FIELD-SCALE VOID DETECTON IN COAL PILES USING

RESISTIVITY IMAGING

Rene Rodriguez, Henry Rodriguez, and Michael Lhamond 

GECHO Exploration, Lexington, KY USA

Jeffrey C. Johnson and Steve Iverson

Spokane Research Laboratory, National Institute for Occupational Safety and Health, Spokane, WA USA

ABSTRACT: To reduce fatalities resulting from personnel falling into voids formed above feeders in coal piles, the National Institute

for Occupational Safety and Health at the Spokane Research Laboratory established a project to monitor coal surge piles to detect the

formation of voids. A resistivity imaging method has been developed by GECOH Exploration, of Lexington, KY, in which a

geophysical technique known as vertical electrical sounding (VES) is used to scan for these voids. VES operates by me asuring the

electrical field produced by a current in the ground at the bottom of the coal pile. This paper summarizes the results obtained from a

field-scale test and shows that mapping voids within a coal pile is possible.

1. INTRODUCTION

Geophysicists have used a technique called vertical

electrical sounding (VES) for more than 100 years to

study subsurface structures in the earth. GECHO,

Inc., Lexington, KY, has developed a technique using

VES [1] that concentrates electrical energy above the

ground surface within a coal pile. An electric current

is supplied to a uniform grid of electrodes installed at

the base of a coal pile before the pile is constructed.

The electrodes are made from steel reinforcement

bars, and the attachment wires are sealed and insulated from the ground. The insulation prevents the electric current from traveling underground and directs it

toward the top of the coal pile. The current induces an

electrical field whose characteristics are highly dependent on layer composition, geometry, and moisture

content of the coal pile. A strong contrast in resistivity

between the air and the coal will delineate the air-coal

interface and identify the presence of a void inside a

coal pile, which can then be mapped.

2. FIELD EXPERIMENT

The field experiment consisted of creating 15 grid

lines, labeled L1 to L15, each containing 24 electrodes. The uniform 24 by 15 grid system made up a

total of 360 electrodes. Each electrode was spaced 0.3

m (1 ft) apart over a total area of 33 m2

 (360 ft2

). The

electrodes were sealed inside a short length of 1.3 cm

(0.5 in) in diameter polyvinyl chloride (PVC) pipe and

secured to a 2- by 4- in wood frame. 

VES measurements were taken by scanning each grid

line. For this small coal pile, the electrical energy was

suppled by a standard 12-V, lead-acid auto battery. A

plastic tarp was used as a ground insulator to prevent

the electric current from discharging into the ground.

To simulate a void inside the coal pile, plastic barrels

were placed in the pile during its formation. Each

plastic barrel was perforated with a large number of

small holes, which allowed moisture in the coal pile

to collect in the barrels. Figures 1 through 9 show the

field layout and instrument set-up. 

The scanner is connected to an auto battery, a ABEM

300-C geoelectrical resistivity instrument, and a laptop computer. Resistivity measurements from the

scanner are transmitted to a computer where software

reduces the data, corrects it, and obtains the inversion

model for each sounding. The inversion of each VES

shows the structure of the coal pile above the electrode. By combining a number of soundings along a

grid line, a cross section of the coal pile is obtained.
Figure 1.—Initial view of site, Lexington, KY. Figure 2.—Assembly of 1.3 cm (0.5 in) in diameter PVC

pipe inserted into 2- by 4-in studs.

Figure 3.—Completed wood frame with PVC pipe

holders.

Figure 4.—Close-up of rebar electrodes sealed in plastic

pipe and attached to electric cable.

Figure 5.—Com pleted grid showing insulator plastic tarp

ground cover, frame reinforced with railroad ties, and

some of 15 electric cable lines

Figure 6.—First layer of coal covering electrode grid and

first perforated plastic barrel being covered horizontally

on coal pile.
Figure 7.—Second perforated plastic barrel being

covered with coal.

Figure 8.—Final coal pile and van containing data

acquisition system.

Figure 9.—Data acquisition system with electrode

scanner (upper right), ABEM 300-C geoelectrical

resistivity instrument (upper middle), auto battery (upper

left), and laptop com puter (lower middle).

3. DATA AND RESULTS USING THE

RESISTIVITY IMAGING METHOD

Measurements are obtained for each grid line and

transferred to the computer via a 232-C interface. The

system takes 23 measurements, one less than the number of electrodes in a line, along each of the 15 lines.

A computer program directs the scanner to sweep the

VES readings systematically in a pre-established order

that maximizes the effectiveness of the electrode

array. The sweeping program compounds readings

using pole-dipole readings at the edges of the grid and

Schlumberger readings in the middle. Tests conducted

at common points showed errors of 1% or less.

Readings taken from each sweep are stored in separate

files. The readings are corrected for geometric factors

associated with each electrode arrangement, and an

apparent resistivity is calculated for each electrode.

The apparent resistivity values are input to an inversion program to compute vertical soundings and

determine coal pile height above each electrode. After

all the vertical soundings are made, the data points are

used to prepare contour maps.

To determine whether a void is present, an empirical

relationship is used to estimate coal pile density from

the total resistivity observed in each VES. The 23

VES data points are inverted, and the coal structure is

imaged to produce a resistivity pseudo-section that

shows the structure of the coal pile for a given cross

section. Figures 10, 11, and 12 show three of the cross

sections. Figure 10 shows the inverted VES data

points along line L-6 and the void produced by the

vertical buried barrel. Figure 11 shows two voids

formed by the vertical and horizontal barrels buried

along line L-7. Figure 12 shows the horizontal barrel

buried along line L-8. 

Figure 13 is a plan view at an elevation of 1.1 m (3.5

ft) above the coal pile base. Note the two voids. The

contouring program SURFER is used to produce the

coal pile height contours shown in Figure 14. These

contours can be used to calculate the total volume of

coal in the pile.

4. CONCLUSIONS

The field experiment showed that voids in coal piles

can be detected using the resistivity imaging method

developed by GECOH. This method can be used to

map not only coal pile voids, but also the volume of

the entire coal pile and density contrasts associated

with moisture content. The images of the plastic
Figure 10.—Inverted VES data along cross section L-6 showing vertical buried

barrel. 

Figure 11.—Inverted VES data along cross section L-7 showing both vertical and

horizontal buried barrels.

Figure 12.—Inverted VES data along cross section L-8 showing horizontal buried

barrel.
Figure 13.—Plan view map of RIM data at elevation of 1.1 m (3.5 ft) above coal

pile base.

Figure 14.—Coal pile height contours created with SURFER program.

barrels are about 15% larger than actual size. The

larger size may be partly explained by the area of high

humidity formed in the plastic barrels. Experimental

errors in this experiment are still within an acceptable

degree of accuracy compared to methods in which

coal piles are mapped using photographs and topographic imaging.

REFERENCE

1. Rodriguez, R., and H. Rodriguez. 1999. Real-time

monitoring of rock-pile volumes by electrical resistivity.

The Leading Edge, Vol. 18, No. 12, pp 1398-1401.