An Introduction to Using Surface Geophysics to 

Characterize Sand and Gravel Deposits 

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

Open-File Report 2006–1257 

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 2006 

For product and ordering information: 

World Wide Web: http://www.usgs.gov/pubprod 

Telephone: 1-888-ASK-USGS 

For more information on the USGS—the Federal source for science about the Earth, 

its natural and living resources, natural hazards, and the environment: 

World Wide Web: http://www.usgs.gov 

Telephone: 1-888-ASK-USGS 

Suggested citation: 

Lucius, J.E., Langer, W.H., and Ellefsen, K.J., 2006, An Introduction to using surface geophysics to 

characterize sand and gravel deposits: U.S. Geological Survey, Open-File Report 2006-1257, 51 p.; [ available 

online at http://pubs.usgs.gov/of/2006/1257/ ]. 

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report. 

Cover photograph: view is looking to the north across a valley south of Steamboat Springs, Colo., U.S.A. 

ii
Contents 

Contents ..............................................................................................................................................................................iii

Figures .................................................................................................................................................................................iii

Tables ..................................................................................................................................................................................iii

Abbreviations .....................................................................................................................................................................iv

Photography Credits .........................................................................................................................................................iv

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

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

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

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

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

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

Additional Information ...................................................................................................................................................6

Seismic Refraction and Reflection ..............................................................................................................................7

DC Resistivity.................................................................................................................................................................15

Ground Penetrating Radar..........................................................................................................................................23

Time-Domain Electromagnetic...................................................................................................................................30

Frequency-Domain Electromagnetic ........................................................................................................................34

Summary and Conclusions .............................................................................................................................................38

References and Selected Additional Reading ............................................................................................................40

Glossary..............................................................................................................................................................................43

Figures 

Figure 1. A portable, rugged computer used for controlling, monitoring, and recording seismic data. ............9

Figure 2. P-wave geophone (red) and seismic cable. .................................................................................................9

Figure 3. Hammer and plate as a P-wave seismic source........................................................................................10

Figure 4. A buried charge used as a P-wave seismic source..................................................................................11

Figure 5. S-wave seismic equipment ...........................................................................................................................12

Figure 6. Generating S-waves with a lightweight S-wave source..........................................................................13

Figure 7. Interpretation of processed S-wave reflection data.................................................................................14

Figure 8. A control unit for a multi-electrode resistivity system..............................................................................17

Figure 9. A metal stake and a “smart” electrode strapped to it for a multi-electrode resistivity system........18

Figure 10. Some electrodes for a multi-electrode resistivity system, at close spacing......................................19

Figure 11. A capacitively coupled resistivity system towed by an all-terrain vehicler. ......................................20

Figure 12. Another view of a capacitively coupled resistivity system....................................................................21

Figure 13. Interpretation of a resistivity model. ..........................................................................................................22

Figure 14. Interpretations of resistivity models collected over a sand and gravel deposit)...............................22

Figure 15. A GPR system with antenna separate from electronics and computer controls. .............................25

Figure 16. A GPR system with antenna and electronics between the cart wheels and monitor and computer 

controls on the handle.....................................................................................................................................................26

Figure 17. GPR equipment mounted on an all-terrain vehicle with antennas towed behind. ............................27

Figure 18. Seventeen GPR antennas are inside the yellow shell. ...........................................................................28

Figure 19. Interpretation of GPR data collected near an active sand and gravel quarry. ..................................29

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Figure 20. Collecting time-domain electromagnetic data. ........................................................................................32

Figure 21. Bedrock depth determined using TDEM soundings on an alluvial flood plain...................................33

Figure 22. A frequency-domain conductivity meter...................................................................................................35

Figure 23. A fixed-frequency conductivity meter with fixed coil spacing on a plastic sled. ..............................36

Figure 24. A fixed-frequency conductivity meter with fixed coil spacing..............................................................37

Figure 25. A conductivity meter utilizing separate coils............................................................................................37

Tables 

Table 1. Physical properties and associated geophysical methods.........................................................................4

Table 2. Typical approximate limits and ranges of selected properties of some common geologic material..5

Table 3. Decision matrix of selected surficial geophysical methods for aggregate investigations. ..................6

Table 4. Comparison of approximate costs for selected geophysical methods...................................................38

 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 (1000 meters; ~3281 feet or ~0.621 mile) 

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

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

mS/m millisiemen 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) 

s second (unit of time) 

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

Photography Credits 

The authors thank these photographers for their contributions. 

Jared Abraham: figure 12. 

Bethany Burton: 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. 

iv
An Introduction to Using Surface Geophysics to 

Characterize Sand and Gravel Deposits 

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

Introduction 

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 non-fuel 

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 State from over 6,300 

operations, ranging in size from those that produce less than 25,000 tons per year to those that 

produce over 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 waste-treatment facilities. Our current quality of life would be impossible to maintain without 

sand and gravel. 

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 to understand better 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 resolution, differentiation, and depth of investigation. 

Third, we required that cultural features, such as 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, 

1
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. Terms that are presented in italics 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 (1994, 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. 

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 aerial 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, as well as 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 m. 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. 

2
Depending on site conditions and the selected method(s), geophysical surveys can provide 

information on the following: 

• aerial 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, like 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 a lack of understanding and 

confidence in the methods, and the subjectiveness of interpretations. We address these issues, and 

more, in the next section. 

3
Surface Geophysical Methods 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. 

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 EM Electrical, electromagnetic 

Dielectric permittivity High-frequency EM Electromagnetic 

Elasticity Elastic waves Seismic 

Radioactivity α, β, gamma rays Radioactivity 

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, as 

well as 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 1500 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 will often be in the 

hundreds of ohm-m. 

Typical values for selected properties of some common geologic material are shown in table 

2. 

4
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). 

Material Conductivity 

(mS/m) 

Resistivity 

(ohm-m) 

Relative Dielectric 

Permittivity 1

P-Wave velocity 

(m/s) 

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

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

Saturated sand & gravel > 20 < 50 Medium 1250 to 1850 

Clay 10 to 200 5 to 100 Medium to high 900 to 2700 

Sandstone 1 to 20 50 to 1000 Low 1800 to 4000 

Shale 20 to 200 5 to 50 Low 2500 to 4300 

Limestone, dolomite < 1 > 1000 Low 2100 to 6100 

Igneous rock < 1 > 1000 Low 4500 to 6000 

Metamorphic rock < 1 > 1000 Low 3000 to 7000 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 mentioned above, 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, there are usually limited intrusion and damage to the environment. 

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 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 this section 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 evaluates 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 

5
present some examples of equipment and results in the figures, which are explained in their 

captions rather than in the text. 

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

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

Geophyicsal 

Method

Aggregate 

Boundaries 

Top of 

Bedrock 

Water 

Table 

Fault 

Detection 

Rippability 

Seismic Refraction S W S S W 

Seismic Reflection W S S S 

DC Resistivity S S S S 

GPR S S S S 

EM S S 

W – Works well in most materials and natural configurations 

S – Works under special circumstances of favorable materials or configurations 

Blank – not recommended. 

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 online general sources include: 

http://www.cflhd.gov/agm/solutionMatrix/index.htm , 

maintained by the Federal Lands Highway Program of the U.S. Department of Transportation, 

Federal Highways Administration, 

http://www.usace.army.mil/inet/usace-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 

6
• 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. 

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, Pwaves 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. Seismographs often can be linked 

together to increase the number of geophones. Portable computers record the seismograph data. 

Field crews for shallow surveys need at least two to four persons. 

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. 

Up to 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. 

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, and 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 

7
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. 

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. 

8
Figure 1. A portable, rugged computer used for controlling, monitoring, and recording seismic data. 

Figure 2. P-wave geophone (red) and seismic cable.

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

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

11
Figure 5. S-wave seismic equipment. The shear-wave source weighs about 31 kg (about 70 lb) and 

has spikes on the bottom to improve coupling to the ground. 

12
Figure 6. Generating S-waves with a lightweight S-wave source. The recording unit, shown in figure 

1, is in the nearby truck. 

13
Figure 7. Interpretation of processed S-wave reflection data from lightweight source along road in 

figure 6. 

14
DC Resistivity 

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) versus 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) versus fine-grained 

(clay) layers, wet versus 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 over 100 metal electrodes are driven a short way into the ground along a 

line. Electric current is introduced in the earth using two of these electrodes. Then electric 

potential is 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 employed 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. 

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. 

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. 

15
The main benefits of the automated-switching resistivity method are 

• of the methods discussed here, generally the least affected by cultural noise, 

• relatively quick site coverage; an experienced crew can collect 1 to 1.5 or more linekilometers of data per day, 

• quick data processing and interpretation; geo-electric sections are generated easily 

using modeling software immediately after measurements are taken, and 

• moderately low cost for equipment, data collection, and data modeling. 

In some situations, the multi-electrode resistivity method can be better suited to determine 

earth electrical layers than electromagnetic methods. Using multi-electrode surveying with closely 

spaced electrodes, good resolution is achieved from a few meters depth to 50-100 m depth. If using 

a capacitively coupled system, 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. 

Multi-electrode surveys are the most efficient way to collect measurements, and produce the most 

continuous and detailed geo-electric images of the subsurface. 

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 non-conductive 

sand that has conductive water occupying the pore space; or, as another example, between sand and 

gravel. There is also the problem of non-uniqueness 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 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 offroad vehicles. Other safety issues include the hazards of operation near roads and the danger of 

lightning strikes on the cables. 

16
Figure 8. A control unit for a multi-electrode resistivity system. 

17
Figure 9. A metal stake and a “smart” electrode strapped to it for a multi-electrode resistivity 

system. 

18
Figure 10. Some electrodes for a multi-electrode resistivity system, at close spacing, and an allterrain vehicle used to deploy the equipment. 

19
Figure 11. A capacitively coupled resistivity system towed by an all-terrain vehicle. 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. 

20
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 

there 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. 

21
Figure 13. Interpretation of a resistivity model from multi-electrode resistivity data collected on 

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

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

Figure 14. Interpretations of resistivity models for multi-electrode resistivity data collected over a 

sand and gravel deposit in a glacial outwash channel in Minnesota (adapted from Anderson, 2000). 

22
Ground Penetrating Radar 

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-m). 

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 below), 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 often 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 are often 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. 

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 nearly real-time results during surveys. Collections of profiles can be used to produce 3-

23
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 using low-frequency antennas. However, 3 to 6 m is more typical with 

mid-sized 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 can often detect 

a reflection from the top of bedrock and the water table. The top of clay layers are usually detected, 

but the bottom of the clay layer and reflectors below the clay cannot be detected. Voids and faults 

can sometimes be detected. 

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, topographic surveys are often required so that antenna 

locations are known. This can add extra expense. 

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 allterrain vehicles off road. 

24
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. 

25
Figure 16. A ground penetrating radar system with antenna and electronics between the cart 

wheels and monitor and computer controls on the handle. 

26
Figure 17. GPR equipment mounted on an all-terrain vehicle with antennas towed behind. 

27
Figure 18. Seventeen GPR antennas are inside the yellow shell and are pushed by a tractor. This 

multi-antenna 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. 

28
Figure 19. Interpretation of GPR data (bottom image) collected near an active sand and gravel 

quarry (top image). Hyperbolic-shaped 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. 

29
Time-Domain Electromagnetic 

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.

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. 

In addition, dc resistivity requires larger electrode spacing for greater depths, while TDEM 

uses larger loop size and later time measurements. Dc resistivity requires much larger electrode 

spacing as TDEM does for the same exploration depth. 

With respect to the FDEM methods (see next subsection), 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 can often detect 

boundaries of the aggregate with bedrock or clay, and sometimes can detect the water table. 

30
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 with high total 

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 

• 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 2-

person crew can lay out extra transmitter loops in advance of moving the transmitter and receiver 

units from loop to loop. Local, strong inhomogeneity 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. 

31
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 m on a side, surrounds the 

receiver coil. 

32
Figure 21. Bedrock depth determined using TDEM soundings on an alluvial flood plain. 

33
Frequency-Domain Electromagnetic 

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 the earth 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. 

Benefits 

This equipment is quite portable, relatively inexpensive, and easy to operate. Because 

contact with the ground is not required, data collection is very 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 often 

detect boundaries of the aggregate with bedrock or clay, and sometimes 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. 

34
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. 

35
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. 

36
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. 

37
Summary and Conclusions 

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 data collection day) can be made. These are presented in table 4. 

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 $1000US 

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-km Refraction: 2 – 3 

Reflection: many

Resistivity ~50 (48 electrodes) 

~80 (100 electrodes) 

2 – 4 1 – 2 line-km 0 – 1 

GPR 25 – 40 1 – 2 1 – 5 line-km 1 – 2 

TDEM 45 – 65 2 6 – 20 soundings 1 – 2 

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

Seismic methods require a lot of fieldwork with relatively expensive equipment, and the 

data cannot generally be interpreted immediately in the field. However, lateral boundaries and 

depth to bedrock can often 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 multi-electrode 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 aerial extent and thickness of deposits, as well 

as 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 

38
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 hole, 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. 

39
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40
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41
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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, parts 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. 

[available online at http://www.usace.army.mil/inet/usace-docs/eng-manuals/em.htm ]. 

Wilcox, S.W., 1944, Sand and gravel prospecting by the earth-resistivity method: Geophysics, v. 9, 

p. 36-46. 

42
Glossary 

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

(1997), http://en.wikipedia.org/ and other general sources. Terms in italics are also defined in this 

glossary. 

Apparent conductivity Apparent conductivity is the electrical conductivity of homogenous 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 the earth 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 homogenous 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. 

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. 

Conduction current Conduction current is an electric current produced by an applied voltage 

difference or an electric field. 

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 x 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 

43
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 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. 

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), i.e., 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), i.e., 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 Pwaves 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 

44
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 (~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 above 0.01 to 0.1 

S/m, 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 ease with which electrical charge 

can be caused to flow) or its reciprocal, electrical resistivity. Because most dry rocks and sediments 

are poor conductors of electricity, 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 conductive ground it always produces both a magnetic field

and an electric field. These fields can cause electrical polarization. The four common 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 molecules with respect to each other), 

orientational (aligning of molecules with asymmetrical charge distributions, like water molecules), 

and interfacial (local accumulations of migrating 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 spontaneous 

potential), induced polarization, and seismoelectric. 

Electrical resistivity Electrical resistivity is that property of a material that resists the flow of 

electric current. Resistance converts electrical energy into heat, leading to a loss of electric 

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 perpendicular to the same two 

surfaces. Resistivity is commonly expressed in ohm-meters (short for ohm-meter2 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. 

45
Electromagnetic field An electromagnetic (EM) field is composed of two related fields, the 

electric field and magnetic field. Typically, 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 vice versa. 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 propagation (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 conduction 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 electromagnetic (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 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 activesource 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. 

46
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 (non-ionizing) 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 figure: •. 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 is then 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. 

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. 

Magnetic field A magnetic field is the effect produced by moving electric charges (called an 

electric current). It is also called magnetic intensity, magnetic induction, or magnetic flux density. 

A magnetic field contains magnetic energy. Unlike electric fields, magnetic 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 permeability 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 

47
magnetizing force. Magnetic permeability affects electromagnetic fields in much the same way as 

electrical conductivity does. 

Magnetic susceptibility Magnetic susceptibility 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. Magnetization is also referred to as magnetic dipole moment per unit volume, 

magnetic polarization, or magnetization intensity. Magnetization is equal to the magnetic 

susceptibility times the magnetizing force. Most common earth materials are called diamagnetic; 

that is, their magnetic susceptibility is slightly negative, 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 relatively large susceptibility. Iron, 

magnetite, and most steels are ferromagnetic. 

Model, modeling With respect to geophysical interpretation, a model is a conceptual 

representation of the distribution of physical properties in the earth that reproduce field geophysical 

measurements. However, agreement 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. Modeling is the process of developing a model. 

P-wave A P-wave is an elastic body seismic wave. Particle motion is in the same direction as 

propagation. A P-wave is also called a primary wave, compressional wave, longitudinal 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 methods measure the Earth’s gravitational or magnetic 

fields and the effects of local objects and structures on those fields. These are natural source or 

passive methods in that the equipment does not produce the field that interacts with the subsurface. 

The gravity method measures variations of the Earth’s gravitational field. These variations 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 

acceleration due to gravity at the Earth’s surface is 980 cm/s2 (~32.15 ft/s2) or 980 Gal. The microgravity method is precise enough to measure variations as small as several microGal (10-6 Gal). 

Unconsolidated sediments such as sand, gravel, silt, clay, and soil have similar densities. The 

various types of rocks are usually slightly denser than sediments. The micro-gravity 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 station, 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 requires a 

skilled technician or scientist and computer software. For these reasons, micro-gravity is not 

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

the other hand, the micro-gravity method could be considered in areas where electrical, 

48
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, 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 versus 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. 

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 is typically not recommended 

for characterization of typical sand and gravel deposits. 

49
Relative dielectric permittivity See dielectric permittivity. 

S-wave An S-wave is an elastic body seismic wave. Particle motion is in a direction 

perpendicular to the direction of propagation. An S-wave is also called a secondary wave, shear 

wave, transverse wave, rotational wave, distortional wave, equivolumnar wave, or tangential 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 

compression and/or shear force in the ground. That force creates seismic wave energy that 

propagates 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 

particular 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 measure 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, pseudo-Rayleigh 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. In this report, we are concerned only with P-waves and S-waves. See 

also elasticity. 

The propagation of seismic waves is a complicated 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), 

50
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 original 

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 perpendicular 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. 

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 characterized by 

periodicity, meaning that maximum and minimum amplitudes or particle displacements occur 

repetitively in time or in space. 

51