Coal Mine Investigation

1. GEOPHYSICS, VOL. 56. NO. 9 (SEWEMBER 1991); P. 1494-1503, IO FIGS., I TABLE.
Premine study of shallow coal seams using high-resolution seismic
reflection methods
Harvey Henson, Jr.* and John L. Sexton*
ABSTRACT
Geological investigationsin the Illinois Basin coalfields
have shown that significant differences in safe and
economical exploitation of coal depends directly on
accurate mapping of the roof rock overlying the seam,
as well as on geological structures in the coal measures.
In roof rock transition zones abovethe Herrin (No. 6) coal
where the nonmarine Energy shale changes to the Anna
shale, a change often occurs from low to high sulfur coal
and from low to high stability roof rocks. In many
instances, use of borehole data alone is inadequate to
locate these features in advance of mining.
High-resolution seismic reflection data collected near
Harco, Illinois were used as part of premine planning to
help predict roof instability, areas of low sulfur coal, and
geologic disturbances. Severalfaults, channels, and facies
changes affecting the Herrin (No. 6) and the Springfield
(No. 5) coal seamsat depths of 137m and 167m, respec-
tively, were interpreted and modeled. One- and two-
dimensional synthetic seismograms calculated from
geological data from drill holes along the seismicline were
used to aid in the interpretion of the seismic reflection
data. Resultsobtained from the high-resolution reflection
surveycombined with drill hole information clearly show
that use of borehole data alone is inadequate to locate
geological featuresthat might affect coal mine operations,
evenif the boreholes were spaced25 m apart. Thus, high-
resolution reflection surveying should be employed
whenever feasible for the safe and economical exploita-
tion of coal deposits.
INTRODUCTION
High-resolution reflection surveyinghasbeen usedsuccessfully
in coal related studies by a number of investigators, for exam-
ple see: Coon et al., 1978; Ziolkowski and Lerwill, 1979; Daly,
1979;Acker and Kumamoto, 1981;Hughes and Kennett, 1983;
Harman, 1984; Lawton, 1985;Greenhalgh et al., 1986; Palmer,
1987;Knapp and Muftuoglu, 1988;Gochioco and Cotten, 1989;
Henson et al., 1989. Use of high-resolution reflection data in
conjunction with drill-hole data is a cost effective method of
mapping coal seams for exploration and exploitation (Daly,
1979).In the Illinois Basin, this method has been usedeffectively,
although not extensively, by several coal companies.
The two principal coal seams mined in the Illionis Basin are
the Herrin (No. 6) and the Springfield (No. 5) coal members
of the Pennsylvanian age Carbondale Formation. In the study
area, averagethickness of each of these seamsis approximately
1.8m with depths of approximately 137m to the No. 6 and 167m
to the No. 5 coal. Common geological features within the coal
seamsinclude minor faults, shalepartings, and channels(Nelson,
1983). Complex lateral facies transitions in the Herrin (No. 6)
roof rocks occur between the soft unstable nonmarine Energy
shaleand the harder more stablemarine Anna shaleand Brereton
limestone sequence (Figure 1). Small sandstone channels, such
as the one depicted in Figure 2, sometimes overlie or interrupt
the coal seamsand are often inaccurately mapped. These struc-
tures and facies changes affect the safe minability of the coal
and cause unstable roof conditions and paths where ground-
water can enter a mine. In areas where the Anna shale overlies
the No. 6 coal seam, sulfur content of the seamis higher, making
the coal less desirable (Krausse et al., 1979). When the gray
Energy shaleis abovethe No. 6 seamand hasa thicknessexceed-
ing 6 m the sulfur content of the coal diminishes significantly.
It is extremely important in premine planning to be able to
map the coal seam,detectstructural features,and delineate facies
changes that may be associated with roof stability and sulfur
content of the coal. It is generally not possible to do this with
drill hole data alone, particularly when distances between drill
holes are hundreds of feet. The results of this study show that
high-resolution seismic reflection surveying can be used suc-
cessfully to detect and map structural and facies changesin the
coal measures between drill holes. Great care must be taken in
everyaspectof the seismicprogram including surveydesign,data
acquisition, data processing, modeling, and interpretation. A
properly designed surveycan result in significant improvements
in safe and economic exploration and exploitation of coal
resources. The purpose of this paper is to present results of a
high-resolution reflection survey along a profile with five drill
Manuscriptreceivedby the Editor April 13, 1990;revised manuscript received April 22, 1991.
*Department of Geology, Southern Illinois University at Carbondale, Carbondale, Illinois 62901.
01991 Society of Exploration Geophysicists. All rights reserved.
1494

2. Premine Coal Seismic Reflection Study 1495
holesthat provide geologicalcontrol. Resultsshow that evenwith from the percussion Betsy to the safer and more reliable, elec-
relatively closely spaced drill holes it is necessaryto useseismic trically fired Betsy Seisgun near shot point 130.
reflection data to determine the geology between drill holes. Data were acquired with a 24-channel DFS-V Texas Instru-
ments recording system configured for high-resolution seismic
DATA ACQUISITION reflection surveying. An off-end array was used with a constant
source to first receiver group offset of 73 m and a group inter-
In July of 1986, eight high-resolution seimic reflection data val of 3 m. The remaining acquisition parameters are given in
sets were collected along two seismic lines near the village of Table 1. Synthetic seismograms calculated using lithology,
Harco, in northwestern Saline County in southeastern Illinois density, and velocity data taken from nearby drill holes provided
(Figure 3). A variety of seismicsourceswere used to collect data preliminary estimates on the reflection times for the coal seams
for a source comparison study. Data and results from seismic and were used as an aid in designing field parameters prior
line one recorded with a Betsy@Seisgun surface source will be to the actual start of the survey. Various field tests were used
discussed in this paper. During recording, a switch was made to further refine the field parameters.
Lawson Shale
FIG. 1. Cross-section showing the complex stratigraphy associated with the Herrin (No. 6) Coal Member. Included in
the figure are splits or partings within the coal, facieschangesin roof rock between the Anna and Energy Shale Members,
and channels. (Modified from Krausse et al., 1979.)
0 0 0 0
II0 0 0
“11, JuSm 610m
LEGEND
Borehole (notto scale)
Actual location of channel
Proposed location of channel
based upon boreholes
FIG. 2. Illustration of a common problem encountered by the
useof boreholes alone in the mapping of channel features.Even
if the ideal borehole surveyis used,the map may not be accurate.
SOUTHERN ILLINOIS
3 20 40 60 80 lOOmc
FIG. 3. Map of the southern Illinois study area in northwestern
Saline County showing general location of seismiclines. Seismic
line one, which is discussed in the text, is the western line.
@TrademarkMAPCO, Inc.

3. 1496 Henson and Sexton
Table 1. Data acquisition parameters.
512 Hz (anti-alias)
(2 shots recorded
DATA PROCESSING
Digital processingof the data was conducted in the Southern
Illinois University at Carbondale (SIU-C) GeophysicsLab using
a Hewlett-Packard 9000 model 550workstation and on a TIMAP
(TexasInstruments Multiple Application Processor)system.Pro-
cessingfor common depth point seismicreflection data wasused
(Sheriff and Geldart, 1985) with great care taken at each step
in the processingsequenceto maximize the quality of the stacked
record sections. The percussion Betsy shot records required a
time consuming application of hand statics to correct for time
differences between initiation of the recording systemand firing
(by hand) of the shots. After editing noisy traces, stackedreflec-
tion records (Figures 4-5) were produced with a nominal 1Zfold
CDP (common-depth-point) coverage. Automatic statics were
also applied to improve the quality of the stacked record. The
seismic data was muted to enhance the reflection information
and eliminate refractions generatedin the thick weathering zone.
A portion of the electric Betsy Seisgun data set was repro-
cessed, without the application of gain control, to produce a
relative amplitude seismicsection (Figure 6). The relative ampli-
tude section displayed variations in reflection amplitude and
seismiccharacterthat wereinterpreted after modeling to indicate
stratigraphic and facies changes within the Herrin (No. 6) coal
roof rock.
WEST
1055-c
1056-C
0
0
SP 25 50 75 100 125
EAST
CDP
.O
D
E
P
‘:
0
T
z
Ii
w .I
(FT)
t 450
550
0' 250’ 500’
i
Om 152m
PERCUSSION BETSY SEISGUN LINE ONE
FIG. 4. First portion of seismic line one recorded using the percussion Betsy Seisgun. The upper figure is the interpreted seismic
section and the lower is the uninterpreted section. Poor data quality (CDPs 25-200) is caused by statics problems.

4. Premine Coal Seismic Reflection Study 1497
w
E

5. WES-I
1054-c
0 1047-c
0
SP
150 175 200 225 250 275 300
Roof Rock StratiaraDhy
m HARD MARINE STRATA OVERLYING THIN
ENERGY SHALE (o-10’)
0 THICK ENERGY SHALE (>20’) WITH ANNA
SHALE-BRERETON LS SEQUENCE ABSENT
m ANVIL ROCK SANDSTONE IN CHANNEL
PHASE
0’
Om
RELATIVE AMPLITUDE SECTION
FIG. 6. Relative amplitude section produced by reprocessing of CDPs 290-650 of the electric Betsy Seisgun data set of
The upper figure is the interpreted seismic section and lower figure is the uninterpreted section.

6. MODELING
Premine Coal Seismic Reflection Study 1499
correlation (Figure 7), while additional drill hole data, such as
density, depth, and velocity, were used to develop the models
for calculation of the I-D and 2-D synthetic seismograms.These
synthetic seismogramsassistedin the identification of observed
reflections. Reflections associated with the Herrin (No. 6) and
Springfield (No. 5) coal seamswere easily interpreted using the
synthetic seismograms from 1-D modeling (Figure 8).
One- and two-dimensional synthetic seismogramswerecalcu-
lated to aid in the interpretation of the seismic data. Methods
outlined by Trietel and Robinson (1966) and by Claerbout (1968)
wereusedto generateconvolutional synthetic seismogramsusing
a Ricker wavelet.Also, a program that calculatesthe near-normal
incidence reflection responseof a velocity-depth model by imag-
ing techniquesasdescribed by Bortfeld (1972) was usedto create
2-D synthetic seismograms. These seismograms calculated for
both the 1-D and 2-D models include only primary reflections
and do not include multiples, shear wavesand converted waves.
Although thesewavetypes are of great importance in coal reflec-
tion studies(Hughes and Kennett, 1983;Fertig and Miiller, 1978;
and Riiter and Schepers, 1978), they were not considered in this
paper becausethe use of geological data from many drill holes
provided reliable calculation of primary reflections.
Models were constructed for severalsmall offset faults, sand-
stonechannels, and facieschangesin the roof rock of the Herrin
(No. 6) coal. Lithologic data obtained from five boreholes
located along the seismicline wereusedto producea stratigraphic
A geological model (Figure 9a) of two Anvil Rock sandstone
channels showsthat a small channel (CDPs 560-580) interrupts
the Anna shale and Brereton limestone roof rock sequence,and
that a larger channel (CDPs 600-630) is in contact with the top
of the Herrin coal and is flanked stratigraphically by thick
Energy shale deposits. The synthetic seismograms (Figure 9b)
calculated from this model usinga 120Hz (dominant freq_uency)
Ricker wavelet are compared to a portion of the Betsy data
(Figure SC)and show that the interpretation of the two chan-
nels on the observed data is confirmed.
Three 2-D models (Figure 10) used to study Anna shale/
Brereton limestone and Energy shale transitions under varying
Anvil Rock sandstone conditions include: (1) Anvil Rock sand-
stone in sheet phase and channel phase (Figures lOa), (2) Anvil
WEST EAST
x1055c #1056C x1054c x1047c xloolc
SHOT PTS.
E
. .
-__. I I Piasa Lsr
I-L,
I I
EXPLANATION
LA - 0COAL
03SANDSTONUVESTON
ml 305m
I3 nnul
-ATED ANNA SHALE
RI Fzl
E-Y SH4I UAYSTOK
FIG. 7. Correlation of stratigraphic data from drill holes along seismic line one. Several sandstone channels are present in the coal
measures, and roof rock facies transitions occur above the Herrin coal between the Anna and Energy shales. Shot points and drill
hole numbers are labeled along the top and are also located on the seismic sections.

7. 1500 Henson and Sexton
Rock in sheet phase only (Figures lob), and (3) no Anvil Rock
sandstone (Figure 10~).Synthetic seismogramsfor these models
were used to help interpret the relative amplitude seismic data
(Figure 6). The synthetic seismograms (Figure 10) reveal a defi-
nite change in seismic reflection character between the hard
Anna/Brereton strata and the soft Energy shale. Roof rock con-
sisting of the Anna/Brereton hard marine sequence is imaged
on the synthetic seismogramas a small-amplitude positive reflec-
tion (located at about 105 ms, two-way reflection time), while
the presence of Energy shale is represented by reflection-poor
zone directly above the negative Herrin (No. 6) coal reflection
(Figures lob and 10~). The base of the Anvil Rock sandstone,
when in channel phase, is imaged on the synthetic as a distinct
positive reflection marked by a phasechange as the channel cuts
into the roof rock (Figure lOa).
model studies of the effects of multiple reflections, shear waves
and converted waves for coal measures using models similar to
those presented in this paper are in early stagesof investigation
and will be the subject of a later paper.
RESULTS
Strong, continuous, and easily identifiable reflections (Figures
4-6) from the coal seamswere recorded as a consequence of the
large acoustic impedance contrast between the low density and
low velocity coal and surrounding limestones, shales, and sand-
stones. Calculation of an estimate of the resolving thickness
(Widess, 1973)for a coal seamwith a velocity of 1524m/s using
a predominant frequency of 120Hz yielded a thicknessof 3.2 m.
The average thickness of the Herrin (No. 6) and Springfield
(No. 5) coal seamsis 1.8m. Therefore, the coal seam reflections
must be composite reflections resulting from the interference
of reflections from the coal seam top and bottom in addition
to various wavesgenerated from other stratigraphic units asso-
ciated with the coal. This is in agreementwith previouscoal seam
model studies that used reflectivity modeling methods (Hughes
and Kennett, 1983;and Rtiter and Schepers;1978).More detailed
Eight faults interpreted from the seismicdata were not evident
from the drill hole correlations. Fault 1, interpreted on the per-
cussion Betsy line (Figure 4), appears to be a 15 m wide zone
of brecciated strata rather than a simple normal fault as sug-
gested by the discontinuous nature of the reflections between
CDPs 245 and 255. The total vertical displacement at fault 1
is estimated to be approximately 3.6 m based upon synthetic
seismogramscalculated from geologic models. Interpreted verti-
cal displacements of the indvidual faults (2-4) near the western
end of the line range from approximately 1.2-3.6 m. Near the
eastern end of the line, four closely spaced normal faults (5-8)
have resulted in a cumulative vertical displacement of the coal
seamsof approximately 10m over a horizontal distanceof about
45 m. These small offset faults may be subisidiary faults of the
Cottage Grove Fault System (Henson et al., 1990), which has
been mapped 3.2 km south of the field area (Nelson et al., 1981).
Several small sandstone channels were interpreted from the
seismic data (Figures 5-6) with the aid of models and synthetic
seismograms. These channels are comprised of a lithologic unit
known as the Anvil Rock sandstone. Thickness of this unit is
typically 9-12 m in the field area, but may increase locally due
to channeling at the base (Nelson, 1983). One channel located
between CDP 560 and 580 is estimated to be more than 30 m
wide and 11 m thick. A second channel located between CDP
600 and 630 is approximately 45 m wide and 14 m thick. Both
channels cut into the roof rock of the Herrin (No. 6) coal seam,
but the channel located near CDP 625 contacts the coal seam.
Uninterpreted and interpreted relative amplitude record sec-
tions (Figure 6) for a portion of line one (shot with the electric
WEST EAST
1055c 1056C 1054c 1047c 1OOlC
SHOT PTS 50 100 150 200 250 300 350 400
.O .O
.2 .2
oBo’
Om 305m
FIG. 8. Plot of 1-D synthetic seismograms produced from the indicated drill holes along the seismic line. The Herrin
(No. 6) and Springfield (No. 5) coal reflections (stippled) and fault zones (numbered l-8) as interpreted on the Betsy
Seisgun data of Figures 4-5 are superimposed on the synthetic traces for comparison. At the depths of the coal seams
10 ms equals approximately 21 m.

8. Premine Coal Seismic Reflection Study 1501
Betsy) show faulting and facies changes affecting the Herrin
(No. 6) coal seam and the overlying roof rocks. Complex facies
transitions in roof rock above the Herrin (No. 6) coal seam were
interpreted from the relative amplitude section (Figure 6) with
the aid of 2-D models and synthetic seismograms (Figure 10).
These transition zones, interpreted to be facieschangesfrom the
nonmarine Energy shale to the marine Anna shale and Brereton
limestone sequence, are likely to be zones of change from low
to high sulfur coal and low to high stability roof rock, respec-
tively. A facies change above the Herrin coal from the marine
sequence to the softer Energy shale occurs between CDPs
450-500 and changes back into the marine sequence near CDP
500. This interpretation was based on the lossof continuity and
amplitude of the reflections in that zone directly abovethe Herrin
coal reflection. Another transition from the hard shale and
limestone to the soft shale occurs near CDP 620 (Figure 6) and
is partially interrupted by an Anvil Rock sandstone channel.
FIG. 9. (a) Two-dimensional geologic model of two channels along the seismic line (CDPs 545-644). The length
of the model is approximately 152 m. (b) Two-dimensional synthetic seismograms produced from the geologic
model with a 120 Hz dominant frequency (c) Betsy Seisgun observed data (CDPs 545-644) to compare to the
synthetic seismograms.

9. 1502 Henson and Sexton
not havebeenpredictedfrom the boreholesalone.Resultsshow
that correlationbetweenevenmore closelyspacedboreholesis
necessaryto adequatelydelineategeologyandthat carefullycon-
ductedhigh resolutionseismicreflectionmethodsrepresentan
effectivemeansof accomplishingthis.The seismicdata canbe
testedby placing new drill holes at selectedlocations where
geologicaldisturbancessuchasfaults,channelsands,and facies
changesin the coal seam are interpreted. These geological
featurescouldhavesignificanteffectson mining operations,and
consequentlytheir detectionis of critical importance.
CONCLUSIONS
Geologicalinformation requiredfor effectivecoalmine plan-
ning isquiteoften not obtainedby drilling practicescommonly
employedin the coal industry. In the researchpresentedhere,
the Herrin (No. 6) and Springfield (No. 5) coals have been
mappedin the field areain the Illinois Basinusinghigh resolu-
tion seismicreflection methodscombinedwith data from five
drill holesalongtheseismicline.Geologicalstructuresassociated
with thecoalseamsandinterpretedusingthe seismicdatacould
GEOLOGIC MODELS
INGFIELD #5 COAL
GY Sh
NGFIELD #S COAL
ISPRINGFIELD 15 COAL
d I
SYNTHETIC SEISMOGRAMS (120 HZ)
(b>(a) 0
FIG. 10.Geologicmodelsof roof rock faciestransitionsbetweenthe Anna and Energyshalesdirectlyabovethe Herrin (No. 6) Coal
Member with varying conditionsof Anvil RockSandstone.Two-dimensionalsyntheticseismogramswerecalculatedfor eachof the
geologicmodelsaboveusinga dominant frequencyof 120Hz sothat thesemodelscouldbecomparedto the Betsyrelativeamplitude
seismicsections.(a) Anvil Rock Sandstonein local sheetand channel phases,(b) Anvil Rock Sandstonein sheetphaseonly, and
(c) Anvil Rock Sandstonereplacedwith a shaleunit.

10. Premine Coal Seismic Reflection Study 1503
ACKNOWLEDGMENTS
The support and drill hole data provided by Marc Silverman
and Peabody Development Company, as well as the release of
these results for publication, are greatly appreciated. Technical
and software support provided by Mr. Clyde Lee of SYTECH,
Inc. made this project possible. Thanks also to Phil Martin for
the use of the electric Betsy Seisgun equipment, and to Don
Steeplesfor the useof a percussionBetsy. The assistanceby Mr.
Scott Wendling sincethe beginning of this study was exemplary.
This research was partially funded by the Illinois Mining and
Mineral Resources Research Institute, grant #G116-4117.
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