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By Brent Lewis, Geologist, BLM

National Applied Resource Sciences

Center (NARSC)

As a result of the closure of the

Bureau of Mines, the National

Applied Resource Sciences Center

(NARSC) acquired a variety of

portable surface geophysical equipment and over the past year, has

implemented several seismic and

resistivity surveys to characterize

geologic settings. These studies

have been in support of environmental issues; however, similar

approaches can be implemented

for assessing mineral deposits or

acid mine drainage issues, the latter being extensively evaluated and

published by the Environmental

Protection Agency (EPA No. 530-

R-95 013a). The surface geophysical methods within NARSC vary

but generally consist of electrical

resistivity, seismic, and electromagnetics. The type(s) to use is

dependent upon the survey.

Surface geophysical methods provide a non-intrusive, cost-effective

means of acquiring subsurface geological information. Such information may encompass lithologic

composition, stratigraphic depths

and thicknesses, groundwater

properties, or properties of the

aquifer. This Resource Note discusses electrical resistivity, whereas

other geophysical methods will be

addressed in future Resource

Notes.

Geologic information can be

inferred from geoelectrical properties of rock and sediment and the

spatial distribution of these properties, both can be acquired by

direct current electrical resistivity.

In simple terms, this method

involves the application of an electrical current to the ground via

two source electrodes with the

potential difference created at the

ground surface being measured

between two receiving electrodes.

The effective depth of a “sounding” is increased by sequentially

increasing the distance between

these electrodes. The further the

distance, the thicker the vertical

interval in which the bulk of the

current flows. Recorded resistivity

values can be considered the

weighted average of the resistivity

of all the beds down to the deepest

bed detected. Differences in resistivity values occur when changes in

subsurface conditions are encountered. These changes are geoelectric boundaries and typically correspond to changes in either lithologic composition, formation

porosity (such as an increase in

fracturing), and/or changes in

groundwater chemistry. Clearly,

the current prefers to flow in a bed

of lower resistivity, such as clay;

conversely, the more sand and

gravel, the greater the resistive

properties.

Depending upon objectives, a

resistivity survey may be employed

via several techniques or a combination of techniques, the most

common being Vertical Electrical

Sounding (VES) and horizontal

profiling. The VES technique is

analogous to electrical drilling

because the result is a one-dimensional interpretation of the subsurface, similar in profile to a common drill log. This technique typically involves the correlation of

multiple sample locations to produce geoelectric cross-sections

from which the general stratigraphy and structural setting are identified. Horizontal profiling provides a two-dimensional profile of

the subsurface, similar to viewing

the wall of an excavated trench.

This technique is typically used

when greater detail is needed.

Effective depths of any investigaNO. 4 DATE 04/30/99

BLM’s Surface Geophysical Capabilities • 

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tion can be customized to achieve

specific objectives, however, based

upon the in-house NARSC equipment, survey depths as great as

250 feet are readily achieved.

Two case studies are provided in the

figures as examples of resistivity

capabilities. The survey at the

Sunrise Mountain Landfill in Las

Vegas, Nevada, consisted of 15 VES

profiles, i.e., electrical drill logs.

The objective of this survey was to

characterize the local stratigraphy

and geological structure, e.g., faulting, in order to design a groundwater monitoring well network.

Geological information from each

VES (Figure 1A is an example of a

single VES data set) was correlated

to produce geologic cross-sections.

An example of a cross-section developed through the correlation of six

VES locations is provided in Figure

1B. In addition to the stratigraphic

information, this survey identified

several fracture zones and a nearby

fault which greatly affect the strategies behind the placement of

groundwater monitoring wells.

The second case study (Figure 2)

utilized the “horizontal profiling”

technique. This survey was implemented at the old Granby Landfill

near Granby, Colorado. The objective of this survey was to identify

local stratigraphy, particularly the

depth to impermeable bedrock.

This required the ability to distinguish between weathered and

unweathered shale [Pierre

Formation]. The depth of a

groundwater monitoring well was

to be completed to the depth of

unweathered bedrock. Resistivity

results suggested this depth to vary

from 24 to 34 feet across the site.

Based upon the drill logs obtained

after the survey, depth to impermeable bedrock was determined to

range from 26 to 32 feet.

Supplementary information

obtained from this survey was the

varied soil types of a nearby wetland.

The in-house resistivity capabilities

of NARSC provide an opportunity

to acquire subsurface geologic information quickly and cost-effectively.

The examples provided are for environmental groundwater objectives;

however, other uses may include

mineral assessments, abandoned

mine investigations, and archeological surveys for buried foundations

or pits. If you would like more

information about resistivity or the

other geophysical methods now

available within BLM, contact

Brent Lewis at 303-236-0550 or email B1Lewis@sc.blm.gov.

Resource Notes are intended to be

early announcements of technical and

informational topics for

Bureau of Land Management personnel

and some of their customers.

Information in this Resource Note

is based on the opinion and experience of

the author and 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.

If you have received a copy of or found out

about Resource Notes in an indirect

way and would like to be included in

future mailings, please send the following:

NAME, TITLE, MAILING ADDRESS

and a list of the two or three subject areas

that you are most interested in or

that most directly relate to your job.

Send this information to

Phil Dittberner,

BLM, RS-140,

P.O. Box 25047,

Denver, CO. 80225-0047

or pdittber@nc1155wp.sc.blm.gov

or FAX 303-236-3508.

If you would like to prepare a

Resource Note for distribution,

or you have an idea and author

in mind for a good Resource Note,

please contact Phil Dittberner at

303-236-1833, FAX 303-236-3508 or

pdittber@nc1155wp.sc.blm.gov

with the topic and the name of writer,

including an address, phone number,

and e-mail address.

Thank you for your interest in

Resource NoteS. Range

Range

Range
Figure 1A. A single VES dataset; illustrating the distribution of the data and depth model.

Figure 1B. Geologic cross section developed from the correlation of six VES locations.

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