Download - Geophysical Technologies – i Geophysical Technologies MODULE

Geophysical Technologies – i

Geophysical TechnologiesMODULE AT A GLANCE

Overview

SLIDE GT-1 – Geophysical Technologies

Magnetic Surveys

SLIDE GT-2 – Magnetic SurveysSLIDE GT-3 – Magnetic Surveys – Primary Tools and ApplicationsSLIDE GT-4 – Magnetic FieldSLIDE GT-5 – Magnetic Surveys – Survey DesignSLIDE GT-6 – MagnetometersSLIDE GT-7 – Magnetic Surveys – InterpretationSLIDE GT-8 – Magnetic Surveys – CostsSLIDE GT-9 – Magnetic Surveys – SummarySLIDE GT-10 – Magnetic Surveys – Advantages and Limitations

Electrical Resistivity

SLIDE GT-11 – Electrical ResistivitySLIDE GT-12 – Resistivity – Common ApplicationsSLIDE GT-13 – Resistivity – Physical BasisSLIDE GT-14 – Resistivity – InstrumentsSLIDE GT-15 – Field Arrays for Resistivity SurveysSLIDE GT-16 – Resistivity (Ohm-Meters) of Common MaterialsSLIDE GT-17 – Vertical Electrical Sounding ExamplesSLIDE GT-18 – Example of Electrical ProfilingSLIDE GT-19 – Resistivity ProfilingSLIDE GT-20 – Resistivity Cross SectionSLIDE GT-21 – Resistivity Profiling – CostsSLIDE GT-22 – Resistivity – AdvantagesSLIDE GT-23 – Resistivity – Limitations

EM Conductivity Surveys

SLIDE GT-24 – Electromagnetic (EM) Conductivity SurveysSLIDE GT-25 – EM Conductivity Surveys – Primary Tools and ApplicationsSLIDE GT-26 – Terrain Conductivity MeterSLIDE GT-27 – EM Conductivity Surveys – Survey Practice

Geophysical Technologies – ii

SLIDE GT-28 – Range of Electrical Conductivities in Natural Soil and RockSLIDE GT-29 – EM Conductivity Surveys – InterpretationSLIDE GT-30 – EM Conductivity Surveys – CostsSLIDE GT-31 – EM Conductivity Surveys – SummarySLIDE GT-32 – EM Conductivity Surveys – Summary (Continued)SLIDE GT-33 – EM Conductivity Surveys – LimitationsSLIDE GT-34 – Time Domain Electromagnetic (TDEM) SurveysSLIDE GT-35 – TDEM Surveys – Primary UsesSLIDE GT-36 – TDEM Surveys – SoundingsSLIDE GT-37 – EM Metal DetectorsSLIDE GT-38 – EM61 SurveySLIDE GT-39 – EM61 – CostsSLIDE GT-40 – TDEM Surveys – CostsSLIDE GT-41 – TDEM Surveys – SummarySLIDE GT-42 – TDEM Survey – Limitations

Borehole Geophysical Methods

SLIDE GT-43 – Borehole Geophysical MethodsSLIDE GT-44 – Borehole – Tools and ApplicationsSLIDE GT-45 – Borehole – ApplicationsSLIDE GT-46 – Borehole – Traditional MethodsSLIDE GT-47 – Borehole – Traditional Methods (Continued)SLIDE GT-48 – Borehole – Advanced TechniquesSLIDE GT-49 – Borehole Image Processing System (BIPS)SLIDE GT-50 – Full Waveform Sonic (FWS) LogsSLIDE GT-51 – Instrument Packages and Coincident SurveysSLIDE GT-52 – Borehole – CostsSLIDE GT-53 – Borehole – Costs (Continued)SLIDE GT-54 – Borehole – SummarySLIDE GT-55 – Borehole – Limitations

Ground Penetrating Radar (GPR)

SLIDE GT-56 – GPRSLIDE GT-57 – GPR Surveys – Physical BasisSLIDE GT-58 – GPR Surveys – InstrumentsSLIDE GT-59 – Low-Frequency Bistatic GPR AntennaSLIDE GT-60 – GPR Surveys – InterpretationSLIDE GT-61 – GPR – CostsSLIDE GT-62 – GPR – Advantages

Geophysical Technologies – iii

SLIDE GT-63 – GPR – LimitationsSLIDE GT-64 – GPR – Summary

Seismic Surveys

SLIDE GT-65 – Seismic Surveys – Interpretation Types of Seismic SurveysSLIDE GT-66 – Seismic Refraction and Reflection – ApplicationsSLIDE GT-67 – Seismic Surveys – Physical BasisSLIDE GT-68 – Refractive Seismic SurveySLIDE GT-69 – Time/Distance PlotSLIDE GT-70 – Refraction AnalysisSLIDE GT-71 – Seismic ReflectionSLIDE GT-72 – Processed Reflection RecordSLIDE GT-73 – Seismic Survey (Refraction) – CostsSLIDE GT-74 – Seismic Survey (Reflection) – CostsSLIDE GT-75 – Seismic Refraction – Advantages and LimitationsSLIDE GT-76 – Seismic Reflection – Advantages and Limitations

Summary

SLIDE GT-77 – Geophysical Technology Selection SummarySLIDE GT-78 – Electromagnetic and Borehole Geophysical Summary

Overview

GT-1Module: Geophysical Technologies

GT-1

Geophysical Technologies

Magnetic Surveys


GT-2

Magnetic Surveys

Magnetometer surveys measure the magnetic field of the earth

Ferrous metals and minerals alter the natural field

Notes:

• A body placed in a magnetic field acquires a magnetization which is typically proportional to thefield. The constant of proportionality is known as the magnetic susceptibility. Susceptibility isvery small for most natural materials. However, typically, iron rich materials have relativelylarge magnetic susceptibilities.

• Materials containing iron have permanent magnetic moments in the absence of externalmagnetic fields. An object that exhibits a magnetic moment is characterized by a tendency torotate into alignment when exposed to a magnetic field. The susceptibility of a rock typicallydepends only on its magnetite content. Certain sedimentary rocks and acid igneous rocks haverelatively small susceptibilities whereas basalts, gabbros, and serpentinites usually have relativelylarger susceptibilities.

• The magnetic field of the Earth originates from electric currents in the liquid outer core. Earthmagnetic field strengths are typically expressed in units of nanoTesla (nT). The Earth’smagnetic field varies during the day because of changes in the strength and direction of theplasma induced currents circulating in the ionosphere—these changes are referred to as diurnalvariations in the Earth’s magnetic field, or the magnetosphere. Sunspot and solar flare activitycan create irregular disturbances in the magnetic field—these changes are referred to asmagnetic storms.

Magnetic Surveys


• In simple terms, magnetometer surveys are used in environmental site assessment for a varietyof objectives as provided in the next slide.

Where would magnetic surveys be considered in site assessment?

Where would you select magnetic surveys over other types of metal detection surveys?

Magnetic Surveys


GT-3

Magnetic Surveys — Primary Tools and Applications

Underground storage tanks

Mapping of landfills

Geologic formations

Detect ordnance

Notes:

• Magnetic surveys have become more widely used in recent years due to the increasedefficiency of magnetometer systems. The most commonly used magnetometers today are calledcesium vapor magnetometers. Other types of magnetometers are available, some withadvanced sampling rates (such as the overhauser proton magnetometer) and the older typeproton precession and fluxgate type systems. These magnetometers measure the magneticfield, in units called the nanoTesla otherwise called the gamma. Measurements can be obtainedwith these systems at a rate of 10 times per second or more, allowing for rapid data collection.

• Magnetometers measure changes in the earth’s magnetic field caused by ferrous objects andgeologic formations.

• Although there are other methods for metal detection, magnetometers are useful for a numberof site objectives. These include location of underground storage tanks, assisting in assessinglandfills for ferrous metal content, assisting in mapping lateral changes in geologic formations,and detection of ordnance in active and inactive military ranges. Another use formagnetometers is the detection of abandoned steel cased wells.

Magnetic Surveys


GT-4

Magnetic Field

Naturally occurring everywhere on earth and varying

Altered in the vicinity of a magnetic body or electrical medium that is carrying current

Net

Ambient

BodyAnomaly Anomaly

Body

NORTH

Notes:

• The earth’s magnetic field induces a magnetic moment per unit volume in buried ferromagneticdebris (bottom), causing a local perturbation (anomaly) in total magnetic field (top).

• The total magnetic field measured is a vector sum of the ambient earth’s magnetic field, pluslocal perturbations caused by buried objects.

• Note the variable spacing of the lines related to the object’s geometry and the implications(gradient or flux) of making measurements in different regions around the object.

Magnetic Surveys


GT-5

Magnetic Surveys — Survey Design

Data collected along transects

Spacing determined by objective

Base Station Measurement

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30

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10

0,0 10 20 30 40 50

NDrum D

iurn

al (

nT)

Time (hrs)

40

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20

10

000 06 12 18 24

Notes:

• Most geophysical surveys are conducted along regularly spaced lines. Surveys are normallyconducted along regularly spaced grid lines. Cesium vapor magnetometers are used inserpentine search or clearance patterns in addition to the grid-based data acquisition approach. Global positioning systems are used more often now to tie the geophysical measurements toground coordinates.

• When total field measurements are being obtained, such as the case with proton and cesiumvapor instruments, a separate stationary base station magnetometer should be used. Thesecond magnetometer should be used to measure diurnal changes in the ambient magnetic fieldas well as possible magnetic storm effects. The next slide shows how the magnetic field varieswith time during the typical day.

Magnetic Surveys


GT-6

Marine Magnetometer SurveyEEGS SAGEEP 2002

Magnetometers

Measure changes in field due to ferrous or magnetically susceptible objects

Also detects geologic variations due to magnetic minerals

Monitor of diurnal changes in the magnetic field

Notes:

• Magnetometers measure changes in the magnetic field due to ferrous objects, magnetic mineralsin soils and rock, and also senses the variability of the natural magnetic field of the earth. Thisfield can vary gradually during the day (10s of nanoTeslas), or can also abruptly change due themagnetic storms (up to hundreds of nanoTeslas). The figure on the right is the result of a marinemagnetometer study mapping changes in sediment magnetic susceptibility. Contaminants in thiscase contained measurable amounts of magnetic oxides (magnetite). Areas of high magnetitecontent are shown in bright colors. These sediments contained significant amounts of runoffmaterials from sewage treatment plants, steel mill waste and storm drain outflow sediment. Thisgeophysical survey was conducted as a rapid and cost effective method to accurately estimatethe distribution of contaminated areas in a basin, prior to the collection of core sample data. Ifcore samples were used as the primary estimating tool to estimate sediments properties andpollutant levels, the distances between sample locations would lead to significant errors in theestimation of the distribution and total volume of sediments required for clean-up.

• The magnetic field is variable during the day requiring monitoring when a detailed magneticsurvey is being conducted. An alternative to using a base station magnetometer would be toconduct a magnetic gradient survey. In this case, two sensors are used, and the difference inmeasurements is calculated.

Magnetic Surveys


GT-7

Magnetic Surveys — Interpretation

Contouring-based interpretation

Notes:

• This graphic represents the common display of geophysical data when collected in either aregular spaced grid, or when using a global positioning system.

• The data presented here are total magnetic field data from one sensor. The data werecorrected for diurnal variations in the earths magnetic field. The objective in this survey was tomap the lateral extent of suspected burial trenches containing mixed waste including ferrousmetals. The total field data usually exhibits a slight horizontal gradient over the ground surface. Using specialized software, this gradient can be removed. The measurements shown in yelloware background values. The metal in the trench causes changes in the earths field that arecaused by the ferrous metal in the trenches. In many cases a positive and a negative “dipole”anomaly is present in the data. This type of anomaly varies depending upon the type of materialin the trench, its orientation, and the location on the earths surface.

• The illustrated magnetic data in this slide is from a survey to delineate the presence of disposaltrenches. The data were collected at Tinker Air Force Base. Contours of total field ormagnetic gradient commonly are used as interpretation tools. Other advanced processing canbe done to minimize the variety of anomaly shapes encountered in magnetic surveys.

Magnetic Surveys


• Survey design for magnetic surveys, and other “grid based “ geophysical methods is animportant consideration. Line spacing and sample intervals are important considerations. These parameters are based upon the expected size of the object or feature being investigated. These parameters may range from defining a single tank or drum or delineating the extent of alandfill or plume.

In this case, the trenches are shown as anomalies with a positive peak to the south, and anegative peak on the north end of the trench. It is quite evident where the ferrous metal hasbeen buried on this site. Using this information, the sampling of soils, placement of wells, andassessment of the extent of the trenching activity was readily determined.

Magnetic Surveys


GT-8

Magnetic Surveys — Costs

Daily rental rate for magnetometer = $60

Daily rental rate for gradiometer = $90

Base station = $20 to $60

Shipping weight = 70 to 150 pounds

Contractor crews and equipment = $1,450 per day

Productivity = 3 to 6 acres per day

Notes:

• Magnetic survey costs vary depending upon the cost of rental, the productivity of the surveyteam, and experience. Magnetometer equipment is generally easy to operate, but requires anunderstanding of proper data collection procedures. The equipment can be rented for anominal cost. Basic software for downloading data is usually included as part of the equipmentrental. Advanced software is available from several vendors including Golden Software, Inc.,Geosoft, Limited and others for data processing (see www.geosoft.com). The data can bedisplayed as either profile maps or contour maps.

• It is recommended that qualified personnel be used when conducting a geophysical survey, orwhen processing and interpreting geophysical data. Links to many of the firms which offergeophysical survey support can be found on the Environmental Engineering GeophysicalSociety (EEGS) website at www.eegs.org.

Magnetic Surveys


GT-9

Magnetic Surveys — Summary

Magnetometer surveys are useful for delineation of buried ferrous metallic objects and in some cases assist in mapping lateral or vertical variations in geologic formations and soils

Advances in equipment allow for rapid surveys

Notes:

• Magnetometers are used to detect buried ferrous metal objects such as drums, tanks and pipes.

• Magnetometers allow for the rapid characterization of a site relative to the presence or absenceof metal objects.

• With current computer capabilities, anomalies can be evaluated in the field on a near-real-timebasis.

• Magnetometers should be considered over metal detection systems in areas where the objectsbeing investigated are expected to be at great depth (greater than 10 feet). The sensing depthof all detection systems for metal detection in dependent upon the size and orientation of theobject.

Magnetic Surveys


GT-10

Magnetic Surveys — Advantages and Limitations

Has the ability to sense objects at greater distances from the sensor up to approximate ___ ft bgs under most operating conditions. The degree of resolution at a given depth will be determined by the size of the object and other site conditions.

Senses only ferrous objects

Not effective near buildings, vehicles, or areas with reinforced concrete

Notes:

• Magnetic surveys can detect large, deeply buried ferrous metal objects. However, smallerobjects at depth may not be detected by this method. Other metal detectors, such as theEM61 (we will discuss this instrument later) cannot detect objects at significant depths belowground surface but magnetic surveys may. Because magnetometers have a greater sensingdistance, they may not be effective near buildings, vehicles, power lines or in areas withreinforced concrete, such as parking lots. An alternative metal detection system such as theEM61 may be an alternative choice in these types of situations.

• The results from magnetic surveys can be difficult to interpret, but recent developments in dataanalysis software have made data interpretation easier and more effective.



GT-11


Direct Current (DC) applied into the ground

Apparent resistivityof the subsurface is derived

Modeling of data to infer geology

Notes:

• Electrical resistivity is one of the most traditional geophysical methods. The method uses directcurrent applied to the ground through electrode pairs. A second pair of electrodes are used tomeasure the resultant voltage from the applied current. Different grid and loop configurationscan be used depending upon the objective of the survey.

• The resistivity of a particular rock type or soil is dependent on the resistivity of the pore fluidand the type of soil. In general, soils have lower resistivity than rock, and clay soils have lowerresistivity than coarse-textured soils.

• Measurement of resistivity in the earth is defined as apparent resistivity because the material intowhich electrodes are usually inserted, is not generally homogenous and continuous between thetransmitting and receiving electrodes.



GT-12

Resistivity — Common Applications

Determine lateral variations of resistivity

Determine depth of fill material

Determine depth to bedrock

Obtain cross-sectional interpretation of site

Assist in determining fracture patterns in bedrock

Notes:

• Direct current resistivity can be a useful tool for mapping lateral variations of earth resistivity forsite characterization of geologic features such as less permeable clay layers or stream channelsthat can be conduits for contaminant migration.

• The method is useful for determining depth of fill and depth to bedrock in certain geologic andsite environments. The most successful areas where depth of fill or depth to bedrock can bederived is in an area where the target geologic unit has a lower resistivity, or a resistivitycontrast that is significant different than the material above it. (For example, a conductivelandfill underlain by a very resistive bedrock, or a bedrock that is primarily clayey material thatis much less resistive than the overlying sands and gravels.) In some cases, a method calledazimuthal resistivity may be used to determine fracture orientations, particularly in shallowbedrock environments.

• Direct current resistivity may be better suited than seismic refraction for determining depth offill. Seismic survey often have difficulty in a fill area because of the unconsolidated nature andlack of compaction of materials. This results in poor energy transfer of the acoustic wavesneeded to provide refraction off of bedrock material.



GT-13

Resistivity — Physical Basis

Electrodes are placed in the ground

Applied direct current into subsurface

Measured resultant voltage

Calculation of apparent resistivity of the subsurface

Current flowlines

Lines ofequal potential

MeasuredpotentialCurrent

source v

Notes:

• The current in a conductor is generally equal to the voltage applied across it divided by aconstant. This constant is the resistance. Resistance (R) is measured in ohms when current (I)is in amps and potential (V) is in volts.

• The resistance of a unit cube to current flowing between opposite faces is termed resistivity. Resistivity is measured in units of ohm-meters. The reciprocal quantity is conductivity,measured in units of milliseimens per meter (mS/m).

• The resistivity of a rock is roughly equal to the resistivity of the pore fluid divided by thefractional porosity. Clays typically have resistivities of less than 102 ohm-meters; loose sandstypically between 102 and 103 ohm-meters. Using this basic information, the geologic materialscan be inferred from the measurements.

• When passing current between a pair of grounded electrodes and measuring the resistivitybetween them, problems related to contact resistances (which are often on the order of 103

ohms) arise. Because of this, a second pair of electrodes with virtually no current flowingbetween them are used to measure resistivity.



GT-14

Resistivity — Instruments

Electrodes and arrays

Notes:

• This slide shows a basic layout of electrodes for a DC resistivity survey. In this case, theoperator has lain out a series of electrodes, has the transmitter and receiver in place, andconducts the survey using only one person. In most cases, 3 person crews are used to conductthe survey, an operator, and 2 persons manning the electrode movements.

• Current electrodes are typically metal stakes. Voltage electrodes can be metal, but aresometimes a potential (called a “pot”) containing nonpolarizing material, such as porcelain orunglazed ceramic. Inside the pot is a copper rod surrounded by copper sulfate solution. Contact with the ground is made via the solution which leaks through the base of the pot. Electrodes are placed in well-defined geometric patterns known as arrays. Commonly usedarrays include the Schlumberger, Gradient, Dipole-Dipole, and Wenner.

• The device that controls and measures current in a resistivity survey is known as the transmitter. Transmitters are usually designed to reverse the direction of current with a cycle time ofbetween 0.5 and 2 seconds. This helps minimize electrode polarization effects. Power isprovided by a dry battery or a generator.

• Voltage measuring units are often referred to as receivers or detectors. Recently manufacturedinstruments generally place the transmitter and receiver equipment in a single housing unit withmicroprocessor control.



GT-15

Field Arrays for Resistivity Surveys

PE PE CECEsurfacea aa

V~

WENNER ARRAY

PE PE CECEsurfaceM AA

V~

SCHLUMBERGER ARRAY

surfacea to 5a aa

DIPOLE-DIPOLE ARRAY

N

CE CE PE PE

~ V

EXPLANATIONPE - Potential electrode

CE - Current electrode

a - Electrode “a” spacing

A,M,N,B - Electrode locations

- Voltmeter

- Current sourceV

~

Notes:

• Several types of electrode arrays are used in DC resistivity surveys. The Wenner,Schlumberger and Dipole-Dipole Arrays are the most commonly used. Each method has itadvantages and limitations, either in terms of resolution or in survey logistics. They allaccomplish generally the same objective. That is a weighted average of the earth’s resistivity toa specified depth depending upon the size of the electrode array.

• The Wenner Array uses equidistant current and voltage electrodes. The array is spread equaldistances for each measurement until the measured voltage is below the acceptable systemmeasurement level. The Schlumberger Array uses a similar approach, however, the voltageelectrodes remain at a fixed position while the current electrodes are moved out at logarithmicdistances until the required depth objective is met, or the signal received at the potentialelectrodes is below acceptable values. The Dipole-Dipole array uses current electrodes andpotential electrodes at a fixed spacing. The separation of the current electrodes to potentialelectrodes are moved out at intervals equal to the spacing, at increasing intervals until the signalis no longer measurable or valid. The current electrodes are moved. This procedure isrepeated until the profile over the area of interest is achieved.



• Wenner and Schlumberger Arrays usually produce a vertical electrical sounding. A verticalelectrical sounding (VES) measures the resistivity at depth by spreading the electrodes from asingle central point, at consecutively greater distances. This results in a measurement at a pointof the resistivity at that point to depths determined by the size of the electrode spread, and thedepth limit of the system. This VES can be linked to adjacent VES measurements and result ina geoelectric sounding. The Dipole-Dipole array results in a “geoelectrical cross section” ortwo dimensional profile of the area surveyed by moving the potential electrodes a number of “a”spacings. The entire electrode set is then moved another “a” spacing, and the potentials aremoved out successive readings again. This process is continued until the profile area iscompleted.



GT-16

Resistivity (Ohm-Meters) of Common Materials

Fine grained materials generally measure low resistivity (10-100 ohm-meters), clays to silts

Resistivity increases in medium grained materials (50-200 ohm-meters), silty-sands to fine sand

Gravels generally have higher resistivity (100-1000 ohm-meters

Competent rock may range from 100 to 1000’s of ohm-meters

Notes:

• Using the DC resistivity method, measured voltages and calculated apparent resistivities canassist in inferring an interpretation of the type of soil and rock within a study area. Tables areusually available using the uniform soil classification, and published data about the ranges ofresistivity of common soil types and rock. There are overlaps in the ranges from each generaltype of soil or rock. The resistivity varies depending upon the moisture content, competencyand type of minerals present in the measured unit. It is useful to have some site informationwhen interpreting the results of resistivity surveys. This may include general geologicinformation, available borehole information and data obtained during the initial site survey.



GT-17

Vertical Electrical Sounding Examples

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1521 1000500200100502010 2000 5000 10,000

1000500200100502010 2000 5000 10,000

Electrode Spacing, AB/2, in feet (ft)

Electrode Spacing, AB/2, in meters

Schlumberger Apparent

Resistivity, P,in Ohm-Meters

Depth in meters

23

1 2

3

130268

268

4040

40

200

Notes:

• This simple example shows three vertical electrical sounding and their associated apparentresistivity curves. Using available modeling software, the measured voltages, geometricelectrode placement information, and some geophysical experience, an interpretation can bederived from the sounding data. In this case a model of 3 horizontal layers was modeled foreach curve.

• Observing apparent resistivity curves it can be noted based upon the character and position ofthe curves within the graphs the approximate layering in the subsurface. In each case, a 3 layersubsurface was modeled. In general, curve 3 is higher in resistivity. This is evident in thevertical position on the graph and position with respect to the calculated apparent resistivity andthe electrode spacing.

• Curves 1 and 2 are similar, however curve 2 is shifted right laterally, indicating that the upperlayer is similar, but has a greater thickness.

• Vertical electrical soundings are principally used to derive information about the layering of thegeologic formation or the depth to bedrock at a site. This method may be used in combinationwith other information including borehole information to evaluate a site using the less costlygeophysical measurement to infer the geologic information about a site.



GT-18

Example of Electrical Profiling

a = 30 feet

b = 60 feet

0 10 20 30 Meters

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0 100 Feet

Sand and Gravel

APPA

REN

T R

ESIS

TIVI

TY,

P, I

N O

HM

-MET

ERS

Horizontal Scale

VES 4

0

20

4010

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20

Clay

Dep

th in

feet

Dep

th in

met

ers

Notes:

• The data shown at the top of the figure are taken at two electrode spacings. Wider spacingusually provides information about deeper subsurface materials.

• Vertical resolution of geologic formations varies depending on the conductance of the strata. As a rule of thumb, it is difficult to resolve a layer that is thinner than the depth from the surfaceto the upper contact with the formation

• Lateral resolution is limited by the spacing at the voltage (potential) electrodes as well as thecurrent electrodes.

• The information at the bottom of the figure is an interpretation of the data. “VES 4” is a verticalelectric sounding. Likely, the data from VES 4 indicate the presence of three geologic layerswithin the depth of investigation of the sounding.



GT-19

Resistivity Profiling

Notes:

• Resistivity profiling is a technique where transects are used to detect lateral changes insubsurface resistivity. Array parameters are varied by separating the voltage electrode pair (V)from the current electrodes (I). The depth of penetration, therefore, varies with changes inelectrode pair separation and with the changes in subsurface materials.

• The resistivity depth-sounding technique uses arrays in which the distances between some or allof the electrodes are systematically increased. Apparent resistivities are plotted against arraygeometry changes. Information regarding the change in resistivity as a function of depth isinferred with this technique.



GT-20

Resistivity Cross Section

Notes:

• Resistivity profiling is a technique where transects are used to detect lateral changes insubsurface resistivity. Array parameters are kept constant. The depth of penetration variesonly with changes in subsurface materials, such as layering.

• This cross-sectional view was created using a Dipole-Dipole resistivity array. The data wasprocessed using software developed for use with DC Resistivity data. The software uses thecalculated apparent resistivity data to calculate and interpolate depths of layers with differentresistivities to produce a cross-sectional view of a site. The interpretator or geophysicist usuallyprovides initial estimates into the program of the number of layers, thicknesses, and resistivitiesanticipated. The program then shifts the initial estimate to match the field calculated dataresulting in a model that represents the measured field data.

• In this survey, several pieces of information were desired. They include the depth of fill and itslateral extent. In many landfill investigations, it is difficult to determine the volume of material ina landfill because the thickness of fill is often not measurable. In this case, both objectives weremet by conducting a series of profile lines over the landfill area.

• Other methods can be more cost effective for the determination of landfill boundaries. We willdiscuss one such alternative method later in this presentation, the electromagnetic induction“terrain conductivity” method.



GT-21

Resistivity Profiling — Costs

Daily rental rate = $50

Accessories = $25

Shipping Weight = 75 to 150 pounds

Contractor crews and equipment = $1,400 to $1,800 per day

Productivity generally in number of soundings per day (2-20) or line mile coverage per day (one-forth mile to 1 mile per day). These productivity rates vary because of site conditions, depth objectives, and desired resolution.

Notes:

• There are a number of vendors that rent geophysical equipment. Links to these vendors areprovided later in this discussion. Typical rates for near surface resistivity equipment are about$75 per day. Shipping weights vary and should be considered in costing a project. If acontractor were hired to perform the work, costs can vary from about $1,400 to $1,800 perday plus mobilization and reporting.



GT-22

Resistivity — Advantages

Surveys are relatively inexpensive

Using several array types, depth soundings, lateral profiling, and geoelectric cross sections

The method is effective in mapping from depths less than several feet in depth to hundreds of feet

Notes:

• Equipment for electrical resistivity measurements can be fairly inexpensive. Commonly knownas the DC resistivity method, a variety of electrode configurations can be used depending uponthe type of subsurface information required. Sounding arrays determine the verticalstratification, profiling provides information about a target at a specific depth, or multiple targetsat differing depths. Profiling essentially provides a “geo-electrical cross-section” of thesubsurface.



GT-23

Resistivity — Limitations

Lateral variations in soil may affect result

Difficult to use in paved and some urban areas

Difficult to place electrodes in rocky soils

Lateral variations in resistivity may result in erroneous interpretations due to data “distortions”

Notes:

• Although the equipment is inexpensive, there are several limitations using this method. Whenused for vertical electrical soundings (VES), the data can be “distorted” by lateral variations ingeologic structure. This may result in erroneous information about the vertical stratification. The method requires sufficient distance away from potential interferences in order to assure thatthe survey data is reliable. Good contact with the ground also is required in order to applysufficient current or retrieve reliable voltage measurements.



GT-24

Electromagnetic (EM) Conductivity Surveys

Active electromagnetic induction techniques

Applications

» Profiling

» Sounding

Coil

GROUND SURFACE

SECONDARY FIELDS FROM CURRENT LOOP SENSED BY

RECEIVER COIL

PRIMARY FIELD

Chart and magnetic tape recorders

Coil

Induced current loops

Transmitter Receiver

Phase sensing

circuits and amplifiers

Notes:

• Conductivity methods use a transmitter coil to generate an alternating electromagnetic fieldwhich induces electric current flows in the earth. The induced currents generate a secondaryelectromagnetic field in the subsurface. This electromagnetic field creates what are called eddycurrents in the subsurface that are sensed by a receiver coil. The character and magnitude ofthe secondary field are governed by the frequency of the transmitted current, thetransmitter-receiver separation and the distribution and magnitude of the electrical properties inthe nearby subsurface.

• The electromagnetic energy transmitted into the ground undergoes attenuation and phase shiftas it propagates. The attenuation is generally inversely proportional to the conductivity of thenearby subsurface. Therefore, it is typically more useful to measure the out-of-phase signal atthe receiver, which is more proportional to subsurface conductivity. However, in certain caseswhere the subsurface conductivity is very high (an area of dense metallic disposal, for example)the attenuation is minimal and in-phase measurements tend to be more proportional tosubsurface conductivity.

• Conductivity methods also are known as active electromagnetic induction techniques, and canbe used to detect both ferrous and nonferrous metallic objects and detect conductivityvariations in soils.



• Profiling is accomplished by making fixed-depth measurements along a traverse line. This isdone by maintaining the transmitter-receiver coil separation, and moving across the surfaceeither by continuously measuring the conductivity or by obtaining measurements at fixed pointsalong a survey line.

• Sounding is accomplished by making measurements at various depths at a fixed location byvarying the coil orientation or separation of the transmitter and receiver. Soundings are nottypically done, but can be accomplished using the EM31 and EM34 instrumentation, the mostcommonly used instruments for this type of survey.

Name several instances where you would consider electromagnetic conductivitysurveys?

Why should you consider an electromagnetic conductivity survey when investigating aleaking underground storage tank site?



GT-25

EM Conductivity Surveys — Primary Tools and Applications

Determine lateral variations of conductivity in soil and rock

Map landfills, placement of wells and soil sampling

Techniques available for shallow and moderate depth

Primary tools include EM31 and EM34

Notes:

• The electromagnetic apparent conductivity measurement technique has been widely used forenvironmental investigations. The EM31 is a rapid technique to determine the lateral extent oflandfills, trenches and waste pits. The EM34 has variable depth capabilities to potentially mapsites with a greater depth that may require characterization. Both techniques can be conductedreasonably quickly without causing soil disturbance.



GT-26

EM31

Terrain Conductivity Meter

The EM method provides a means of measuring the electrical conductivity of subsurface soil and rock

Notes:

• This slide shows an EM31 terrain conductivity meter.

• The EM31 is a fixed geometry instrument. The transmitter is located on one end and thereceiver is on the other, about 4 meters apart. In the center, near the operator, the electronicsand data logger can be found.

• The EM31 can be used for bulk conductivity measurements up to about 18 feet (6 meters) indepth in the standard vertical dipole position. A bulk conductivity measurement to about 7.5(about 3 meters) ft can be obtained by rotating the instrument such that the transmitter andreceiver coils are in the horizontal dipole position. Advantages of using the horizontal dipolemode are the more detailed conductivity measurement of the near surface, less than 7.5 feet (3meters). The horizontal mode is not commonly used because of the need to rotate theinstrument.



GT-27

EM Conductivity Surveys — Survey Practice

Data acquisition

Conductivity is influenced by soil and rock properties

Effects of cultural features

Variable depth investigations

EM34

Notes:

• Profiles are continuously or station-based conductivity measurements obtained along a transect. Measurements may be obtained along a grid of transects to support contouring of the data. Multiple measurements at different coil spacings may be made with variable geometryinstruments (typically the Geonics EM34) at a single station to support depth-dependentconductivity relationships. Data are typically recorded in a data logger and transferred to acomputer for processing.

• The measured apparent conductivity from the electromagnetic instrument can be used to inferthe type of soil and rock within a survey area. Using information about known ranges ofconductivity will assist in the data interpretation.

• The presence of surface conductors (for example, rail road tracks) must be carefully noted inthe survey log because of the influence in data collection. Underground utilities and overheadpower may cause interferences in the data. Often, underground utilities may be one of thetargeted objectives of a EM31 or EM 38 surveys.



GT-28

Range of Electrical Conductivities in Natural Soil and Rock

Clay and Marl

Loam

Top Soil

Clayey Soils

Sandy Soils

Loose Soils

River Sand and Gravel

Glacial Till

Chalk

Limestones

Sandstones

Basalt

Crystalline Rocks

Conductivity (milliSiemens/meter)

103 102 101 1 10-1 10-2 10-3

Notes:

• This illustration shows the range of conductivity that can be encountered in various materials. Ingeneral, clays or fine-grained materials have high conductivity values, gravels, and sand havemoderate conductivity values, and consolidated bed rock has low conductivity values.

• The important point from this slide is that geologic materials may have a wide range ofconductivity values, and “cross over” into the range of values for other types of material. Inorder to provide the most accurate interpretation about the geology of a site, some knowledgeof the potential geologic units within a site are necessary. Also, if monitor wells are present,geophysical logs using natural gamma and induction may be useful in helping to understand thephysical properties of the subsurface. Induction logs require an open or PVC cased hole. Natural gamma logs can be conducted in steel cased holes.



GT-29

EM Conductivity Surveys —Interpretation

Data contouring

Notes:

• Various means to obtain spatial representations, such as magnetic surveys, also can be appliedto conductivity measurements. Color contour maps usually are represented using availablesoftware packages. This illustration is of the same site illustrated in the magnetometer example,Tinker Air Force Base, Oklahoma. The objective of the survey was to determine the locationsof waste pits and also areas where waste pits were absent.

• Apparent conductivity measurements show modeled responses to the trenches, lateralvariations which could result from the presence of utilities, roadways and/or other site features. The objective has been met over this site of determining the location of trenches and filloperations.



GT-30

EM Conductivity Survey — Costs

Daily rental rate = $50-$70/day

Shipping Weight = 75-100 pounds


Productivity = 1 to 3 line miles per day (EM34)

Productivity = 4 to 8 line miles per day (EM31)

Notes:

• There are a number of vendors that rent geophysical equipment. Links to some vendors areprovided later. Typical rates for EM31, and other conductivity instrument equipment, are about$50 to $70 per day. Shipping weights vary and should be considered in costing a project. If acontractor were hired to perform the work, costs can vary from about $1,350 per day plusmobilization and reporting. Productivity varies from up to 8 miles per day with the EM31 anddown to about 1 to 4 line miles per day for the EM34.

• For more information concerning geophysical survey costs, see Table __ .



GT-31

EM Conductivity Surveys —Summary

Key Uses» EM31 surveys are useful in determining lateral

variations in soil conductivity

» Mapping limits of landfills, trenches and waste pits

» Has some capability of metal detection

» EM34 has variable and greater exploration depth, up to 30 meters

» The EM34 resolution decreased with increased coil separation

(continued)

Notes:

• Electromagnetic conductivity measurement surveys are usually a cost effective method for a firstlook at an environmental site. The method is highly successful in determining lateral variations insoil conductivity. The method is highly successful in many cases at determining the lateral limitsof landfills, trenches and waste pits.

• In some instances the method may be useful in metal detection, however, there are othermethods that are a better choice for metal detection.

• Using the fixed and variable coil separation methods can provide some information forconductivities at various depths at a site. The EM34 can be effective to depths up to about 30meters. Targeting of deep depths can result in the loss of resolution for intermediate layersbetween the surface and the depth of exploration.



GT-32

EM Conductivity Surveys —Summary

Considerations

»Site conditions, such as access, vegetation, topography and man made features are critical to the success of the method

»Understanding the objective of the investigation

»These methods can be highly effective in site characterization

Notes:

• When determining the use of electromagnetic conductivity surveys for site assessment, it isnecessary to evaluate site conditions to identify issue that could be related to access, vegetation,topography and cultural (man-made) features that may be critical to the success and costeffectiveness of the survey. It is necessary to evaluate what constraints could limit conductingand interpreting results from a proposed geophysical survey.

• The objective of the investigation is important. Although the electromagnetic conductivitymethod is widely used in site assessment, there are some applications where other instrumentsmay be more appropriate.

• The electromagnetic conductivity method measures variations in ground conductivity as itrelates to changes in soil type and rock type or in some limited cases to a contaminant type. Itis most successful in mapping lateral variations in soil types, limits of landfills, and other areaswhere soil may have been disturbed. Sometimes it also can be used for metal detection oridentifying the locations of utilities.



GT-33

EM Conductivity Surveys —Limitations

Surveys provide an apparent conductivity measurement

The equipment is somewhat cumbersome

Survey data may be affected by utilities

Resolution of subsurface soil units decrease with increased depth of investigation

Notes:

• The measurement obtained with this method is considered an “apparent conductivity” becausethe system may be measuring in non-homogeneous or laterally changing soils. For example, ifthe sensors are crossing over the ground from a gravel area into a silty area, the sensors may bein both soil types resulting in an average reading. Geophysicists have termed these fieldmeasurements as an apparent conductivity measurement.

• Using increased coil separations for greater depth information decreased resolution. One canlook at the measurement as a “bulk conductivity” measurement where the conductivity of all soilwithin the influence of the transmitter and receiver coil is averaged. In other words, theconductivity measurement is an average conductivity of the subsurface within the transmittingand receiving distance of the system. This is important when considering the objective of asurvey.

• Considerations of the complexity or simplicity of the geology of the site should be made alongwith realistic expectations on what can be derived from the survey. In all geophysical methodsincluding EM conductivity the accuracy or resolution of the technique is dependant upon thedepth of the object or geologic target. When the target is at greater depth, resolution ordetection may be decreased because of the required physical parameters of the instrumentationmay need to be altered, sensor separation increased, etc.



• The equipment is somewhat cumbersome due to batteries, coils and coil booms. The operatormay require more frequent rest periods.

• It is important to obtain the location of subsurface utilities and metallic objects. This will assistin interpretation of the electromagnetic data. In some cases, it may be useful to conduct thesetypes of surveys to determine the location of unknown subsurface utilities and structures. Thismay assist in the interpretation of other site geophysical survey data.

• In the vicinity of utilities, the equipment may be ineffective, as with many other geophysicalmethods.



GT-34

Time Domain Electromagnetic (TDEM) Surveys

Electromagnetic method using time varying signal

Electromagnetic Induction method

Two principle types of Surveys

Vertical Electromagnetic Soundings

Metal Detection

Notes:

• The time domain electromagnetic (TDEM) method was developed initially for use in explorationfor the presence of mineral deposits. Advances in computer software and electroniccomponents has advanced this technology. The method utilizes the principle of electromagneticinduction. The difference between the time domain EM method and the electromagneticconductivity method discussed earlier is the use of a pulsed electromagnetic signal rather than acontinuous transmission.

• The TDEM method transmits a pulse that is emitted into the ground. The EM current is turnedoff in the transmitter, and secondary currents that are created in the ground are sensed in thereceiver coil while the primary current is off. This minimizes the need to deal with the primarysignal interference.

• The method is used for two principle surveys for environmental investigations, verticalelectromagnetic soundings and at a smaller scale, metal detection. The figure on the right isfrom a survey in the vicinity of an abandoned oilfield evaporation pond. The pond was notlined. Time domain electromagnetic soundings were conducted over the area, and downgradient along profile lines. The data indicate the presence of a conductive zone, shown in red,that is interpreted to be the brine plume.



GT-35

TDEM Surveys — Primary Uses

Sounding method has greater lateral and vertical resolution than DC resistivity method

Metal detection equipment has superior resolution of metal objects over other metal detection methods

Notes:

• The TDEM sounding method is superior resolution of horizontal earth layers over the DCResistivity method. Lateral variations in geology do not affect measurements to the degree ofDC soundings. Therefore, lateral resolution of geologic formations is greater than othersounding methods.



GT-36

TDEM Surveys — Soundings

Square transmitter loop increasing in size for increasing depth of investigation

Lower frequencies for greater depth of exploration

Data presented in apparent resistivity vs. time

Notes:

• The TDEM sounding technique uses a square transmitter loop laid out on the surface. Severalsystems are available. Geonics Limited of Canada produces the Protem System (seewww.geonics.com). This system uses 3 types of transmitting systems. The EM47 transmitter isused most commonly for environmental investigations. This system is effective using a portabletransmitter for exploration to about 200-300 feet. Zonge engineering has a competitivegeophysical system called the nanoTEM (see www.zonge.com).

• Greater transmitter loop size and lower frequencies allow for greater depth of investigation. These systems use large transmitter loops and motor powered generators, which can be usedfor exploration to depths of up to about 2 kilometers.

• Data presentation consists of an apparent resistivity curve versus time. Modeling of theapparent resistivity curves is similar to DC resistivity processing, resulting in an interpretation ofthe subsurface. A series of soundings can be used to produce what is called a geo-electriccross section.



GT-37

EM Metal Detectors

Used to locate buried metal containers, drums, tanks, and utilities

Includes time domain (EM61) and frequency domain metal detectors

EM 61

Notes:

• The Geonics EM61 is a time domain metal detector used to detect subsurface metals of alltypes.

• The system transmits a time varying electromagnetic pulse in the subsurface. The receivermeasures secondary signals in the subsurface created in metallic objects. The measuredquantity is the millivolts. Higher value measurements are related to either near surface or largeobjects.

• The Geonics EM61 is a cart mounted system with two coils. The lower coils act as atransmitter and receiver. The upper coils act as a secondary receiver used to estimate thedepth of burial. The operator carries a back pack and a data logger. The wheel systemcontains an odometer that triggers data collection at about 0.6 feet per measurement. Theposition of each measurement is automatically recorded.



GT-38

EM61 — Survey

Sidewalk

Side

wa

lk

Building

Elevated asphaltarea surroundedby concretecurb

Pump Island

Pump Island

Light Pole Light Pole

Light Pole

TelephonePole

TelephonePole

Traffic SignalControl Box

WaterMeterCover

0 30Scale

TelephonePole

(0,0) (110,0)

(0,125)

0

7000

Response(mV)

ProbableBuried Tanks

Abandoned Gas Station, East St. Louis, Illinois

Underground Storage Tanks

Notes:

• This map shows EM61 metal detector data from an underground storage tank survey at a gasstation. The bar graph on the upper right shows the range of measured millivolts signaling. Thecontours clearly increase in magnitude in the vicinity of the underground storage tanks.



GT-39

EM61 — Costs


GPS option = $50

Shipping Weight = 130 pounds


Productivity = 1 to 3 acres per day

Notes:

• The EM61 is available in the original version that contains a basic data logger, and singlemeasurement at each point. This system is good for a variety of metal detection tasks.

• The EM61 MK2 system contains a data logger that readily accepts global positioning systemdata.

• As with all geophysical techniques, the crew and equipment rates are similar, and dependantupon crew size, usually two persons for the simple survey equipment such as the EM61. Productivity of the survey varies depending upon the site conditions such as terrain andtopography.

• For more information concerning geophysical survey costs, see Table ___.



GT-40

TDEM Surveys — Costs

Daily rental rate = $200-400/day

Shipping Weight = 150-500 pounds


Productivity = 10 – 25 soundings per day

Notes:

• TDEM sounding equipment is available from several vendors including the instrumentmanufacturer. The weight of the equipment for environmental investigations is about 300pounds including a heavy receiver coil and receiver and batteries. Electrical wire is used mostcommonly for the transmitter, and can be purchased at an electrical supply outlet. Somewherebetween 12 or 14 gauge insulated wire is normally used. Approximately a 30 meter by 30meter transmitter loops are usually used for investigations. Smaller loops are usually multiturnthat can be supplied by the rental company. Multi-turn transmitter loops require less effort todeploy and provide equivalent results. Use of this type of transmitter loop may beadvantageous in areas where larger single turn transmitter loops are difficult to lay out. Themulti-turn loops are heavy and would increase shipping costs. Whereas, the single turn loopcan be purchased at an electrical supply store. The loop is simply 10, 12 or 14 gauge insulatedelectrical wire. A contractor would charge about $1,600 per day of field work, and this mayinclude field processed data. Productivity is about 10 to 25 sounding per day depending uponterrain and other site conditions.

• For more information concerning geophysical survey costs, see Table.



GT-41

TDEM Surveys — Summary

TDEM Soundings

» Does not require electrode placement

» Superior resolution of depth and layer resistivity

» Equipment is portable, but somewhat cumbersome

» Method is not greatly affected by lateral changes in geology

» Limited use in areas with underground utilities or overhead power

Notes:

• The time domain electromagnetic method has superior capabilities for mapping horizontallayering in the subsurface over the DC resistivity method. The equipment does not requireintrusive probes, and can be moved fairly rapidly from location to location on a site. Themethod is affected by underground utilities and power lines.

• The time domain metal detectors are regarded as a superior metal detector over other methods. The detector senses all types of metals.



GT-42

TDEM Survey — Limitations

TDEM sounding method is not as effective at shallow depth (less than 5 to 15 meters)

Limited use in areas with utilities and power lines

The EM61 metal detection method has limited depth capabilities

Notes:

• The TDEM method, because it is a time based measurement, is not effective in mostapplications for mapping shallow features. For shallow investigations conducted to detecttargets at between 5 to 15 meters direct current methods like a conductivity surveys could bemore appropriate.

• The TDEM sounding method does have limits of investigation. The method cannot resolvegeologic information in the surface less than about 10 meters.

• The TDEM metal detectors have a high accuracy of detection and resolution of metallic targetsover the magnetometer method, but has a limit of about 3-4 meters for detection depth.

• As with other electromagnetic and magnetic methods, utilities, underground water lines andother man made structures will effect the measurements. In areas where these features arepresent, the seismic method may be an alternative for the investigation.

• The EM61 can detect from some metal targets laterally such as buried automobiles. However,the detection of objects at depth is limited. In fact, a small object may be missed that is nearthe surface. The method is best suited for environmental site investigations for the detection oftanks and drums. For a single drum, keep in mind, that the survey line must be nearly over thetop of the drum. A magnetometer survey may be a better alternative for drum detection.



GT-43


Delineation of geologic and hydrogeologic units

In situ analysis of various physical parameters

Site-specific and inter borehole applications in combination with surface method results

Colog

Mount Sopris Instruments

Notes:

• Many borehole geophysical survey tools are available for examination of site condition. Toolsare available that can measure and collect a continuous data, or data at a specific locationwithin a borehole. The data are represented graphically as a geophysical log. Multiple logs aretypically collected to take advantage of a joint analysis of the physical characteristics of theborehole.

• Measurements obtained in a borehole can provide information about the well construction,lithology, presence and orientation of fractures, permeability and porosity, water quality, and anumber of other parameters.

• With borehole geophysical data, rapid interpretation is possible. When combined with surfacegeophysics, borehole geophysical methods can be used to develop a three-dimensional (3-D)understanding of site conditions.

• Selection of the suite of geophysical tools used at a site should be considered carefully. Factorssuch as project goals, geophysical information desired, instrumentation, and surface andsubsurface conditions will impact the effectiveness of any logging program.

• Borehole equipment for shallow environmental investigations is usually portable, and can beeasily brought to a job site in a small van or pick-up truck.



GT-44

Borehole — Tools and Applications

Electrical tools

Gamma

Neutron

Televiewers

Caliper

Fluid temperature

Fluid conductivity

Borehole radar

Lithology, water quality

Lithology, clay content

Porosity, moisture content

Fractures, bedding

Hole conditions, lithology

Gradients, flow

Fluid contamination

Fractures, lithology

Notes:

• There are an number of borehole tools that are available, some traditional, and some thatrequiring special equipment, expertise, and special processing. There are too many to list in thisslide. A table is provided that summarizes many of the available techniques, applications,required hole conditions and limitations. Please refer to this table, and look up the links to theliterature, firms that conduct these types of surveys, and the tools that are available to conductborehole work.



GT-45

Borehole — Applications

Assists in refining the project teams understanding of geologic and other site conditions

Assists in the selection of assumptions used for surface geophysical surveys

Identification of small scale features, such as fractures of preferred pathways between boreholes

Notes:

• Borehole geophysical surveys are useful for the determination of specific details about ageologic formation that may be missed using traditional visual observation methods.

• Borehole tools can provide detailed information about the physical properties of the subsurface. These physical properties can assist in the selection of the proper input assumptions, such asconductivity or porosity of a formation used to configure and interpret surface geophysicalsurveys. Consideration of borehole techniques should be conducted in advance of constructionof monitoring wells or well completion. Some borehole techniques can only be applied touncased holes. Polyvinyl chloride (PVC)-cased holes can be surveyed using natural gammaand electromagnetic induction conductivity. Steel-cased holes allow for the use of only alimited number of borehole techniques.



GT-46

Borehole — Traditional Methods

Natural gamma ray

Single point resistance

Spontaneous potential

Normal resistivity

Electromagnetic induction

Mount Sopris Instruments (continued)

Notes:

• Traditional geophysical techniques are borehole methods that are conducted in a singleborehole that are available either through a well logging service company, other geophysicalsurvey firms, or with minimal training. Therefore, these techniques can be conducted by sitepersonnel using rented equipment. There are other traditional techniques, a table containingmost borehole techniques and a summary is provided at the end of the material presented.

• The traditional and most common borehole logs include the natural gamma, single-pointresistance, and spontaneous potential. These measurements are commonly housed in one tool. Measurement of natural gamma is surveyed during one “run” up the hole, while the single pointresistance and spontaneous potential (SP) are surveyed during a second run up the hole. Resistance and SP are performed in an open fluid filled hole. Measurements are usuallyconducted coming out of the hole in the case of potential obstructions that may be in the hole.

• Natural gamma Logs can be conducted in open or cased holes. Natural gamma radiationemitted by the rocks surrounding the borehole is detected by the instrument. The mostsignificant naturally occurring sources of gamma radiation are potassium-40 and daughterproducts of the uranium-thorium decay series. Clay and shale bearing rocks commonly emitrelatively high amounts of gamma radiation. They include weathered components of potassiumfeldspar and mica, and tend to concentrate uranium and thorium by ion absorption andexchange.



• Single point resistance logs measure electrical resistance of the formation rock. In general, theresistance increases with an increase in grain size and decreases with increasing boreholediameter, fracture density, and dissolved solids concentration in the water. This survey must beconducted in a water filled or drilling fluid filled hole.

• Spontaneous potential logs record potentials (voltage) that are developed between the boreholefluid and the surrounding rock and fluids. Spontaneous potential logs can be used to determinelithology in the borehole and water quality. This survey must be conducted in a water filled ordrilling fluid filled hole.

• Normal resistivity logs record the electrical resistivity of the borehole environment andsurrounding rocks and water as measured by variably spaced potential electrodes on thelogging probe. Typical spacing for the potential electrodes are 16 inches for “short-normal”and 64 inches for “long normal” resistivity. Normal resistivity logs are affected by bedthickness, borehole diameter and borehole fluid. These surveys must be conducted in a waterfilled or drilling fluid filled hole.

• Electromagnetic induction is an important technique for logging information about theconductivity of the geologic material in a borehole. This method is extremely useful because themethod can be performed in uncased or PVC cased holes. In addition, it is not necessary tohave fluid in the hole.



GT-47

Borehole — Traditional Methods

Caliper

Fluid resistivityor conductivity

Temperature

Colog

Notes:

• Several other commonly used and important borehole techniques include the fluid conductivitymethod, caliper and temperature probes. Caliper logs record the diameter of the borehole. Changes in borehole diameter can be related to well construction and the competence of thegeologic formation. The caliper survey measures the diameter of the hole mechanically. Itprovides information about the geology, fracturing, or caving along the borehole wall. Becauseborehole diameter commonly affects log response, the caliper log is useful in analysis of othergeophysical logs that may be influenced by poor contact with the formation. Borehole surveysthat may be affected include single point resistance and neutron.

• The Fluid conductivity probe records the electrical conductivity of the water in the borehole. Changes in conductivity reflect differences in dissolved solids concentration of water. Thesesurveys are useful for delineating water bearing zones and identifying the vertical flow in aborehole.

• A fluid temperature log records the water temperature in the borehole. These logs are usefulfor delineating water bearing zones and identifying vertical flow between zones of differinghydraulic head.



GT-48

Borehole — Advanced Techniques

Acoustic televiewer

Borehole image processing

Full waveform sonic

Variable density

Borehole radar

Flow meter

Video camera

Colog

Notes:

• There are a variety of more advanced techniques that will be discussed in the next few slides. A table is provided at the end of this section that lists most of the techniques used today.

• Borehole radar, along with some of the traditional and other advanced techniques, can be usedto determine lithology and fractures in the borehole.

• Two other advanced techniques that may be considered include the Acoustic Televiewer andBorehole Image Processing System (BIPS).

• The illustration on the right portion of this slide is an acoustic televiewer (ATV) image. This isan ultra sonic imaging device that provides high resolution information used for measuring theorientation and distribution of borehole fractures and other features. Recent advances incomputer technology have improved the quality and accuracy of ATV data and thepresentation of the ATV images. The method is useful for formation evaluation, distribution andfracture orientation, and borehole inspections for casing or well bore breakouts. This methodrequires a fluid filled hole, calibration to insure precision and accuracy including repeat runs inthe hole. During the repeat runs, adjustments are made to resolve the data using variable colorscales for the image output (this can be done as a post processing step), variations in the loggingspeed and rotation of the transducer is conducted to improve the data.



• The optical televiewer method provides a very high resolution oriented borehole image data set. This is an excellent alternative for borehole imaging where the turbidity of the well bore fluidprevents use of the higher resolution Borehole Image Processing System (BIPS) data. TheATV data can also be acquired at a faster rate that the BIPS at about 10 feet per minute. Because this is an acoustic measurement, it functions only in fluid filled portions of the borehole.



GT-49

Borehole Image Processing System (BIPS)

Colog and Mount Sopris Instruments

Notes:

• BIPS provides the highest resolution images of the borehole wall. The BIPS images areessentially digitized video signals and are more realistic than those provide by the ATV. Because this is an optical tool, the BIPS works only in clear fluid filled or air filled boreholes. The logging speed is restricted to about 2.5 feet per minute. The field deliverable for the BIPSis a VHS tape showing a color two-dimensional (2-D) waterfall display of the borehole wall. Calibration is performed to insure precision and accuracy using a calibration chamber with anoriented color chart. Compass calibrations also are performed to obtain accurate orientation offractures and bedding in the borehole.

• Data analysis and final presentation of the BIPS data includes a 3-D processed image of theborehole that can allow precise measurements of fracture and bedding orientations. Data that isprovided can include the images as shown above, animated 3-D rotation, fracture tables, andstatistics about fracture density.



GT-50

Full Waveform Sonic (FWS) Logs

Mount Sopris Instruments

Notes:

• Full waveform sonic (FWS) logs measure how sound is transported through a formation in anopen hole. The information and borehole must be saturated to apply this technique. The FWSlogs can be used much like a down hole seismic survey to identify fractures, lithologic, and rockproperty analysis. Physical properties such as porosity, permeability, competency, and rockstrength can be estimated. The probe also can be used in the fluid filled portion of the boreholeto determine if the well grout has bonded sufficiently to the well casing.

• The full waveform sonic log can be used to assist during the design and interpretation of surfaceseismic surveys.



GT-51

Instrument Packages and Coincident Surveys

Notes:

• It is a common practice to combine several instruments (for example, caliper, gamma ray, andneutron) in one “package” and obtain measurements simultaneously in a single down holelogging run. Similar approaches are used for surface geophysics where two or more sensorsmay be mounted to a survey vehicle in a package format. Both down hole and surfacegeophysics use multiple sensors or instruments to obtain measurements during a single run ortransect. Two or more sensors may be used in the same borehole or along the same transect,but not simultaneously. This is often referred to as coincident surveying.

• In many surveys, apparently conflicting data is obtained from package or coincident surveys. Ameans of resolving these “conflicts” is to analyze sensor characteristics (such as signal to noiseratios) to develop error probabilities. Weightings are developed to “fuse” the data into a singleinterpretation. This process is sometimes done qualitatively when budgets do not allow formore rigorous, quantitative analysis.

• This data presentation provides a summary of a number of the types of borehole surveys thathave been discussed. By showing this type of summary plot, a better understanding of theresponse of the various instruments can be realized.

• Typically, this number of borehole tools are not within the budget of a project or may not befully effective because of physical limitations at a site. This could include logging inunconsolidated formations requiring cased holes, or simply ineffective resolution of somephysical parameters in the borehole.



GT-52

Borehole — Costs

Daily rental rates

»Winch electronics cable (>500 feet) = $100

»Natural gamma, resistivity probe = $70

»Induction conductivity probe = $50

»Sonic probe = $175

»Flow rate = $100


(continued)

Notes:

• Daily rental rates are wide ranging for borehole systems. A common system includes thewinch, basic natural gamma – resistivity - self potential probe and costs about $170 per day. Additional probes cost from about $50 to about $175 per day. Shipping costs are aconsideration due to the length of probes and weight of the system. A contractor may chargeabout $1,150 per day plus logging fees per foot. Typical logging rates are about 10 feet perminute depending on resolution and probe used.



GT-53

Borehole — Costs

Logging fees = $0.30 to 3 per foot

Shipping weight = 100 to 400 pounds

Productivity varies with probe selection, number of runs or probes, hole depth, and speed of logging. Usually about 10 to 20 feet per minute, plus mobilization time.

Notes:

• Logging fees vary depending upon the type of survey desired, the condition of the borehole,and other considerations regarding site logistics. If you are going to conduct the survey usingrental equipment, the weight of the system can be up to 400 pounds. Logistics play a key rollwhen productivity is estimated including the depth of the hole, access to the hole, and weather. The type of probe employed may require a slow logging speed or a number of repeat runs. Typical logging speed can range from 10 to 20 feet per minute. Mobilization fees aredependant upon the number of holes logged by the vendor and the distance from the vendorslocation.



GT-54

Borehole — Summary

Provides additional detailed information that cannot be obtained using visual observations

Provides physical data for optimization of surface geophysical techniques

Can provide inter borehole data on features such as fractures and other potential preferred pathways

Notes:

• Borehole geophysical methods can be an important tool in defining important small features orunrecognizable features in boreholes that may assist in correlating information between holesthat may assist in site characterization. Borehole geophysical tools are often useful indetermining physical properties of the subsurface to assist in selection or interpretation of othersurface geophysical methods.

• A variety of tools are available that may be used in either PVC cased, open, or steel casedholes.



GT-55

Borehole — Limitations

Most probes can only be used in open holes

Some probes require drilling fluid or water in the borehole

The effective radius of measurement is within a small distance of the borehole

Multiple boreholes are necessary to increase interpretation of an area

Notes:

• Limitations of borehole surveys include the necessity of a number of techniques that require anopen hole for measuring the physical properties. This could result in a collapse of the hole inunconsolidated formations.

• Electrical probes require an open fluid filled hole in order to obtain information about theelectrical properties of the borehole.

• The measurement of nearly all physical parameters is only within a small radius of the borehole.

• Multiple boreholes provide a better understanding of the subsurface and allow some confidencein the formations between boreholes when borehole techniques are applied.

• Borehole geophysical surveys are fairly rapid, however, these surveys result in downtime of thedrilling contractor.

Ground Penetrating Radar


GT-56

GPR

GPR is useful for estimating the configuration or depth to shallow bedrock

Notes:

• Consider a reported single 55-gallon drum, small cluster of drums or an abandoned well headat an unknown location in a field. How or where should characterization be conducted? Whatis the approach you would use to locate the feature? Using a portable cesium vapormagnetometer, a metal detector or ground penetrating radar? What method is mostappropriate? What method would be most effective to quickly locate objects at the site? Without using geophysical techniques, locating the drum would probably be done on atrial-and-error basis which could be costly, expensive, and may or may not be successful.

• In the following sections, the applications, advantages and limitations, costs, of yet another setof valuable geophysical tools will be discussed. Several case histories for site assessment usingGPR and seismic geophysical methods also will be provided to give users a more practical feelfor what tools can be used and what limitations might be expected.



GT-57

GPR Surveys — Physical Basis

Operates as an electromagnetic method

Propagation and scattering of the electromagnetic wave

Reflections off subsurface dielectric differences

Produces a cross sectional representation

Site specific technology that is not effective in conductive clay or silt rich soil

Highly effective in clean sands and even terrain

Notes:

• GPR transmitter antennas are designed to radiate a broadband electromagnetic pulse of only afew nanoseconds’ duration. Because the pulse is broadband, energy is transmitted over arange of frequencies. Transmitters are identified by the center frequency of the band (forexample, a 500 MHz transmitter or a 300 MHz transmitter).

• The propagation of electromagnetic waves in the subsurface is primarily dependent on the wavefrequency, ground conductivity, and relative permittivity of the subsurface. Abrupt changes inconductivity or relative permittivity create interfaces in the subsurface where scatteringphenomena, such as reflection or refraction, may occur. Scattering of the incident pulse resultsin energy partitioning and wave attenuation.

• GPR technologies use the transmission of pulses of electromagnetic energy (radar waves) intothe ground. The signals transmitted travel into the ground and are reflected by buried objects. Reflected signals travel back to the receiving unit, are recorded, and are processed into animage.

• GPR transmitter antennas are designed to radiate a broadband pulse of only a fewnanoseconds’ duration when excited. For optimal performance, the GPR antenna should bepositioned perpendicular to the ground surface.



• The propagation of electromagnetic waves in the subsurface is primarily dependent on thefrequency of the wave, conductivity of the ground, soil moisture content, and relativepermittivity of the subsurface. Abrupt changes in conductivity or relative permittivity createinterfaces in the subsurface where reflection or refraction can occur.

• Under optimal conditions, GPR technologies are capable of detecting both metallic andnonmetallic objects. These objects include PVC piping, transite waster pipe (fiberglassreinforced concrete) and, in some cases, buried wooden structures. Another application maybe using the method to detect changes in soil caused by historic human activity such as anarcheologic site or historic trench. In these cases, the soil conductivity and dielectric constantare changed from the surrounding areas, possibly allowing detection.

• The main limitation of GPR is its reduced effectiveness in highly conductive soil (such as wetclay soil).



GT-58

Instrumentation

Costs

POWER SUPPLY

TRANSMITTER-RECEIVER SWITCH

TRANSMITTER

GRAPHICRECORDER

TAPERECORDER

GROUND SURFACE

LAYERED MATERIAL

ANTENNA

TRANSMITTED IMPULSES

REFLECTED IMPULSES

ISOLATED REFLECTOR

RECEIVER

GPR Surveys — Instruments

Notes:

• GPR instruments consist of a control unit, antennas and cables, a printer or digital datarecorder, and a power supply. The control unit generates timing signals to key the transmitteron and off and synchronize this keying with the receiver. It controls the scan rate, the timerange over which echoes are compiled, and the gain that can be applied to the echoes toimprove resolution.

• The transmit antenna or transducer emits an electromagnetic pulse in the subsurface. Aseparate receiver is used to sense reflected pulses from near surface targets and later receiveddeeper reflectors.

• Transmit antenna frequencies of 300 MHz or 500 MHz are most often used for environmentalwork.



GT-59

Low-Frequency Bistatic GPR Antenna

Notes:

• The typical system consists of a real-time color display and controller that is carried by theoperator, a battery pack, a printer, and a GPR antenna. Higher- frequency antennas aregenerally used to detect shallow targets — for example, to determine the thickness ofreinforced concrete pavement.

• The various antenna configurations and frequency are important. Higher frequency antennaeprovide better resolution, but may not penetrate as deeply as the lower frequency systems. Low frequency antennae may penetrate the ground more deeply, but the resolution is reduced. Separate transmitter and receivers may be useful to use to estimate depths by varying theseparation.

• This illustration shows a pair of 100 MHz GPR antennae. The antennae are designed fordeeper exploration. An operator is shown pulling the antennae across the ground surface. Also shown in the foreground is the cable that supplies power to the antennae and transmitssignals back to the controller-receiver.



GT-60

GPR Surveys — Interpretation

Profiles and qualitative signal for pattern recognition

Appr

oxim

ate

Dep

th (F

t.)

Distance (Ft.)

0

1

2

3

4

5

6

0 10 20 30

Radar Profile ñ 500 mHz antenna

Fill

Tank Tank Tank

Fill

Diffraction Patterns

Notes:

• The interpretation of unprocessed GPR data is relatively subjective and yields only approximateinformation about the shape and location of buried objects. The depth of investigation typicallyis no greater than 10 to 15 meters and may be considerably less in soil that exhibits relativelyhigh electrical conductivity. For example, a near-surface steel underground storage tank (UST)buried in sand could be completely invisible to GPR if it were covered by a 6-inch layer ofhighly conductive, clay-rich soil. The equipment is relatively bulky and typically requires a carbattery as a power source.



GT-61

GPR — Costs

Daily rental rate = $105 to $225

Accessory antenna = $25



Productivity varies from 1,200 feet per day to more than 20 miles per day depending upon logistics and objectives

Notes:

• There are a number of vendors that rent geophysical equipment. Links to these vendors areprovides later in this discussion. Typical rates for GPR equipment are about $105 to $225 perday. Shipping weights vary and should be considered in costing a project. If a contractor werehired to perform the work, costs can vary from about $1,400 per day plus mobilization andreporting. An additional antenna is fairly inexpensive to have as a backup or to assist inobtaining information at a different frequency for better resolution or depth information.



GT-62

GPR — Advantages

GPR can provide a good cross-sectional representation of the subsurface

GPR is useful for a number of applications including shallow bedrock, water table

Identifying buried utilities, tanks and possibly contaminants

Detection of non metallic features that cannot be detected with other methods

Notes:

• GPR surveys can provide a good cross-sectional representation of the subsurface. The methodis useful for a number of applications such as detecting drums, utilities, shallow bedrock,fractures, and so on. The method can, in specific instances, be the most effective method ofproviding detailed information about the subsurface.

• GPR can be used for delineation of shallow bedrock and, in some cases, be effective indetecting the water table.

• The method may be effective where other methods have interferences such as overhead lines,overhead awnings in the detection of buried utilities and tanks.

• There have been projects where GPR has been effective in detecting either directly (rarely) orindirectly, the location of petroleum byproducts in the soil.

• The method may be the only method that is effective in some cases to detect polyvinyl chloride(PVC) lines, concrete pipes, and other non-metallic features in the subsurface.



GT-63

GPR — Limitations

The method is not effective in conductive soils

Depth interpretation may be difficult for detection of groundwater in unfavorable geology

Estimating the depth of ground water in low conductivity soils such as clays and siltstones

Identify buried utilities, ferrous and non-ferrous metal objects

Notes:

• The method is effective only in low conductivity soils such as sands. Clay and silt (conductivesoils) attenuate GPR signals and limit the depth of investigation. Lateral variations in soil affectthe performance of the system. These lateral variations also will affect the interpretation aboutdepth of measurement.



GT-64

GPR — Summary

Highly site-specific method

Very effective in sandy soil

Ineffective in clay soil

Useful for detection of tanks and drums

May be used for mapping subsurface geology if favorable conditions exist

Notes:

• GPR is useful only under certain site conditions.

• Favorable conditions are areas where the soil has a low conductivity, or is relatively low withhomogeneity of the soil. Unfavorable conditions include areas where the ground is uneven, treeroots are present, grass is long and wet, or clay soils predominate.

• The method requires relatively low conductivity soils such as sand, gravel, or shallowcompetent bedrock.

• There have been instances where GPR has been effective (such as the sands of Cape Cod) tosee up to 100 feet in depth, mapping bedrock features.

• The GPR method is not effective in clay soils or in areas where reinforced concrete is present. The clay soil or moisture causes an attenuation of the signal. This results in poor penetration ofthe GPR signal.

• GPR can be useful for the detection of subsurface tanks, drums or for mapping subsurfacegeology, if favorable conditions exist.

Seismic Surveys


GT-65

Seismic Surveys — InterpretationTypes of Seismic Surveys

Seismic refraction

Seismic reflection

Notes:

• There are two types of seismic surveys employed for environmental investigations, seismicrefraction and seismic reflection.

Seismic Surveys


GT-66

Seismic Refraction and Reflection — Applications

Refraction

»Determination of depth of geologic feature

»Estimate compotency in support of engineering cost estimates

Reflection

»Identification of subsurface geologic features such as bedrock highes and faults

Notes:

• Seismic refraction surveys are less costly than reflection surveys. Many parameters about thesubsurface may be derived from the data including competency, “rippability,” degree ofweathering and fractures. Acquisition and processing costs are less than seismic reflectionsurveys. The method does however require an increased soil or rock velocity with depth. Hidden layers, layers imbedded within a low velocity layer, may be hidden, within the data, butmay not be defined.

Seismic Surveys


GT-67

Seismic Surveys — Physical Basis

Types of elastic waves

Seismic velocities

Interface effects

Where velocity, V2 > V1

Shot Point Recording Geophones

Direct waves and ground roll

Reflected waves

Refracted waves

Overburden Bedrock

V1

V2

Notes:

• When a sound wave travels in air, the molecules oscillate backwards and forwards in thedirection of energy transport. This type of wave is referred to as a P-wave.

• P-waves propagate in solid matrix as do S-waves, which are oscillations at right angles to thedirection of energy transport. P and S wavefronts expand throughout the matrix and aretermed body waves.

• As is the case with electromagnetic and sonic waves, scattering phenomena (including refractionand reflection) and energy partitioning occur when a seismic wave encounters an interfacebetween two different rock types with differing P and S wave travel velocities.

Seismic Surveys


GT-68

Refractive Seismic Survey

GEOPHONES

SEISMOGRAPH

ENERGY SOURCE

DIRECT WAVE

REFRACTED WAVE

SOIL

BEDROCK

Notes:

• Refracted waves travel down through the overburden, are critically refracted below theinterface between the overburden and bedrock. These waves then travel upward through theoverburden to the geophone. Along those paths, refracted waves travel at bedrock velocitiesbelow the interface of the overburden and the bedrock and at overburden velocities along theupward and downward paths.

• This slide illustrates a simplified example of refractive seismic survey.

• A generalized refraction seismic survey uses 12 geophones as a minimum. Typical seismicsources include a sledge hammer, mechanical elastic wave generator or explosives.

Seismic Surveys


GT-69

Time/Distance Plot

Depth Layer1

Layer2L2

L1

L1 = Layer 1L2 = Layer 2V1 = Velocity of Layer 1 = 1/Slope of L1V2 = Velocity of Layer 2 = 1/Slope of L2Xc =Critical Distance

XcSource to Geophone Distance

Tra

vel T

ime

(mill

isec

onds

)

V2 -V1

V2+V1

Xc

2D =

Notes:

• This slide shows a graphical representation of refraction data and calculation of differentgeologic material wave velocities.

Seismic Surveys


GT-70

Refraction Analysis

1000 500 0 -500

0

50

100

DE

PT

H (

ft)

DISTANCE (ft)

1250 ft/sec

3500 ft/sec

5000 ft/sec

Notes:

• The profile shows variations in velocity among three layers. Low velocity likely representsunconsolidated sediments, while the high velocity represents bedrock.

Seismic Surveys


GT-71

Seismic Reflection

shot point

Seismograph

Geophone

V1

V2

Notes:

• Reflected waves travel down to the interfaces of the overburden and the bedrock and arereflected back up at the same angle to the geophone. Reflected waves travel at overburdenvelocities along their entire paths.

• Seismic reflection surveys usually require additional equipment including an increased number ofrecording geophones and a good energy source for generating signal. This may include avibration source.

Seismic Surveys


GT-72

Processed Reflection Record

Notes:

• The final product of a seismic reflection survey is obtained after processing of the data. Distance versus time plots are converted into distance versus depth plots. Often, layers orstructures are plotted on the interpretation map.

Seismic Surveys


GT-73

Seismic Survey (Refraction) — Costs


Accessories = $100 to $300



Productivity varies from about 1,200 linear feet per day to several miles depending upon logistics, objectives, and depth of investigation

(continued)

Notes:

• Daily rental rates range from about $135 for seismograph and $100 to $300 for geophones,cables and impact source. Several vendors are available to provide equipment. Shippingweighs are a major consideration due to cost factors. A contractor crew may charge about$2,000 or more per day plus the processing and report costs. Productivity and cost varies dueto desired resolution and site conditions.

Seismic Surveys


GT-74

Seismic Survey (Reflection) — Costs

Daily rental rate = $150 to $300

Accessories = $100 to $1,000

Shipping Weight = equipment often is transported


Productivity varies from about 1,200 linear feet per day to several miles depending upon logistics, objectives, and depth of investigation

Notes:

• Daily rental rates range from about $150 to $300 for seismograph and $100 to $1,000 forgeophones, cables and impact source, including hammer, elastic wave generator and vibratorsources. Several vendors are available to provide equipment. Shipping weighs are a majorconsideration due to cost factors. A contractor crew may charge about $2,000 or more perday plus the processing and report costs.

Seismic Surveys


GT-75

Seismic Refraction — Advantages and Limitations

Advantages

» Determination of depth and soil/rock velocity

» Infer soil competency, weathering, fractures

» Acquisition and processing less expensive than reflection

Limitations

» Resolution less than reflection surveys

» Large impact source may be required

» Increased rock velocity with depth required

» “Hidden layers” may be detected, but possibly not interpreted

Notes:

• Seismic refraction surveys are less costly than reflection surveys. Many parameters about thesubsurface may be derived from the data including competency, “rippability,” degree ofweathering and fractures. Acquisition and processing costs are less than seismic reflectionsurveys. The method does however require an increased soil or rock velocity with depth. Hidden layers, layers imbedded within a low velocity layer, may be hidden, within the data, butmay not be defined.

Seismic Surveys


GT-76

Seismic Reflection — Advantages and Limitations

Advantages

» Resolution greater than seismic refraction

» Geometry between impact source and geophones smaller than refraction

» Details within a soil or bedrock can be mapped

Limitations

» Site conditions dictate data quality

» Rock properties are not derived

» Expensive acquisition cost for near surface objectives

» Processing cost greater than refraction

Notes:

• Seismic reflection surveys can provide a great resolution of the subsurface. The method issomewhat site specific in that data quality may be in question in some situations such as urbanrequirements, areas where the impact source has problems such as in loose soils or high watertable. Acquisition and processing can be costly.

Summary


GT-77

Geophysical Technology Selection Summary

Environmental ProblemMetal detection

Waste pits, trenches, landfill boundariesContaminant plumes (or preferential pathway)Depth to bedrock

Void detection (solution cavities, mine workings)Fault and fracture mapping

Geophysical MethodMagnetometer (ferrous metal), EM61 (all metals)EM conductivity (EM31), GPREM conductivity (EM31), GPRDC resistivity, seismic refraction, TDEM soundingsGPR, seismic reflection

Seismic reflection, GPR, DC resistivity

Notes:

• There are a number of geophysical technologies available for environmental sitecharacterization. Some have dual or multiple applications. This slide provides a number ofenvironmental problems, and a listing of geophysical methods. In many cases, there may be amultiple number of instruments that can solve the problem, but the advantages and limitations ofa particular instrument may come into play when determining the geophysical method of choice.

Summary


GT-78

Electromagnetic and Borehole Geophysical Summary

In fractured or other complex geologic settings, electromagnetic and borehole geophysics can increase the information content available at a reasonable cost upon which to develop a sound systematic plan for a site

Geophysical methods can result in substantial cost savings by helping to focus monitoring and measurement activities

Using both surface and subsurface methods with differing capabilities is the best means for assuring the defensibility of project decisions

Notes:

• When applying the Triad approach at complex sites or where little is known about the geologyand hydrogeology at a site, geophysical methods offer high information value at a reasonablecost. Intrusive sampling activities can be focused and remedial goals achieved more quicklyand effectively when some information is used to guide follow-up activities. It is important tohave a good handle on the potential interference that can impact the results before selecting andimplementing a geophysical program to help guide systematic planning and development of aCSM. Expending more time and money during the front end-planning portions of a programand considering the use of geophysical methods will, in the long run, prove to be a wise use oftime and funds.

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