U.S. Department of the Interior

U.S. Geological Survey

Circular 1310

An Introduction to Using Surface Geophysics 

to Characterize Sand and Gravel Deposits
1 Collecting capacitively coupled resistivity data 

on alluvium in Nevada.

2 View is looking north across a valley south of 

Steamboat Springs, Colorado.

3 Monitoring seismic data collected on alluvium 

in Nevada.

4 Gravel deposits near Red River, New Mexico. 5 A four-electrode resistivity survey near Red River, 

New Mexico

Cover Photographs 13

245
An Introduction to Using 

Surface Geophysics to Characterize 

Sand and Gravel Deposits

By Jeffrey E. Lucius, William H. Langer, and Karl J. Ellefsen

Circular 1310

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Lucius, J.E., Langer, W.H., and Ellefsen, K.J., 2007, An introduction to using surface geophysics to characterize sand 

and gravel deposits: U.S. Geological Survey Circular 1310, 33 p. [available online at http://pubs.usgs.gov/circ/c1310/ ].
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Scope of Report.....................................................................................................................................1

Prospecting Techniques ......................................................................................................................2

Traditional Techniques................................................................................................................2

Geophysical Surveys...................................................................................................................3

Surface Geophysical Methods to Characterize Sand and Gravel Deposits........................................4

Physical Properties...............................................................................................................................4

Geophysical Characterization ............................................................................................................5

Additional Information .........................................................................................................................7

Seismic Refraction and Reflection ....................................................................................................8

DC Resistivity.......................................................................................................................................12

Ground Penetrating Radar ................................................................................................................16

Time-Domain Electromagnetic .........................................................................................................20

Frequency-Domain Electromagnetic...............................................................................................22

Summary and Conclusions.........................................................................................................................24

References and Selected Additional Reading........................................................................................25

Glossary.........................................................................................................................................................27

Figures

1–6. Photographs showing:

 1. A portable, rugged computer used for controlling, monitoring, and recording 

 seismic data ...........................................................................................................................8

 2. P-wave geophone and seismic cable................................................................................8

 3. Hammer and plate as a P-wave seismic source..............................................................9

 4. A buried charge used as a P-wave seismic source........................................................9

 5. S-wave seismic equipment................................................................................................10

 6. Generating S-waves with a light-weight S-wave source.............................................10

7. Interpretation of processed S-wave reflection data from light-weight source along

road in figure 6. ...........................................................................................................................11

8–12. Photographs showing:

 8. A control unit for a multielectrode resistivity system....................................................12

 9. A metal stake and a “smart” electrode strapped to it for a multielectrode 

 resistivity system .................................................................................................................13

 10. Some electrodes for a multielectrode resistivity system, at close spacing, 

and an all-terrain vehicle used to deploy the equipment.............................................13

 11. A capacitively coupled resistivity system towed by an all-terrain vehicle ...............13

 12. Another view of a capacitively coupled resistivity system .........................................14

13. Interpretation of a resistivity model from multielectrode resistivity data collected 

on alluvium on a flood plain ......................................................................................................15

14. Interpretations of resistivity models for multielectrode resistivity data collected 

over a sand and gravel deposit in a glacial outwash channel in Minnesota...................15
iv

15–18. Photographs showing:

 15. A ground-penetrating radar system with antenna separate from electronics 

and computer controls .......................................................................................................17

 16. A ground-penetrating radar system with antenna and electronics between 

the cart wheels and monitor and computer controls on the handle ..........................17

 17. Ground-penetrating radar equipment mounted on an all-terrain vehicle with 

antennas towed behind......................................................................................................17

 18. A 17-antenna ground-penetrating radar system............................................................18

19. Interpretation of ground-penetrating radar data collected near an active sand 

and gravel quarry........................................................................................................................18

20. Photograph showing collection of time-domain electromagnetic data............................20

21. Bedrock depth, in meters, determined using time-domain electromagnetic 

soundings on an alluvial flood plain ........................................................................................21

22–25. Photographs showing:

 22. A frequency-domain conductivity meter.........................................................................22

 23. A fixed-frequency conductivity meter with fixed coil spacing on a plastic sled......23

 24. A fixed-frequency conductivity meter with fixed coil spacing ....................................23

 25. A conductivity meter utilizing separate coils..................................................................23

Tables

1. Physical properties and associated geophysical methods...................................................5

2. Typical approximate limits and ranges of selected properties of some common 

geologic material ..........................................................................................................................5

3. Decision matrix of selected surficial geophysical methods for aggregate 

investigations ................................................................................................................................6

4. Comparison of approximate costs for selected geophysical methods .............................24

Abbreviations

cm centimeter (10–2 meter)

DC direct (constant) electrical current

EM electromagnetic (coupled electric and magnetic fields)

FDEM frequency-domain electromagnetic (measurements are taken at a particular frequency)

GHz gigahertz (109 Hz)

GPR ground penetrating radar

Hz hertz (one cycle per second, a measure of frequency)

kHz kilohertz (103 Hz)

km kilometer (1,000 meters; about 3,281 feet or about 0.621 mile)

m meter (about 3.281 feet, a measure of distance)

m/s meter per second (a measure of velocity)

mS/m millisiemens per meter (same as 10–3 mho/meter, a measure of electrical conductivity)

mV millivolt (10–3 volt, a measure of electrical potential)

MHz megahertz (106 Hz)

ohm-m ohm-meter

s second (unit of time)

TDEM time-domain electromagnetic (measurements are taken sequentially in time)
Photographic Credits

The authors thank these photographers for their contributions.

Jared Abraham: pages 16, 22, figure 12

Bethany Burton: cover, figures 11, 23

Wade Kress: figures 20, 22

Jeffrey Lucius: cover, figures 1, 2, 3, 4, 5, 6, 8, 9, 10, 15, 16, 17, 18, 19, 24, 25

This report is an introduction to surface geophysical techniques that aggregate producers can use to characterize known 

deposits of sand and gravel. Five well-established and well-tested geophysical methods are presented: seismic refraction and 

reflection, resistivity, ground penetrating radar, time-domain electromagnetism, and frequency-domain electromagnetism. 

Depending on site conditions and the selected method(s), geophysical surveys can provide information concerning areal extent 

and thickness of the deposit, thickness of overburden, depth to the water table, critical geologic contacts, and location and correlation of geologic features. In addition, geophysical surveys can be conducted prior to intensive drilling to help locate auger 

or drill holes, reduce the number of drill holes required, calculate stripping ratios to help manage mining costs, and provide 

continuity between sampling sites to upgrade the confidence of reserve calculations from probable reserves to proved reserves. 

Perhaps the greatest value of geophysics to aggregate producers may be the speed of data acquisition, reduced overall costs, and 

improved subsurface characterization.

Sand and gravel are components of a commodity 

referred to as natural aggregate, which also includes crushed 

stone. Together, sand, gravel, and crushed stone are the largest 

nonfuel mineral commodities produced in the United States 

and in the world. Annual production of aggregate in the United 

States during 2005 is about 2.91 billion tons (Bolen, 2006; 

Tepordei, 2006).

Of the total aggregate production, about 1.26 billion tons 

(43 percent) is construction sand and gravel. Sand and gravel 

are produced in every State in the United States from more 

than 6,300 operations, ranging in size from those that produce 

less than 25,000 tons per year to those that produce more than 

2,500,000 tons per year (Bolen, 2006; Tepordei, 2006).

Sand and gravel are primarily used in construction to 

build and maintain urban, suburban, and rural infrastructures including highways, roads, bridges, parking lots, and 

sidewalks; commercial and residential buildings; factories 

and power-generating facilities; and water-supply and wastetreatment facilities. Our current quality of life would be 

impossible to maintain without sand and gravel.

An Introduction to Using 

Surface Geophysics to Characterize 

Sand and Gravel Deposits

By Jeffrey E. Lucius, William H. Langer, and Karl J. Ellefsen

Scope of Report

This report presents an introduction to surface geophysical techniques that aggregate producers can use to better 

characterize known deposits of sand and gravel. In some cases, 

geophysics might assist in the initial exploration for sand and 

gravel, but that is not the focus here. We assume the producer 

has already identified a potential deposit and needs a better 

understanding of its extent and quality. Nevertheless, geophysical surveys could be conducted during the test pitting and test 

drilling phases of exploration and characterization. In addition, 

geophysical measurements might be useful if problems are 

encountered during the extraction phase.

We focus on five well-established and well-tested surface 

geophysical methods that we judged suitable for characterizing sand and gravel deposits. To be considered, we required 

the method first to provide useful information; for example, 

the property measured is relevant to distinguishing a sand 

and gravel deposit from nearby deposits or rock. Second, we 

required the method to provide the correct combination of 

Abstract

Introduction
resolution, differentiation, and depth of investigation. Third, 

we required that cultural features (buildings, road traffic, radio 

transmitters, fences, buried utilities, surface power lines, and 

construction and manufacturing equipment) not have a significant effect on the method. Fourth, we required that the cost of 

data acquisition (labor and time), data processing, and equipment be reasonable. Finally, we required that useful information from the measurements be produced quickly.

This report is intended for geologists, geotechnical 

engineers, and aggregate producers who are unfamiliar with 

geophysical methods or with their application to the characterization of sand and gravel deposits. Readers are presented 

with an organized collection of information and options, but 

recommendations for specific sites or geologic environments 

are not made. Nevertheless, the information here is useful as 

an introduction to and summary of the field equipment, limitations, and benefits of the selected techniques. Some terms are 

defined in a Glossary at the end of this report.

Prospecting Techniques

Traditional Techniques

A number of manuscripts describe approaches to prospect 

for and characterize aggregate resources. Some older documents include those by Thoenen (1932), Lenhart (1960), Dunn 

and Cutcliffe (1971), Dunn (1983), Goldman and Reining 

(1983), and Langer (1988). More recent documents include 

Dunn (1991), Timmons (1994a–c, 1995), Goldman (1994), 

Langer and Knepper (1998), and Langer (2006).

Determining preliminary target areas for sand and gravel 

exploration commonly takes into account economic and social 

factors such as markets, transportation, land use, zoning regulations, and property ownership. Geologic exploration for sand 

and gravel deposits within target areas commonly begins with 

desktop studies utilizing existing data such as geologic maps and 

reports, satellite imagery, and aerial photographs. Field studies 

are conducted to collect new data and verify desktop analyses.

An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits
If reconnaissance studies indicate a target area worthy 

of further investigation, more detailed field studies may be 

conducted at specific sites. Systematic sampling is conducted 

to determine the areal extent, thickness, stratigraphic variation, 

and physical properties of deposits. Test pits, truck-mounted 

augers, and other truck-mounted drills commonly are used to 

determine these factors and to obtain samples to determine the 

properties of the sand and gravel. A power hoe can sample to 

about 4- or 5-meter depths, whereas truck-mounted augers can 

drill to depths of about 60 meters. Deposits having cobbles 

and boulders are difficult for augers to penetrate and may 

require the use of other drilling equipment. Sample spacing 

depends on the desired level of detail and confidence and typically ranges from 30 m in highly complicated areas to as much 

as 500 m in large areas of very simple geology.

Geophysical Surveys

In the most general sense, geophysics measures all 

aspects of the physics of the Earth, atmosphere, and outer 

space. This report focuses on surface geophysical methods that 

obtain information about the spatial distribution of subsurface 

physical properties that can be used to characterize sand and 

gravel deposits.

Only the electrical resistivity geophysical method was 

used to explore for sand and gravel prior to the late 1930s 

(Patterson, 1937). Since then, geophysical techniques and 

data processing have advanced significantly. Now, a number of geophysical methods can be used to help detect, map, 

and characterize sand and gravel deposits. In addition to 

ground-based geophysics, airborne remote sensing techniques 

have been applied to aggregate resources; see, for example, 

Knepper and others (1995).

Currently, ground-based geophysical methods to characterize sand and gravel deposits include electrical methods, 

electromagnetic methods, ground penetrating radar, and 

seismic methods (Ellefsen and others, 1998; Ellefsen and 

others, 2005). These will be discussed in the next section. We 

consider potential field methods and radioactivity methods

generally unsuitable for sand and gravel characterization, and, 

except in the Glossary, we do not discuss them in this report.

Depending on site conditions and the selected method(s), 

geophysical surveys can provide information on the following:

areal extent and thickness of the deposit,

thickness of overburden,

depth to the water table,

critical geologic contacts, and

location and correlation of geologic features.

•

•

•

•

•

Additionally, geophysical measurements can be taken where 

closely spaced geological changes might be undetected by 

drilling, such as areas of suspected buried channels.

Ground-based geophysical surveys can be conducted 

prior to intensive drilling to help:

locate auger or drill holes,

reduce the number of drill holes required,

calculate stripping ratios to help manage mining 

costs, and

provide continuity between sampling sites to upgrade 

the confidence of reserve calculations from probable 

reserves to proved reserves.

The surveys can be conducted to provide subsurface 

detail where digging test pits, boring, or drilling are encumbered because of limited access, such as in areas of active 

cropland or forested land, or where auger trucks or drill rigs 

are unable to navigate due to the ground surface being soft, 

muddy, or covered with water. They can also be conducted 

where the geologic properties of the target deposit may restrict 

boring; for example, where deposits contain cobble-sized 

or larger clasts. Additional problems related to drilling are 

described by Timmons (1995).

The results of geophysical surveys are maps or cross sections showing the spatial variations in geophysical properties, 

such as the speed of seismic waves. Additional information 

is needed to translate geophysical properties into meaningful 

geologic properties such as the thickness of sand or gravel. 

This additional information may come from driller’s logs or 

geologic mapping, for example. Thus, geophysical surveys 

may be considered as a complement to, but not a replacement 

for, traditional methods of exploration.

Perhaps the greatest value of geophysics to aggregate producers may be the speed of data acquisition, reduced overall 

costs, and improved subsurface characterization. The greatest 

deterrent to the use of geophysics for sand and gravel characterization is likely both a lack of understanding and confidence 

in the methods and the subjectiveness of interpretations. We 

address these issues, and more, in the next section.

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•

•

Surface Geophysical Methods to Characterize Sand and Gravel Deposits
An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits

Physical Properties

Table 1 presents selected physical properties and their related field or phenomenon and the 

generalized geophysical method associated with each one.

Sand and gravel deposits have specific combinations of physical properties. These combinations may or may not be sufficiently different from other types of adjacent sediment or rock for 

geophysical methods to identify the sand and gravel. The properties that most commonly distinguish sand and gravel from surrounding material are electrical conductivity, dielectric permittivity, 

seismic velocity, and density. Magnetic susceptibility and radioactivity might differentiate the sand 

and gravel deposit if the source rock for the sand and gravel is much different from the surrounding rock. Differences in electrical conductivity are usually caused by clays, porosity, the amount of 

water filling the pore space, and the conductivity of the pore water. The bulk dielectric permittivity 

of the ground is most influenced by small changes in water content and by packing or layering. 

Seismic velocity is affected by the stiffness or hardness of the ground (described using elastic properties) and density, which quite often are very different between unconsolidated sand and gravel 

and indurated bedrock.

Dry sand and gravel have low electrical conductivity (5 mS/m or less, or a resistivity of 

200 ohm-m or more), low relative dielectric permittivity (RDP) (below 6 to 9), and low seismic 

velocity (often less than 1,000 m/s for P-waves).

Saturated sand and gravel have a relatively high electrical conductivity (20 mS/m or more, or 

resistivity of perhaps 50 ohm-m or less), intermediate relative dielectric permittivity (above 14 to 

16), and relatively high seismic velocity (greater than 1,500 m/s for P-waves).

Bedrock usually has a seismic velocity of several thousand m/s (P-waves). However, the electrical conductivity and dielectric permittivity may be similar to or different from that of sand and 

gravel. If the bedrock is limestone or sandstone, the electrical resistivity commonly will be in the 

hundreds of ohm-meters.

Typical values for selected properties of some common geologic materials are shown in table 2.

Surface Geophysical Methods 

to Characterize Sand and Gravel Deposits
Table 1. Physical properties and associated geophysical methods.

Physical property Field or phenomenon Geophysical method

Density Gravity Gravity

Magnetic susceptibility Magnetism Geomagnetic

Electrical conductivity Electricity, low-frequency electromagnetism Electrical, electromagnetic

Dielectric permittivity High-frequency electromagnetism Electromagnetic

Elasticity Elastic waves Seismic

Radioactivity alpha, beta particles and gamma rays Radioactivity

Table 2. Typical approximate limits and ranges of selected properties of some common geologic material (adapted from Sheriff, 1991; 

Reynolds, 1997; and ASTM Standard D 5777–00).

[mS/m, millisiemens per meter; ohm-m, ohm-meter; m/s, meter per second; <, less than; >, greater than]

Material Conductivity

(mS/m)

Resistivity

(ohm-m)

Relative dielectric 

permittivity1

P-wave velocity

(m/s)

Soil <10 to >50 <20 to >100 Low to high 100 to 500

Dry sand and gravel <5 >200 Low 450 to 950

Saturated sand and gravel >20 <50 Medium 1,250 to 1,850

Clay 10 to 200 5 to 100 Medium to high 900 to 2,700

Sandstone 1 to 20 50 to 1,000 Low 1,800 to 4,000

Shale 20 to 200 5 to 50 Low 2,500 to 4,300

Limestone, dolomite <1 >1,000 Low 2,100 to 6,100

Igneous rock <1 >1,000 Low 4,500 to 6,000

Metamorphic rock <1 >1,000 Low 3,000 to 7,000

1

For RDP, low is generally less than 10, medium is 10 to 36, and high is greater than 36.

Geophysical Characterization

Geophysical characterization of a sand and gravel deposit 

involves measuring the physical properties previously mentioned, or contrasts of these properties, in order to determine 

the thickness of overburden, depth to bedrock, and thickness 

and lateral extent of the deposit. Geophysical characterization 

can help determine the limits and volume of the deposit, depth 

to the water table in or below the deposit, and sometimes stratigraphy and depositional environment related to the deposit. 

In some cases, geophysics can determine deposit quality with 

respect to the amount of fine material and porosity.

Geophysical methods can be faster and less expensive 

than additional drilling and trenching and can provide continuous profiles and three-dimensional information. The depth of 

investigation for most methods is suitable for exploration and 

characterization of many sand and gravel deposits. In addition, 

intrusion and damage to the environment usually are limited.

There are limitations to the information surface geophysics can provide. Generally, geophysical measurements 

cannot define precise rock or sediment lithology, mineralogy, 

hardness, or chemical reactivity. On the other hand, geophysics can provide some physical property information, such as 

the distribution of electrical resistivity and the location of 

Surface Geophysical Methods to Characterize Sand and Gravel Deposits


Table 3. Decision matrix of selected surficial geophysical methods for aggregate investigations (adapted from 

U.S. Army Corps of Engineers, 1995).

Geophysical

method

Aggregate

boundaries

Top of

bedrock

Water

table

Fault

detection Rippability

Seismic refraction S W S S W

Seismic reflection W S S S

Direct current resistivity S S S S

Ground-penetrating radar S S S S

Electromagnetic S S

W–Works well in most materials and natural configurations.

S–Works under special circumstances of favorable materials or configurations.

Blank–not recommended.

lithologic or hydrologic boundaries. However, a particular 

set of field measurements often cannot be associated with 

a unique model of subsurface conditions; that is, there is an 

inherent ambiguity to all geophysical measurements. This 

ambiguity can be reduced with the use of multiple geophysical methods and with additional geological information such 

as borehole data.

Given the limitations and benefits of geophysical methods, successful application of a particular method is always 

site specific and quite often dependent on the experience and 

judgment of the geophysicist.

In subsequent sections we describe five surface geophysical methods that can be practical for sand and gravel deposit 

characterization. The information here is drawn from our own 

experience and from many of the references listed at the end 

of this report. Table 3 provides an evaluation of the applicability of the selected methods for some of the problem types 

encountered when characterizing sand and gravel deposits. 

Some of the basic principles and theory for each method are 

discussed here, with additional details in the Glossary. We 

show how each method can be used to define or reveal some 

characteristic of sand and gravel deposits. We do not discuss 

the mathematical theory behind the method, the details of 

performing a survey, processing and interpreting field data, or 

the cost of equipment or services. We do discuss the specific 

benefits and limitations of each method. We also present some 

examples of equipment and results in the figures, which are 

explained in the figure captions rather than in the text.

An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits
Surface Geophysical Methods to Characterize Sand and Gravel Deposits 

Additional Information

Because equipment models change regularly, software 

is improved or replaced, and prices change or are negotiated, 

we have refrained from mentioning specific equipment or 

software manufacturers, listing prices of equipment or rental 

fees, or providing a directory of retailers. Readers can collect 

some of this information by using popular search engines on 

the Internet. However, the geophysical methods themselves 

are enduring. The reader can learn more about the theory and 

application of geophysics in “applied geophysics” books such 

as Telford and others (1990), Reynolds (1997), Sharma (1997), 

Butler (2005), and Burger and others (2006). Some on-line 

general sources are:

http://www.��� cflhd.gov��/agm��/solu��tionMa����trix��/index����.htm

maintained by the Federal Lands Highway Program of 

the U.S. Department of Transportation, Federal Highways 

Administration,

http://www.�us��� ace.���� army.��� mil/��� inet/�us������� ace-docs/���������� eng-manuals/

em1110-1-1802/toc.htm

provided by the U.S. Army Corps of Engineers, and

http://galitzin.mines.edu/INTROGP/

which is a technical introduction available at the Colorado 

School of Mines Web site. The “References and Selected 

Additional Reading” section of this report lists some additional sources.

The American Society for Testing and Materials (ASTM, 

http://www.astm.org/) publishes guides for using many geophysical techniques, including, for example, the following.

D6429–99 Standard Guide for Selecting Surface 

Geophysical Methods

D6431–99(2005) Standard Guide for Using the 

Direct Current Resistivity Method for Subsurface 

Investigation

D6432–99(2005) Standard Guide for Using the Surface 

Ground Penetrating Radar Method for Subsurface 

Investigation

D5777–00 Standard Guide for Using the Seismic 

Refraction Method for Subsurface Investigation

D6639–01 Standard Guide for Using the Frequency 

Domain Electromagnetic Method for Subsurface 

Investigation

D6820–02 Standard Guide for Use of the Time Domain 

Electromagnetic Method for Subsurface Investigation

D7128–05 Standard Guide for Using the Seismic 

Reflection Method for Shallow Subsurface 

Investigation

These guides summarize the technique, equipment, field 

procedures, data processing, and interpretation methods 

for the various geophysical techniques.

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Figure 2. P-wave geophone and seismic cable.

Figure 1. A portable, rugged computer used for controlling, 

monitoring, and recording seismic data.

An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits

Seismic Refraction

and Reflection

Overview

Seismic methods were developed early in the 20th century. However, it was not until the 1980s and the advent of 

affordable digital signal processors and personal computers 

that seismic investigations could be conducted for environmental and engineering purposes. Outside of oil and mineral 

exploration, seismic methods are used to determine depth and 

lateral extent of layers, thickness and volume of deposits, and 

thickness of overburden. Potentially, rock quality (rippability) 

can be determined from seismic velocity.

Seismic methods depend on the transmission of strain 

energy, as an elastic wave (called a seismic wave) through 

a medium. Elastic waves are generated by an external force 

being applied to a medium. The external force in shallow seismic methods is often a sledgehammer striking a plate or block, 

a weight dropped or accelerated onto the ground or plate, a 

slug or bullet fired into a shallow hole, or a small explosive 

charge buried in a hole. Seismic wave energy is a form of 

mechanical energy (particle motion).

The two types of seismic waves discussed here are P-waves 

and S-waves. In all media, P-waves travel faster than S-waves. 

The velocities of these waves are determined by both the elastic

properties and density of the ground. Seismic waves travel 

slower in unconsolidated sediments and faster in rock. The elastic properties and density of the ground influence seismic waves.

Seismic wave detectors are called geophones and convert 

ground vibrations (or motion) into an electrical signal. Geophones are usually distributed in straight lines across the study 

area. The electrical signal from the geophones is converted 

into digital values using modular 12-, 24-, or 48-channel, 

high-resolution, signal-enhancement seismographs. Some 

seismographs can be linked together to increase the number 

of geophones. Portable computers record the seismograph 

data. Field crews for shallow surveys need a minimum of two 

to four persons (figs. 1–7). In the reflection method, the interpreter must find in the 

seismic record waveforms that are reflections from subsurface 

layers. Generally, these reflections occur in the record later 

than refractions, and they are often mixed with surface waves 

and noise. This can make reflections hard to identify, so the 

data must be processed much more extensively than refraction data. In addition, after the data are processed to find the 

reflection, additional processing must occur to determine the 

seismic velocities and then to transform the data into a cross 

section showing the geometry of the subsurface structures. 

When bedrock is 20 m or deeper, reflection may be more 

appropriate than refraction.

For both methods, interpretation is in terms of subsurface 

boundaries between regions or layers with different seismic 

velocities that result from changes in elastic properties and 

(or) density.

Refraction and reflection are two seismic techniques 

for interpreting seismic wave measurements. In the refraction 

method, the arrival time of the first waves to reach the geophones is measured. From these first arrivals, the interpreter 

determines the depth to the refractor (an underground layer) 

and seismic wave velocity in the media at various places along 

the survey line. As many as three or four layers can usually 

be identified using standard interpretation methods. An image 

of continuously variable subsurface seismic velocities can be 

produced using inversion techniques. With current technology, refraction is often better than reflection for determining 

bedrock depths less than 20 m.
Surface Geophysical Methods to Characterize Sand and Gravel Deposits 

Figure 3. Hammer and plate as a

P-wave seismic source.

Figure 4. A buried charge used as a P-wave seismic source.
10

Figure 5. S-wave seismic equipment. 

The shear-wave source weighs about 

31 kilograms (about 70 pounds) and has 

spikes on the bottom to improve 

coupling to the ground.

Figure 6. Generating S-waves with a light-weight S-wave source. The recording 

unit, shown in figure 1, is in the nearby truck.

An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits
11

Figure 7. Interpretation of processed S-wave reflection data from light-weight source along road 

in figure 6.

Benefits

Despite the generally much higher cost of the seismic 

method, the results can be very useful in characterizing 

sand and gravel deposits, especially for determining depth 

to bedrock. Seismic methods can produce near-continuous, 

detailed coverage of sections of the ground, detecting both 

lateral and vertical variations. Resolution can be as small as 

several meters, depending on source and geophone frequency, 

the method used, and the ground conditions. Seismic methods 

can be very effective in mapping bedrock under alluvial fill, 

as well as some stratigraphic and depositional features, and 

sometimes the water table (if using P-waves). When additional 

information is available, the methods may be able to tell the 

difference between clay-rich and clay-poor sediments. In areas 

that are too conductive for effective ground-penetrating radar 

use, and where electromagnetic techniques cannot provide 

adequate resolution, seismic methods can be an alternative 

geophysical method for exploration.

Limitations

Despite some significant advantages, seismic methods 

have noteworthy disadvantages. For example, ambient sound 

and vibrations from equipment, vehicles on roads, persons 

walking near the geophones, wind, and rain add significant noise to the recorded data. One of the most important 

disadvantages can be the expense of processing and interpreting large volumes of reflection data. Refraction data 

require somewhat less processing. Seismic equipment also 

can be relatively expensive and requires labor-intensive field 

deployment techniques. In fact, seismic data collection is the 

most labor intensive of the methods discussed here. In general, 

the seismic methods can be much more expensive to use than 

the other methods mentioned in this report because of large 

crew sizes, expensive equipment, and relatively slow data collection and data processing. Data collection can be faster and 

less expensive with the refraction method because there are 

often far fewer seismic source locations.

The seismic methods also have a safety issue that other 

methods do not have: the seismic source. Because sources 

must direct substantial force into the ground, caution and 

training are required, particularly with projectile sources 

(shotgun or rifle), powered sources (heavy, machineaccelerated drop weights), and buried charge sources (explosives). Even with a sledgehammer and plate, the operator 

must wear hearing protection and beware of hammer impacts 

as well as moving a heavy target plate or block. Other safety 

issues include the hazards of operation near roads, operating vehicles off road, and the danger of lightning strikes on 

the cables.

Surface Geophysical Methods to Characterize Sand and Gravel Deposits
12 An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits

Figure 8. A control unit for a multielectrode resistivity system.

Overview

The resistivity method was developed early in the 1900s 

and was in widespread use because of its simplicity and good 

results. Four metal-stake electrodes are driven into the ground. 

Electric current is introduced using two of the electrodes, 

and the electric potential is measured between the other two 

electrodes. With the help of computer modeling, a representative resistivity structure of the earth is determined. Within the 

limits of the technique’s resolution, the method is proven and 

reliable in identifying areas that are relatively conductive (for 

example, water-saturated sediments or fine-grained/clay-rich 

regions) compared to areas that are relatively resistive (such as 

relatively dry sediments or coarse-grained regions).

Presently, resistivity surveys are used in a wide variety of 

mineral, engineering, geologic, and environmental site investigations. With regard to sand and gravel deposits, the resistivity 

method can detect depth and lateral extent of coarse-grained 

(sand or gravel) compared to fine-grained (clay) layers, wet or 

dry sediments, depth to water table, and depth to bedrock.

The arrival of the personal computer in the 1970s and 

1980s made interpretation easier and gave better quantitative 

results than visual curve-matching. Data-collection systems 

were developed in the 1990s that utilize dozens or even 

hundreds of electrodes with automated switching between 

electrode sets. From 24 to more than 100 metal electrodes are 

driven a short way into the ground along a line. Electric current 

is introduced into the earth by using two of these electrodes. 

Electric potential is then measured simultaneously between 

pairs of other electrodes. For an eight-channel system, one 

channel is used for the pair of current electrodes, and potential 

is measured simultaneously between pairs of potential electrodes on the remaining channels (up to seven at one time). 

Measurements between sets of electrodes are repeated in many 

combinations along the line of electrodes. The arrangement of 

electrodes (that is, where the current and potential electrodes 

are in relation to each other) and the distance between electrodes can vary depending on the electrode configuration used 

and the goal of the survey. Depth of investigation is increased 

by increasing the separation of the electrodes. Field crews 

consist of from two to four persons (figs. 8–10).

Around 1999, a capacitively coupled resistivity system 

was developed and marketed. This system does not use the 

metal-stake, or point, electrodes of the traditional resistivity 

system. Instead, pairs of insulated wire cables, called dipoles, 

are towed along the ground surface. Each cable in a dipole is 

called a line electrode. An alternating current is produced in 

the earth by the transmitter dipole, and voltage is measured 

simultaneously by several receiver dipoles. Variations of 

resistivity with depth can be determined by making multiple 

passes over the same area using different separations of the 

transmitter and receivers and different dipole lengths. Apparent 

resistivity and an earth resistivity model are determined in a 

standard manner (figs. 11–14).

DC Resistivity
13

Figure 9. A metal stake and a “smart” electrode strapped to it for 

a multielectrode resistivity system.

Figure 11. A capacitively coupled resistivity system towed by an 

all-terrain vehicle (ATV). There is a global-positioning system (GPS) 

antenna mounted on the back of the ATV. The control unit for the 

resistivity system is in front of the driver on the front rack; the GPS 

control unit is mounted on the left handlebar.

Figure 10. Some electrodes for a multielectrode 

resistivity system, at close spacing, and an allterrain vehicle used to deploy the equipment.

Surface Geophysical Methods to Characterize Sand and Gravel Deposits
14 An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits

Figure 12. Another view of a capacitively coupled resistivity 

system. The transmitter is the isolated, single cylindrical 

unit on a skid near the lower right of the picture. Closer to the 

all-terrain vehicle are five receivers. The wires on either side 

of each transmitter or receiver constitute a dipole antenna to 

induce or respond to, respectively, an alternating electric 

field in the earth. The separation between receivers and 

transmitter, as well as the dipole length, can be varied.

Benefits

With supportive geologic or borehole information, the 

resistivity method, in a favorable environment, can detect the 

contact between aggregate and bedrock, the presence of clay 

layers, and the water table.

The main benefits of the automated-switching resistivity 

method are listed:

generally the least affected by cultural noise (of the 

methods discussed here),

relatively quick site coverage; an experienced crew 

can collect 1 to 1.5 or more line-kilometers of data 

per day,

quick data processing and interpretation; geoelectric 

sections are generated easily by using modeling software 

immediately after measurements are taken, and

moderately low cost for equipment, data collection, 

and data modeling.

•

•

•

•

In some situations, the multielectrode resistivity method 

can be better suited to determine earth electrical layers than 

electromagnetic methods. By using multielectrode surveying 

with closely spaced electrodes, good resolution is achieved 

from a few meters’ depth to 50–100 m depth. If a capacitively coupled system is used, exploration depth is limited to 

10–20 meters. However, data collection with a capacitively 

coupled system is very fast and nearly continuous across the 

surface; tens of line-kilometers can be collected in a day when 

the system is towed by an all-terrain vehicle. Multielectrode 

surveys are the most efficient way to collect measurements and 

produce the most continuous and detailed geoelectric images 

of the subsurface.
15

Figure 14. Interpretations of resistivity models for multielectrode resistivity data 

collected over a sand and gravel deposit in a glacial outwash channel in Minnesota 

(adapted from Anderson, 2000).

Figure 13. Interpretation of a resistivity model from multielectrode resistivity data collected on 

alluvium on a flood plain. Limestone bedrock has electrical resistivity of about 150 ohm-meters and 

higher. The alluvium is mostly sand and gravel with clayey or sandy loam soil at the surface.

Surface Geophysical Methods to Characterize Sand and Gravel Deposits

Limitations

Different materials may have the same resistivity, so 

measurements alone should not be interpreted uniquely in 

terms of soil type or lithology. Supplementary geologic or 

geophysical information is required to differentiate between, 

for example, conductive clay and nonconductive sand that 

has conductive water occupying the pore space; or, as another 

example, between sand and gravel. There is also the problem 

of nonuniqueness or equivalence, in that various combinations 

of layer thickness and resistivity will be measured identically 

by the equipment.

Considerable distance may be required for an array of 

electrodes. Depending on the electrode separation and the 

sediment resistivity, the array length may be many times the 

depth to a target, such as bedrock.

There are many sources of noise that can affect the quality and accuracy of resistivity measurements, including:

cultural features such as pipelines, buried utility cables, 

grounded power lines, and metal fences;

lightning or natural earth (telluric) currents, which may 

induce voltage into the cables;

lateral geologic variations, especially near-surface 

electrical variations; and

leakage in the current wire.

Safety issues with electrical methods are limited to moving 

heavy cables, pounding stakes into the ground, electric current in 

the cables, and perhaps operation of all-terrain and other off-road 

vehicles. Other safety issues include the hazards of operation 

near roads and the danger of lightning strikes on the cables.

•

•

•

•
16 An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits

Overview

Ground penetrating radar (GPR) is a relatively new technique that was commercially developed in the 1970s for use 

in shallow earth exploration. GPR is an electromagnetic method 

that depends on the propagation of high-frequency (10 MHz to 

3 GHz) electromagnetic (EM) energy in the ground. Because of 

this, the ground must have reasonably low conductivity, usually 

below 40 mS/m (or a resistivity above 25 ohm-meters).

A transmitting antenna radiates an electromagnetic wave 

into the ground. When the electromagnetic wave encounters 

a change in electromagnetic properties, some of the wave is 

reflected back toward the receiving antenna, where the amplitude of the electric field and arrival time are detected. As radar 

waves travel through the ground they lose amplitude due to 

spherical divergence (the energy is spread out over an increasingly larger sphere), intrinsic attenuation (the production of 

electric current by the electric field and the conversion of that 

current to heat), and scattering by heterogeneity.

The most common cause of reflections is a change in 

water content, which causes a change in bulk dielectric permittivity. Water content variations are associated with sedimentary 

structures, lithology, grain size, density (porosity), and saturation. Contrasts in electrical conductivity, from clay or metallic 

objects, also cause reflections.

Because of the high frequencies used, the GPR method 

has exceptional resolution of subsurface features, often much 

less than a meter. However, depth of investigation in sediments 

can be quite limited, with 15 to 20 meters the typical maximum 

depth in very low conductivity, nonfrozen materials, and less 

than a meter in conductive materials.

While GPR may not be commonly used to characterize 

sand and gravel deposits because of its limitations (discussed in 

the following “Benefits” section), there may be occasions that its 

higher resolution compared to other geophysical techniques may 

be useful at particular sites. Generally, when the environment is 

suitable (that is, where the surface and subsurface are relatively 

free of clay), GPR can be used to map the vertical and horizontal 

limits of sand and gravel to determine the volume of the deposit. 

It can be used also to detect the top of lenses of fine-grained 

(clayey) materials, to map stratigraphy within the sand and 

gravel, to determine the depth to water table, and to determine 

thickness of overburden. The GPR method usually cannot be 

used to differentiate sand from gravel unless there is additional 

geologic information.

Several commercial systems are available today that are 

relatively inexpensive to own or rent. The equipment is portable, 

rugged, and easy to operate, though a trained person is required 

to interpret the data. GPR equipment consists of one or two 

antennas, some cables, a power source, and a control device 

that also records data. The antennas commonly are mounted on 

a sled or in a cart and moved by hand or with an all-terrain or 

other small vehicle along a straight path on the ground. The control unit may be with the antennas or separated from them. The 

antennas come in different sizes, with larger antennas having 

lower frequencies. Lower frequency antennas have lower spatial 

resolution, typically 0.5 to 1.5 m, but can detect deeper objects 

or layers. Depending on the equipment and antennas, a one- or 

two-person field crew is required (figs. 15–19).

Ground Penetrating Radar
17

Figure 16. A ground-penetrating radar system 

with antenna and electronics between the cart 

wheels and monitor and computer controls on 

the handle.

Figure 17. Ground-penetrating radar equipment mounted on an all-terrain 

vehicle with antennas towed behind.

Figure 15. A ground-penetrating radar system 

with antenna separate from electronics and computer 

controls. This system can be mounted in a cart or 

towed by hand.

Surface Geophysical Methods to Characterize Sand and Gravel Deposits
18

Figure 18. A 17-antenna ground-penetrating radar system. 

Antennas are inside the yellow shell and are pushed by a 

tractor. This multiantenna system collects data across a 

2-meter-wide swath, and data are processed into a 3-D 

volume. The position of the tractor is determined using an 

automated laser-tracking theodolite.

Figure 19. Interpretation of ground-penetrating radar data (bottom image) 

collected near an active sand and gravel quarry (top image). Hyperbolicshaped reflections near the left side of the radar image are from large cobbles 

and boulders, similar to those seen in the lower left area of the upper image.

An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits

Benefits

Ground can be covered relatively quickly using GPR, 

resulting in detailed, near-continuous profiles (or vertical sections 

of data). Where profiles are near boreholes or other sources of 

stratigraphic information, the raw data images (if scaled correctly) can often be interpreted directly, providing almost realtime results during surveys. Collections of profiles can be used to 

produce three-dimensional blocks of data, from which horizontal 

or vertical slices of information can be extracted.

Over ground that has low conductivity and low scattering losses, surveys can potentially reach depths of 15 to 20 m 

when using low-frequency antennas. However, 3 to 6 m is more 

typical with midsized antennas. Compared to other geophysical 

techniques, GPR resolution is very good with decimeter-scale 

resolution of subsurface features using antennas with a frequency of 500 MHz or higher. When antenna frequency is close 

to 100 MHz, resolution is at the meter scale.

In a favorable environment and with supportive geologic 

information, GPR often can detect a reflection from the top of 

bedrock and the water table. The tops of clay layers are usually detected, but the bottoms of the clay layers and reflectors 

below the clay cannot be detected. Voids and faults sometimes 

can be detected.
Surface Geophysical Methods to Characterize Sand and Gravel Deposits 19

Limitations

Success with the GPR method is site specific. Ground electrical conductivity is the major factor limiting the GPR depth of 

investigation. Survey depth may be extremely limited (much less 

than 1 m) if ground conductivity is high (above 30 to 40 mS/m). 

If the target area is covered with a clayey soil, the soil will have 

to be removed before GPR can be used effectively. If conductive 

layers or lenses occur within the deposit, GPR cannot “see” past 

them. Another factor limiting investigation depth is scattering of 

the radar wave energy due to large cobbles and boulders. This 

can be minimized with the use of lower-frequency antennas. 

Finally, for the limits of the deposit to be detected, there must 

be sufficient contrast in permittivity or conductivity between the 

sand and gravel and adjacent sediment or rock.

In order to focus the radar waves into the ground, the 

antenna must remain in close contact with the ground surface. 

Therefore, the terrain over which the survey is conducted 

cannot be too rough or contain numerous objects that must 

be avoided, such as trees, boulders, and fences. Because of 

the relatively high resolution, extra expense can be added if 

topographic surveys are required so that antenna locations 

are known.

Safety issues with GPR are relatively minor and are 

associated with typical cautions as applied to any field work. 

There may be some electric shock issues if the equipment is 

operated in the rain or wet environments. Other safety issues 

include the hazards of operating trucks and all-terrain vehicles 

off road.
20

Figure 20. Collecting time-domain electromagnetic data. 

The control unit is in the foreground. The receiver coil is in 

the background. A wire square transmitter loop, 40 meters 

on a side, surrounds the receiver coil.

An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits

Overview

Time-domain electromagnetic (TDEM), also called 

transient electromagnetic or TEM, is a sounding method 

that determines the resistivity at different depths below the 

measurement station. It was developed in the 1920s and 

1930s in Europe. Commercial equipment has been widely 

available since the 1960s and 1970s. It is a popular method 

in ground-water and environmental studies, mineral 

exploration, and geologic mapping. The application of 

TDEM for sand and gravel deposits is similar to that of 

DC resistivity.

Time-domain methods take one or more measurements 

as a function of time. The transmitter is an ungrounded, 

large, square or rectangular loop of insulated wire. A direct 

current is passed through the wire for a brief time, producing a magnetic field about the loop. The current is then 

abruptly stopped, and the decaying magnetic field induces 

electric current in the ground under the loop, roughly in 

the shape of the loop. This current diffuses into the ground, 

causing progressively larger and deeper loops of current, 

called eddy currents. As the current diffuses downward and 

outward into the ground, it produces a secondary magnetic 

field. The voltage produced in the receiver by these secondary magnetic fields is called a transient and can be interpreted with respect to the electrical resistivity structure of the 

ground. This voltage is measured at sequential time intervals, 

with later time intervals containing information from deeper 

layers. Because the transient response is measured over a 

very short time period, the measurements are repeated many 

times and an average determined to improve the signal. The 

maximum depth of exploration is one to two times the transmitter loop diameter. To determine lateral variations, both 

the transmitter and receiver are moved to new stations, which 

may be along a straight path or situated at various locations 

in the study area.

TDEM equipment is quite portable and easy to operate but moderately expensive. Two persons is the practical 

size for a TDEM survey crew. An experienced geophysicist uses computers and modeling to determine the variation of resistivity with depth from the field measurements 

(figs. 20–21).

Time-Domain Electromagnetic

Benefits

In comparison to the DC resistivity sounding method, 

TDEM has the following advantages:

faster data collection for deep soundings,

more sensitive to small variations in conductivity,

does not have contact resistance problems with very 

resistive surface layers,

less sensitive to varying surface topography and to 

dipping layers, and

better at detecting very conductive targets.

•

•

•

•

•
21

Figure 21. Bedrock depth, in meters, determined using 

time-domain electromagnetic soundings on an alluvial 

flood plain.

Surface Geophysical Methods to Characterize Sand and Gravel Deposits

In addition, DC resistivity requires larger electrode spacing for greater depths, while TDEM uses larger loop size and 

later time measurements. Direct current resistivity requires 

much larger electrode spacing as TDEM does for the same 

exploration depth.

With respect to the FDEM methods (see "FrequencyDomain Electromagnetic" section), TDEM has better layer 

resolution and definition. Because measurements are made 

after the transmitter is turned off, TDEM is generally insensitive to small coil misalignments. FDEM is very sensitive 

to coil misalignment because measurements are taken with 

transmitter turned on. Both methods usually can detect boundaries of the aggregate with bedrock or clay and sometimes can 

detect the water table.

Limitations

The TDEM method is poor at differentiating relatively 

resistive targets, such as the freshwater level in sand or gravel. 

However, it can detect conductive ground water (water containing large concentrations of dissolved solids). TDEM is also 

poor at resolving very shallow targets, generally those less 

than 10 meters deep.

Sources for noise in TDEM measurements include the 

following:

50-Hz and 60-Hz power lines;

nearby radio and radar transmitters;

thunderstorms;

high wind, which may cause the receiver coil to 

vibrate in the Earth’s magnetic field;

metallic structures, including fences and power lines;

buried metallic objects such as pipes and cables; and

rarely, induced polarization from clayey soils.

Depending on loop size and sounding spacing, data collection can be relatively slow; generally, fewer than 10 soundings can be collected in a day. To speed up data collection, a 

two-person crew can lay out extra transmitter loops in advance 

of moving the transmitter and receiver units from loop to loop. 

Local, strong heterogeneity can be identified by placing the 

receiver at various places around the transmitter loop.

Safety issues with TDEM are minimal and are associated 

with typical cautions as applied to any field work. Caution 

must be taken to eliminate the possibility of lightning strikes 

on the wire loops. Other safety issues include the hazards of 

operating trucks and all-terrain vehicles off road.

•

•

•

•

•

•

•
22

Figure 22. A frequency-domain conductivity meter. The 

meter has fixed coil spacing but measures several frequencies 

at once. A global positioning system antenna is on the operator’s 

back to record station location.

An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits

Overview

Like TDEM, frequency-domain electromagnetic (FDEM)

methods were developed in the 1920s and 1930s in Europe. 

Commercial equipment has been widely available since the 

1960s. FDEM instruments determine the electrical conductivity 

of earth materials at one or more frequencies. Here we are only 

interested in what are commonly referred to as ground conductivity meters or terrain conductivity meters, and not VLF (very 

low frequency) instruments or metal detectors. Ground conductivity meters may be more useful in the exploration stage for 

sand and gravel than in producing detailed characterization of 

a deposit. FDEM equipment is portable, relatively inexpensive, 

and easy to operate. Data are easy to interpret.

In general, a small transmitter coil is energized with 

an alternating current, which produces a time-varying primary 

magnetic field. The magnetic field penetrates into the earth 

and produces alternating electric currents (eddy currents) in 

conductive targets. These eddy currents produce secondary 

magnetic fields, which are sensed, along with the primary 

magnetic field, by a small receiver coil. The instrument calculates an apparent conductivity of the ground; this calculation 

depends upon the spacing and orientation of the coils, the 

frequency of the primary alternating current, and the ratio of 

the secondary to primary magnetic fields.

One person can perform surveys with the smaller conductivity meters, which contain transmitter and receiver coils 

in the same housing. Two persons are required for instruments 

where the coils can be separated; one person must hold each 

coil. The smallest meters can even be used on vertical faces 

of excavations (figs. 22–25).

Frequency-Domain 

Electromagnetic

Benefits

This equipment is quite portable, relatively inexpensive, and easy to operate. Because contact with the ground is 

not required, data collection is rapid, and large areas can be 

covered quickly. Little or no data processing is required for 

preliminary interpretations, which can be made in the field. 

Site conductivity maps are made easily using the raw data. 

Large aerial surveys may indicate prospective sand and gravel 

deposits. Both the FDEM and TDEM methods can usually 

detect boundaries of the aggregate with bedrock or clay and 

sometimes can detect the water table.

Limitations

These instruments may not measure conductivity accurately in highly resistive ground. In highly conductive ground 

the calculated apparent conductivity may be substantially 

different from the true conductivity of the ground. Often, 

only qualitative vertical layering information can be obtained; 

quantitative information can be minimal. Sometimes modeling 

the data and using DC resistivity measurements to calibrate 

the data can improve interpretation.

Power lines, fences, and buried metallic pipes can produce noise in the data. Relative misalignment of the transmitter and receiver coils will introduce errors in the data.

Safety issues with FDEM are minimal and are associated 

with typical cautions as applied to any field work, including 

hazards of operating vehicles off road.
23

Figure 23. A fixed-frequency conductivity 

meter with fixed coil spacing on a plastic 

sled. The operator is using a data logger 

to record measurements and note when 

the meter is passing a flagged station. 

The flagged stations are topographically 

surveyed to determine the measurement 

locations.

Figure 24. A fixed-frequency conductivity meter with fixed coil spacing. A data 

logger can be attached to record readings.

Figure 25. A conductivity meter utilizing separate coils.

Surface Geophysical Methods to Characterize Sand and Gravel Deposits
24

Table 4. Comparison of approximate costs for selected geophysical methods. This does not include the cost of software for data 

processing and visualization, which can be hundreds of dollars to thousands of dollars.

Method Equipment (new)

in $1,000 US

Field

persons

Data collection

per day

Data-processing

days per field day

Seismic 21–37 (24 geophones)

40–75 (48 geophones)

0.1 (hammer source)

10–20 (powered source)

2–4 0.2–0.6 line-kilometer Refraction: 2–3

Reflection: many

Resistivity about 50 (48 electrodes)

about 80 (100 electrodes)

2–4 1–2 line-kilometers 0–1

GPR 25–40 1–2 1–5 line-kilometers 1–2

TDEM 45–65 2 6–20 soundings 1–2

FDEM 20–30 1–2 1–5 line-kilometers 1–2

An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits

When considering the use of geophysics to characterize 

sand and gravel deposits, the cost and efficiency of the various 

methods must be considered. It is beyond the scope of this report 

to tabulate and present all costs associated with conducting geophysical surveys. However, estimates for new equipment prices 

(year 2006), data-collection costs (in terms of number of field 

personnel and data-collection rates), and data-processing costs 

(in terms of processing and interpretation days for every datacollection day) can be made. These are presented in table 4.

Seismic methods require a lot of field work with relatively expensive equipment, and the data cannot generally 

be interpreted immediately in the field. However, lateral 

boundaries and depth to bedrock can usually be determined. 

TDEM equipment is relatively expensive and data collection is relatively slow, but site coverage can be excellent and 

data can be quickly processed. FDEM methods are good for 

rapid data collection and identifying lateral changes in ground 

conductivity with relatively inexpensive equipment, but 

determining depths of layers can be problematic. GPR equipment is relatively inexpensive and data collection is rapid, 

but depth of investigation can be shallow. Even though the 

cost of multielectrode resistivity equipment is relatively high, 

especially for a 100-channel system, data collection is relatively quick and uncomplicated when compared to traditional 

4-electrode surveys. However, setting up the electrodes and 

taking measurements (even though automated) can take hours. 

Initial data interpretation of lateral boundaries and layers can 

be performed by the field crew automatically and within minutes after a section is completed.

Safety issues and their costs also should be considered 

for each method. Some of these issues were mentioned in the

previous sections devoted to each technique. However, we do 

not address all safety concerns associated with the field operations of the techniques.

The geophysical techniques discussed here allow for 

nearly continuous profiles of detailed information that provides continuity between sampling sites or boreholes. With 

resolution ranging from a few centimeters to many meters, 

depending on the technique and specific equipment used, geophysical surveys can provide information on the areal extent 

and thickness of deposits, overburden thickness, and depth to 

water table. However, resolution decreases with depth for all 

surface geophysical methods. In addition, estimates of layer 

thickness can be different from actual thickness because of 

uncertainties in the measurement process and model ambiguity. It is very important to calibrate measurements and interpretations using borehole and test-pit data to guarantee the 

most accurate interpretations. Interpretations are improved 

with advanced knowledge of local stratigraphy, hydrology, 

and cultural conditions.

Data collection over broad areas can be relatively quick, 

and equipment ownership or rental, maintenance, and deployment can be relatively inexpensive. The techniques generally are 

minimally invasive and do not damage the environment or property. Because of this, geophysical methods might be allowed 

where permission for drilling or excavation has been refused.

Because of their advantages, and despite their limitations, geophysical methods have proven to be useful supplements to traditional geologic mapping and sampling in 

the characterizing sand and gravel deposits and planning 

quarry development. Geophysical measurements can extend 

information beyond control points; support and augment 

ground-truth data from drill holes, pits, and laboratory 

measurements; and help to define or to revise the volume estimates (reserve and probable reserve calculations). In general, 

the geophysical methods discussed here are noninvasive, fast, 

and quiet.

Summary and Conclusions
Summary and Conclusions 25

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Goldman, H.B., 1994, Sand and gravel, in Carr, D.D., ed., 

Industrial minerals and rocks, 6th ed.: Littleton, Colo., 

Society for Mining, Metallurgy, and Exploration, Inc., 

p. 869–877.

Goldman, H.B., and Reining, D., 1983, Sand and gravel, in

Lefond, S.J., ed., Industrial minerals and rocks, 5th ed., v. 2: 

NewYork, American Institute of Mining, Metallurgical, and 

Petroleum Engineers (AIME), p. 1151–1166.

Jacobson, R.P., 1955, Geophysical case history of a commercial gravel deposit: Mining Engineering, v. 7, p. 158–162.

References and Selected 

Additional Reading
26 An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits

Jol, H.M., Parry, D., and Smith, D.G., 1998, Ground penetrating radar—Applications in sand and gravel exploration, 

in Bobrowsky, P.T., ed., Aggregate resources—A global 

perspective: Rotterdam, Netherlands, A.A. Balkema, Inc., 

p. 295–306.

Knepper, D.H., Jr., Langer, W.H., and Miller, S., 1995, A 

survey of natural aggregate properties and characteristics 

important in remote sensing and airborne geophysics: Nonrenewable Resources, v. 4, p. 99–120.

Langer, W.H., 1988, Natural aggregates of the conterminous 

United States: U.S. Geological Survey Bulletin 1594, 33 p., 

2 plates, scale 1:5,000,000.

Langer, W.H., 2006, Construction sand and gravel, in Kogel, 

J.E., Trivedi, N.C., Barker, J.M., and Krukowski, S.T., eds., 

Industrial minerals and rocks, 7th ed.: Littleton, Colo., 

Society for Mining, Metallurgy, and Exploration, Inc., 

p. 159–169.

Langer, W.H., Drew, L.J., and Sachs, J.S., 2004, Aggregate 

and the environment: Alexandria, Va., American Geological 

Institute, 64 p.

Langer, W.H., and Glanzman, V.M., 1993, Natural aggregate—Building America’s future: U.S. Geological Survey 

Circular 1110, 39 p.

Langer, W.H., and Knepper, D.H., Jr., 1998, Geologic characterization of natural aggregate—A field geologist’s guide to 

natural aggregate resource assessment, in Bobrowsky, P.T., 

ed., Aggregate resources—A global perspective: Rotterdam, 

Netherlands, A.A. Balkema, Inc., p. 275–293.

Lenhart, W.B., 1960, Sand and gravel, in Gillson, J.L., ed., 

Industrial minerals and rocks, 3d ed.: New York, The 

American Institute of Mining, Metallurgical, and Petroleum 

Engineers (AIME), p. 733–758.

Loke, M.H., 1999, Electrical imaging surveys for environmental and engineering studies—A practical guide to 2-D and 

3-D surveys: Geometrics, Inc. [accessed February 28, 2007, 

at ftp://geom.geometrics.com/pub/GeoElectric/Literature/

Lokenote.pdf].

Lucius, J.E., Olhoeft, G.R., Hill, P.L., and Duke, S.K., 1992, 

Properties and hazards of 108 selected substances, 1992 ed.: 

U.S. Geological Survey Open-File Report 92–527, 554 p.

McCuaig, S.J., and Ricketts, M.J., 2004, Ground-penetrating 

radar—A tool for delineating aggregate-resource deposits: Newfoundland and Labrador Department of Mines 

and Energy, St. John’s, Newfoundland, Geological Survey 

Report 04–1, p. 107–115.

McNeill, J.D., 1980, Electromagnetic terrain conductivity measurement at low induction numbers: Mississauga, 

Ontario, Geonics, Ltd., Technical Note TN–6, 15 p.

Nabighian, M.N., ed., 1991, Electromagnetic methods in 

applied geophysics, v. 2, Applications, parts A and B: Tulsa, 

Okla., Society of Exploration Geophysicists, Investigations 

in Geophysics, no. 3, 972 p.

Odum, J.K., and Miller, C.H., 1988, Geomorphic, seismic, 

and geotechnical evaluation of sand and gravel deposits 

in the Sheridan, Wyoming, area: U.S. Geological Survey 

Bulletin 1845, 32 p.

Patterson, S.B., 1937, Crushed and broken stone, chapter 42, 

in Industrial minerals and rocks: New York, American Institute of Mining and Metallurgical Engineers, p. 795–836.

Reynolds, J.M., 1997, An introduction to applied and environmental geophysics: Chichester, England, Wiley, 796 p.

Saarenketo, T., and Maijala, P., 1994, Applications of geophysical methods to sand, gravel, and hard rock aggregate 

prospecting in northern Finland, in Luttig, G.W., ed., Aggregates—Raw materials giant: Report on the 2d International 

Aggregates Symposium, Erlangen, Germany, October 22–

27, 1990, p. 109–123.

Sharma, P.V., 1997, Environmental and engineering geophysics: Cambridge, England, Cambridge University Press, 

475 p.

Sheriff, R.E., 1991, Encyclopedic dictionary of exploration 

geophysics, 3d ed.: Tulsa, Okla., Society of Exploration 

Geophysicists, 376 p.

Telford, W.M., Geldart, L.P., and Sheriff, R.E., 1990, Applied 

geophysics, 2d ed.: Cambridge, England, Cambridge University Press, 790 p.

Tepordei, V.V., 2006, Stone, crushed: U.S. Geological Survey 

Mineral Commodity Summaries, p. 158–159.

Thoenen, J.R., 1932, Prospecting and exploration for sand and 

gravel: U.S. Bureau of Mines Information Circular 6668, 

259 p.

Timmons, B.J., 1994a, Prospecting for natural aggregates—An 

update, part 1: Rock Products, v. 97, no. 8, p. 43–45.

Timmons, B.J., 1994b, Prospecting for natural aggregates—An 

update, part 2: Rock Products, v. 97, no. 9, p. 23–25.

Timmons, B.J., 1994c, Prospecting for natural aggregates—An 

update, part 3: Rock Products, v. 97, no. 10, p. 43–46 and 54.

Timmons, B.J., 1995, Prospecting for natural aggregates—An 

update: Rock Products, v. 98, p. 31–37.

U.S. Army Corps of Engineers, 1995, Geophysical exploration for engineering and environmental investigations: 

Engineer Manual EM 1110–1–1802, USACE CECW–EG, 

multiple pagination [accessed February 28, 2007, at 

http://www.usace.army.mil/inet/usace-docs/eng-manuals/

em.htm].

Wilcox, S.W., 1944, Sand and gravel prospecting by the earthresistivity method: Geophysics, v. 9, p. 36–46.
27

A

alternating current Alternating current 

is an electric current where magnitude and 

direction of the current changes (reverses) 

cyclically over time. Alternating current 

always produces an alternating magnetic field. 

There will be an associated alternating electric 

field in nearby conductors. Alternating current 

is commonly used in houses and offices. See

also direct current.

apparent conductivity Apparent conductivity is the electrical conductivity of 

homogeneous ground that will produce the 

same response as was measured in the field 

at a single set of measurement parameters. 

For a frequency-domain electromagnetic 

instrument, measurement parameters might 

include excitation frequency, coil geometry, 

and intercoil separation. An apparent conductivity can be defined for each set of these 

parameters. The actual ground conductivity may be very different from the apparent 

conductivity. This is due in part because the 

conductivity of earth materials can change 

both laterally and vertically. However, a single 

measurement from a conductivity meter cannot discern these variations. By collecting 

measurements at different frequencies and(or) 

different coil separations, and by understanding the construction and limitations of the 

meter, an electrical conductivity model of the 

earth can be constructed beneath a measurement station.

apparent resistivity Apparent resistivity 

is the electrical resistivity of homogeneous 

ground that will produce the same response 

as was measured in the field using a single 

set of measurement parameters. Apparent 

resistivity and apparent conductivity are the 

reciprocal of each other. Apparent resistivity allows comparison of measurements 

from one area to another and provides a first 

approximation to the actual earth resistivity. Modeling is required to determine the 

representative electrical resistivity structure 

of the earth.

Glossary

Definitions have been adapted from Sheriff (1991), Lucius and others (1992), Reynolds 

(1997), http://��en.w�ik�ip���� edia.��� org/ and other general sources.

C

conduction current Conduction current is 

an electric current produced by an applied 

voltage difference or an electric field.

D

density Density is mass per unit volume. 

Common measurement units are gram per 

cubic centimeter (g/cm3), kilogram per cubic 

meter (kg/m3), pound per cubic foot (lb/ft3), and 

ton per cubic yard (ton/yd3). For two objects of 

the same size, the denser one will weigh more 

or feel heavier. Density of sand and gravel is in 

the range of 1.7 to 2.3 g/cm3; for sandstone 1.6 

to 2.7 g/cm3; for shale, 1.75 to 3.2 g/cm3; and 

for limestone, 1.9 to 2.9 g/cm3.

dielectric permittivity Dielectric permittivity 

(also called dielectric constant) is a measure of 

a material’s ability to store electric energy by 

separating opposite polarity charges in space 

when an external electric field is applied. It 

is measured in farad per meter (F/m). Often, 

relative dielectric permittivity (RDP) is used 

to describe this property. RDP is the ratio of a 

material’s dielectric permittivity to the dielectric permittivity of free space (like in outer 

space), which is 8.854 × 10–12 F/m. RDP is 

unitless. Air has an RDP of about 1.003. The 

RDP of water is about 80, depending on temperature. The RDP of common dry earth materials is often between 2 and 6. However, when 

water is present in the material, the dielectric 

permittivity usually increases as water content 

increases. A mixture of relatively dry sand 

and gravel has an RDP between 3 and 6. The 

RDP of saturated sand and gravel is usually 

between 16 and 36, depending on porosity. 

The speed of radar waves (or any electromagnetic wave) in a material is determined 

mostly by the RDP. Radar waves (and light 

waves) travel faster in air and slower in water. 

If a material has a very low RDP, it is called a 

dielectric or an insulator.

diffusion Diffusion is a physical process 

where matter (particles) or energy is spontaneous spread from high concentration to 

Glossary
28

low concentration. In geophysics, it can refer 

to the transport of an electric field or magnetic field, or to the movement of charges in 

response to an applied electric or magnetic 

field. Diffusion causes rapid, exponential 

decay of the electric and magnetic field with 

distance and time. See also propagation, 

which is a different physical process.

Direct current Direct current is an electric 

current where the flow of electric charge 

(such as electrons or ions) through a conductor is in one direction from high to low electric potential. Direct current always produces 

a magnetic field. Direct current is sometimes 

called galvanic current. Direct current is commonly used to run certain types of equipment 

and is commonly produced by batteries. See

also alternating current.

E

elastic, elasticity Elastic means that an 

object returns to its original shape after having 

been deformed by an external force. An object 

that does not return to its original form is 

called plastic. Elasticity is defined by the various combinations of stress, strain, and the type 

of force being applied. The stress-strain properties of elastic earth materials are described 

by elastic moduli. The velocities of seismic 

waves can be expressed in terms of elastic 

moduli and density. See elastic properties.

elastic properties Elastic properties of 

an isotropic material (where the properties 

are the same measured in any direction) are 

described using three elastic moduli, one 

constant, and one ratio of strain. The bulk 

modulus (k) is the stress-strain ratio under 

uniform (same in all directions) pressure or 

compression, ∆P/(∆V/V), that is, the change 

in pressure divided by the fractional change in 

volume. The bulk modulus is also called the 

modulus of compression. The shear modulus

(μ) is the stress-strain ratio for simple shear, 

(∆F/A)/(∆L/L), that is, the change in force per 

unit area divided by the fractional change in 

length. The shear modulus is also known as 

the rigidity modulus. Young’s modulus (E) is 

a measure of the stiffness of a material and is 

the stress-strain ratio when a rod is pulled or 

compressed, (∆F/A)/(∆L/L). Lamé’s constant

equals (k – 2μ/3). Poisson’s ratio is the ratio 

of transverse strain to longitudinal strain. For 

example, when a rod is compressed, its length 

is shortened (longitudinal strain) and its width 

is increased (transverse strain). The velocity 

of P-waves and S-waves can be defined in 

terms of the bulk and shear moduli and the 

density. See also stress, strain, and elasticity.

electric current Electric current is the 

movement of electrically charged particles, 

such as electrons or ions. Electric current 

produces a magnetic field. Because electrical charge is involved there is also an electric 

field. The unit for current is the amp. See also

alternating current, direct current, electrical 

methods, and electromagnetic methods.

electric field An electric field is produced 

by the presence of electrical charge, such as 

an electron or ion. An electric field contains 

electrical energy. An electric field can be 

static (strength does not change over time) or 

varying (strength changes over time). Common units for an electric field are volt per 

meter or Newton per coulomb.

electric potential Electric potential is the 

stored energy (or potential energy) per unit 

electric charge associated with a static (or 

constant) electric field. It is measured in volts. 

Electrical potential (or voltage) is only physically meaningful when measured between two 

different locations.

electrical conductivity Electrical conductivity is the ability of a material to conduct electric current and is the reciprocal of electrical 

resistivity. Units are siemens per meter (S/m). 

Electrical conductivity affects the attenuation 

of electromagnetic fields. Where electrical 

conductivity is near zero, such as in air (about 

10–13 S/m), electromagnetic (EM) waves can 

travel great distances, but electric current generally cannot flow. In the earth, where electrical conductivity is often greater than 0.01 to 

0.1 Siemen per meter, EM waves propagate 

relatively short distances, but electric current 

flows readily.

electrical methods Electrical methods 

measure electric potential (voltage) produced 

by electric current, or electric charge flow, 

in the ground. Current in earth materials 

is controlled by the fundamental material 

property called electrical conductivity (the 

An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits
2

9

ease with which electrical charge can be 

caused to flow) or its reciprocal, electri-

cal resistivity. Because most dry rocks and 

sediments are poor conductors of electric-

ity, conductivity is generally determined by 

the quality and quantity of water in the pore 

space, the connectedness of the pores, and the 

relative abundance of clay minerals. Electric 

conduction in conductive rocks and sediments 

principally occurs in two ways: electrolytic 

(or ionic), such as the movement of ions in 

water, and electronic (or ohmic), such as the 

movement of charge carriers (for example, 

electrons) in metals or clays. A third form 

of limited conduction, called displacement 

or dielectric conduction, occurs in media 

with low conductivity.

As electrical current passes through conduc-

tive ground it always produces both a magnetic 

field and an electric field. These fields can 

cause electrical polarization. The four com-

mon forms of electrical polarization include 

electronic (net displacement of the electron 

cloud with respect to the nucleus of an atom), 

ionic (displacement of charged atoms or mol-

ecules with respect to each other), orientational 

(aligning of molecules with asymmetrical 

charge distributions, like water molecules), 

and interfacial (local accumulations of migrat-

ing charge against a chemical or electrical 

barrier). The electrical methods focus on the 

electric field and sometimes on polarizations 

produced by charge flow.

Electrical methods include direct current 

resistivity, self potential (also called sponta-

neous potential), induced polarization, and 

seismoelectric.

electrical resistivity Electrical resistivity is 

that property of a material that resists the flow 

of electric current. Resistance converts electri-

cal energy into heat, leading to a loss of elec-

tric potential or electric current. Electrical 

resistivity is the ratio of the electric field 

intensity between two parallel surfaces of a 

cube to the electric current density perpen-

dicular to the same two surfaces. Resistivity is 

commonly expressed in ohm-meters (short for 

ohm-meter

2

 per meter). The resistivity of earth 

material covers a broad range. In general, soil, 

clay, and shale have low resistivity; sand and 

gravel have high resistivity; and limestone 

and some igneous rocks can have even higher 

resistivity. However, the amount and type of 

water in the material can drastically affect a 

material’s bulk resistivity.

electromagnetic field An electromagnetic 

(EM) field is composed of two related fields, 

the electric field and the magnetic field. Typi-

cally, the strength of the two fields changes 

over time, alternating in strength in a cyclic 

and synchronized manner. A changing electric 

field produces a changing magnetic field, and 

the reverse. The number of cycles per second 

is called the frequency (usually measured in 

hertz [Hz], which is one cycle per second). 

Electromagnetic properties affect the propaga-

tion (or transmission) and attenuation of EM 

fields. An EM field cannot propagate for any 

great distance in conductive media like typical 

rocks and sediments because the electric field 

leads to electric current (also called conduc-

tion current) and rapid dissipation of the 

energy into heat. When an EM field occurs in 

media with low conductivity, such as air, ice, 

or resistive sediments, it travels or propagates 

as an electromagnetic wave; a radio wave is a 

common example.

electromagnetic methods The electromag-

netic (EM) methods are the most complex and 

varied in terms of technique and equipment. 

EM methods involve alternating electric and 

magnetic fields with frequencies from about 

10–5 Hz to about 1010 Hz. When an EM field 

occurs in media with low conductivity, such 

as air, ice, or resistive sediments, it travels 

or propagates as a wave; a radio wave is a 

common example. Generally, below about 

100 kHz in conductive media, EM energy 

transport is governed by a diffusion process 

and conduction currents dominate. Above 

about 10 MHz, EM propagation in resistive 

media is governed by a wave process. With the 

exception of ground penetrating radar (GPR), 

which operates at frequencies from 10 MHz to 

4 GHz), EM methods generally work best over 

moderately conductive ground.

The EM methods differ from the electrical 

methods in that the electromagnetic fields 

in the ground are produced from electric or 

magnetic fields generated above the ground. 

Through a process called inductive coupling, 

which does not require direct contact of the 

transmitting source with the ground, currents 

are caused to flow in the ground. This can be 

Glossary
30

an advantage over the electrical methods that 

require electrodes inserted into the ground 

(called galvanic coupling) by allowing much 

quicker surveying over an area. EM methods 

often measure the magnetic field and(or) the 

electric field, making the equipment, and sometimes the interpretation, more complicated.

Some EM methods are passive in that the 

equipment used does not have to produce 

the field that interacts with the subsurface. 

These include the telluric, magnetotelluric, 

and VLF (very low frequency) methods. The 

frequency-domain EM, time-domain EM, 

and GPR methods are active-source methods 

that use a transmitter to produce an EM field. 

Transmitters usually consist of a large single 

loop of wire (often configured as a square), a 

small coil of wire with many turns, or a dipole 

(a straight length of wire or sheet of metal), 

through which is passed an alternating electric 

current or a pulse of current.

Alternating current in a loop of wire produces 

an alternating magnetic field, which induces 

current flow in nearby conductors. These 

induced currents, in turn, produce a secondary 

magnetic field. Alternating current in a dipole 

produces an alternating electric field, which 

then produces an alternating magnetic field 

that induces current flow in the earth. In either 

case, whether using an electrically shorted 

loop or an open dipole, alternating EM fields 

are produced in the earth. For the receiver 

using a loop, the secondary magnetic fields 

produce current flow in the receiver loop and 

a measurable voltage. For a receiver using a 

dipole, the secondary electric fields produce a 

voltage that is measured.

electromagnetic properties Electromagnetic 

properties affect the propagation and attenuation of electromagnetic fields in the earth. The 

three properties are electrical conductivity, 

dielectric permittivity, and magnetic permeability. The speed of EM waves is determined 

mostly by dielectric permittivity. The attenuation (or reduction in strength) of EM waves is 

determined mostly by electrical conductivity.

electromagnetic wave An electromagnetic 

wave is an electromagnetic disturbance (in this 

case an electromagnetic field) that propagates 

(moves or travels) in a periodically repeating 

manner, transferring energy from one place 

to another. Radiation (nonionizing) is a term 

synonymous with electromagnetic wave. The 

directions of the magnetic field and electric 

field are at right angles to each other and 

perpendicular to the direction of energy flow. 

Consider this cross shape: +. The electric field 

varies in strength (or amplitude) on the horizontal bar with positive amplitude to the right. 

The amplitude of the magnetic field changes 

on the vertical bar, with positive amplitude to 

the top. The direction of propagation then is 

out of and perpendicular to this page.

energy Energy is the capacity to do work. 

All physical systems possess this fundamental 

quantity. Energy is defined as the amount of 

work required to change the state of a system 

from one position or level to another position 

or level.

equivalence Equivalence means that a 

layer can produce nearly the same geophysical response for a variety of combinations 

of thickness and property magnitude. This is 

particularly true with the electrical, electromagnetic, and potential-field methods. For 

example, nearly the same electric current 

flows in two different resistive layers if the 

product of their thickness and resistivity are 

similar. Equivalence issues may be overcome 

using additional geologic or geophysical 

information. Equivalence is less of a problem 

with wave-based methods such as ground 

penetrating radar and seismic methods.

I

inversion Inversion is the process of 

determining a model, or distribution, of earth 

properties that reproduces the observed field. 

The model or distribution may not be unique.

isotope Isotopes are different forms of the 

same element that have different atomic 

weights; that is, the number of protons in the 

atom is the same, but the number of neutrons 

is different. Unstable isotopes decay, releasing 

radioactivity, until they form stable isotopes 

of either the same element or a different one.

M

magnetic field A magnetic field is the effect 

produced by moving electric charges (called 

an electric current). It is also called magnetic 

An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits
31

intensity, magnetic induction, or magnetic 

flux density. A magnetic field contains 

magnetic energy. Unlike electric fields, mag-

netic fields have no related “magnetic charges.” 

A magnetic field that changes in strength over 

time produces an electric current in nearby 

conductors. The unit of magnetic field strength 

is either the tesla or gauss.

magnetic permeability Magnetic perme-

ability is a measure of the degree to which a 

substance may be magnetized. It is the unitless 

ratio of the magnetic field strength to the 

external magnetizing force. Magnetic field 

strength is also called magnetic field, magnetic 

intensity, magnetic induction, or magnetic flux 

density. Magnetic field strength is equal to the 

magnetic permeability times the magnetizing 

force. Magnetic permeability affects elec-

tromagnetic fields in much the same way as 

electrical conductivity does.

magnetic susceptibility Magnetic suscep-

tibility is a measure of the degree to which 

a substance may be magnetized. It is the 

unitless ratio of magnetization of a material 

to the external magnetizing force. Magneti-

zation is also referred to as magnetic dipole 

moment per unit volume, magnetic polariza-

tion, or magnetization intensity. Magnetiza-

tion is equal to the magnetic susceptibility 

times the magnetizing force. Most common 

earth materials are called diamagnetic; that is, 

their magnetic susceptibility is slightly nega-

tive and so produces a very small magnetic 

dipole moment (or strength) in a direction 

opposite to the inducing magnetizing force. 

Paramagnetic materials have a very small 

positive susceptibility and produce a magnetic 

dipole moment that slightly strengthens the 

magnetizing force. Examples of paramagnetic 

materials include the elements aluminum, 

calcium, platinum, sodium, uranium, and 

oxygen. A third class of materials is called 

ferromagnetic and has a positive and rela-

tively large susceptibility. Iron, magnetite, 

and most steels are ferromagnetic.

model, modeling With respect to geophysi-

cal interpretation, a model is a conceptual 

representation of the distribution of physical 

properties in the earth that reproduce field 

geophysical measurements. However, agree-

ment between the actual field data and the 

computed data from a model does not prove 

or validate the model. Models usually lack 

uniqueness and typically are simplifications 

of the actual distribution of properties in the 

earth. A model is used to provide a better 

understanding of the field observations. Mod-

eling is the process of developing a model. P

P-wave A P-wave is an elastic body seismic

wave. Particle motion is in the same direc-

tion as propagation. A P-wave is also called a 

primary wave, compressional wave, longitu-

dinal wave, pressure wave, dilatational wave, 

rarefaction wave, or irrotational wave. P-waves 

travel faster than S-waves in the same material.

potential field methods Potential field meth-

ods measure the Earth’s gravitational or mag-

netic fields and the effects of local objects and 

structures on those fields. These are natural 

source or passive methods in that the equip-

ment does not produce the field that interacts 

with the subsurface.

The gravity method measures variations of 

the Earth’s gravitational field. These varia-

tions occur because of the density distribution 

of local masses (such as hills, valleys, and 

lakes) as well as deep geologic structure and 

even the position of the Moon with respect 

to the Earth. The average value of accelera-

tion due to gravity at the Earth’s surface is 

980 cm/s

2

 (about 32.15 ft/s

2

) or 980 gal. 

The microgravity method is precise enough 

to measure variations as small as several 

microgal (10–6 gal). Unconsolidated sedi-

ments such as sand, gravel, silt, clay, and soil 

have similar densities. The various types of 

rocks are usually slightly denser than sedi-

ments. The microgravity method might detect 

variations of the bedrock surface elevation, 

cavities in rock, or lateral changes between 

sediments or fill and rock. Gravimeters that 

are sensitive at the microgal level are relatively 

expensive and require a skilled and patient 

operator. The gravimeter measurement sta-

tion, as well as local land and cultural masses, 

must be precisely located with a topographic 

survey, increasing the cost and time of gravity 

surveys. The processing and interpretation 

of gravity data require a skilled technician 

or scientist and computer software. For these 

Glossary
32

reasons, microgravity is not considered in 

this report as a practical method for characterizing most sand and gravel deposits. On the 

other hand, the microgravity method could be 

considered in areas where electrical, electromagnetic, and seismic methods fail due to 

electrical or acoustic noise or cultural features 

such as buried or overhead utilities.

The geomagnetic method measures variations in the Earth’s magnetic field, either its 

total intensity or the intensity of one or more 

individual components (such as vertical or 

horizontal). Variations of the magnetic susceptibility of sediments and rocks perturb this 

field. Ferrous metal objects cause the strongest perturbations. The Earth’s magnetic field 

also varies in time, in position on the Earth 

(due to the dipole nature), and from magnetic 

storms. Magnetometers of high quality are 

relatively inexpensive, and their operation is 

easy to learn. Skilled technicians or scientists 

and computer software may be required for 

processing and interpretation. Geomagnetism 

is often used on a small scale to detect buried 

metal objects and on a large scale for regional 

geologic mapping. Generally, there will be 

insufficient contrast in magnetic susceptibility 

of sand and gravel compared to fines or clay 

and most sedimentary rocks to differentiate 

them. If bedrock has a strong susceptibility 

compared to the fill or overburden, then the 

geomagnetic method may be able to determine variations in depth to bedrock. Nevertheless, the geomagnetic method is considered 

here as an impractical method for characterizing most sand and gravel deposits.

propagation, propagate Propagation, in 

geophysics, refers to the movement of energy 

through a material. Seismic propagation implies 

the periodic and directional variation of elastic 

disturbance to transfer strain energy from place 

to place through wave motion. Electromagnetic 

propagation transfers electromagnetic energy 

from place to place by either wave motion or 

diffusion. The dominant energy transmission 

process depends upon the conductivity of the 

material, the excitation frequency, and a scale 

factor. At higher frequencies, wave propagation 

dominates, while at lower frequencies diffusion 

dominates. See also diffusion.

R

radar wave A radar wave is an electromagnetic wave. Most ground penetrating radar 

antennas radiate a single pulse of an electromagnetic field. This field is called a radar 

wave as it propagates (or moves) away from 

the antenna. When the radar wave encounters 

a change in electromagnetic properties, part 

of the wave may be reflected off the boundary 

and part may continue through the boundary.

radioactivity, radiation Radioactivity is the 

emission of particles or energy as part of the 

decay of isotopes into isotopes of the same 

or different element. This ionizing radiation 

is typically alpha particles (equivalent to a 

helium nucleus: two protons and two neutrons), beta particles (actually electrons), and 

gamma rays (high energy electromagnetic 

waves like X-rays).

radioactivity methods Radioactive methods 

make use of the radioactive decay of certain 

elements. During the decay process, radiation is emitted. This radiation is typically 

alpha particles, beta particles, and gamma 

rays. Geiger counters detect alpha and beta 

particles. Scintillation meters detect gamma 

rays and beta particles.

Though there are many naturally occurring 

radioactive elements, only uranium, thorium, 

and potassium-40 are important in surface 

radioactivity detection methods (Telford and 

others, 1990). Radioactive elements, potassium 

in particular, tend to accumulate in clay and 

shale. If a sand and gravel deposit is bounded 

by shale or clay, a borehole radioactive method 

such as natural gamma logging might be useful 

for characterizing its thickness. This method 

typically is not recommended for characterization of typical sand and gravel deposits.

relative dielectric permittivity See dielectric 

permittivity.

rippability Rippability is a measure of the 

ability to excavate earth or rock materials with 

conventional excavating equipment. A material 

may be classified as nonrippable, marginally 

rippable, or rippable.

S

S-wave An S-wave is an elastic body 

seismic wave. Particle motion is in a direction 

perpendicular to the direction of propagation. 

An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits
33

An S-wave is also called a secondary wave, 

shear wave, transverse wave, rotational wave, 

distortional wave, equivolumnar wave, or tan-

gential wave. S-waves can be polarized (the 

direction of particle motion) in the vertical 

(SV-wave) or horizontal (SH-wave) direction.

seismic methods Seismic methods depend 

on the transmission of strain energy (called 

seismic energy) as a wave through a medium. 

Strain occurs because of an external force 

being applied to a medium. The ratio of that 

force to the area it is applied to is called 

stress. If a body returns to its original shape 

after being subjected to stress, it is said to be 

elastic. A seismic source produces compres-

sion and(or) shear force in the ground. That 

force creates seismic wave energy that propa-

gates from the source in all directions, in both 

the ground and the air.

The two main types of seismic waves are 

body waves, which travel through layers and 

objects, and surface waves, which travel in 

the interface between layers, such as at the 

Earth’s surface. For this report, we discuss 

only body waves. The two types of body 

waves are P-waves and S-waves. In a par-

ticular medium, P-waves travel faster than S-waves. The velocity of these waves is 

determined by both the elastic properties and 

density of the ground. Seismic waves travel 

slower in unconsolidated sediments and faster 

in rock. Seismic wave detectors are called 

geophones, which convert ground vibrations 

(or motion) into a voltage response that is 

recorded by seismographs and computers.

seismic speed Seismic speed is a mea-

sure of how fast seismic waves travel (or 

propagate) in the ground without regard to the 

direction of that propagation. The speed of 

these waves is affected by both the elasticity, 

or stiffness, and density of the ground.

seismic velocity Seismic velocity is a 

measure of how fast seismic waves travel (or 

propagate) in the ground and the direction of 

that propagation. See also seismic speed.

seismic wave A seismic wave is an elastic 

disturbance that propagates from one place 

to another. Seismic waves are waves of 

elastic strain energy traveling through the 

ground. There are many types of seismic 

waves: (1) two types of body waves: P-waves

and S-waves; (2) several types of boundary 

or surface waves: Rayleigh waves, pseudoRayleigh waves (called ground roll), Love 

waves, Stonely waves, and tube waves; 

(3) channel waves; (4) air waves (or shock 

waves); and (5) standing or stationary waves. 

this report, we are concerned only with 

P-waves and S-waves. See also elasticity.

The propagation of seismic waves is a com-

plicated phenomenon. Seismic wave energy is 

a form of mechanic energy (particle motion). 

When the seismic wave encounters changes in 

elastic properties and(or) density, some of the 

energy is transmitted through the boundary and 

some is reflected back to the surface where it 

is measured as ground motion by geophones. 

The actual transmission and reflection of 

seismic waves is complicated and is often 

accompanied by transformation from one type 

of wave to another. As the waves travel through 

the ground, they lose energy due to spherical 

divergence (the energy is spread out over an 

increasingly larger sphere), intrinsic attenuation 

(absorption of elastic energy by the ground), 

and scattering (propagation to and away from 

the geophones). See also seismic methods.

strain Strain is the ratio of a change in length 

(or volume) of a medium or object to the origi-

nal length (or volume) due to an external force, 

or stress. Strain is written as ∆L/L or ∆V/V, 

where ∆ means “change in.”

stress Stress is the ratio of force to the area 

to which it is applied (written as F/A). Pressure 

is an example, with units of pound-force per 

square inch. A normal force is applied perpen-

dicular to a surface and tends to compress an 

object or material. A tangential force (or shear 

force) is applied parallel to a surface and tends 

to shear the object or material. W

wave A wave is a disturbance that propagates 

through space (vacuum or no medium), in a 

material or medium, or at the interface between 

two media. A wave travels and transfers energy 

without permanent displacement of particles in 

the medium: that is, there is no associated net 

mass transport; instead, particles oscillate about 

fixed positions. Waves are usually character-

ized by periodicity, meaning that maximum and 

minimum amplitudes or particle displacements 

occur repetitively in time or in space.

Glossary
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Lucius and others—An Introduction to Using Surface Geophysics to Characterize Sand and Gravel Deposits—U.S. Geological Survey Circular 1310