Geophysical Technologies for Detecting Underground Coal Mine Voids, Lexington, KY, July 28-30, 2003
APPLICATIONS OF THE ELECTRICAL RESISTIVITY METHOD
FOR DETECTION OF UNDERGROUND MINE WORKINGS
William J. Johnson – D’Appolonia, Monroeville, PA
The need to understand the location of abandoned workings has recently been high-
lighted by the failure of the Martin County Coal Corporation tailings impoundment near
Inez, Kentucky on October 11, 2000 and the July 24, 2002 Quecreek Mine inundation
that trapped nine miners for 77 hours in Somerset County, Pennsylvania.
On a more
routine basis, subsidence from abandoned mines represents a significant public cost in
Just in Pennsylvania, the State has invested over $100 million to address
mine subsidence problems and estimates that 200,000 acres of high priority subsidence
prone land still remain to be stabilized.
Abandoned mines are often difficult to locate.
Detailed mine maps may be unreliable or
Conventional exploration (drilling) can easily miss targets as small as a mine
Nevertheless, geophysical methods are seldom employed to help map aban-
doned mines, possibly because geophysics surveys are perceived to be too expensive
or will not help to solve the problem at hand.
In actuality, there are situations where a
geophysical survey can be expected to be effective and other times when the results
may be more problematic.
This paper reviews both the theoretical and practical aspects
of electrical resistivity methods to define abandoned mine workings with the intent of de-
fining conditions where the technique can be expected to produce useful results and the
limitations of electrical measurements.
DC Electrical Surveys
The purpose of a DC electrical survey is to
determine the subsurface resistivity distribu-
tion of the ground, which can then be related
to physical conditions of interest such as
lithology, porosity, the degree of water satu-
ration, and the presence or absence of voids
in the rock.
The basic parameter of a DC
electrical measurement is resistivity.
tivity is not to be confused with resistance.
Resistance (R), measured in ohms, is the result of an electrical measurement, where
according to Ohm’s Law:
V = I/R or R = V/I
where V = voltage in volts and I = current in amps.
Sketch of parameters to define resistivity