United States Offi ce of Emergency and July 2014 Environmental Protection 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 geology
Characterize hydrogeology Locate metal targets and voids
Physical Properties Measured
Velocity Seismic Radar Electrical Impedance Electromagnetics Resistivity Magnetic Magnetics Density Gravity
Overview of Environmental Geophysics 1 Overview of Geophysical Methods
Magnetics
Measures natural magnetic field
Map anomalies in magnetic field Detects iron and steel
Geometrics Cesium Magnetometer
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 Metal Detector
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 gravitational field
Used to determine density of materials under instrument
Maps voids and intrusions
Scintrex Gravity Meter
Ground Penetrating Radar
Transmits and receives electromagnetic energy
Maps geology
Locates cultural targets
Has very high resolution
Overview of Environmental Geophysics 5 Overview of Geophysical Methods
Noggin Ground Penetrating Radar Unit
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
Geophysical Methods Advantages
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 Geophysical Survey 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
Ground Penetrating Radar Gradient Magnetometer
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
Linking Data to a Location
• Define X and Y • X, line or longitude • Y, position or latitude • 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
nickel 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 magnetometers • 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 – Earth’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
Gradient Configurations: Adjoining or Remote
Option A Gradient Mode
Option B Base Station Mode
Overview of Environmental Geophysics 7 Magnetic Methods
Gradient Readings
• 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
Why is Gradient Data Significant?
• 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 Weather 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).
Gradient Measurements (Vertical Gradient)
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
Bad
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
EM-34 receiver
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/Limitations of FDEM Detectors
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 tides
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
Advantages
• 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 tide 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 (groundwater) • Top of bedrock • 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
Refraction Advantages
• 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
Reflection Advantages
• No hidden bed problem • Less spread out arrays • Higher resolution
Reflection Disadvantages
• 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 physics 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 mineralogy 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 = 2a 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
Electrical Resistivity Advantages
• 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.)
Groundwater Straddle Packer Testing
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
Know Your Borehole !
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
Ground‐Penetrating Radar
Ground Penetrating Radar
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
TV/Radio X-Rays Visible EMI GPR
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
GPR Energy Radiation
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
Example of Energy Radiation
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
Adjust shape
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
Radom Data Collection 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