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United States Ofﬁ ce of Emergency and July 2014 Remedial Response www.epa.gov/superfund Agency Washington, DC 20460

Superfund

Introduction to Environmental Geophysics

Student Manual

Overview of Geophysical Methods

OVERVIEW OF GEOPHYSICAL METHODS

Geophysical Surveys

 Characterize

 Characterize  Locate metal targets and voids

Physical Properties Measured

 Velocity  Seismic  Radar  Electrical Impedance  Electromagnetics  Resistivity  Magnetic  Magnetics  Density 

Overview of Environmental Geophysics 1 Overview of Geophysical Methods

Magnetics

 Measures natural

 Map anomalies in magnetic field  Detects iron and steel

Geometrics Cesium

Electromagnetics (EM)

 Generates electrical and magnetic fields

 Measures the conductivity of target  Locates metal targets

Overview of Environmental Geophysics 2 Overview of Geophysical Methods

EM-31

Marion Landfill, Marion, IN

EM-61

Geonics EM-61 EM

Resistivity

 Injects current into ground

 Measures resultant voltage  Determines apparent resistivity of layers

 Maps geologic beds and water table

Overview of Environmental Geophysics 3 Overview of Geophysical Methods

Sting Resistivity Unit

Seismic Methods

 Uses acoustic energy

 Refraction - Determines velocity and thickness of geologic beds

 Reflection - Maps geologic layers and bed topography

Seistronix Seismograph

Overview of Environmental Geophysics 4 Overview of Geophysical Methods

Gravity

 Measures

 Used to determine density of materials under instrument

 Maps voids and intrusions

Scintrex Gravity Meter

 Transmits and receives electromagnetic energy

 Maps geology

 Locates cultural targets

 Has very high resolution

Overview of Environmental Geophysics 5 Overview of Geophysical Methods

Borehole Geophysical Methods

 Variety of downhole tools available

 Optical and acoustic televiewers  Parameters measured

 Temperature  Flow direction  Conductivity and Resistivity  Density  Gamma

Borehole Geophysical Methods

Resistivity

Video

Gamma / Temperature

Caliper

Overview of Environmental Geophysics 6 Overview of Geophysical Methods

Modeling for Interpretation

 Two kinds of simple models  Forward  Inverse  Models depend on input conditions and data  Models can be very helpful in visualizing the site

 Models are not reality

 Non-intrusive

 Rapid data collection  Detects a variety of targets

 Screens large areas

 Fills in data gaps

Correct Interpretation

Overview of Environmental Geophysics 7 Overview of Geophysical Methods

Geophysical Methods Limitations

 Methods require a specialist

 Interpretations are non-unique  May be expensive

 Physical contrasts must exist

 Resolution varies by method and depth of target

Problematic Interpretation

Overview of Environmental Geophysics 8 Design

Y

X

A Good Survey Results In…

• A record of useful information – Background data to support survey – Rationale for methods used – Survey data - maps – Conclusions in lay terms • Efficient use time - money • A document that maintains its value

Survey Design Rationale

• Establishes a plan • Find potential pitfalls • Maximize benefit • Minimize surprises – Property line issues – Archeological sites – Utility lines • Customize requests

Overview of Environmental Geophysics 1 Geophysical Survey Design

Pre-survey Planning: Garbage IN – Garbage OUT

• Inadequate background information & planning dooms a survey before it starts: – Requires more time in the field – Increases costs – Missed targets – Questionable data

Define Problem

• List issues of concern • Can geophysics help? • Data confirmable? • How will results benefit your plan?

Background Paperwork Review

• Site history • Previous studies • Geology • Geohydrology • Geographic issues • Health, safety & QAPP issues

Overview of Environmental Geophysics 2 Geophysical Survey Design

Background Map Review

• Sanborn or other Public Maps – Historical site records & buildings

• Topographic Maps – Terrain conditions

• Geologic Maps – Indirect conditions

Sanborn Maps: Anacortes, Washington State

Feb. 1897 Nov. 1907

Nov. 1950 Oct. 1925

Sanborn UMI

Topographic & Geologic Maps

Overview of Environmental Geophysics 3 Geophysical Survey Design

Background Photo Review

Recent Site Photo Historical Site Photo

Recent Aerial Photo Historical Aerial Photo

Photo Interpretation

May 7, 1981: Color Infrared Sept 25, 1936: B & W

U.S. EPA Environmental Lammers Barrel Photographic Beavercreek, Ohio Interpretation Center April 5, 1988: Color

Other Issues To Consider

• Property boundaries • Consent for access • Traffic & pedestrians • Vegetation status • “Noise” issues • Utility location • Archeological sites

Overview of Environmental Geophysics 4 Geophysical Survey Design

Utility Locating

• Utility services require several days notice • Service provides “dig” number for site area • All utilities may not be members of service Courtesy: Ohio State University • Have service remark Dial 811 area if necessary on your phone • Know tolerances of for local utility service provider location service

National Historic Preservation Act

• Why should we care? – It’s the law – Regulations require it – It’s EPA’s policy – It’s a good idea

Public Law 89-665; 16 U.S.C 470 & Subsequent Amendments EPA HQ Contact: [email protected] - 202.564.6646 State Contacts: www.ncshpo.org

Code of Federal Regulations (CFR) “Handling Drums & Containers”

• 1910.120 ( j ) (1) (x) “A ground-penetrating system or other type of detection system or device shall be used to estimate the location and depth of buried drums or containers”

Overview of Environmental Geophysics 5 Geophysical Survey Design

Analyze Background Information to Determine..

• Area to be surveyed • Size - number of suspect targets • Potential problems • Site reconnaissance needed?

Match Most Favorable Geophysical Techniques to Problem

• What method(s) contrast most from background? • Note depth confines • “Noise” issues

Seismic Refraction

Electromagnetic Electromagnetic GEM Unit EM-61 Unit

Overview of Environmental Geophysics 6 Geophysical Survey Design

Optimize Data Collection

• Establish how data will be collected – Traverse pattern – Grid spacing – Axis labeling – Data Location ID

Key Issues For Collecting Data • Systematic collection (grid or lines) • Spacing dependent on target size • Accurate grid or line establishment • Method to ensure location accuracy • Label grids or lines reasonably • Maintain good field notes • Take plenty of photographs!

Data Collection (magnetics, electromagnetics, ground penetrating radar)

Overview of Environmental Geophysics 7 Geophysical Survey Design

Consider Analogy Between Data Density & Photographic Pixels

Detection Probability (Using Individual Station Measurements)

At = Area of Target 4,3564335

As = Area of Site 43,560

Probability of As/At As/At As/At Detection =10 =100 =1000

100 16 160 1600 98 13 130 1300 90 10 100 1000 75 8 80 800 50 5 50 500

Number of data points required (modified from Benson et al., 1988)

Determining Grid Spacing

2 Area of Site in ft 2 = a in ft Area of Target in ft 2 a x Probability Factor = Sampling Points (Approx.) Area of Site in ft 2 = b Sampling Points

b = Grid Spacing in Feet Probability Factors 100% = 1.625 75% = 0.8 98% = 1.3 50% = 0.5 90% = 1.0

Overview of Environmental Geophysics 8 Geophysical Survey Design

Typical Acquisition Traverses

• Alternating mode • Areas broken into – Most often used rectangular shapes • Random mode • Irregular boundaries – Used for small or – Use multiple rectangles large areas • Positioning methods • Parallel mode – Station – Irregular shaped – Timed – collection sites – Wheel encoder –GPS

Random Survey Pattern (Small Area)

boulder start end fence

Random Survey Using GPS (Large Area)

• Maximize productivity • Data linked to GPS • Best in obstructed areas • Areas must be free of: – Vegetative canopies – Tall buildings – Major power lines

Overview of Environmental Geophysics 9 Geophysical Survey Design

Random Survey GPS Issues

(One dot per 5 data points) • Data locations from Mag on ATV • Dots show data points • Note N-S dot spacing due to speed changes • Note data gaps

Alternating Traverse No GPS

Start

End

Alternating Traverse Grid Setup No GPS

• Layout grid markers at desired spacing – Flagging (plastic) Site boundary – Spray chalk or paint – Ropes Tapes & markers – Alignment placards

– Wooden stakes Traverse • Large sites require directions multiple marker lines 0,0

Overview of Environmental Geophysics 10 Geophysical Survey Design

Alternating Traverse Parallel Swath GPS

Start

End

Parallel Swathing GPS

• Initialize start & end points of line • GPS maintains parallel lines • Operator follows cursor on lightbar • Lat. - Long. output to sensor data

Photo: Geometrics

Lightbar Guidance

• Center: on line • Left: move left • Right: move right • Outer edges yellow: nearing line end • Outer edges red: at line end • Advances to next spacing

Overview of Environmental Geophysics 11 Geophysical Survey Design

Parallel Traverse – No GPS

End

Start

• Define X and Y • X, line or • Y, position or • Several data collection options for tagging X, Y – Data recorder sets method

Data Recorder Methods

• Station position • Time – distance • Encoder wheel •GPS

Overview of Environmental Geophysics 12 Geophysical Survey Design

Correcting for Position (Y)

• Time-distance issue – Must correct for pace •GPS – Correct for errors – Use proper datum • Wheel Encoder – Resolve distance errors

Continuous Data Acquisition Issues for Y Axis

12 9 9 pts • Operator inputs start pts pts & end points per line • Unit auto “fits” data Reality to input distance Vs Processed – Assumes same pace • Obstacles usually slows pace • Use data pause Even Off Posted pace: pace: features as needed real real Line 1 Line 2 Line 2

Global Positioning Systems

• Accuracies vary by method & equip. used

• Example: some on a scale to locate an airport

• Example: others on a scale to find center of runway

Overview of Environmental Geophysics 13 Geophysical Survey Design

Several GPS Methods

• Stand alone GPS receiver • Differential correction (DGPS) – Real time using beacons, base stations • Post processing GPS values • 3 Grades of GPS accuracy – Recreational, mapping, survey

How a Differential GPS Service Works

Rover

Drawing modified from Omnistar corp. graphic Fixed Bases

Ground Based Local Positioning & Data Collection System

Overview of Environmental Geophysics 14 Geophysical Survey Design

System Overview

• Laser beam tracking • Line-of-site system • Merges & stores – Total station data + – Geophysical data or – Radiological data • Positioning options – guidance or tracking • Real-time displays

Auto Tracking & Guidance

How It Works

• Laser tracks optical target • Collects data – Position x, y, z data – Sensor data • Computes coordinates • Merges data into one file • Transmits to rover • Displays data/position on HUD

Overview of Environmental Geophysics 15 Geophysical Survey Design

2 Screen Views of HUD

• Blue Arrow shows direction related to X- axis baseline • Coordinates • Target path line • Current path • Distance L/R line • Data readouts • Recording indicator

Screen Color On Rover’s HUD Has Meaning

Red Zone White: Normal Yellow Zone Yellow: Near End

Red: Plan Passed End View Grid Area Blue: Lost Tracking Link

Pre-Planning for Seismic Survey

• Length of line required • Number of lines & orientations • Ambient “noise” issues • Topography-elevation changes • Good consistent ground coupling • Line protection (traffic, etc.)

Overview of Environmental Geophysics 16 Geophysical Survey Design

Which Method is Applied First?

• Dependent on site goals • Generally...... First – Methods having larger sensing areas – Rapid data collection times • Generally...... Second – Methods with more definitive sensing capabilities

Check List For Considering Geophysical Survey

• Define problem • Will geophysics help? • Research history • List methods that will • Find area of concern show most contrast • Note site conditions • How will you use this • Describe target(s) information? • Estimate depth

A Note About Contracting Geophysical Jobs

• Use source that is knowledgeable about all geophysical methods • Write contract to assume several “what if” scenarios to deal with special issues • Obtain copies of raw data & notebooks • Be aware that interpretation & reports may be optional

Overview of Environmental Geophysics 17

Check List For Considering Geophysical Survey

• Define problem • Will geophysics help? • Research history • List methods that will • Find area of concern show most contrast • Note site conditions • How will you use this • Describe target(s) information? • Estimate depth A Note About Contracting Geophysical Jobs

• Use source that is knowledgeable about all geophysical methods • Write contract to assume several “what if” scenarios to deal with special issues • Obtain copies of raw data & notebooks • Be aware that interpretation & reports may be optional Magnetic Methods

Metal Detector ≠ Magnetic Method

photo credit: Wikipedia

METAL DETECTORS use internal power to create a electromagnetic field to locate metal

MAGNETOMETERS are passive instruments and only sense ambient magnetic fields

The Magnetic Method

• Senses presence of iron (ferrous metals) • Measures magnetic fields • Easy to apply and interpret

Overview of Environmental Geophysics 1 Magnetic Methods

Ferrous & Non Ferrous Metals

iron cobalt aluminum copper

iron alloys steel

titanium brass stainless steel

Why Is Magnetics Important?

• Non-invasive, passive detection method • Quantitative results • Large masses detectable at significant depths • Complements other geophysical methods

Optimal Detectable Features Unique to Magnetics

• Buried drums, tanks, pipes, valves • Steel casing (abandoned wells) • Mixed ferrous wastes (landfills) • Steel reinforced foundations • Natural occurring ferrous minerals • Fired clays (bricks, clay pots)

Overview of Environmental Geophysics 2 Magnetic Methods

Why Are Baked Clays Magnetic? • Magnetic force microscopy image showing magnetic domains • Heated beyond curie point & when cooled domains realign

NDT Resource Center

Fire Pit Negev Desert

What Tools are Used to Measure Magnetic Fields?

• Instruments called • Several types & configurations available • Measures strength of magnetic intensities

Overview of Environmental Geophysics 3 Magnetic Methods

Magnetic Survey Tools for Hazardous Waste Sites

• Generally 1 of 3 tool types used – Proton precession – Overhauser precession – Alkali vapor (cesium) • All measure magnetic intensity • Detects ferrous materials - minerals • Tools are portable - independent systems

Selecting An Instrument

• Proton precession – Slow sampling cycle times: 3 – 6 seconds – Rugged system can be linked to GPS • Overhauser – Faster cycle times: 1s – GPS link possible – Sensors sensitive to extreme heat (120°) • Alkali Vapor – Fastest cycle times: 0.1s – Sensors have high sensitivity but are fragile & \$ – Most systems have direct hook-ups for GPS

Magnetometer Tool Options

• Several types available – Proton precession (2 types) Selection Options – Alkali vapor Standard precession • Each configurable Overhauser precession Alkali Vapor – Total field mode – Gradient mode Total field (TF) TF + Base Station – Base station mode Gradient* • Sensor option alignments * Vertical gradient –Vertical * Horizontal gradient – Horizontal

Overview of Environmental Geophysics 4 Magnetic Methods

What Exactly Is Measured?

• An integration of magnetic properties – ’s magnetic field intensity – Natural magnetic intensity rock/soil – Cultural magnetic intensities • Values either attractive or repulsive – Represented by + or - numbers – (+) values same direction of inducing field – (-) values oppose direction of inducing field

Earth’s Magnetic Field

• Always present • Invisible to senses • Viewed as background • Sensitive to other ferrous influences • Changes with latitude

Earth’s Magnetic Background

A A

B Strongest

B C

Moderate

C

Weakest

Overview of Environmental Geophysics 5 Magnetic Methods

Ferrous Interactions • Ferrous metal has its own magnetic field • Capable of altering Earth’s field • Limited influence • Easily measured • Provides accurate location method

Measurement Units

• Units measured in gammas or nano Teslas • 1 gamma = 1 nano Tesla – 55 gallon drum lid about 40  or nT – 250 gallon tank about 1000  or nT

Sensor Configurations

• Most systems can operate 1 or 2 sensors at same time • 1 sensor – Obtains total field data • 2 sensors – Collects total field & gradient data

Overview of Environmental Geophysics 6 Magnetic Methods

Total Field Configuration: One Sensor • Intensity measured from a single sensor • Tool’s latitude defines background • Anomalies: > or < than background • Solar activity will influence data

Photo: Geometrics

Gradient Configuration: Two Sensors • Intensity measured from two sensors • Background is defined as “0” • Anomalies: > or < than background • Solar activity will not influence data

Option B Base Station Mode

Overview of Environmental Geophysics 7 Magnetic Methods

• Total field (bottom sensor) minus vertical gradient (top sensor) noted as  or nT per unit of distance between sensors • 55,900 - 55,200 = 700  /meter or nT/M • Negative values are also possible

• Earth’s background fluctuates due to solar disturbances • Failure to neutralize a rapid background change will result in misleading data • Gradient data ignores solar changes

Solar Disturbances

Solar Forecasts: http://www.swpc.noaa.gov/today.html

Overview of Environmental Geophysics 8 Magnetic Methods

NOAA / Space Prediction Center

3-day Report of Solar and Geophysical Activity

Last 75 Reports Today's Space Weather Space Weather Now

• Joint USAF/NOAA Solar Geophysical Activity Report and Forecast SDF Number 126 Issued at 2200Z on 06 May 2013

• IA. Analysis of Solar Active Regions and Activity from 05/2100Z to 06/2100Z: Solar activity has been at low levels for the past 24 hours. The largest solar event of the period was a C2 event observed at 06/0205Z from Region 1739 (N12E30). There are currently 8 numbered sunspot regions on the disk.

• IB. Solar Activity Forecast: Solar activity is expected to be low with a chance for M-class flares on days one, two, and three (07 May, 08 May, 09 May).

55,000 55,200 65,200

N N N

55,000 55,900 65,900

0 gamma 700 gammas 700 gammas Typical Typical Anomaly With Background Anomaly Solar Disturbance

Cesium Magnetometer

• Ionizing light “pumps” electrons to higher energy levels • Magnetic fields affect rate energy gain/loss • Constant “pumping” allows continuous data acquisition • Accuracy of .1 gamma (detect several nails)

Overview of Environmental Geophysics 9 Magnetic Methods

Cesium Mag Measurements

More Energy Emitted In Strong Ambient Field

Photo-Matic

Cesium Ionizing Vapor Photocell Light Chamber

Less Emitted In Weak Ambient Field

(modified from Bloom, 1960)

Alkali Vapor Sensor Orientation

Tilt =  on horizontal plane parallel to direction of travel Good Rotate =  on vertical plane perpendicular to direction of travel

0º Tilt 0 º Rotate

0º Tilt 45 º Rotate

Data Interpretation

• Data analyzed by computer program • Typically by some contouring method – Lines connecting equal values at specific intervals • Displayed as 2D or pseudo 3D graphic

Overview of Environmental Geophysics 10 Magnetic Methods

Data Values

• Location over target affects data • Strongest values closest to target

Data Signatures

Pt. A Pt. B Pt. C Pt. D Pt. E measurement surfaces r r r N-S S-N r r N-S r rS-S rN-N N S-S S-N N S N rN-N

r = r N-N S-S rS rN-N> rS-S r = N-S rS-N r S N-S > rS-N = but opposite forces exist r r strong positive field Pt. D symmetrically @ Pt. A & Pt. B N < S weak negative field Pt. E measured field everywhere negative

Alan Witten

Data Display Options

Alen Witten – In Press

Examples of synthetic magnetic data for a horizontally oriented dipole

a) Contour plot: dashed = (-); Solid = (+)

b) Gray-scale plot: dark = (-); light = (+) c) Surface plot (3D)

Overview of Environmental Geophysics 11 Magnetic Methods

+

-

Modified from The University of Melbourne

Estimating Target Depths

½ Max. d

a) 2.4 m 3.12 m b) 1.0 m 1.30 m c) 1.6 m 2.08 m

Credit: Alan Witten

Depth Estimate Calculation From Contour Map

horizontal orientation = 45 degrees • Solid & open circles are vertical orientation = 0 degrees locations of max. value & + ½ max. value: 3.6m (as measured from map scale) • Contour interval 20 nT • Target = horiz. metal bar – Depth: actual = 5m - – Depth: est. = 4.68m

Credit: Alan Witten

Overview of Environmental Geophysics 12 Magnetic Methods

Another Depth Estimate

horizontal orientation = 90 degrees • Solid & open circles are vertical orientation = 90 degrees locations of max. value & ½ max. value: 1.8m + (as measured from map scale) • Contour interval 20 nT • Target = vert. metal bar – Depth: actual = 3m – Depth: est. = 2.34m

Credit: Alan Witten

Mag Data Example – 3D From Chicago Test Site

Multiple Magnetic Sources

Pavement - Concrete & Rebar

Soil

Buried Waste

Drum Mass ≈ Rebar Mass: Difficult to Distinguish Drum Mass > Rebar Mass: Easier to Distinguish

Overview of Environmental Geophysics 13 Magnetic Methods

Dealing With Noise Issues

• Accounting for un- wanted Interferences – Power lines, fences, cars • Apply a “walk-away” test – Start at source – Walk-away until readings normalize – note distance

Unwanted Magnetic Noise Examples

Data Interpretation Pitfalls

• Incorrect grid spacing • Contour interval too large or small • Cultural noise not properly addressed • No data maps or reference points • Use of color maps in reports that are photocopied in B&W

Overview of Environmental Geophysics 14 Magnetic Methods

Mag Anomaly Example 1

• 1 Crushed drum Contour Map (lying vertical) 10 • Depth: -4.5’ to -8.5’ • Values: +26 to -54 • Contour interval: 10 0 • Blues: pos. values

• Reds: neg. values 10

10 0 10 Feet

Example 1 Source

Mag Anomaly Example 2

20 Contour Map • 5 Crushed drums • Depth: -5’ to -6’ 10 • Values: +78 to -171 0 • Contour interval: 35 • Blue: pos. values 10 • Reds: neg. values

20 10 0 10 20 Feet

Overview of Environmental Geophysics 15 Magnetic Methods

Example 2 Source

Mag Anomaly Example 3

Contour Map • 1 Drum (horizontal) 10 • Depth: -3’ to -6’ • Values: +111 to -572 • Contour interval: 35 0 • Blues: pos. values

• Reds: neg. values 10

10 0 10 Feet

Mag Anomaly Example 4

20 • 2 Iron pipes: 10’ x 4” Contour Map

• Depth: -1.7’ to -2’ 10 • Values: +129 to -238

• Contour interval: 35 0 • Blues: pos. values • Reds: neg. values 10

2010 0 10 20 Feet

Overview of Environmental Geophysics 16 Magnetic Methods

Mag Anomaly Example 5

• Two 500 gal. tanks Contour Map • Depth: -2’ to -7’ 10 • Values: +1114, -120 • Contour interval: 35 0 • Blues: pos. values • Reds: neg. values 10 20 10 0 10 20 Feet

Tank Removal

In-Situ This Geophysical Stuff Actually Works!

500 Gallon Tanks

Mag Anomaly Example 6

Overview of Environmental Geophysics 17 Magnetic Methods

Example 6 Tank Removal

Mag Anomaly Example 7

Mag Anomaly 7 Removal

Overview of Environmental Geophysics 18 Magnetic Methods

Confirmatory Methods for Magnetics

Magnetics Rapid Data Collection Establish Amount of Mass General Lateral Dimensions

GPR Electromagnetics Depth to Target Detailed Lateral Dimensions Top of Target Shape Generalized Depth Information (dependent on soil conditions) (dependent on Tx & Rx range)

Example of Landfill Mag Data

Overview of Environmental Geophysics 19 Magnetic Methods

More Landfill Mag Data

Vacant Field Mag Data

Marine Cesium Magnetometer

• Towed by boat • X-Y location control by GPS • Depth control by line & speed or floatation device

Geometrics

Overview of Environmental Geophysics 20 Magnetic Methods

Surface Mag in Aluminum Boat

Marine Applications

• Lake George Channel • Indiana Harbor Canal • Looking south Indianapolis blvd. bridge

Marine Cesium Magnetometer Data

Overview of Environmental Geophysics 21 Magnetic Methods

Search for CSS Hunley Using Magnetics

CSS H. L. Hunley – USS Housatonic

August 29th 1862 - February 17th 1864 U.S. Naval historical Center Photograph

Battle Site Mag Anomalies

A B

A, B, C Courtesy: Submerged Cultural Resources Unit - National Park Service – Santa Fe, NM

C

Morris + Bailey

Overview of Environmental Geophysics 22 Magnetic Methods

What’s Wrong With This Picture?

Requesting A Survey (Questions Provider Should Ask You) • How big is the site • Composition of targets • Orientation & size of targets • Depth or burial method of targets • Describe terrain & site conditions • Explain special circumstances

Provider Submits Plan (Questions You Should Ask)

• Why are selected method(s) appropriate? • What tool & configurations will be used? • Method to ensure data location accuracy? • What deliverables will be provided? • Will data be presented for the layperson? • How can I relocate area at a later date?

Overview of Environmental Geophysics 23 Magnetic Methods

Limitations

• Subject to cultural noise • Detection of small objects reduced with depth • Depth estimates most difficult for non- homogenous masses • Masses cannot be uniquely characterized

Summary & Conclusion

• Magnetometers detects ferrous metal & fired clays • Non-invasive, passive detection method • Quantitative results relative to amount of mass • Large masses detectable at significant depths • Complements other geophysical methods • Note: Magnetometers are different from metal detectors – metal detectors emit energy to detect metal – magnetometers passively measure ambient conditions

Overview of Environmental Geophysics 24 Electromagnetic Methods

ELECTROMAGNETIC (EM) METHODS

Module Goals

 Describe electromagnetic methods in general  Explain the differences between these two types of electromagnetic instrumentation  Describe the application of the two types in the field of environmental geophysics

EM Methods

 Often used with magnetics  Fast and inexpensive  Measures conductivity  Frequency Domain  Time Domain

Overview of Environmental Geophysics 1 Electromagnetic Methods

Frequency Domain EM (FDEM)

 Fixed Frequency - Fixed Depth  Multiple Frequency - Variable Depth  Reads Conductivity Directly  Metal Detection

Frequency-Domain EM

T R

Varying electric field

Varying magnetic field

Eddy currents

Frequency-Domain EM

T R

Varying electric field

Varying magnetic field

Eddy currents

Overview of Environmental Geophysics 2 Electromagnetic Methods

FDEM Signal Components

 The secondary magnetic field has two components  Quadrature phase - used to measure ground conductivity - 90° out of phase with primary field  In-phase - used to detect excellent conductors (metal) - 180° out of phase with primary field

EM-31

 ~ 4.5 meter maximum depth (3.66 m coil spacing)  Operating frequency 9.8 kHz  Soil conductivity (mS/m) - quadrature phase  Metal detection (ppt) - in-phase component

EM-31

Marion Landfill, Marion, IN

Overview of Environmental Geophysics 3 Electromagnetic Methods

Depth of Penetration

 ~1.5 x coil spacing for vertical dipole  ~.75 x coil spacing for horizontal dipole

EM-34

 Three coil spacings –  10 m. (6.4 kHz)  20 m. (1.4 kHz)  40 m. (0.4 kHz)  Soil conductivity - quadrature phase  Coil spacing - in-phase component

Air National Guard Base, Alpena, MI

EM-34 transmitter

Overview of Environmental Geophysics 4 Electromagnetic Methods

Gem-2 and 3

 Multi-frequency signal  Variable depth of investigation  Output is secondary magnetic field (ppm) to the primary magnetic field

Gem-2

Overview of Environmental Geophysics 5 Electromagnetic Methods

Conditions Affecting Conductivity

 Soil type  Moisture  Cultural debris  Pore fluid

 Advantages  Fast, inexpensive  Reasonable lateral resolution  Limitations  Limited depth of penetration  Sometimes difficult to interpret  Many noise sources

Frequency Domain EM

Examples

Overview of Environmental Geophysics 6 Electromagnetic Methods

Vacant lot in Toronto

Overview of Environmental Geophysics 7 Electromagnetic Methods

Lot after excavation

EM-31 In-phase survey

EPA Gem2 data Northridge, IL 21030 Hz In phase data

Overview of Environmental Geophysics 8 Electromagnetic Methods

. GEM 2

Apparent Conductivity

GEM-2

Magnetic Susceptibility

Time Domain EM (TDEM)

 Square Wave signal - Variable Depth  Conductivity at depth  Metal Detection

Overview of Environmental Geophysics 9 Electromagnetic Methods

Time Domain EM (TDEM)

TR

PRIMARY MAGNETIC FIELD DECAYING SECONDARY MAGETIC FIELD

TDEM TR SURFACE

TIME AND DEPTH

TDEM Metal Detector

Overview of Environmental Geophysics 10 Electromagnetic Methods

TDEM Metal Detector

EM-61 MK2

Site near Dayton, OH

Time Gates EM-61 MK2

 Channel 1 – 216 μ seconds (bottom coil)  Channel 2 – 366 μ seconds (bottom coil)  Channel 3 – 660 μ seconds (bottom coil)  Channel 4 – 1266 μ seconds (bottom coil)  Channel T – 660 μ seconds (top coil)

Overview of Environmental Geophysics 11 Electromagnetic Methods

EM-61 Data Display

35

TDEM Metal Detector

 One transmitting coil  Two receiving coils  Ability to discriminate depth and screen surface metal  Depth of detection about 3.5 meters

Advantages and Limitations of TDEM Detectors

 Advantages  Fast and inexpensive  Easy to interpret  Excellent lateral resolution  Unaffected by conductive soil  Limitations  Limited depth of penetration - 3.5 meters  No geologic data

Overview of Environmental Geophysics 12 Electromagnetic Methods

Time Domain EM

Examples

Overview of Environmental Geophysics 13 Electromagnetic Methods

Overview of Environmental Geophysics 14 Gravity Method

Gravity Method

GRAVITY

Gravity

• Theory and measurement • Targets • Surveys

Overview of Environmental Geophysics 1 Gravity Method

Gravitational Equation g=GM/R² Where: • g = acceleration due to gravity • G = Universal Gravitational Constant • 6.67191 x 10-11 m3 kg-1 S-2 • M = mass of Earth • R = radius of Earth

Unit of g gal

1 µ gal = 1 x 10-6 cm sec2

Field Measurements

R elevation

g is measured g is related to density

Overview of Environmental Geophysics 2 Gravity Method

Types of Measurements

Absolute - high accuracy fixed instrument actual acceleration

Relative - good precision mobile instrument related to density

Gravimeters

• Falling body • Pendulum • Metal zero-length spring •L & R • Quartz zero-length spring •Scintrex

Overview of Environmental Geophysics 3 Gravity Method

Corrections to Measurements

• Latitude • Free air • Bouger slab • Terrain correction • Earth

Gravity Corrections

BOUGER SLAB FREE AIR

DATUM

Gravity Targets

• Density changes • Voids

Overview of Environmental Geophysics 4 Gravity Method

Density Change

3 1.8 gm/cm INTRUSION

BURIED VALLEY

3 2.3 gm/cm3 2.8 gm/cm

Voids

TANK CAVE 2.2 gm/cm3

0 gm/cm3 0 gm/cm3 1 gm/cm3

2.5 gm/cm3

Overview of Environmental Geophysics 5 Gravity Method

Noise Sources

• Cultural sources •Wind • Microseisms

• Unique property measured • Void detection • Low noise

Limitations

• Limited environmental targets • Expensive • Skilled operator needed • Need a target

Overview of Environmental Geophysics 6 Gravity Method

GRAVITY SURVEYS

Types of Surveys

• Transects • Areal surveys

Survey Setup

• Know the target • Note linearities on maps, etc. • Karst areas, note sinkholes, etc. • Obtain elevations • Establish base station if necessary • Obtain Earth data

Overview of Environmental Geophysics 7 Gravity Method

Instrument Setup

• Let instrument stabilize • Place station markers • Determine marker elevation • Set up instrument • Measure height • Take measurement

Let Instrument Stabilize

Scintrex Gravimeter

Place Station Markers

Overview of Environmental Geophysics 8 Gravity Method

Set Up Instrument

Measure Height

Take Measurement

Data Interpretation

• Data processed • Data plotted • Compared with known features • Modeled

Overview of Environmental Geophysics 9 Gravity Method

Data Confirmation (Ground Truth)

• Drilling • Trenching • Further geophysical surveys

Quick tides screen shot

Overview of Environmental Geophysics 10 Seismic Methods

Seismic Methods

SEISMOGRAPH

GEOPHONES

ENERGY SOURCE

DIRECT WAVE

REFRACTED WAVE SOIL

BEDROCK

Module Goals

• Describe subsurface acoustic energy travel • Describe site geology effect on the seismic method • Determine site subsurface information using seismic data • Determine effectiveness of seismic method in varying conditions • Operate seismic equipment

Overview of Environmental Geophysics 1 Seismic Methods

Seismic Method

Geophysical method that involves inducing sound waves (energy) into the ground and recording the time it takes for the energy to travel through the subsurface

Seismic Method (cont.)

Because of different acoustic velocities, sound waves reflect or refract as they cross geologic boundaries

Reflection vs. Refraction

Refraction

Reflection

Overview of Environmental Geophysics 2 Seismic Methods

Subsurface Acoustic Wave Travel

Subsurface Wave Travel

Governed by principle of Index of Refraction (Snell's Law)

Subsurface Wave Travel Snell's Law

Describes how acoustic waves react when they encounter the boundary of two layers having different velocities

Overview of Environmental Geophysics 3 Seismic Methods

Subsurface Wave Travel (cont.)

As an acoustic wave encounters a layer having a higher velocity, part of the wave is reflected and part is refracted

ENERGY SOURCE DIRECT WAVE Surface REFLECTED WAVES

V = 1500 ft/sec Unconsolidated sand and gravel

V = 10,000 ft/sec Limestone

ENERGY SOURCE DIRECT WAVE Surface

V = 1500 ft/sec Unconsolidated sand and gravel

V = 10,000 ft/sec Limestone

Overview of Environmental Geophysics 4 Seismic Methods

ENERGY SOURCE DIRECT WAVE Surface

V = 1500 ft/sec Unconsolidated sand and gravel

V = 10,000 ft/sec Limestone

ENERGY SOURCE DIRECT WAVE Surface

V = 1500 ft/sec Unconsolidated sand and gravel

V = 10,000 ft/sec Limestone

ENERGY SOURCE DIRECT WAVE Surface

V = 1500 ft/sec Unconsolidated sand and gravel

V = 10,000 ft/sec Limestone

Overview of Environmental Geophysics 5 Seismic Methods

ENERGY SOURCE DIRECT WAVE Surface

REFRACTED V = 1500 ft/sec WAVES Unconsolidated sand and gravel

TRANSMITTED WAVES V = 10,000 ft/sec Limestone

ENERGY SOURCE DIRECT WAVE Surface REFRACTED WAVES

V = 1500 ft/sec Unconsolidated sand and gravel

TRANSMITTED WAVES V = 10,000 ft/sec Limestone

Direct Wave B First Arrival Geophones SOURCE 10 20 30 40 50 60 70 80 90 100

V = 1500 ft/sec

V = 10,000 ft/sec

Overview of Environmental Geophysics 6 Seismic Methods

Direct Wave – First Arrival Geophones SOURCE 10 20 30 40 50 60 70 80 90 100

V = 1500 ft/sec

V = 10,000 ft/sec

Wave Signatures

10 9080706050403020 0 10 20 30 40 50 60 70 80 90 100 110 120

Determine Subsurface Information

Overview of Environmental Geophysics 7 Seismic Methods

Time Travel

The time required for a wave to travel a given distance is based on the velocity of the geologic unit or units

Curve Plotting

By plotting arrival time vs. geophone distance from source, velocities and depths to geologic units can be determined

SHOT POINT 12 6543 78 9101112

V = 1500 ft/sec

V = 10,000 ft/sec

V = 10,000 ft/sec

Intercept time (ti)

Crossover distance (XC)

10 20 30 40 50 60 70 80 90 100 110 120 Distance (in feet)

Overview of Environmental Geophysics 8 Seismic Methods

Velocity Calculations B Curve Plotting

Geologic unit velocities and depths can be derived from the time-distance plot using mathematical or graphic methods

Mathematically-derived Velocities

x Upper layer V = t

Second layer V= x t – t

Third layer V = x

(t1 –t2)

Mathematically-derived Depths

•Intercept-time formula •Crossover distance formula

Overview of Environmental Geophysics 9 Seismic Methods

Intercept-time Formula

To determine depth to Layer 2, (Z1):

x V2 –V1 (Z1) = 2 V2 + V1

Crossover Distance Formula

To determine depth to Layer 2, (Z1):

t i V1 V2 (Z1) = 2 2 2 (V2) –(V1)

Intercept-time Formula

To determine depth to Layer 3, (Z ):

2 2 2Z1 (V3) –(V1) V V (Z ) = ½ t2 – 3 2 2 2 2 V V (V3) –(V2) ( 3 1 )

Overview of Environmental Geophysics 10 Seismic Methods

Common Velocity Ranges

Sand and gravel (dry) 1,500 - 3,000 ft/sec Sand and gravel (saturated) 2,000 - 6,000 ft/sec Clay 3,000 - 9,000 ft/sec Water 4,800 ft/sec Sandstone 6,000 - 13,000 ft/sec Limestone 7,000 - 20,000 ft/sec Metamorphic rock 10,000 - 23,000 ft/sec

Reference: Bison Instruments, Inc.

Effectiveness of Seismic Refraction Method

Seismic Refraction

Seismic refraction is used to determine: • Depth to saturated zone () • Top of • Facies variation in an aquifer

Overview of Environmental Geophysics 11 Seismic Methods

Seismic Refraction (cont.)

Seismic refraction is generally used for: • Mapping unconsolidated alluvial • Investigation of limestone or sandstone aquifer • Unsaturated consolidated deposits underlaid by saturated consolidated deposits

• Less expensive • Fast interpretation • Shallow investigations • Useful in a wide variety of geological environments

Refraction Limitations

• Problems identifying •Drums • Thin beds • Velocity reversal • Limited number of rock units identified • Complex data in steeply dipping formations • Poor lateral resolution of velocity variations

Overview of Environmental Geophysics 12 Seismic Methods

Seismic Reflection

• Acoustic energy encounters a boundary between two geologic layers • If the acoustic impedance contrast is large enough some of the energy is reflected and the rest is transmitted • Resolution of the thickness may be difficult for thin beds

• No hidden bed problem • Less spread out arrays • Higher resolution

• More involved to interpret • More expensive than refraction • Works only in special cases for shallow investigations • Generally used for deeper investigations (greater than 50 feet)

Overview of Environmental Geophysics 13 Seismic Methods

Seismic Equipment

Seismic Equipment Components

•Energy source •Geophones •Seismograph

Seismic Equipment Energy Sources

Energy sources include:

•Hammer •Weight drop •Shotgun slug (10 Gauge) •Explosives

Overview of Environmental Geophysics 14 Seismic Methods

Energy Sources Sledge Hammer

Advantage: • Very Portable Disadvantages: • Strenuous operation • Limited Depth -20- 60 feet of unsaturated material -Maximum Depth ~ 100 feet below surface

Energy Sources Weight Drop and Shotguns

Advantage: • Good depth range (100 - 300 feet)

Disadvantage: • Not as field portable

Seisgun

Overview of Environmental Geophysics 15 Seismic Methods

Noise Sources

• Vehicular traffic • Construction zones • Wind blowing in trees • People walking

Seismic Equipment - Geophones

• Electromagnetic transducers that detect vibrations • Placed into ground at appropriate spacing • Spacing array determined by survey

Seismic line of geophones with cable

Overview of Environmental Geophysics 16 Seismic Methods

Geophone →

Cable connector

Seismograph

Stores and displays time when each signal is received at each geophone

Laptop with Geophone switch seismic software box Cable to geophones

Overview of Environmental Geophysics 17 Seismic Methods

Seistronix control panel from laptop

Seismograph (cont.)

When energy source is repeated, the signals will be averaged or stacked. This is done to improve the signal to noise ratio and to cancel out random noise.

Seismic refraction line interpretation

From MN DNR

http://www.dnr.state.mn.us/maps/geophysics.html

Overview of Environmental Geophysics 18 Seismic Methods

Tomographic Survey Lines

Tomographic Survey Diagram

Raw Seismic Data

Overview of Environmental Geophysics 19 Seismic Methods

Processed Data Plot

Target Revealed

Overview of Environmental Geophysics 20 Electrical Resistivity Method

ELECTRICAL RESISTIVITY METHOD

I

V

C C2 1 na P1 P2 na (A) (M)a (N) (B)

Module Goal

Determine the applicability of electrical resistivity in locating various targets

Module Objectives • Describe the method • Simple of resistivity • Effects of the earth media • Various arrays available – 1D, 2D, 3D and 4D • Interpretation methods - models • Describe advantages and limitations of the method • Field and interpretation complications • Operate the instrument

Overview of Environmental Geophysics 1 Electrical Resistivity Method

Electric Flow in Geologic Materials

Flow controlled by the matrix and the amount of moisture in interstitial pores

Terms

Coulomb – Fundamental unit of charge Current – Charge/time (in amps) – I Voltage – Work/charge (potential drop) – V Ohm – Measurement of resistance ( friction) – R

Ohm's Law V = IR Where: V = Potential difference between two surfaces of constant potential I = Current in a conducting body R=Resistance between surfaces V R = I

Overview of Environmental Geophysics 2 Electrical Resistivity Method

Electrical Flow in Geologic Materials

Wenner Array T

V

C1 a P1 aaP2 C2 A a

Apparent Resistivity ( app)

If earth were uniform, apparent resistivity would not vary.

Variations in resistivity can be interpreted as deviations from a uniform earth model.

Apparent Resistivity Calculation V  app = K I

Wenner: K = 2a Schlumberger: K = n(n+1)a Dipole-Dipole: K = n(n + 1)(n+2)a

Overview of Environmental Geophysics 3 Electrical Resistivity Method

Resistivity Controlling Factors

• Porosity • Saturation • Dissolved salts in groundwater • Clay content

Resistivity General Conditions

• Most soil and rock minerals are electrical insulators with high resistivity (exceptions: clay, magnetite, graphite, pyrite) • Electrical current is transmitted through rock by ions in pore waters • Low resistance contaminants may decrease apparent resistivity

Resistivity Method

Measurement of electric fields caused by current introduced into the ground as a means for studying the electrical resistivity structure of the earth

Overview of Environmental Geophysics 4 Electrical Resistivity Method

Resistivity Profiling

Fixed electrode spacing used to obtain horizontal changes in resistivity

Wenner

I

V

a a a C1 P1 P2 C2 (A) (M) (N) (B)

Resistivity Profiling

Measurement 1 C1 P1 P2 C2

Measurement 2 C1 P1 P2 C2

Overview of Environmental Geophysics 5 Electrical Resistivity Method

Profiling in Appropriate Terrain

Resistivity Sounding

Electrode spacing increased to obtain resistivity changes from successively greater depths (electric drilling)

Schlumberger

I

V

na na C1 P1 P2 C2 (A) (M) (N) (B)

Overview of Environmental Geophysics 6 Electrical Resistivity Method

Resistivity Sounding

C1 P1 P2 C2 Measurement 1

C1 P1 P2 C2 Measurement 2

Sounding in Appropriate Terrain

Dipole-dipole

I V

C1 a C2 na P1 a P2 (A) (B) (M) (N)

Overview of Environmental Geophysics 7 Electrical Resistivity Method

Dipole-dipole Data Collection

I V I V

1a 2a C1 a C2 P1 a P2 C1 a C2 P1 a P2

I V I V

3a 4a C1 a C2 P1 a P2 C1 a C2 P1 a P2

Data Points from Dipole-dipole Array

Data Points from Dipole-dipole Array

Overview of Environmental Geophysics 8 Electrical Resistivity Method

Sting Resistivity Unit

Mini-Res Resistivity Unit

Summary of Surveys

 1 D survey – Schlumberger, Wenner

 2 D survey – Dipole-Dipole  3 D survey – multiple Dipole-Dipole or grid of electrodes on surface

 4 D survey – 3 D surveys carried out over successive time periods

Overview of Environmental Geophysics 9 Electrical Resistivity Method

Modeling Examples

 Forward Modeling

 1 D inverse model  2 D inverse model

 3 D inverse model

Ra Data Chart 1 D survey

104 2.43% RMS

103

102

10 Apparent resistivity (ohm-m) resistivity Apparent

1 L 1 10 102 103 104 Electrode spacing (m)

Resistivity (ohm-m) 102 103 104 R 10–1

1 1 D Model Chart

10 Depth (m) 102

103

D

Overview of Environmental Geophysics 10 Electrical Resistivity Method

2 D Inversion model

0.5 2.9 6.5 9.0 12.2

16.1 INVERSE MODEL RESISTIVITY SECTION

44.0 82.2154 287 536 1001 1869 3491 Resistivity in ohm.m Unit electrode spacing 3.0 m.

SOIL 3.0 6.0 9.0 CAVERN LIMESTONE

2 D Forward and Inverse Models

• Quantitative modeling is possible • Can estimate depths, thicknesses, and resistivities of subsurface layers • May estimate resistivity of saturating fluid • Finds void locations

Overview of Environmental Geophysics 11 Electrical Resistivity Method

Electrical Resistivity Limitations

• Cultural noise • Large, clear area required • Labor intensive (2- to 3-person crew) • Lack of resolution in some cases

Can Target be Resolved?

• Media • Target • Contrast

Resistivity Targets

• Buried stream channels • Freshwater/saltwater interface • Depth to groundwater/bedrock interface • Cavities/sinkholes

Overview of Environmental Geophysics 12 Borehole Geophysics

Borehole Geophysics Applied to Bedrock Hydrogeologic Evaluations

Don Bussey, CPG-08847 USEPA/ERT – Las Vegas, Nevada [email protected]

Borehole Geophysical Tools:

• Natural Gamma • Temperature • Caliper • Conductivity/Resistivity • Borehole Video • Heat-Pulse Flowmeter • Optical and Acoustic Televiewer • Borehole Deviation • Casing Collar Locator (Magnetic) • Cement Bond Log (Sonic)

Overview of Environmental Geophysics 1 Borehole Geophysics

Natural Gamma and Temperature Logging

Gamma logging is useful in evaluating stratigraphic sequences and for borehole to borehole correlation.

Temperature logging can aid in detection of groundwater flow in or out of a borehole.

Overview of Environmental Geophysics 2 Borehole Geophysics

Caliper Logging

Caliper logging measures borehole diameter, useful in detecting fractures or voids in open-hole bedrock boreholes.

Overview of Environmental Geophysics 3 Borehole Geophysics

Electrical Conductivity Logging

Resistivity/Conductivity Logging

1 2 4 3

Borehole Video Logging

Borehole video logging provides a visual picture of borehole conditions.

Useful in identifying fractures, voids, cascading water, well/boring blockage and other downhole trouble shooting.

Overview of Environmental Geophysics 4 Borehole Geophysics

Borehole Video Log

Overview of Environmental Geophysics 5 Borehole Geophysics

Heat-Pulse Flowmeter Logging

Heat-pulse Flowmeter logging is used to measure vertical flow within a well at discrete vertical intervals (> 0.1 gpm). Useful in determining depths where water may be entering or leaving a borehole.

Optical and Acoustic Televiewer Logging

• Televiewer logging presents a 360-degree acoustic or optical digital borehole representation. • Useful in evaluating fractures, bedding, and voids. • Strike and dip of fractures can also be calculated.

Overview of Environmental Geophysics 6 Borehole Geophysics

Formation Gamma Caliper ATV OTV Resistiv Temp Flow

Overview of Environmental Geophysics 7 Borehole Geophysics

Virtual Core Using Optical Televiewer Data

Borehole Deviation Logging

• Useful to determine borehole deviation

• Useful to evaluate whether packer assemblies can be utilized downhole

Overview of Environmental Geophysics 8 Borehole Geophysics

Well Depth >700 feet Well Base Elev. Difference 650 feet Deviation 143 feet 0.215 ft/ft

Overview of Environmental Geophysics 9 Borehole Geophysics

Oil & Gas Well Abandonment Applications Casing Collar Locator and Cement Bond Logging

Used in the oil patch during oil and gas well abandonment.

Casing Collar logs (magnetic) used to identify casing collars for targeting during casing shoot offs.

Cement Bond logs (sonic) identify presence of cement behind logged casing – useful during casing perforating.

Cement Bond Logs important in Underground Injection well certification.

Overview of Environmental Geophysics 10 Borehole Geophysics

Borehole Geophysical Data - Uses

• Groundwater Sampling Strategy

• Packer Test Design

• Discrete-zone Multi-level Assembly Design (Westbay, Flute, Solinist , etc.)

Overview of Environmental Geophysics 11 Borehole Geophysics

Acoustic and Optical Televiewer Logs

Heat-Pulse Flowmeter Logs

Case Study Interpretations

Overview of Environmental Geophysics 12 Borehole Geophysics

Santana Community Production Well

Corozal, Puerto Rico

Cayuga County Groundwater Contamination Site

Cayuga County, New York

Overview of Environmental Geophysics 13 Borehole Geophysics

Geophysical, Stratigraphic, and Flow-Zone Logs EPA-1

Formation Gamma Caliper AcousticLithologic Optic Resistiv Temp Flow L

Ground water at the 630 monitor wells in the 630 Onondaga Limestone 635 flows NW and NE 640

560

555 Ground water at the EPA test wells in the Bertie Fm. 550 flows South then SW 545

Overview of Environmental Geophysics 14 Borehole Geophysics

Conclusion

Borehole Geophysics can help Understanding Geology, Hydrogeology, and Chemistry in Bedrock Geologic Settings

Don Bussey, CPG-08847

USEPA-ERT/Las Vegas

Overview of Environmental Geophysics 15

References / Sources Borehole Geophysics

• Geophysical logging instruments • Mount Sopris Instruments, www.mountsopris.com • Applications of Borehole Geophysics to Water-Resources Investigations, USGS • http://pubs.usgs.gov/twri • http://ny.water.usgs.gov/projects/bgag/intro.te xt.html

What is GPR?

 Technique to collect & record information about subsurface  Non-intrusive method for near surface zones  Provides most detail of all geophysical methods  Many parameters involved to apply method optimally Credit: GSSI & SERDP

GPR Basics

 Acronym for Ground Penetrating Radar  “Ground” can be soil, ice, rock, concrete, water, wood - anything non-metallic

 Emits a pulse into the ground

 Records echoes

 Builds an image from the echoes

Overview of Environmental Geophysics 1 Ground‐Penetrating Radar

GPR Similar to Fish Finder - Echo Sounder Processes

 Sends pulsed focused signal

 Signal scattered back from obstacles in path; fish, bottom

 Result: single record has 2 blips

Credit: Sensors & Software at different times

Fish Finder & Echo Sounder Decoding Processes

 Continuous data collection

 Recordings displayed side by side

 Result: a 2D cross- section of the subsurface

Credit: Sensors & Software

Where Similarities End Between GPR & Echo Sounder Processes

Echo Sounder GPR

 Acoustic pulse vs  Electromagnetic pulse  Sound wave vs  Electromagnetic wave propagation propagation  Only for aquatic vs  Limited multi-matrix conditions applications  Analyzes reflections  Analyzes reflections

Overview of Environmental Geophysics 2 Ground‐Penetrating Radar

Typical GPR System

 Digital video logger  Transmitter & Receiver antenna  Odometer controlled  GPS

GPR Theory

GPR: A Wave-Based Technique

• Wave energy travels at a characteristic wave speed • Dependent on the material through which it travels • This is the main difference between GPR and EMI.

Overview of Environmental Geophysics 3 Ground‐Penetrating Radar

Wave Properties The wavelength of a wave is the distance between any two adjacent corresponding locations on the wave train.

Wavelength

Time

Frequency refers to how many waves are made per time interval. This is usually described as how many waves are made per second, or as cycles per second.

What Are Nanoseconds?

 GPR time is measured in units of nanoseconds

 1 nanosecond is 1 billionth of a second =1/1,000,000,000 second

 GPR signals travel 1 ft (0.3m) in air in 1 nanosecond

 ns is the abbreviation for nanosecond

Electromagnetic Spectrum

Top Scale Short wavelength Wavelength in Meters Long wavelength

10-12 10-10 10-8 10-6 10-4 10-2 1 100 104 106 108

1020 1018 1016 1014 1012 1010 108 106 104 100 1 (GHz) (MHz) (KHz) High frequency Bottom Scale Low frequency Frequency in Hertz

GPR = 10 to 1000 MHz range

Overview of Environmental Geophysics 4 Ground‐Penetrating Radar

The Process Begins

 Data collected along closely spaced transects within a grid

 An active process of transmitting EM pulses from surface antennas into the ground

 Measures time elapsed between when pulses are sent and received at the surface

 Pulses transmitted through various matrices, target features, cause velocities to change

 Changes in velocities plotted to create image

Transmitted Pulses & Why Their Velocities Change

 Physical & electrical attributes of a medium control how fast electromagnetic waves travel

 Physical attributes  Moisture, density, porosity, chemical properties  Electrical attributes affecting energy propagation  Relative Dielectric Permittivity (RDP)  Controls wave speed  Electrical conductivity  Signal attenuation  Reflection Strength

Relative Dielectric Permittivity (RDP)

 Also called: dielectric Typical RDP Values (k) constant Air 1 Ice 3-4  Dry Sands 4 Measure of ability of a Granite 5 material to store a Limestone 6 Saturated Sands 25 charge from an applied Silts 5-30 Clays 5-40 EM field and then Water 81 transmit that energy  The greater the RDP, the slower radar energy will move through it

Overview of Environmental Geophysics 5 Ground‐Penetrating Radar

Electrical Borehole Conductivity Conductivity Data

 Ability of a material to conduct electric current

 Conductivity increases with increase in water and/or clay content

 Higher conductivities limit depth

 Conversion of EM energy to heat

Reflection Strength (2 Layer Model)

k 2 - k 1 r =

k 2 + k 1

k 1 = relative dielectric permittivity of first layer

k 2 = relative dielectric permittivity of second layer

Reflection Strength

r = 0 to 0.2 weak reflections

r = 0.2 to 0.3 moderate reflections

r = greater than 0.3 strong reflections

Metal reflects nearly 100% of a radar wave

Overview of Environmental Geophysics 6 Ground‐Penetrating Radar

Conjoined Electrical & Magnetic Waves EM Energy Transmission

Credit: SERDP

• Dielectric mediums allow passage of significant electromagnetic energy without dissipating energy • The more electrically conductive a material: – the less dielectric it is – energy will attenuate at a much shallower depth • In highly conductive mediums – the electrical component of the propagating electromagnetic wave is conducted away in the ground – when this happens the wave as a whole dissipates • For propagation to occur the electrical and magnetic waves must constantly "feed" on each other during transmission.

Types of Factors Limiting Radar Penetration

 Minerals in medium creating free ions & increase electrical conductivity  Sulfates, carbonate minerals, iron & salts or any charged elemental mineral = high conductivity  Iron rich soils have high magnetic permeability  Radar energy will not penetrate metal, it is reflected 100%

Pulse & Echo Traverses Through Soil Matrix & Target

From Ground Surface to Increasing Depths

Single Transmitted Pulse

Tx & Rx Antenna Dry Wet Air Soil Soil Clay At Ground Drum Surface

Multiple Echo Pulses From Target

Overview of Environmental Geophysics 7 Ground‐Penetrating Radar

Transmitted Pulses Have Frequency Options

 Operator has choices to select limited range of frequency options  Each frequency option has trade-offs  Higher freq. = better target resolution  Lower Freq. = greater depth of penetration

Antenna Characteristics

Frequency Depth Resolution (MHz) (feet) (feet)

250 5-45 0.5

500 1.5-12 0.3

1000 0-1.5 0.05

Examples of Differing Frequency & RDP Options

Wavelength Wavelength Wavelength Antenna Wavelength in medium in medium in medium Frequency Frequency with with with RDP=5 (MHz) (meters) RDP=15 RDP=25 (meters) (meters) (meters)

1000 0.3 0.13 0.08 0.06 900 0.33 0.15 0.09 0.07 500 0.6 0.27 0.15 0.12 300 1 0.45 0.26 0.2 120 2.5 1.12 0.65 0.5 100 3 1.34 0.77 0.6 80 3.75 1.68 0.97 0.75 40 7.5 3.35 1.94 1.5 32 9.38 4.19 2.42 1.88 20 15 6.71 3.87 3 10 30 13.42 7.75 6 Credit: SEDRP

Overview of Environmental Geophysics 8 Ground‐Penetrating Radar

Antenna Frequency

1000 500

250 MHz Credit: Sensors & Software

Viewing Data

What Creates Reflections?

 Changes in EM impedance associated with material property variations  Impedance reduces to resistance in circuits carrying steady direct current  Changes in material property: density, porosity, texture  Changes in physical properties: dielectric permittivity, electrical conductivity and magnetic permeability  Greater changes in properties, the more signal reflected

Overview of Environmental Geophysics 9 Ground‐Penetrating Radar

Individual Waveform Reflection or Reflection Trace

Reflection pulse from A composite of ground surface many wavelets Many bed boundaries & recorded from other discontinuities many depths reflect a wavelet of energy to the surface & produces a series recorded of reflections generated at one Attenuated reflections location, called a reflection trace Credit: SERDP Composed of 512 Digital Samples

Two-Way Travel Time

 Amount of time for the radio wave to make round-trip from the surface down to the reflector and back  Greater for deeper objects  Can be converted to depth if velocity is known  Measured in nanoseconds

Frequencies & Interface Reflections

1. 2. 3.

Credit: SERDP

1. Small λ at top (A) & bottom (B) produces a reflection, composite reflection trace of the two (C) can define both interfaces 2. Longer λ barely has enough definition from the top & bottom (D & E) to produce a composite reflection trace (F) that exhibits both interfaces 3. Low λ produces a wave reflecting off both interfaces (G & H), but composite reflection trace affected by constructive & destruction interference of two waves, only top interface is visible in composite reflection trace (I) λ = wavelength

Overview of Environmental Geophysics 10 Ground‐Penetrating Radar

GPR Record Traces Widely Spaced Distance along ground surface Two-Way Travel Time Travel Two-Way

GPR Display/Record Closely Spaced Distance along surface Depth Two-way travel time travel Two-way

 Energy from GPR is NOT a pencil-like beam  Footprint size varies as a function of:  Relative dielectric permittivity of material  Antenna Frequency

Elliptical Cone

Credit: SERDP

Overview of Environmental Geophysics 11 Ground‐Penetrating Radar

RDP Affecting Energy Radiation  Radar energy Tx cone becomes focused while traveling thru layers of increasing RDP, common in most soil conditions

Soil Profile Cross Section

Example of RDP Increasing with Depth

Credit: SERDP

 Cone of Tx cone becomes broader with depth when RDP is low  High RDP matrices - Tx cone is narrower

Credit: SERDP

Energy Focusing & Scattering

Small Features

a) large area presented, most energy directed back b) target presents small cross-section and scattered signal is not directed back to Rx Large Features Credits: SERDP

Overview of Environmental Geophysics 12 Ground‐Penetrating Radar

Energy & Antenna Coupling

A Street in Southern Portugal

• Cobbles had poor coupling properties, only propagated to 40-50 ns • When antennas crossed onto asphalt (A) coupling improved • Reflections visible at 60 & 90 ns at right are barely visible (B) • What would happen moving from saturated soils to large paved parking lot?

Rounded Target Signatures

• Conical projection of energy allows waves to travel in oblique direction to buried point source (1) as seen in (A) • 2-way time (t) recorded & plotted in depth below the antenna where it was recorded (2) • As many reflections are recorded when antennas move to and from buried object, the result is a reflection hyperbola (3), when all traces are viewed in profile as seen in (B)

GPR Suitability Map

Overview of Environmental Geophysics 13 Ground‐Penetrating Radar

Examples & Applications

GPR Applications

 Mapping near subsurface geology - Bedrock - Water Table - Faults and Fractures  Locating cultural objects - Drums and Tanks - Landfills and Pits - Contamination - plumes

GPR Unit Options

Credits: Sensors & Software

Overview of Environmental Geophysics 14 Ground‐Penetrating Radar

Underground Storage Tanks

Underground Storage Tanks

1 Cross-Section Slice

Post Processed Plan View Time Slices

Underground Storage Tanks

Tank Tank

0 - -0

-3

Time, ns Time, -6 Depth, feet

40 - -9 Maine 500 MHz

Geophysical Survey Systems, Inc.

Overview of Environmental Geophysics 15 Ground‐Penetrating Radar

Trench with Drums

0 - Time, ns Time,

120 - 120 MHz

Geophysical Survey Systems, Inc.

Contaminated Groundwater

Position, m 0 200 0 - 0m

10m

Time, ns Time, 600 - 20m Original survey

0 - Time, ns Time, 600 - Five years after remediation

Sensors & Software Inc.

Water Table Mapping

Dry sand & gravel Water table Wet sand & gravel

Bedrock

0 Distance, feet 75 0 -

-0 , ns Time Depth, m 400 - -15

Glacial sand and gravel deposits near Lake Superior, Ontario, Canada, 100 MHz, Sensors and Software, Inc.

Overview of Environmental Geophysics 16 Ground‐Penetrating Radar

Sinkholes

Sinkhole at Winter Park, Florida

0 Distance, feet 31 0 - Time, ns Time,

900 - 80 MHz GPR data showing developing sinkhole, Florida. GSSI

Saltwater Intrusion

Credits: Sensors & Software

 Freshwater constantly recharged by rainfall producing positive pressure head, forcing freshwater down & out to shoreline

 A boundary between fresh & saline water occurs & is often abrupt

Water-borne GPR

Antenna

Control Unit

Overview of Environmental Geophysics 17 Ground‐Penetrating Radar

Post Processing GPR Data

Survey Grid

Y-Axis

X-Axis

Series of GPR Profiles

Grumman Exploration

Overview of Environmental Geophysics 18 Ground‐Penetrating Radar

3-D GPR

Ground Surface Sand and Gravel

Water Table Sand and Gravel Gravel lenses

Bedrock

Marquette, MI 30 m by 6 m area 6 - 8 m depth Grumman Exploration

3-D GPR

Time Slices

 GPR dataset from Forum Novum site in the Tiber Valley, Italy.  Site is a Roman market place and church that were built in the 2nd century A.D.  GPR time slices revealed buried walls and foundations from the ancient Roman buildings.

Dean Goodman

Overview of Environmental Geophysics 19 Ground‐Penetrating Radar

Gas Station- Petroleum Product

Pump Elec. Island Pad Lines Fuel Piping

Time Slices

Jeff Daniels and Dave Grumman, OSU

Creosote Pit 0 100

0

Creosote -filled pit 11.5 North

Color Assignment -3,000 3,000 Relative Amplitude

Estimating Target Depth

Overview of Environmental Geophysics 20 Ground‐Penetrating Radar

Depth Calibration: 3 Methods to Measure Depth

 Estimate matrix method

 Depth to known target

 Point target hyperbola matching

Depth Calibration: Method 1- Matrix Estimate

 Measure travel time

 Need material speed

 Depth = velocity x time / 2

 How …….?

Method 1 Matrix Estimate Material Velocity (ft/ns) Air 1.0 Ice 0.56 Dry Soil 0.43 Dry Rock 0.39 Moist Soil 0.33 Concrete 0.33 Wet Soil 0.22

Better Depth Capabilities Depth Better Water 0.11

Overview of Environmental Geophysics 21 Ground‐Penetrating Radar

Method 2 Depth to Known Target

 Known depth

 Adjust velocity using inst. controls

Method 3 Point Target Hyperbola Matching

 Benefit wide beam  Localized features  Hyperbolas (inverted U’s)  Shape determines velocity  Inst. can curve match

Method 3 Point Target Hyperbola Matching

Dean Goodman

Overview of Environmental Geophysics 22 Ground‐Penetrating Radar

Method 3 Point Target Hyperbola Matching

 Determines velocity

Method 3 Point Target Hyperbola Matching

Example of shape fitting to a target response on a Sensors & Software PulseEKKO field monitor.

Optional Quick & Easy Method Estimating Exploration Depth

35 Depth =  meters

 = conductivity in mS/m

Overview of Environmental Geophysics 23 Ground‐Penetrating Radar

Planning or Reviewing a GPR Survey

Two Methods to Collect Data

• Radom Method • Grid or Systematic Method

Systematic Grid Data Collection Method

Y-Axis

X-Axis

Overview of Environmental Geophysics 24 Ground‐Penetrating Radar

Survey Design

 Proper design of GPR surveys is critical to success.  The most important step in a GPR survey is to clearly define the problem.  There are 5 fundamental questions to be asked before deciding if a radar survey is going to be effective.

Equipment Medium Variables1, Constraints2 Constraints2

1  Frequency  Composition2  Energy Radiation  Electrical properties 2  Focusing & Scatter  Water Content2 2  Energy Transmission  Signal Absorption2 2  Ground Coupling  Shape, profile2

1: What is the Estimated target depth?

 The answer to this is usually the most important

 If the target is beyond ? the range of ideal GPR conditions, GPR can be ruled out

Overview of Environmental Geophysics 25 Ground‐Penetrating Radar

2: What is the target geometry?

 Most important target factor is size

•height  If target is non- •length •width spherical, target •strike orientation should •dip •etc. be qualified

3: What are the target electrical properties?

 What is relative dielectric permittivity (K) and electrical conductivity ( ) of k  target?

4: What is the host material?

 Electrical properties of the host needs to be defined

 Need contrast in electrical k properties with host  environment

 Variations of electrical properties in the host material can create noise

Overview of Environmental Geophysics 26 Ground‐Penetrating Radar

5: What is the survey environment like?

 GPR sensitive to surroundings

 Extensive metal structures

 Radio frequency EM sources & transmitters

 Site accessibility

GPR Summary

 Reflection technique which uses radio waves to detect changes in subsurface electrical properties  Limited exploration depth in conductive soils  GPR provides the highest resolution* of any surface geophysical method  The most important step in a GPR survey is to clearly define the problem

Disclaimers

 Disclosure of product names in this report is not an implied or direct recommendation of the equipment used for this survey. It is only provided for its scientific value related to a specific method or tool used.  SERDP citations or credits refer to Strategic Environmental Research & Development Program, a Department of Defense Environmental Research Program.

Overview of Environmental Geophysics 27

GPR Suitability Map

Geophysical Instrumentation Companies

Purchase

GEM Systems Markham, Ontario http://www.gemsys.ca/ Magnetometers

Seistronix Rancho Cordova, CA http://seistronix.com/ Seismic units

Gisco Minneapolis, MN http://giscogeo.com/ EM, Gravity, Resistivity units

GSSI Salem, NH http://geophysical.com/ GPR

Mount Sopris Denver, CO http://www.mountsopris.com/ Downhole logging tools

ABEM Sundbyberg, Sweden http://www.abem.se/ Resistivity, Seismic units

L & R Instruments Incline Village, NV http://l-and-r.com/ Resistivity units

Geonics Mississagua, Ontario http://geonics.com/ EM and AMT units

Terraplus Richmond Hill, Ontario http://www.terraplus.ca/index.html Magnetometers, Resistivity u

AGI Austin, TX http://www.agiusa.com/index.shtml Resistivity units

Sensors and Software Mississagua, Ontario http://www.sensoft.ca/index.html GPR

Geometrics San Jose, CA http://www.geometrics.com Magnetometers

Rental

K.D. Jones Austin, TX http://www.kdjonesinstruments.com

Exploration Instruments Austin, TX http://www.expins.com/index.html Geomatrix Leighton Buzzard, UK http://www.geomatrix.co.uk

Environmental Equipment Harrisburg, PA http://www.envisupply.com

SJ Geophysics Delta, British Columbia http://www.sjgeophysics.com/rentals

Northwest Geophysics Redmond, WA http://www.northwestgeophysics.com/index.cfm

RT Clark Oklahoma City, OK http://www.rtclark.com/PAGE/RentalCategories.html

Apollo Geophysics Bellingham, WA http://apollogeophysics.com/index.php?page=home

Geometrics San Jose, CA http://www.geometrics.com