Applications 

of Electrical 

Resistivity: A Surface Geophysical

Method

By Brent R. Lewis, Geologist

BLM, National Science and 

Technology Center

Background

Bureau of Land Management

(BLM) geologists, hydrogeologists,

and hazardous materials specialists

must frequently obtain subsurface

geologic information while working

under time and budget constraints.

It is possible to image the subsurface quickly and inexpensively

through the application of various

nonintrusive surface geophysical

methods. I discuss here one method

known as direct current electrical

resistivity, which is used on a variety

of BLM projects such as:

• Performing geologic and stratigraphic mapping 

• Mapping leachate leaking from

evaporation ponds

• Estimating volumes of tailing

piles and tar pits

• Optimizing the placements of

water supply wells and groundwater monitoring wells

• Conducting validity exams of

sand and gravel deposits

Electrical resistivity technology has

recently evolved with the increased

sophistication of electrical hardware

and software. Formerly, two methods of subsurface imaging using

electrical resistivity were available:

electrical sounding for investigating depth and electrical trenching

for assessing lateral variations.

Electrical soundings presented a

one-dimensional vertical profile of

limited lateral control, whereas

electrical trenching generated a

lateral profile but was limited to a

constant depth and the bulk resistivity of only that depth interval. It

is easier using today’s instruments to

acquire the data as a set of soundings comprising a two-dimensional

cross-section or profile of the subsurface. On the basis of the distribution of resistivity within a profile,

an accurate interpretation of the

subsurface geologic setting can be

made. This information can be

used in the field to optimize the

location of a well or excavation.

Discussion

Electrical resistivity surveys measure

the electrical resistive properties of

earthen materials. Resistance is the

ability of the material to inhibit an

electrical current; it is the opposite

or reciprocal of electrical conductivity. This method considers the

subsurface geologic setting as a

series of electrical resistors that naturally inhibit an electrical current.

The success of electrical resistivity

surveys lies in their ability to detect

changes in the electrical field caused

by these resistors and, subsequently, to determine their locations, depths, and thicknesses.

The two-dimensional electrical

resistivity surveys are employed

by using a linear array of metal

electrodes connected by a cable.

An electrical current is applied to

the ground through two source

electrodes along this array, and the

potential difference (voltage drop)

created at the surface is measured

between two receiving electrodes

located at a known location and

distance. The potential difference

produced as the current encounters earthen material (a resistor) is

measured and used to determine

the resistivity of the material

between the electrode pairs. To

produce a cross-section composed

of many resistivity measurements,

the position along the array and

the distances separating the electrode pairs must be varied for

each measurement. 

The effective depth of a measurement is sequentially increased by

increasing the distance between the

source electrodes and the receiving

electrodes. The farther this distance,

the greater the vertical interval in

which the bulk of the current

flows. To acquire data from a new,

lateral position, the lateral location

of these electrode pairs is sequentially moved down the array after

each measurement, and a new measurement is collected at a new lateral

location. As the electrode separation

is incrementally changed or moved

and a new resistor or different geoelectric property is encountered,

differences or contrasts of resistivity

are noted. The electrical current

tends to flow in areas of low resistivity such as clay, resulting in a relatively lower resistivity value than

sand. The apparent resistivity value

is recorded by the instrument and is

typically measured in ohm-meters or

ohm-feet. 

Although the magnitude of resistivity and cause is important, the

information obtained from the lateral and vertical changes, (i.e., spatial distributions of the data) is

often more diagnostic. These spatial variations are boundaries from

which inference can be made

regarding the distribution of the

stratigraphy or sediment facies.

These changes are inferred as

geologic boundaries that may actually be caused by one or more of

the following: a change in lithology,

sediment grain size, or chemical

NO. 62 DATE 02/04/03

Range

Soils/geology

Resource Note 62

page 1
boundaries resulting from changes

in water chemistry or moisture

content. In all instances, the user

must determine if the results are

possible given the existing knowledge of site conditions. It is important, therefore, to have conceptual

ideas of the geologic setting and the

effects it may have on the geophysical signature for a more accurate

interpretation. 

The sequential changes in electrode

arrangements can be performed

manually—or automatically if specialized equipment is used. The

BLM uses an automated, multielectrode system consisting of a Sting R1

resistivity meter and a Swift automatic switching box and multielectrode cable (Advanced Geosciences

Inc., Austin, Texas; Figure 1).

Interpretations made from the raw,

unprocessed resistivity field data

(pseudosections) can be difficult.

Potentially more accurate methods

rely on a computer processing

method known as data inversion.

The inversion process accounts for

many of the variables,

such as the array, current, voltage potential, and the matrix

of readings, and suggests the true resistivity rather than the

apparent resistivity

presented in the

pseudosection.

Inversion can be further controlled with

known information

such as a drill log. If

such control is not

available, the inversion process can still

be used. In either

situation, a best-fit

model is generated

from which interpretations are

made. 

The BLM now uses inversion

processing software (IP2DI version 4.12 from Interpex, Golden,

Colorado). The National Science

and Technology Center (NSTC)

is developing the integration of

two-dimensional profiles into

three-dimensional models by

intersecting survey lines and using

earthVision software (Dynamic

Graphics, Inc., Alameda, California;

Figure 2).

The in-house resistivity equipment

now being used has 60 electrodes

and is capable of acquiring data

from depths as great as 200 feet.

The vertical accuracy in the upper

50 feet is typically within 2 feet; for

depths between 50 and 100 feet,

vertical accuracy is about 4 feet. A

single line typically takes 1.5 hours

to set up, acquire, process, and plot

in the field. 

National Science and Technology

Center staff have collected and

interpreted data from a variety of

applications of electrical resistivity

on BLM projects.

Contact

Brent Lewis

National Science and Technology

Center

Denver Federal Center

Building 50

P.O. Box 25047

Denver, CO, 80225-0047

Phone: 303-236-0550

Fax: 303-236-3508

Email: Brent_Lewis@blm.gov

Range

The Bureau of Land Management Resource Notes,

which are posted on this

Web site

(http://www.blm.gov/nstc/resourcenotes/resnotes.html),

are early announcements of technical and informational

topics for BLM personnel and their customers. Information

contained in this Resource Note has not been 

peer-reviewed. Conclusions and opinions expressed herein

do not necessarily represent those of BLM. Use of trade

names does not imply U.S. government 

endorsement of commercial products.

Technical Editor - Deborah Harris

(Debbie_Harris@blm.gov)

Science Editor - Brian St. George

(Brian_St_George@blm.gov)

Graphic Designer - Ethel Coontz (Ethel_Coontz@blm.gov)

If you would like to prepare a Resource Note

for electronic distribution, or if you have an idea or 

can suggest an author for a Resource Note, 

contact the Science Editor (e-mail:

Brian_St_George@blm.gov; phone: 303-236-1930) with

the topic and the name of the writer, including 

mailing address, phone number, and e-mail address.

Thank you for your interest in Resource NoteS.

Resource Note 62

page 2

Figure 1. A Sting R1 resistivity meter and

a Swift automatic switching box and multielectrode cable are used to perform sequential

changes in electrode arrangements.

Figure 2. EarthVision software aids in the integration of twodimensional profiles into three-dimensional models.