrain the void space in the Columbus limestone north of Bellevue, Ohio.

(2). Gravity and electrical measurements can detect changes in water levels in this karst and

(3). Electrical resistivity profiles can map the ‘underground rivers’ that carry water from the Bellevue, Ohio area and discharge into Castalia’s Blue Hole and Sandusky Bay.

Geophysical probing of karst features depends on the contrast in the physical properties between the void and host rock. Gravity and electrical resistivity have been widely used in prospecting concealed karst. Both methods complement each other as gravity works best in karst filled with air or low density material and with large sized voids and it weakens as the density of the fill material nears that of the host rock, while electrical resistivity will work as long as there is a significant contrast in electrical conductivity regardless of the density of the infill material (Yuhr et al., 1993). Thus, measurements made when the water table is low and repeat measurements when the water table is high should change density and electrical conductivity of the voids in this karst.

1.1.5 Previous Studies

Several geological, hydrogeological and geophysical studies were previously conducted in the Bellevue-Castalia karst plain. The first historical records about the Blue

Hole of Castalia dates to 1760, when Major Robert Rogers of New Hampshire passed by the Castalia springs and estimated that the discharge at the spring was ten hogsheads (1 hogshead= 238.48 liters) per minute (VerSteeg & Yunck, 1932).

VerSteeg and Yunck (1932) and Daugherty (1941) report that heavy rainfall in the springs of 1913, 1930 and 1936 flooded low ground north of Bellevue, where water issued from the ground, and sinkholes acted like fountains. Both natural and man-made sinkholes were once used to dispose of waste water, and this practice led to the contamination of the carbonate aquifer. The construction of the sewage treatment facility in 1971 in Bellevue has stopped most of the pollution from sewage (Sikora, 1975).

Tintera (1980) mapped over 200 visible sinkholes and depressions in the Bellevue-Castalia region, and found that groundwater flow predominantly along the northeast direction. The flow of groundwater toward the northeast resulted in widening of the northeast oriented joints by dissolution. He also noted that the karst features have similar alignment and are limited to those areas of the Columbus Limestone where the drift thickness ranges between

20-30 feet (6-9 m).

Kihn (1988) conducted a hydrogeological study of the karst plain with an emphasis in the

Seneca Caverns. Later Drane (1993) carried hydrogeological and geophysical investigations in Thompson Township, Seneca County and found a good agreement between potentiometric lows and residual gravity low suggesting groundwater flow is controlled by solution cavities. Chaffee (1995) also showed the residual gravity is disturbed around the sinkholes and in agreement with the general trend of the potentiometric surface.

Ludwikoski (1993) studied the geomorphology of the region while Forster (1997) carried geochemical and flood pulse analysis. Carlson (1992) hypothesized that an intrastratal karst in the Bass islands region developed by a subaerial exposure and dissolution of the G evaporite during post-Silurian and pre-Middle Devonian. The intrastratal karst was exposed by glacial erosion and reactivated during the Quaternary. Dinsmore (2011) carried a spatial analysis of sinkhole distribution in the Bellevue-Castalia karst plain and his results agree with Carlson (1992). He also speculated that cycles of isostatic elevation change due to glaciations in the Quaternary resulted in enhancement of fracture permeability.

During the spring 2008 flooding, the ODNR (2009) conducted groundwater monitoring.

ODNR reported that spring discharge at Rockwell Springs Trout Club, Sandusky County, was responsive to the heavy precipitation during that season. Aden (2013) mapped 997 karst features using a digital elevation model (DEM) and available geological data, followed by field verification.

Chapter 2

Geology, Hydrogeology and Karst Review

2.1 Geology

The geology of the area spans from Precambrian basement rocks into Pleistocene sediments. Basement rocks form the Cincinnati-Findlay Arch (Herdendorf et al., 2006) that separates the Illinois and Appalachian Basins (Figure 1-1). A thin layer of Paleozoic rocks cap the crest and gets thicker farther from the flanks. Starting in the Cambrian, the sea transgressed into land and deposited different sediments depending on the depth of the waters and paleogeography. The ensuing orogenies caused the regression of the sea and erosion of the mountains and earlier deposited sedimentary rocks (Coogan, 1996).

Moreover, the lowering of sea level and evaporation formed evaporites. The Silurian formations and Mississippian rocks are the oldest and youngest rocks (respectively) exposed in the region. Uplift, weathering and erosion have removed any evidence of deposition between the late Mississippian and the Quaternary Period. At the beginning of the Quaternary, climate began to cool and a series of glaciers advanced from Canada shaping the current geomorphology of the region.

2.1.1 Bedrock Geology

The stratigraphy of the study area spans from Middle Silurian to Quaternary as shown in Table 2.1. These units (Figure 2-1) constitute the east flank of the Findlay Arch and dip South East toward the Appalachian Basin (Figure 1-1). At the base of these sedimentary formations is the Rochester Shale. The Rochester Shale is overlain by the undifferentiated Lockport formation, which ranges from whitish gray coarse crystalline and fossiliferous to brown microcrystalline dolomite (Janssens, 1977). The Greenfield

Dolomite lies unconformably on the undifferentiated Lockport formation. This dolomite is a light brown microcrystalline to very fine crystalline (ODNR, 1970) and is overlain by the Tymochtee Dolomite. The Tymochtee formation is a grayish black microcrystalline dolomite (ODNR, 1970). The uppermost Silurian Formations is the Bass Islands

Dolomite described as calcareous to argillaceous dolomite (Ulteig, 1964) and microcrystalline dolostones (Carlson, 1992).

Unconformably overlying the Bass Islands Dolomite is the Devonian Amherstburg and

Lucas Dolomite of the Detroit Group. The Amherstburg Dolomite is a massive to thick- bedded dolomite varying from clay to fine sand size in texture (Sparling, 1988). The

Lucas Dolomite lies conformably over the Amherstburg Dolomite and is thinly bedded.

Above the Detroit Group, the Columbus Limestone lies disconformably and is divided into three units starting from bottom: crinoidal mudstone, fossiliferous grainstone and

Lime mudstone (Hatfield, 1988). It crops out between Erie County in the north and the

Pickaway County in central Ohio (Figure 1-1). It has a thickness range of 18-32 m

(Judge, 1998) and chemical composition of around 90% CaCO3 (ODNR, 2009). The high 9

proportion of the calcite makes it highly susceptible to acid water dissolution and development of karst features.

Table 2.1: Stratigraphy of the Bellevue area (modified from Forster, 1997; Chaffee, 1995)

System Series Group Formation Average Thickness ft (m) Quaternary Pleistocene Wisconsin Till 20 (6) Devonian Erian Delaware Limestone 35 (11)

Ulsterian Columbus 60 (18) Limestone Detroit Lucas Dolomite 35 (11) River Amherstburg 50 (15) Dolomite Silurian Cayugan Salina Bass Islands 100 (30) Dolomite Tymochtee 150 (46) Dolomite Greenfield Dolomite 45 (14) Niagaran Lockport Undifferentiated 225 (69)

Rochester Shale 25 (8)

The Delaware Limestone is the youngest Devonian bedrock in the area consisting of crinoidal and cherty mudstone with thin shale layers, bioclastic grainstones and corals from bottom to top (Hatfield, 1988). The high silt content gives the bluish gray color hence the name “Blue Limestone”. The full stratigraphic section of both the Columbus and Delaware limestone is exposed at the Parkertown quarry (Figure 1-1). The thicknesses of the Columbus and Delaware Limestone at the quarry are 59 ft (18 m) and

49 ft (15 m) respectively (Hatfield, 1988). 10

Figure 2-1: Geologic map of the Bellevue region (USGS, 2005).

Figure 2-2: Bedrock elevation map modified from ODNR (2003).

2.1.2 Surficial Geology

A series of glaciations and deglaciations occurred in North America starting in the middle Pleistocene. The glaciers modified the landscape by scouring the bedrock (Figure

2-2) and filled low-lying areas. However, effects of earlier glaciations and deglaciations were destroyed or concealed by the most recent glaciation (the Wisconsinan glaciation).

The study area is categorized as a lacustrine ground moraine of Late Wisconsinan-Late

Figure 2-3: Drift thickness of the Bellevue area (extracted from DEM and figure 2-2).

Woodfordian ice deposits (Ludwikoski, 1993) characterized by thin clay till and silty to sandy lacustrine sediments (ODNR, 1998). The drift thickness in the area ranges from 0 to over 100 ft (30 m) (Figure 2-3).

Glacial lakes formed at the ice margins of retreating glaciers and reworking of the glacial till formed the beach ridges of Maumee III, Warren and Whittlesey (Dean et al., 1991).

Beach ridge Maumee III is a boundary between the glacio-lacustrine sediments near Lake

Erie and the till plains in the south. Glacial scouring of the carbonate bedrock also resulted in the formation of the Columbus cuesta in northern Ohio.

2.1.3 Joint orientation

Two sets of joints exist in the area with the dominant set trending between N50°E and

N70°E and few trending N35W (Dean et al., 1991). The joints were formed as a result of the arching of Findlay Arch (Verber & Stansbery, 1953). They serve as a conduit for groundwater movement and facilitate the dissolution of the limestone. Sinkholes with elongated axis have a similar orientation as the solution-widened joints. The presence of solution-widened joints at the bottom of the sinkholes explains the role of the joints in sinkhole formation (Tintera, 1980). To the south, the Seneca Cavern (Figure 1-1) fracture extends 3.2 km and trends N68°W with apparent dip of 40°NE (Ruedisili et al., 1990).

2.2 Hydrogeology

The carbonate aquifer which is approximately 720 foot thick, consists of Devonian and Silurian limestone and dolomite (Table 2.1). These carbonates are part of the regional aquifer that discharges into the Sandusky Bay. These carbonates are underlain by the

Rochester Shale, which serves as an aquitard, is overlain by Pleistocene glacio-lacustrine drift. The carbonate aquifer is unconfined, semi-confined or confined depending on the overburden thickness. The water yield characteristics of the formations are given in Table

2.2.

Table 2.2: Bedrock water yield of the study area (modified from Dinsmore, 2011; Chaffee, 1995)

System Formation or Water Yielding Characteristics Well Group Yields (gal/min) Devonian Delaware Although the entire thickness yields Limestone water, primary zones are 10-30 ft (3- 9m) below the formation Columbus Although the entire thickness yields 500- Limestone water, primary zones are 20-40 ft (6- 1000+ 12m) below the top of the formation Lucas/ Most wells produce at a depth of 40- 500-600 Amherstburg 60 ft (12-18m) below the top of the Dolomite group Silurian Bass Islands Most wells produce at 20-50 ft (6- Dolomite 15m) above the Salina Group

Tymochtee Wells yield at 30-70 ft (9-21m) above Up to 600 Dolomite the Greenfield Formation (A1- equivalent) Greenfield The zone 20-30 ft (6-9m) above the Dolomite Lockport Dolomite has producing wells Lockport Dolomite Yields minimal amounts of water. 5-100 Primary zones are 50-80 ft (15-20m) below the contact with the Salina Group (Greenfield). May be mineralized at depth.

Figure 2-4: Potentiometric surface map of the Bellevue area and its surrounding counties (ODNR, 2013a).

Figure 2-5: Drainage pattern of the Bellevue region (U.S. Census Bureau, 2011).

Groundwater flows north toward Lake Erie and is governed by the bedrock topography as indicated by a potentiometric low (Figure 2-4) coinciding with the preglacial Erigan river valley (Kihn, 1988). The average hydraulic gradient is 10 ft/mile with an increase to

40ft/mile over the Erigan River valley (Ludwikoski, 1993). The main type of flow is 17

diffuse flow, where groundwater travels through primary porosity and fracture porosity.

The diffuse flow is demonstrated by the absence of significant specific conductance at the start of storm events (Forster, 1997). Diffuse flow is also indicated by the rapid increase in static water level during episodes of precipitation followed by a slow decrease in static water level after storm events. The lack of turbidity at the springs during episodes of precipitation and the hydraulically interconnected water bearing zones at different depths show that the water flow is deep and slow (Kihn, 1988).

Recharge occurs mainly through a conduit flow via sinkholes resulting absence of surface drainage (Figure 2-5). In conduit flow, groundwater flows in turbulent mode through conduits. Groundwater is discharged through several springs around Bellevue,

Castalia, and Sandusky Bay. During periods of excessive precipitation, the water carrying conduits fill and gush water through the sinkholes, flooding low lying areas.

Moreover, perched water is common in the depressions during the rainy season.

Seasonal fluctuations in the water table at USGS monitoring well north of Bellevue are displayed in (Figure 2-6). The monitoring well (Figure 1-1) is 135 feet in depth and is completed in the Columbus Limestone (USGS, 2013). During precipitation, the well responds by a rapid rise in water table followed by a gradual drop in water table. This well was crucial in monitoring the water table conditions during the study.

Figure 2-6: Water level at USGS monitoring well north of Bellevue. The rapid rise in water table followed by a gradual decrease is typical of karst aquifer (USGS, 2013).

2.3 Karst Development

The region is characterized by a hummocky plain with karst features such as sinkholes, springs and sinking streams which are confined mainly in the Columbus Limestone subcrop covered with a thin lacustrine drift (ODNR, 1998). The karst development is polygenetic in origin which is of collapse and dissolution type with a source that lies deeper within the

Silurian formations (Carlson, 1992).

During the late Silurian, these rocks were sub-aerially exposed and the infiltration of water facilitated conversion of the anhydrites to gypsum. The expansion of anhydrites (G evaporite) to gypsum caused an increase in volume which resulted in the doming of the overlying Bass Islands Dolomite (Carlson, 1992). Eventually subrosion of gypsum formed voids. During the Quaternary period, it was reactivated in response to the glacial loading

and unloading (Carlson, 1992; Dinsmore, 2011). The collapse of the overlying formations formed caves along the margins of the collapsed block (Carlson, 1992). The absence of the

G evaporite in the Bass Islands and its presence to the south east explains that dissolution was active in the up dip section of the formation (Janssens, 1977).

The caves of Put-in-Bay and Seneca Caverns are of a collapse type in origin. At Seneca

Caverns, the caves developed in the Columbus Limestone and Lucas Dolomite. The caves lowermost depth is unknown as the lower levels are filled with water. The floor of the caves seems to fit with the ceiling similar to the South Bass Islands caves (Ruedisili et al., 1990).

The presence of underwater stalactites in Put-in-Bay caves suggests that these caves existed when the water level was low (Verber and Stansbery, 1953). But the numerous closed depressions that exist in the karst plain are likely dissolution type sinkholes (Tintera, 1980).

Chapter 3

Methodology

3.1 Gravity

Gravity is the sole geophysical method that is not influenced by the content or structure of materials that fill karst features except for a difference in density (Yuhr et al., 1993).

The typical infill materials in karst are air, water and clay which have a significant contrast in density with limestone. By measuring gravity along a profile, a horizontal

(lateral) variation in density can be detected. Once the density contrast is detected, it is possible to estimate the mass of the limestone replaced with infill material and volume of the infill material.

A LaCoste & Romberg model “G” (#1109) gravity meter that measures gravitational acceleration difference between stations was used to collect the data. Before and after every survey, the gravity base station at the Bowman-Oddy Laboratories, established in

2004, was occupied. This base station is located in the south courtyard, east of the second pillar from the door (Stierman, 2004). Observed gravity at this base station is 980235.563 mGal. A field base station was selected on a concrete driveway at Strecker Road,

Bellevue, OH and was occupied at the start and end of each field day. Gravity station’s

name, time, dial readings and location were recorded in a field book. Reading resolution of the L&R Gravimeter is ≈ 0.01 mGal, estimated to 0.001 mGal.

3.1.1 Data Reduction

Gravity depends on elevation, latitude, Earth tides, Bouguer slab density and density of the mass below the point of measurement. To determine the lateral variation in gravity due to geology, the effect of the shape of the earth, rotation, density and elevation must be accounted for. The dial reading of a balanced meter is recorded at each gravity station.

This was converted to mGal reading using the dial conversion constant supplied by the gravity meter manufacturer, where each G-meter is calibrated. Gravity effect due to the tide of the moon was computed using tide correction utility program (Mega Systems Ltd,

2013) and the resulting tide correction in mGals was subtracted from the raw data.

The drift correction was made by linearly interpolating instrumental drift between repeat measurements at the base station. Once the measurements were tide and drift corrected, the observed gravity is obtained. A terrain correction was not applied due to absence of significant topographic relief in the area.

( ) ( )

where gobs and tobs are the observed gravity and time at any station, gb2 is the observed gravity at base station at the end of the survey (tb2), gb1 is observed gravity at the base station at the start of the survey (tb1).

3.1.1.1 Latitude Correction

Gravity of the Earth increases with the increase of latitude due to the Earth’s elliptical shape and rotation. This correction corrects the influence of the earth’s gravity due its ellipsoidal shape and angular rotation. The theoretical gravity using IGF 1967 formula

(Bremaecker, 1985) is reduced from the observed gravity.

[ ( ) ( )]

where φ is the latitude in degrees.

3.1.1.2 Free Air Correction

This corrects the decrease of gravity from as the elevation increases. This correction does not include the earth material between sea level and the gravity station. The elevation of each gravity station was obtained from OSIP’s Digital Elevation Model obtained from Lidar survey (OGRIP, 2013). These data have a 2.5ft*2.5ft spatial resolution and ± 1 foot vertical accuracy with RMSE of 0.5 foot at 95% confidence interval (OIT, 2007). The following formula is used to determine the gravity effect due to elevation:

where h is the elevation in meters from the mean sea level.

3.1.1.3 Bouguer Density Correction

This correction is applied to compensate the pull of gravity on the station by the mass of the Earth between the geoid and the gravity station. The density is assumed as 2670 kg/m3. This is calculated with:

where G is the universal gravity constant (6.67*10-11 N·m2·Kg-2), h is elevation above mean sea level and ρ is density of the slab.

3.1.1.4 Simple Bouguer Anomaly

Once the latitude, free air and Bouguer corrections were applied on the observed gravity, a simple Bouguer anomaly was obtained. It encompasses gravity due to both shallow and deeper sources.

where Gobs is the observed gravity, Gref is the theoretical gravity, FAC is the Free-Air

Correction and BC is the Bouguer slab factor.

3.1.2 Regional - Residual Gravity Separation

The simple Bouguer anomaly obtained above consists of gravity effect due to deep earth structures and shallow structures of interest. Gravity due to the deep structures is called regional gravity and is characterized by a longer wavelength while the gravity due to the local structures is called a residual gravity is represented by a short wavelength

anomaly. Figure 3-1 shows a gravity signature of a buried cylinder with the same radius

(a = 2m) and density (air-filled) varying from narrower to broad wavelength with increasing depth (from 10 to 40m). The regional gravity masks the gravity field of shallow structures of interest and has to be removed. In this study, to separate the residual gravity, a polynomial least squares fitting was attempted. However, it caused an overestimation of the residual gravity because the gravity anomaly continues in north- south direction of the study area. Thus, sixteen east-west profiles at 150m spacing were plotted and the regional gravity was isolated by graphical smoothing. The residual gravity was determined by subtracting the regional gravity from the simple Bouguer gravity as follows:

3.1.3 Gravity Modeling

A model was used to test feasibility of void detection using gravity. A buried horizontal cylinder of equal radius and equal depth filled with air and water was used to test variations in gravity. The difference in gravity response between each scenario is due to a change of density of the infill material (Figure 3-1). Comparison of gravity anomaly of the same buried horizontal cylinder due to a change in infill material (from air to water) versus error in gravity that may arise from use of Lidar derived digital elevation model (DEM) and handheld GPS receiver to extract elevation for free air correction was also made. The model shows an error in elevation of +1m due to uncertainties from DEM and GPS receivers would not overshadow the gravity change that result from a change in the infill materials (Figure 3-1). 25

Figure 3-1: Gravitational field plotted versus horizontal distance over a buried horizontal cylinder. It shows changes in gravity due to a change in density, depth and error in elevation.

( ( ))

where G is the universal gravity constant, ρ is density, z is the depth to the center of the cylinder, a is the radius and x is the distance from a point on the surface right above the center of the cylinder (Telford et al., 1990, pp. 37).

3.1.4 Void Volume Estimation

To estimate the void spaces within the karst, Gauss’s law of anomalous mass was applied. This method uses the Gauss’s mathematical theorem and calculates the anomalous mass from the residual gravity (Grant and West, 1965). It shows a direct relationship between the residual gravity and the mass of the body causing a gravity anomaly. However it doesn’t provide information about the nature of the geologic body and is independent of depth (LaFehr, 1965). The residual gravity is divided into a grid of cells each of area a, and average residual gravity in each cell is determined. The anomalous mass is calculated as follows:

∑

where M is the anomalous mass, G is universal gravity constant, and g is residual gravity anomaly over area a (Grant and West, 1965, pp. 269-273). The estimation of the missing mass enables the calculation of the volume of the deficient mass using

( )

where V is the volume of the deficient mass, M is the anomalous mass, ρc and ρi are density of the country rock and infill material respectively.

3.1.5 Repeat Gravity Measurement

To detect water table elevation changes within the karst, microgravity survey was carried at Strecker road when the water table elevation was low and high respectively. During the initial measurement, the stations were marked with flags. However, the flags were missing during the repeat measurement and tape measurements from known markers were used to locate the gravity stations. A USGS monitoring well located 2.5 km south of

Strecker road was used to monitor the water table fluctuations. Gravity measurement when the water table is high is expected to show a higher gravity than it was during low water table.

3.2 Electrical Imaging

Electrical resistivity work well where there is a significant contrast in electrical conductivity between the limestone and the infill materials in the karst [e.g., Roth et al.,

2000; Zhou et al., 2000; Kruse et al., 2006]. While water and clay have high electrical conductivity compared to limestone, air-filled void can have similar electrical conductivity to a massive limestone that lacks moisture. Electrical conductivity depends on porosity, presence of ions, electrolytes and water saturation. Typical resistivity values of some Earth materials are given in Table 3-1.

Electrical resistivity measurement is based on Ohm’s law. Ohm’s law states that a current flowing between two points is directly proportional to a difference in voltage between the points.

where, I is the current, V is the voltage difference and R is the resistance of the medium.

Current is injected to the ground and the change in voltage is measured. The injected current to the surface is known and the voltage is measured by the resistivity meter.

Using Ohm’s law, the resistance of the medium is obtained. The resistance of an object depends on the resistivity of the material it is composed of, distance the current travels and shape of the object (Figure 3-2).

Figure 3-2: Geometry of a uniform cylinder with length L, cross-sectional area A and electrical resistivity of ρ.

where ρ, L and A are the resistivity, length and area of the material respectively. The unit of resistivity is ohm-meter (Ωm).

The instrument that measures electrical resistivity is made up of a D.C. power source, voltmeter and two pairs of electrodes (Figure 3-3). Current is injected to the ground through electrode A and flows toward electrode B. The voltmeter measures the voltage difference between electrodes M and N.

In resistivity prospecting, the apparent resistivity at the surface is measured. This assumes

a homogenous Earth. For a given ratio (both measured), apparent resistivity depends on

the electrode configuration.

where ρa is the apparent resistivity, G is the geometric factor of the electrode array, V is the voltage drop and I is the current.

Figure 3-3: A simple diagram illustrating electrical resistivity measurement setup. Electrode A is the current source and electrode B is the current sink, while M and N are potential electrodes.

There are different types of electrode configurations. Some of them map solely vertically or horizontally while others do both. Mapping a vertical section is called sounding. It is carried by moving away potential and current electrode from the midpoint of the survey line. Profiling is the lateral mapping of the subsurface and it involves moving the electrodes at a constant spacing along the profile line. By employing multi-electrodes both vertical sounding and horizontal profiling can be acquired at the same time and give

a good representation of vertical and lateral heterogeneity. Such a method is called a 2D resistivity array. The selection of a method depends on the target of the study.

One of the 2D array electrode configurations is the dipole-dipole (Figure 3-4). In this method, electrode dipoles have an equal spacing and a switching device determines which four sets of the electrodes will be used for each measurement. In this study, the dipole-dipole array was used. The equation below calculates the apparent resistivity of a dipole-dipole measurement.

( )( )

Figure 3-4: A general set-up of a dipole-dipole resistivity array. a is the electrode spacing.

A number of studies have used dipole-dipole array in mapping karst and it showed good spatial resolution (Labuda and Baxter, 2001; Neawsuparp and Soisa, 2007; Zhou et al.,

2008). It also clearly defined the bottom and structure of the sediment filled sinkholes.

The smaller the particle size of the sediment, the more effective was resistivity in outlining the structure of sinkholes (Neawsurparp & Soisa, 2007).

Table 3.1: Resistivity of some earth materials (Telford et al., 1990).

Material Resistivity range (Ωm) Unconsolidated wet clay 20 Clays 1-100 Soil water 100 Dolomite 3.5*102-5*103 Limestone 50-107

In our study, Advanced Geosciences, Inc. SuperSting R1 resistivity meter (Figure 3-5)

and a Swift automated electrode system equipped with 28 electrodes was used. The

electrode separation was decided after acquiring resistivity signature at the Seneca

Caverns (Figure 1-1). The dipole-dipole resistivity data were inverted using a least

squares resistivity inversion method (RES2DINV program, ver. 3.3 (Loke, 1998)) to

generate the resistivity profiles which are color contoured (Figure 3-6).

Figure 3-5: AGI Supersting resistivity meter, switching box and cable/electrode connections.

Figure 3-6: Inversion of a dipole-dipole electrical profile. (a) Shows the measured apparent resistivity pseudosection, (b) is the calculated apparent resistivity pseudosection generated using a model that fits the measured apparent resistivity pseudosection and (c) shows the inverted resistivity section based on the best fit between the measured apparent resistivity and the calculated apparent resistivity. Horizontal distance is in meters.

3.2.1 Repeat resistivity measurement

To monitor temporal changes in subsurface resistivity, a dipole-dipole resistivity profile measurement was made at Strecker road during low and high water table. During the first measurement, the road was dry (Figure 3-7) but during the repeat measurement, there was ponded water in the fields (Figure 3-8). However, an empty sinkhole (Figure 3-

9) adjacent to the profile and ponded water suggests that the surface water is not indicative of the water table.

Figure 3-7: Photo of Strecker road during first resistivity measurement in October 2012.The fields to the right we dry.

Figure 3-8: Photo of ponded surface water in a field adjacent to Strecker road during repeat resistivity measurement in August 2013. 34

Figure 3-9: An empty sinkhole next to ponded surface water in Figure 3-8.

3.3 Electrical resistivity signature of Seneca Caverns

In order to obtain a geophysical signature that can be used to interpret measurements in areas that have not been previously probed, a dipole-dipole resistivity measurement was done over known fracture type cave, the Seneca Caverns. The dipole-dipole resistivity showed approximately 5 meter thick drift cover (blue) overlying the limestone bedrock

(green). The bedrock contains several pockets of high resistivity zones (yellow-red) and a low resistivity zone (deep blue) located between 22-50 m at a depth of 11-19 m, interpreted as air- and water-filled cavities respectively (Figure 3-10). Analysis of the data gave information needed to design electrode appropriate for prospecting for concealed karst north of Bellevue.

Figure 3-10: Dipole-dipole resistivity profile at the Seneca Caverns and its interpretation. Horizontal distance is in meters.

3.4 Well Logs

Well logs from ODNR were reviewed to investigate the subsurface geology. Although the well logs were prepared by drillers, they can be used to determine the bedrock elevation. Ten well logs in the area documented broken and porous limestone, and limestone with mud seam (Figure 3-11 and Table 3.2-4).

Table 3.2: Well log reporting mud seam within the carbonate bedrock (ODNR, 2013b). Drift Mud Well Easting Northing Thickness Thickness Depth to Top of ID No (m) (m) (m) (m) Clay/Mud(m) A 412098 346779 4573220 3.0 0.3 13.7

Table 3.3: Well logs reporting broken limestone within the carbonate bedrock (ODNR, 2013b). Drift Broken Depth to top of the Well Easting Northing Thickness Limestone Broken Limestone ID No (m) (m) (m) (m) (m) B 417893 346957 4575045 6.1 4.0 6.1 C 762407 346883 4576123 5.2 4.3 13.1 D 964303 344233 4573647 3.0 3.0 3.0 E 399069 347128 4576748 3.0 3.0 3.0 F 459001 346904 4576700 7.6 7.6 4.6 G 482741 347342 4576587 7.0 7.0 4.3 H 514082 347440 4576575 7.6 7.6 4.6

Table 3.4: Well logs reporting contain porous Limestone within the carbonate bedrock (ODNR, 2013b). Depth to Top of Drift Porous Porous Well Easting Northing Thickness Limestone Limestone/Mud ID No (m) (m) (ft) Thickness (m) (m) I 59764 345756 4573622 4.0 0.3 22.6 J 40351 344321 4574298 12.5 3.0 12.5

Figure 3-11: Wells with logs that documented broken, porous and mud seam containing limestone (Table 3:2-4)

Chapter 4

Results

4.1 Gravity

346 newly acquired gravity stations (Figure 4-1) were used to prepare a Bouguer gravity anomaly map (Figure 4-2). Appendix A shows the coordinates, elevation and observed gravity for each station. The measurements were focused in areas that were mapped as suspect sinkholes by Aden (2013) with microgravity survey at State Route

269, Strecker and Southwest roads at spacing that ranges between 20-50 m. The survey was largely restricted to roads due to lack of broader accessibility as data were collected during the growing season.

4.1.1 Simple Bouguer gravity

The Bouguer gravity shows a regional decrease from -49 mGal to -57 mGal in SE-

NW direction. However as it approaches the contact of Columbus and Delaware limestone from the east, it trends north-south until it reaches the center of the Columbus limestone and then shifts to Northeast-Southwest (Figure 4-2). It has a west-east gradient of 0.357 mGal/km. The gravity low in the northwest of the study area beyond the contact 39

of Lucas dolomite and Columbus limestone corresponds to thicker overburden sediments as drift thickness increases toward Lake Erie.

Figure 4-1: Map of new gravity stations.

4.1.2 Residual gravity

Residual gravity was determined using graphical smoothing of a series of E-W

Bouguer gravity profiles. Sixteen east-west cross-sections were plotted at a 150 m northing difference to separate the regional gravity. Residual gravity ranges from 0.20 to

-1.04 mGal (Figure 4-4). It is lowest at the center of the study area within the Columbus 40

limestone and it approaches zero mGal near the contact of the Columbus limestone with both Lucas dolomite and Delaware limestone. The negative residual anomaly is also seen in the northwest of the study within the Lucas dolomite. Six of the east-west cross sections are shown in Figure 4-3.

Figure 4-2: Bouguer gravity contour map.

Figure 4-3: Elevation and Bouguer gravity of six of the several E-W profiles along the estimated regional gravity. Line 1 is south of Co Rd 205 while lines 2 and 3 are between Co Rd 205 and Bilman Rd. Line 4 is adjacent to and crosses Bilman Rd and Line 5 crosses a topographic depression at State Route 269. Line 6 is adjacent to Strecker Rd. The low elevation at Line 6 is Parkertown quarry.

Figure 4-4: Residual gravity of the study area.

4.1.3 Microgravity

Microgravity surveys were conducted at Strecker Rd, State Route 269 and Southwest

Rd. At State Route 269, the measurement was focused on a topographic depression that is

4 km2 in area and believed to be a large sinkhole complex. This sinkhole complex appears to extend from Strecker road west to Southwest road. Removal of the regional gravity showed gravity low at all the three profiles with the maximum amplitude of -0.26 mGal detected in State Route 269.

Four prominent residual gravity lows are detected in state route 269 (Figure 4-5). Gravity low I extends for 580m along the road and has maximum amplitude of ~ -0.20 mGal and is related to a topographic depression and is followed by gravity low II which runs for 43

780 m with maximum amplitude of -0.26 mGal and associated with topographic depression. Well logs adjacent to gravity low I show an overburden thickness ranging between 0 and 15 ft (4.5 m). Active and suspect sinkholes are mapped adjacent to this gravity low. Gravity low III has a maximum amplitude of -0.135 mGal and has a width of

195 m. Gravity low IV of -0.076 mGal that extends for 175 m is detected southwest of gravity low III.

Four gravity lows are mapped at Strecker road (Figure 4-6). Gravity low V with maximum amplitude of -0.075 mGal is adjacent to a tree patch that has sinkholes and a spring (Figure 4-7). Gravity low VI has maximum amplitude of about -0.085 mGal and is related with sinkholes adjacent to the road. Gravity low VII which is the larger gravity low has a -0.167 mGal in maximum amplitude. It drops -0.1 mGal within a distance of 20 m. Repeat measurement gave the same result. Although the road has a higher relief, the field adjacent to this gravity low has low elevation. Another prominent gravity low is gravity low VIII. It has a width of 600 m and maximum amplitude of -0.14 mGal.

Another gravity low (gravity low IX) with maximum amplitude -0.145 mGal and 850 m in width is detected (Figure 4-8) at intersection of Southwest and Bilman roads. The gravity low is associated with a topographic low where active sinkholes are present.

Figure 4-5: Microgravity profiles along State Route 269. (a) South of intersection with Strecker Rd while (b) is north of intersection with Knauss Rd, (c) is between intersections with Knauss and Hale roads.

Figure 4-6: East-west microgravity profile along Strecker road.

Figure 4-7: Spring adjacent to Strecker road near gravity low IV. Photo is taken looking Southwest in June 2013.

Figure 4-8: Microgravity profile along Southwest road.

4.1.4 Volume of Voids

With the objective of understanding the extent of dissolution within the carbonate bedrock, Gauss’s theorem of anomalous mass (Grant and West, 1965) was used to estimate mass deficit from the negative residual gravity.

∑

where M is the anomalous mass, G is universal gravity constant, and g is residual gravity anomaly over area a (Grant and West, 1965, pp. 269-273). The negative residual gravity was gridded into 12158 cells of equal area (50m*50m) and the average residual gravity per cell is calculated. The sum of the average residual gravity in 12158 cells is

∑

Figure 4-9: Map showing distribution of mass calculated from the residual gravity. The cell size is 50m*50m. The boundary of the mass deficit polygon is a zero residual gravity contour.

A total mass deficit of 3.17*108 metric tons was found. The distribution of the missing mass in cells of 50m*50m in size is shown in Figure 4-9. The deficit mass in return is used to calculate the volume of the missing mass. A density contrast of 2669, 1670 and

460 kg/m3 was assumed for air, water and clay filled voids respectively. The volume of the infill material that is responsible for the negative residual gravity ranges between 0.11 and 0.69 km3 depending on the density of the material (Figure 4-10 and Table 4.1).

Table 4.1: Calculated range of void volume along the density contrast of the infill materials. Fill Material Density contrast (kg/m3) Void Volume (km3) Air 2669 0.12 Water 1670 0.19 Clay 460 0.69

Figure 4-10: Map showing the volume of carbonate bedrock that is missing due to the dissolution. Cell size is 50m*50m. The boundary of the volume of voids polygon is a zero residual gravity contour.

4.1.5 Repeat gravity measurement

A repeat gravity measurement (Figure 4-11) was carried along Strecker road when the water table at the USGS monitoring well was ~ 13 ft (4 m) higher than it was during the 49

first measurement. Flags put at the station during the initial measurement were missing and a tape measurement was used to locate the locations. However, repeat measurement when the water table was high showed a decrease in gravity.

Figure 4-11: Gravity measurement at Strecker road when the water table was low (June) and high (August).

Ten gravity measurements were collected at a field base station located at Strecker road during summer 2013 at irregular intervals. Table 4.2 shows the measured Bouguer gravity at the field base station along respective water table level at the monitoring well.

The difference in Bouguer gravity between consecutive surveys was used to determine changes in gravity due to water table fluctuations at the field base station. Six of the nine changes in gravity between consecutive measurements showed a similar trend with water table variations at the monitoring well. In other words, an increase or decrease in the measured gravity was related with an increase or decrease in water table respectively at the monitoring station. However, the rate of change in gravity relative to water table at the monitoring station was not uniform. The rest have shown poor correlation.

Gauss’s potential theorem (Grant and West, 1965) was applied to estimate the gravity- derived mass and volume change of groundwater (Figure 4-12b-d) beneath the base station and its surroundings. In a similar manner as in section 4.1.4, a cell of area

50m*50m where the field base station is located was used as the areal extent to calculate the gravity-derived change in mass and volume of groundwater. The summary of the result is given in

Where ( )

( )

∑

The calculated change in mass (Ma) between consecutive surveys is then used to calculate the change in volume (Va) as follows.

( )

Figure 4-12: Temporal water table and gravity changes between consecutive surveys with calculated mass and volume changes at a field base station at Strecker road in Bellevue (a) Change in water table at the monitoring well, (b) changes in observed gravity, (c). Change in mass, and (d) Change in volume of water at the field base station. 52

Table 4.1: Summary of the gravity, mass and volume changes between consecutive measurements at Strecker field base station. Observed Water Gravity Table Δg 3 Survey Date (mGal) (m) (mGal) ΔWT (m) Ma(ton) Va(m ) 1 6/17/2013 -54.224 207.2 2 6/18/2013 -54.238 207.1 -0.014 -0.1 -847 -507 3 6/19/2013 -54.215 207.1 0.023 0.0 1388 831 4 6/24/2013 -54.341 206.8 -0.126 -0.3 -7520 -4503 5 7/16/2013 -54.243 211.0 0.097 4.2 5812 3481 6 7/30/2013 -54.216 212.9 0.027 1.8 1613 966 7 7/31/2013 -54.232 212.8 -0.015 -0.1 -913 -546 8 8/31/2013 -54.226 211.3 0.006 -1.5 347 208 9 9/7/2013 -54.372 210.7 -0.146 -0.6 -8696 -5207 10 9/8/2013 -54.348 210.6 0.023 -0.1 1392 834

4.2 Electrical Resistivity

Four dipole-dipole electrical resistivity profiles with three traverses trending E-W and one trending N-S along the Strecker and Hale roads were conducted (Figure 4-13). The resistivity surveys were mainly restricted to roads due to lack of access to fields under cultivation. The E-W profiles were selected on the assumption that most subterranean karst streams run south to north. One dipole-dipole profile at Strecker road was repeated to investigate changes with water table changes. The total length of the profiles is 2 km.

The 2D resistivity inversion at Strecker road when the water table was low (Figure 4-14a) can be categorized into blocks of high resistivity (red-yellow), a medium resistivity zone

(green) transitioning to a low resistivity zones (deep blue to light blue). Repeat measurement (Figure 4-14b) when the water table was high, showed a decrease in depth to a low resistivity zone near the base of the zone investigated by our electrode array.

At Hale road (Figure 4-15), a continuous block of high resistivity was imaged below the low resistivity zone near the surface. This high resistivity zone transitions to a moderate resistivity zone followed by a low resistivity region. Two dipole-dipole resistivity measurements made north of Strecker West Road showed a pocket of high resistive zone located near a sinkhole, and, zones of low and moderate resistivity (Figures 4-16 and 4-

17).

Figure 4-13: Dipole-dipole electrical resistivity profiles locations.

Figure 4-14: Dipole-dipole inversion results at Strecker road: (a) measurement in October 2012 when the static water level at USGS monitoring well was at 9.5m below the surface, (b) Repeat measurement in August 2013 when the water table was at 5.5m below the surface, and (c) interpretation of the dipole-dipole result along lithologic stratigraphy.

Figure 4-15: Dipole-dipole resistivity at Hale road taken in Jun 2012. The static water level at the nearby USGS monitoring well was at 9.7 m below the surface.

Figure 4-16: Resistivity inversion profile and interpretation north of Strecker road west.

Figure 4-17: A north-south resistivity inversion profile and interpretation north of Strecker road west.

Chapter 5

Discussion

5.1 Gravity

To separate the gravity due to shallow structures, the regional gravity was removed from the Bouguer gravity. The Bouguer anomaly trends north-south and it decreases from east-southeast to west-northwest (Figure 5-1). The gravity anomaly extends to the north and south of the study area and using least squares polynomial fitting to remove the regional gravity was not practical due to the concentration of data points along roads. Thus, sixteen east-west profile lines at 150 m interval were used to remove the regional gravity using graphical smoothing.

Separation of the regional gravity showed a large swath of negative anomaly showing the spatial extent of missing mass due to dissolution. The residual gravity ranges between 0.2 and -1 mGal. Most of the negative residual gravity is within the

Columbus limestone (Figure 4-4). However, the gravity low also continues to the

Lucas dolomite in the northwest. This can be explained by the increased drift thickness in the northwest of study area reaching up to 90 ft (Figure 5-5). The gravity low corresponds well with the distribution of sinkholes (Figure 5-4).

Nine gravity lows were delineated from the microgravity survey at State Route 269,

Strecker and Southwest roads. Gravity low I (Figure 4-5a) which extends for 580 m at

State Route 269 has maximum amplitude of -0.2 mGal and is associated with a topographic depression. Gravity low II (Figure 4-5a) which is also in State Route 269 is associated with a significant topographic depression, part of a feature interpreted as a sinkhole complex 4 km2 in area (Figure 5-2). Gravity low III (Figure 4-5b) is located south of gravity low I. A fourth gravity low, gravity low IV (Figure 4-5c) is located between the intersections of State Route 269, Knauss and Hale roads (Figure 5-3). The location of this gravity low matches with the location of the suspect sinkholes mapped by Aden (2013).

At Strecker road, four gravity lows were detected (Figure 4-6). Two of them (gravity lows V and VI) are close to sinkholes near the road. Gravity low V could be related to a group of sinkholes and a spring located south of it (Figures 5-2 and 4-7). At gravity low VI, two suspect sinkholes and one verified sinkhole exist on both sides of the road

(Figure 5-2). There is a topographic depression adjacent to gravity low VII while the fourth gravity low, gravity low VIII lies on an extension of the topographic depression at Strecker road west. Gravity low IX is delineated near the intersection of Southwest road and Bilman road (Figure 4-8), and a northeast-southwest trending active sinkholes cross the gravity low (Figure 5-2).

Figure 5-1: Simple Bouguer gravity and sinkholes overlain over overburden thickness. Sinkhole data from Aden (2013).

Using Gauss’s theorem of anomalous mass, 3.17*108 Metric ton of mass deficit within an area of 36.7 km2 was calculated from the negative residual gravity within the carbonate bedrock. The mass deficit is the amount of the carbonate bedrock that was removed by dissolution and it was used to estimate the volume of the voids using

3 density contrast of air, water and mud which ranges between 0.12 and 0.69 km .

Figure 5-2: Map showing location of gravity lows II, V-IX and sinkholes on a topographic depression that extends between Strecker road west and Southwest road. Sinkhole data from Aden (2013).

Repeat gravity measurement at Strecker road resulted in lower gravity during the wet season (Figure 4-11) with an average difference between initial and repeat measurements of -0.017 mGal. The difference between the surveys is uniform between stations and the lower gravity during the high water could be attributed to

either the change in gravity was close to the gravity meter resolution or error in elevations of stations as flags put during the initial measurements were missing during the repeat measurement. Location of the stations was determined using tape meters from known land markers, and a 10 cm change in elevation would result in

0.02 mGal change.

Figure 5-3: Location of gravity lows I, III and IV and sinkholes at topographic depression near the intersection of State Route 269 and Knauss road. Sinkhole data from Aden (2013).

Figure 5-4: Sinkholes overlain over residual gravity. Most of the sinkholes lie over the negative residual gravity. Sinkhole data from Aden (2013).

The field base station at Strecker road showed a change in gravity that is similar to the change in the water table at the monitoring well (Figure 4-12a, b). The trend of gravity change between six consecutive surveys correlated well with the reported water table changes at the USGS monitoring well. However, direct comparison of the changes in volume of water estimated from the observed gravity and water table level was not possible. The monitoring well is located 2.5 km from the field base station and it is difficult to infer the water table conditions at the base station due to a heterogeneous karst development. Moreover, there is no knowledge of storativity of the aquifer below the base station.

Figure 5-5: Overburden thickness (ft) from well logs (ODNR, 2013b) overlain over the residual gravity.

5.2 Electrical resistivity

A total of 2 km of dipole-dipole electrical resistivity profiles with electrode spacing of 8 or 10 m were collected along four lines. A repeat measurement was taken along one line when the water table was higher (Figure 4-14b). The dipole-dipole profiles show that the low resistivity zones are below the Columbus Limestone within the

Detroit Group formations (Figure 4-14c).

The dipole-dipole imaging at Strecker road can be classified as 8 m thick drift cover over a massive limestone and fractured bedrock. Repeat measurement during the wet season showed temporal variation of resistivity. Repeat measurement (Figure 4-14b)

showed that the high resistive section of the profile between 0 and 360 m shrunk from below 54 m to 28 m below the surface and was replaced by intermediate resistivity suggesting the presence of fractured bedrock. The rise of the water table caused it to decrease in resistivity. Between X=330 and 350m, the low resistivity (13-69 Ωm) rose up to 32 m below the surface (Figure 4-14). This resistivity decrease is interpreted as saturated sediment karst. Another decrease in depth to low resistivity between X=520 and 540 m was also noted where it reaches up to 40 m below the surface. The depth of saturated sediment varies up to 8 m within a distance of 170 m inferring that the water table variation could be monitored using electrical resistivity.

Once good locations for monitoring are determined, closer electrode spacing would improve depth resolution. Moreover, three developed and one incipient sinkhole are delineated (Figure 4-14c) from the dipole-dipole imaging at Strecker road. The developed sinkholes are located between 260 and 280 m, 330 and 350 m, and 520 and

540 m while the incipient sinkhole is between 420 and 440 m.

At Hale road (Figure 4-15), the high resistivity block is interpreted as a competent rock though it might have some air-filled voids and the low resistivity zones as water or clay filled voids. Another two profiles were run north of Strecker road west (Figures

4-16 and 4-17). The low resistivity represents the drift cover while the high resistive pocket is interpreted as air-filled void while the moderate resistivity zones could be porous limestone as four well logs from the vicinity of the profiles documented the presence of broken limestone at a depth range of 6 -12.2 m (Table 3:2-4 and Figure 3-

11) which is in good agreement with intermediate resistivity section (Figure 4-16).

Chapter 6

Conclusion and Recommendations

6.1 Conclusion

The goal of the study was to delineate concealed sinkholes and conduits using gravity and electrical resistivity within the Columbus limestone subcrop north of Bellevue, OH.

For this purpose, gravity and dipole-dipole electrical resistivity data were collected.

Repeat measurement was also carried using both methods to monitor temporal variations in water table.

Gravity data were used to delineate nine gravity lows and estimate karstic void volume using Gauss’s theorem of anomalous mass. Moreover, gravity measurements at the field base station detected water table variations within the karst. Electrical resistivity has delineated sinkholes and fractured zones and shown the low resistivity zones are related with moisture content. The fractured zones correlate with depths of adjacent well logs that contain broken limestone. However, it was not possible to distinguish between water filled and saturated clay filled voids. The repeat dipole-dipole measurement when the water table was high enabled distinction between massive and fractured limestone. Both

gravity and dipole-dipole electrical resistivity methods showed a good agreement as three sinkholes detected by resistivity at Strecker road fall within the gravity lows.

The study was limited to roads due to accessibility as the fields were planted during the survey season. The gravity monitoring lacked accurate information on the water table level since the monitoring well was located 2.5 km from the field base station, making the comparison of the water volume calculated from the observed gravity with the water table condition of the field base station difficult. Moreover, drought of summer 2012

(NOAA, 2012) delayed the repeat survey.

The finding from this study supports that the karst development lies deeper than the

Columbus limestone and the absence of surface expression of karst to the west of the study area can be explained by the presence of thick drift cover. Changes in gravity at the field base station seem to be associated with the hydrological mass changes. The calculated volume of the void spaces shows the extent of bedrock dissolution. The dipole- dipole proved to be a good method to map sinkholes by delineating an outline of the low resistivity zone while the repeat measurement was helpful to discern between massive and fractured bedrock, and as well as between dry and saturated sediment fill.

The study achieved the objectives of delineating void spaces and monitoring hydrologic variations but no ‘underground rivers’ were detected by the dipole-dipole electrical resistivity. The likely reason is that the ‘rivers’ lie below the depth of penetration of the dipole-dipole electrical resistivity. Knowledge of the extent of the mass deficit can be used to delineate areas prone to hazard.

6.2 Recommendations

The study was conducted during a farming season and was limited to roads. To expand the study off-roads, the following suggestions should be considered. Future studies should require permission to work in fields and private properties before spring planting and after harvest. A field base station has to be established outside the karst region where the effect of mass changes associated with water table variation is assumed minimal. A

G-meter with an aliod electronic feedback system that has a better resolution and accuracy can give a reliable measurement of water table variations.

Moreover, a water table monitoring has to be applied adjacent to locations where repeat measurements are carried to detect local water table variations. In this study, the monitoring well used was located 2.5 km away from where the repeat gravity and electrical measurements were carried. Although, the monitoring well was helpful in showing the water table variations in the area, the inferred water table from the electrical resistivity profiles was deeper than the water table measurements at the well.

References

Aden, D. J. (2013). Karst of the Bellevue Quadrangle and portions of the Clyde and Castalia Quadrangles, Ohio. 59 maps. Ohio Department of Natural Resources, Division of Geological Survey, (Open-File Report 2013-1).

Ahmed, S., and Carpenter, P. J. (2003). Geophysical response of filled sinkholes, soil pipes and associated bedrock fractures in thinly mantled karst, east-central Illinois. Environmental Geology, 44(6), 705-716.

Bremaecker, J. C. D. (1985). Geophysics: The Earth's Interior: Krieger Publishing Company, 342pp.

Carlson, E.H. (1991). Minerals of Ohio. Ohio Division of Geological Survey Bullettin 69, 1–155.

Carlson, E. H. (1992). Reactivated interstratal karst-example from the Late Silurian rocks of western Lake Erie (U.S.A). Sedimentary Geology, 76(1992), 273-283.

Chaffee, M. C. (1995). Geophysical prospecting of karst features in the Bellevue- Castalia region of North-Central Ohio. (Master's Thesis), University of Toledo, 101pp.

Coogan, A. H. (1996). Ohio's surface rocks and sediments. Fossils of Ohio: Ohio Division of Geological Survey Bulletin, 21pp.

Daugherty, R. R. (1941). The Geography of the Bellevue Area. Ohio Journal of Science, 41(5), 366-379.

Dean, S. L., Kulander, B. R., Forsyth, J. L., and Tipoton, R. M. (1991). Field Guide to Joint Patterns and Geomorphological Features of Northern Ohio. Ohio J. Sci., 91(1), 2-15.

Dinsmore, M. A. (2011). Origin and Evolution of Sinkholes on the Bellevue-Castalia Karst Plain, North-Central Ohio. (Master's Thesis), University of Akron, 139pp.

Drane III, L. A. (1993). Hydrogeology and Geophysics of Karst Terrain in Thompson Township, Seneca County, Ohio, with Investigation of the Ground-water Non- 68

point Source Pollution from Bacteria, Nitrates, and Pesticides. (Master's Thesis), University of Toledo, 200pp.

Forster, K. M. (1997). A Geochemical and Flood Pulse Analysis Through Continuous Electronic Monitoring of Springs in the Bellevue-Castalia Area, North-central Ohio. (Master's Thesis), University of Toledo, 162pp.

Grant, F. S., and West, F. W. (1965). Interpretation theory in Applied Geophysics: McGraw-Hill, 583pp.

Hatfield, C. B. (1988). Middle Devonian Carbonate Rocks and Shales of North-Central Ohio. Ohio J. Sci., 88(1), 18-22.

Herdendorf, C. E., Klarer, D. M., and Herdendorf, R. C. (2006). The Ecology of Old Woman Creek, Ohio: An Estuarine and Watershed Profile (2nd edition). Ohio Department of Natural Resources, Division of Wildlife, Columbus, Ohio, 452pp.

Janssens, A. (1977). Silurian rocks in the subsurface of northwestern Ohio, Ohio Department of Natural Resources, Division of Geological Survey Report of Investigations, 96pp.

Judge, S. (1998). Stratigraphy, sedimentology, and paleontology of the Bellpoint member of the Columbus limestone (Devonian), Central Ohio. (Master's Thesis), Ohio State University, 84pp.

Kihn, G. E. (1988). Hydrogeology of the Bellevue-Castalia area, North-Central Ohio with emphasis on Seneca Caverns. (Master's Thesis), University of Toledo, 163pp.

Kruse, S. E., Grasmueck, M., Weiss, M., and Viggiano, D. (2006). Sinkhole structure imaging in covered karst terrain. Geophysical Research Letters, 33(LI64056), 1-6.

Labuda, T. Z., and Baxter, A. C. (2001). Mapping Karst Conditions Using 2D and 3D Resistivity Imaging Methods Symposium on the Application of Geophysics to Engineering and Environmental Problems 2001 (pp. GTV1-GTV1).

LaFehr, T. R. (1965). The Estimation of the Total Amount of Anomalous Mass by Gauss's Theorem. J. Geophys. Res., 70(8), 1911-1919.

Loke, M. H. (1998). Res2dinv ver 3.3, Rapid 2D resistivity & IP inversion using the least-squares method (Wenner, dipole-dipole, pole-pole, pole-dipole, Schlumberger, rectangular) on land, underwater and cross-borehole surveys, released by Advanced Geosciences, Inc., Austin, Texas.

Ludwikoski, D. J. (1993). Geomorphology of parts of Sandusky, Seneca, Erie, and Huron Counties, Ohio. (Master's Thesis), University of Toledo, 219pp.

Mega Systems Ltd. (2013). Tide Correction Utility. from http://www.megsystems.ca/webapps/tidecorr/tidecorr.aspx

Neawsurparp, K., and Soisa, T. (2007). Application of resistivity survey to investigate sinkhole and karst features in southern Thailand. Proceedings of International Conference on Geology of Thailand: Towards sustainable Development and Sufficiency Economy, 19-24.

NOAA. (2012). Summer 2012 Drought Update. http://www.ncdc.noaa.gov/news/summer-2012-drought-update

NOAA. (2013). Annual climatological Summary, Sandusky OH. http://www.ncdc.noaa.gov/cdo- web/datasets/ANNUAL/stations/COOP:337447/detail

ODNR. (1970). Ground water Planning in Northwest Ohio: A Study of the Carbonate Rock Aquifer Ohio Water Plan Inventory Report, 98pp.

ODNR. (1994). Impact of Best Management Practices on Surface Runoff and Groundwater Quality in a solutioned Limestone Area, Thompson Township, Seneca County, Ohio, 54pp.

ODNR (Cartographer). (1998). Physiographic regions of Ohio [Map].

ODNR (Cartographer). (2003). Bedrock-Topography data for Ohio.

ODNR. (2009). Groundwater induced flooding in the Bellevue Ohio area spring and summer 2008, 23pp.

ODNR. (2013a). Ohio Potentiometric Surface Maps. from http://ohiodnr.com/water/gwpsurface/County_List/tabid/3621/Default.aspx

ODNR. (2013b). Water well logs. from http://www.dnr.state.oh.us/water/maptechs/wellogs/app/

OGRIP. (2013). OSIP II. from http://ogrip.oit.ohio.gov/ProjectsInitiatives/StatewideImagery.aspx

OIT. (2007). Raster Digital Data. State of Ohio Office of Information Technology, Ohio Geographically Referenced Information Program. from http://gis3.oit.ohio.gov/ZIPARCHIVES/ELEVATION/GRIDMOSAICS/metadata .xml 70

Roth, M. J. S., Nyquist, J. E., and Guzas, B. (2000). Locating subsurface voids in karst: a comparison of multi-electrode earth resistivity testing and gravity testing. Proc. From the Symposium on the Application of Geophysics to Engineering & Environmental Problems, 359-365.

Ruedisili, L. C., Kihn, G. E., and Bell, R. C. (1990). Geology of Seneca caverns, Seneca County, Ohio. Ohio Journal of Science, 90(4), 106-111.

Sikora, G. S. (1975). Groundwater Quality of a Carbonate Aquifer in the Bellevue, Ohio Area. (Master's Thesis), University of Toledo, 47pp.

Sparling, D. R. (1988). Middle Devonian Stratigraphy and Condont Biostratigraphy, North-Central Ohio. Ohio J. Sci., 88(1), 2-18.

Stierman, D. (2004). BO Gravity Base Station. from http://www.eeescience.utoledo.edu/Faculty/stierman/Admin/gbase.htm

Telford, W. M., Geldart, L. P., and Sheriff, R. E. (1990). Applied Geophysics: Cambridge University Press, 770pp.

Tintera, J. J. (1980). The identification and Interpretation of Karst Features in the Bellevue-Castalia Region of Ohio. (Master's Thesis), Bowling Green State University, 110pp.

U.S. Census Bureau. (2011). 2011 TIGER/Line® Shapefiles: Water.

Ulteig, J. R. (1964). Upper Niagaran and Cayugan stratigraphy of northeastern Ohio and adjacent areas Ohio Department of Natural Resources, Geological Survey Report of Investigations, 48pp.

USDA. (2006). Soil Survey of Erie County, Ohio. http://soildatamart.nrcs.usda.gov/Manuscripts/OH043/0/OHErie6_7_06.pdf USGS. (2005). Ohio Geologic Map Data. http://mrdata.usgs.gov/geology/state/state.php?state=OH USGS. (2013). USGS current conditions for the nation., from http://waterdata.usgs.gov/nwis/nwisman/?site_no=411819082493900&agency_cd =USGS

Verber, J. L., and Stansbery, D. H. (1953). Caves In The Lake Erie Islands. Ohio J. Sci., 53(6), 358-362.

VerSteeg, K., and Yunck, G. (1932). The Blue Hole of Castalia. Ohio Journal of Science, 32(5), 425-435.

Yuhr, L., Benson, R., and Butler, D. (1993). Characterization of karst features using Electromagnetics and Microgravity: a strategic approach. Symposium on the application of Geophysics to Engineering & Environmental Problems, 209-228.

Zhou, W., Beck, B. F., and Stephenson, J. B. (2000). Reliability of dipole-dipole electrical resistivity tomography for defining depth to bedrock in covered karst terrains. Environmental Geology, 39, 760-766.

Appendix A

Observed Gravity

Easting Northing Elevation Observed Gravity Station (m) (m) (m) (mGal) BEL01 346223.7 4576610.1 217.7 980190.38 BEL02 346992.0 4576576.9 218.3 980190.66 BEL03 347031.7 4576573.6 217.9 980190.77 BEL04 347065.6 4576571.4 216.5 980191.06 BEL05 347105.9 4576567.5 215.5 980191.27 BEL06 347167.2 4576566.1 215.1 980191.35 BEL07 347197.0 4576564.1 215.1 980191.37 BEL08 347226.6 4576562.6 215.3 980191.38 BEL09 347249.4 4576561.9 215.5 980191.36 BEL10 347286.2 4576563.1 215.7 980191.34 BEL11 347314.0 4576558.2 215.6 980191.40 BEL12 347346.3 4576557.4 215.5 980191.44 BEL13 347368.6 4576555.7 215.6 980191.45 BEL14 347404.0 4576554.2 215.6 980191.45 BEL15 347428.6 4576550.4 216.0 980191.42 BEL16 347461.2 4576550.2 216.2 980191.40 BEL17 347484.7 4576548.8 216.3 980191.42 BEL18 347522.2 4576548.1 216.5 980191.42 BEL19 347578.3 4576543.5 217.1 980191.28 BEL20 347598.2 4576543.4 217.7 980191.26 BEL21 347635.9 4576540.9 217.9 980191.28 BEL22 347657.8 4576540.3 217.9 980191.30 BEL23 347693.7 4576538.8 218.1 980191.32 BEL24 347721.4 4576539.2 218.0 980191.37 BEL25 347758.5 4576534.8 217.0 980191.63 BEL26 347786.1 4576532.9 216.7 980191.69 BEL27 346886.0 4576579.0 217.3 980190.91

Easting Northing Elevation Observed Gravity Station (m) (m) (m) (mGal) BEL28 346641.9 4576602.7 217.6 980190.62 BEL29 346621.2 4576603.8 217.6 980190.56 BEL30 346601.6 4576606.1 217.4 980190.51 BEL31 346581.4 4576605.7 217.9 980190.48 BEL32 346561.4 4576606.7 218.0 980190.45 BEL33 346540.6 4576609.0 218.0 980190.45 BEL34 346521.5 4576608.2 217.9 980190.47 BEL35 346501.5 4576608.7 217.8 980190.53 BEL36 346482.5 4576610.0 217.3 980190.63 BEL37 346459.5 4576611.5 216.7 980190.74 BEL38 346441.3 4576613.6 216.4 980190.76 BEL39 346421.3 4576615.0 216.3 980190.73 BEL40 346402.0 4576614.4 216.5 980190.71 BEL41 346381.7 4576616.5 216.2 980190.70 BEL42 346362.4 4576615.8 216.6 980190.70 BEL43 346339.1 4576618.0 216.4 980190.69 BEL44 346320.9 4576616.8 216.9 980190.66 BEL45 346302.6 4576621.0 216.9 980190.58 BEL46 346282.8 4576619.9 217.4 980190.51 BEL47 346260.3 4576621.3 217.5 980190.48 BEL48 346241.8 4576623.8 217.5 980190.45 BEL49 346222.2 4576624.7 217.6 980190.42 BEL50 346201.7 4576625.2 217.8 980190.38 BEL51 346181.0 4576628.4 217.8 980190.37 BEL52 346162.3 4576626.9 217.8 980190.33 BEL53 346140.6 4576629.1 217.7 980190.33 BEL54 346121.3 4576630.1 218.0 980190.33 BEL55 346101.6 4576630.8 217.8 980190.34 BEL56 346082.4 4576629.9 218.0 980190.32 BEL57 346060.5 4576631.5 218.1 980190.29 BEL58 346043.4 4576631.8 218.3 980190.24 BEL59 346021.1 4576633.4 218.5 980190.21 BEL60 346002.0 4576636.5 218.6 980190.18 BEL61 345981.2 4576638.8 218.9 980190.17 BEL62 345961.3 4576638.0 218.6 980190.19 BEL63 345941.1 4576640.3 218.7 980190.20

Easting Northing Elevation Observed Gravity Station (m) (m) (m) (mGal) BEL64 345921.0 4576640.0 218.6 980190.19 BEL65 345902.0 4576641.4 218.9 980190.15 BEL66 345882.6 4576641.7 219.3 980190.03 BEL67 345862.5 4576641.8 220.4 980189.88 BEL68 345843.0 4576644.8 220.6 980189.77 BEL69 345816.5 4576646.6 221.4 980189.69 BEL70 345786.5 4576649.1 221.8 980189.69 BEL71 346681.8 4576591.7 217.7 980190.62 BEL72 346720.9 4576588.8 217.8 980190.64 BEL73 346752.0 4576588.4 217.9 980190.64 BEL74 346791.1 4576588.6 217.8 980190.69 BEL75 346816.6 4576585.5 217.8 980190.71 BEL76 346853.9 4576582.6 217.3 980190.83 BEL77 346863.1 4576565.6 217.3 980190.83 BEL78 346861.1 4576548.7 217.1 980190.83 BEL79 346862.3 4576526.5 216.7 980190.86 BEL80 346864.7 4576508.8 216.2 980190.95 BEL81 346861.3 4576486.3 215.7 980191.00 BEL82 346863.1 4576468.0 215.4 980191.01 BEL83 346864.1 4576445.6 215.2 980191.01 BEL84 346863.8 4576427.4 215.1 980191.02 BEL85 346862.6 4576407.5 214.9 980191.00 BEL86 346860.8 4576387.4 215.0 980190.98 BEL87 346861.8 4576368.2 215.1 980190.94 BEL88 346861.9 4576347.9 215.3 980190.86 BEL89 346862.5 4576326.9 215.5 980190.81 BEL90 346861.5 4576307.9 215.8 980190.74 BEL91 346863.2 4576287.8 216.3 980190.65 BEL92 346862.3 4576267.4 216.5 980190.58 BEL93 346862.5 4576247.1 216.9 980190.52 BEL94 346863.2 4576228.4 216.9 980190.49 BEL95 346861.7 4576207.7 216.7 980190.53 BEL96 346861.9 4576186.0 216.3 980190.55 BEL97 346862.8 4576167.4 215.7 980190.62 BEL98 346860.3 4576150.3 215.3 980190.75 BEL99 346862.9 4576129.4 214.9 980190.84

Easting Northing Elevation Observed Gravity Station (m) (m) (m) (mGal) BEL100 346862.2 4576110.2 214.5 980190.85 BEL101 346863.5 4576089.6 214.4 980190.86 BEL102 346861.9 4576070.4 214.3 980190.87 BEL103 346864.5 4576051.3 214.4 980190.84 BEL104 346864.9 4576028.3 214.6 980190.79 BEL105 346863.7 4576010.5 214.8 980190.73 BEL106 346863.1 4575991.2 215.3 980190.64 BEL107 346862.8 4575971.3 215.6 980190.51 BEL108 346860.1 4575949.6 215.7 980190.43 BEL109 346863.8 4575928.2 215.9 980190.39 BEL110 346860.9 4575910.2 216.0 980190.36 BEL111 346864.3 4575890.7 216.1 980190.32 BEL112 346862.5 4575870.4 216.1 980190.31 BEL113 346862.9 4575848.3 216.3 980190.30 BEL114 346863.7 4575828.5 216.2 980190.30 BEL115 346861.5 4575808.3 216.1 980190.31 BEL116 346862.0 4575787.8 215.9 980190.28 BEL117 346860.8 4575769.5 215.7 980190.31 BEL118 346862.6 4575749.3 215.7 980190.33 BEL119 346863.2 4575729.9 215.7 980190.33 BEL120 346864.0 4575709.6 215.6 980190.34 BEL121 346862.8 4575688.1 215.7 980190.33 BEL122 346864.8 4575667.4 215.8 980190.34 BEL123 346865.1 4575647.5 215.9 980190.35 BEL124 346864.8 4575627.7 215.8 980190.34 BEL125 346865.7 4575607.3 215.8 980190.33 BEL126 346863.1 4575589.5 215.6 980190.35 BEL127 346864.6 4575567.7 215.7 980190.33 BEL128 346863.9 4575547.8 215.8 980190.30 BEL129 346864.1 4575528.7 215.9 980190.26 BEL130 346862.7 4575506.4 216.1 980190.20 BEL131 346863.0 4575487.1 216.2 980190.16 BEL132 346861.6 4575466.7 216.5 980190.12 BEL133 346862.3 4575442.1 216.8 980190.01 BEL134 346862.3 4575419.9 217.3 980189.88 BEL135 346862.6 4575394.8 217.8 980189.78

Easting Northing Elevation Observed Gravity Station (m) (m) (m) (mGal) BEL136 346862.0 4575370.3 218.1 980189.66 BEL137 346865.3 4575341.4 219.0 980189.47 BEL138 346865.5 4575317.2 219.5 980189.38 BEL139 346864.5 4575293.3 219.7 980189.31 BEL140 346866.8 4575269.4 219.9 980189.24 BEL141 346865.6 4575241.5 220.2 980189.19 BEL142 346865.1 4575217.7 220.4 980189.17 BEL143 346866.4 4575207.0 220.4 980189.14 BEL144 346865.2 4575119.3 220.6 980189.05 BEL145 346863.5 4575027.4 221.2 980188.81 BEL146 346865.7 4574954.3 221.2 980188.72 BEL147 346865.5 4574926.9 220.3 980188.73 BEL148 346866.0 4574904.6 220.3 980188.81 BEL149 346864.6 4574878.2 219.6 980188.87 BEL150 346867.5 4574851.1 218.9 980188.93 BEL151 346863.4 4574827.8 219.0 980188.91 BEL152 346863.3 4574803.3 218.9 980188.91 BEL153 346864.6 4574777.4 219.1 980188.88 BEL154 346865.1 4574753.3 219.3 980188.81 BEL155 346861.8 4574729.1 219.5 980188.71 BEL156 346863.9 4574703.2 220.0 980188.61 BEL157 346862.9 4574678.1 220.3 980188.54 BEL158 346862.9 4574654.2 220.3 980188.50 BEL159 346862.5 4574628.7 220.4 980188.47 BEL160 346862.1 4574604.4 220.5 980188.45 BEL161 346862.2 4574578.4 220.7 980188.42 BEL162 346862.8 4574553.1 220.2 980188.42 BEL163 346862.9 4574528.4 220.3 980188.44 BEL164 346861.4 4574503.2 220.1 980188.44 BEL165 346861.9 4574478.7 220.1 980188.43 BEL166 346862.1 4574452.6 220.1 980188.43 BEL167 346862.5 4574427.5 219.9 980188.42 BEL168 346862.9 4574403.9 219.7 980188.46 BEL169 346860.7 4574377.2 219.2 980188.53 BEL170 346861.6 4574351.5 218.9 980188.61 BEL171 346862.7 4574376.7 219.0 980188.59

Easting Northing Elevation Observed Gravity Station (m) (m) (m) (mGal) BEL172 346863.3 4574351.4 218.8 980188.69 BEL173 346863.0 4574325.8 218.7 980188.69 BEL174 346861.6 4574300.6 218.6 980188.73 BEL175 346861.9 4574275.3 218.3 980188.72 BEL176 346862.1 4574250.8 218.5 980188.69 BEL177 346862.1 4574225.7 218.6 980188.65 BEL178 346862.6 4574200.8 218.7 980188.62 BEL179 346862.1 4574175.9 218.8 980188.58 BEL180 346862.7 4574147.8 218.8 980188.51 BEL181 346862.1 4574125.9 219.0 980188.40 BEL182 346860.2 4574100.8 219.3 980188.35 BEL183 346857.1 4574076.4 219.5 980188.30 BEL184 346852.8 4574051.8 219.8 980188.22 BEL185 346847.9 4574025.3 219.5 980188.17 BEL186 346841.0 4574003.1 219.9 980188.10 BEL187 346835.6 4573979.1 220.1 980188.07 BEL188 346827.0 4573955.1 220.3 980188.09 BEL189 346816.4 4573932.8 220.2 980188.06 BEL190 346807.8 4573908.9 220.4 980188.04 BEL191 346799.1 4573885.6 220.4 980188.00 BEL192 346789.4 4573865.4 220.4 980187.99 BEL193 346778.6 4573842.7 220.3 980187.95 BEL194 346770.7 4573818.8 220.1 980187.94 BEL195 346760.1 4573795.6 220.0 980187.93 BEL196 346748.7 4573773.1 219.9 980187.95 BEL197 346731.1 4573727.3 219.8 980187.95 BEL198 346720.2 4573701.7 219.7 980187.95 BEL199 346699.8 4573657.8 219.5 980187.94 BEL200 346690.7 4573634.1 219.6 980187.88 BEL201 346679.9 4573611.4 219.7 980187.85 BEL202 346672.0 4573590.7 219.7 980187.83 BEL203 346661.4 4573561.3 220.0 980187.75 BEL204 346640.4 4573514.7 221.1 980187.52 BEL205 346564.2 4573344.5 222.1 980187.14 BEL206 346365.1 4572881.5 224.5 980186.23 BEL207 347037.8 4573943.0 220.6 980188.17

Easting Northing Elevation Observed Gravity Station (m) (m) (m) (mGal) BEL208 347174.5 4573935.9 221.7 980188.08 BEL209 347245.1 4573931.7 219.2 980188.60 BEL210 347304.2 4573929.1 219.2 980188.67 BEL211 347375.4 4573926.0 219.8 980188.60 BEL212 347476.8 4573919.0 221.4 980188.34 BEL213 347777.7 4573903.6 222.4 980188.40 BEL214 348278.3 4573878.7 222.2 980188.90 BEL215 348672.8 4573858.7 221.7 980189.32 BEL216 348923.9 4574036.1 221.2 980189.72 BEL217 348920.6 4574338.7 220.7 980190.03 BEL218 348917.4 4575494.9 218.5 980191.38 BEL219 348914.4 4576349.4 218.3 980191.92 BEL220 348601.8 4576499.9 218.2 980191.89 BEL221 348300.0 4576514.9 217.0 980191.95 BEL222 348071.5 4576527.1 216.9 980191.80 BEL223 346873.8 4576809.7 217.8 980190.95 BEL224 346871.8 4577254.4 218.7 980191.04 BEL225 346872.0 4577570.5 219.1 980191.12 BEL226 346868.7 4578102.5 224.4 980190.48 BEL227 346857.3 4578742.2 218.6 980192.11 BEL228 348022.3 4578548.9 215.5 980193.48 BEL229 348681.4 4578282.1 214.6 980193.94 BEL230 347477.2 4578338.1 218.1 980192.35 BEL231 345749.0 4578434.3 234.2 980188.02 BEL232 345743.4 4578119.4 233.0 980188.07 BEL233 345749.6 4577797.6 229.7 980188.47 BEL234 345755.7 4577361.4 223.7 980189.54 BEL235 345756.5 4576969.6 224.2 980189.39 BEL236 345767.2 4576519.5 220.4 980189.66 BEL237 345769.9 4576223.4 219.9 980189.49 BEL238 346538.0 4573553.2 219.6 980187.77 BEL239 346031.7 4573581.5 223.2 980186.84 BEL240 345799.5 4573934.8 222.6 980187.10 BEL241 345791.3 4574447.5 222.8 980187.43 BEL242 345785.3 4574911.6 219.5 980188.46 BEL243 345785.1 4574961.9 218.9 980188.62

Easting Northing Elevation Observed Gravity Station (m) (m) (m) (mGal) BEL244 345783.5 4575014.4 218.7 980188.66 BEL245 345783.0 4575067.3 218.9 980188.65 BEL246 345782.9 4575116.7 218.2 980188.85 BEL247 345782.0 4575171.4 217.2 980189.07 BEL248 345781.4 4575225.2 216.7 980189.24 BEL249 345782.5 4575275.2 216.2 980189.34 BEL250 345783.3 4575325.9 215.9 980189.41 BEL251 345782.8 4575377.2 215.9 980189.40 BEL252 345782.1 4575429.7 216.3 980189.43 BEL253 345782.4 4575482.8 215.9 980189.48 BEL254 345781.2 4575531.4 216.4 980189.49 BEL255 345781.8 4575581.6 217.0 980189.42 BEL256 345780.6 4575630.1 217.8 980189.33 BEL257 345780.2 4575682.6 217.4 980189.42 BEL258 345780.1 4575730.3 217.5 980189.44 BEL259 345778.9 4575779.4 218.4 980189.35 BEL260 345779.7 4575833.2 219.8 980189.13 BEL261 345767.2 4576442.4 219.1 980189.80 BEL262 345769.2 4576355.2 218.6 980189.82 BEL263 345771.9 4575977.6 219.7 980189.27 BEL264 345294.2 4575212.6 219.9 980188.44 BEL265 345403.4 4575209.6 220.0 980188.39 BEL266 345496.5 4575210.1 217.1 980188.99 BEL267 345612.4 4575206.4 217.3 980188.94 BEL268 345716.0 4575205.2 216.2 980189.19 BEL269 346594.6 4573550.2 220.5 980187.62 BEL270 346497.0 4573557.8 219.5 980187.69 BEL271 346451.9 4573560.9 220.3 980187.55 BEL272 346411.3 4573560.2 220.8 980187.40 BEL273 346365.7 4573564.6 222.2 980187.11 BEL274 346323.5 4573565.3 223.4 980186.85 BEL275 345952.3 4573586.7 223.4 980186.72 BEL276 345862.8 4573590.0 223.5 980186.64 BEL277 345682.9 4573596.9 223.7 980186.56 BEL278 346096.0 4576078.4 218.9 980189.64 BEL279 346155.0 4576201.4 218.4 980189.87

Easting Northing Elevation Observed Gravity Station (m) (m) (m) (mGal) BEL280 346156.4 4576281.8 218.3 980189.94 BEL281 346151.1 4576391.3 216.1 980190.47 BEL282 346199.0 4576485.0 215.9 980190.60 BEL283 345550.1 4573598.0 223.9 980186.41 BEL284 345207.5 4573607.8 221.8 980186.66 BEL285 344726.8 4573618.3 223.4 980186.14 BEL286 344398.3 4573627.9 224.1 980185.89 BEL287 344343.6 4573816.0 222.7 980186.37 BEL288 344353.8 4574167.7 222.1 980186.78 BEL289 344370.8 4574656.3 220.4 980187.52 BEL290 344382.2 4575060.9 220.3 980187.86 BEL291 344578.4 4575231.0 220.4 980188.06 BEL292 344910.3 4575222.7 217.9 980188.53 BEL293 344395.6 4575598.8 223.9 980187.47 BEL294 344425.5 4576324.5 219.9 980188.83 BEL295 344449.4 4577249.1 221.6 980189.19 BEL296 344165.8 4577452.6 224.3 980188.57 BEL297 343268.6 4576959.9 223.1 980188.20 BEL298 342777.7 4575885.3 223.5 980187.61 BEL299 342978.9 4575281.3 224.3 980187.06 BEL300 343639.7 4575262.0 220.8 980187.67 BEL301 344168.6 4575243.8 221.6 980187.72 BEL302 343865.5 4576152.5 222.9 980187.85 BEL303 342729.8 4574501.9 225.3 980186.39 BEL304 342111.6 4573677.1 221.3 980186.54 BEL305 341217.9 4573704.1 224.6 980185.78 BEL306 340681.5 4573187.3 226.6 980184.99 BEL307 339599.1 4573748.9 228.2 980184.66 BEL308 340747.2 4575246.3 221.7 980187.30 BEL309 341883.1 4575299.8 224.2 980187.08 BEL310 342787.5 4576019.0 224.1 980187.50 BEL311 342004.6 4576199.7 222.6 980187.78 BEL312 342700.7 4573179.6 227.7 980184.86 BEL313 343589.4 4573649.9 224.9 980185.85 BEL314 344303.7 4572375.3 224.4 980184.81 BEL315 344735.6 4577752.8 224.5 980188.83

Easting Northing Elevation Observed Gravity Station (m) (m) (m) (mGal) BEL316 346617.7 4578348.1 225.3 980190.21 BEL317 345730.9 4579799.2 210.7 980193.67 BEL318 346571.9 4581682.7 208.1 980195.93 BEL319 342894.5 4578933.1 212.6 980191.60 BEL320 340821.5 4578233.9 208.1 980191.32 BEL321 339592.0 4577218.7 211.6 980190.06 BEL322 337943.2 4575410.1 213.5 980188.22 BEL323 338048.7 4579113.3 196.1 980194.06 BEL324 340673.3 4582207.2 188.0 980198.30 BEL325 337402.2 4571037.9 231.9 980181.74 BEL326 340997.1 4569203.3 235.1 980180.49 BEL327 342616.9 4569266.7 238.4 980180.21 BEL328 345845.8 4568577.2 238.7 980180.57 BEL329 344735.6 4577752.8 225.3 980185.88 BEL330 346617.7 4578348.1 222.2 980187.06 BEL331 345730.9 4579799.2 223.9 980187.15 BEL332 346571.9 4581682.7 223.8 980188.01 BEL333 342894.5 4578933.1 225.3 980187.32 BEL334 340821.5 4578233.9 219.7 980191.20 BEL335 339592.0 4577218.7 219.8 980192.00 BEL336 337943.2 4575410.1 221.7 980192.08 BEL337 338048.7 4579113.3 219.6 980193.22 BEL338 340673.3 4582207.2 220.3 980192.02 BEL339 337402.2 4571037.9 215.9 980195.13 BEL340 340997.1 4569203.3 198.6 980200.92 BEL341 342616.9 4569266.7 210.1 980197.07 BEL342 345845.8 4568577.2 218.7 980194.21 BEL343 349128.2 4568321.0 240.0 980181.67 BEL344 352203.7 4568941.4 232.8 980184.79 BEL345 356082.9 4572100.5 218.6 980191.82 BEL346 352599.0 4572665.0 226.6 980189.45