Application Of Seismic Refraction Survey To Delineate The Extent
Of Hard Cover Over Under Ground Coal Mine Workings: Some Case
Studies
A.K.Das, S.K.Gupta and N.P.Srivastava
Exploration Division CMPDI, Ranchi, IndiaABSTRACT
Precise estimate and knowledge of hard rock cover thickness over
the shallow coal seam workings in under ground coal mines is of
prime concern for careful planning and layout access opening for
efficient mining activities and safety measures. The technique of
seismic refraction survey was identified and introduced in some
shallow under ground coal mining areas of Indian coal fields for
the determination of seismic velocities of subsurface layers with
respective thickness which in turn correlated with compactness of
the formation. The seismic refraction method yields velocity values
as part of the interpretation and data can provide nearly
continuous depth to bedrock profiles in a cost effective manner and
estimate the compactness of the formation. The accurate
determination of seismic velocities is important in engineering
consideration since the velocity is controlled by the fundamental
parameters of elastic strength and density. This paper reviews
practical aspects of seismic refraction survey technique to
determine the hard rock cover thickness over shallow coal mine
workings with the intent of defining conditions where the technique
can be expected to produce useful results. The method applied with
similar objective to different mining areas of Coal India and the
results found to be very encouraging. Results of some of this
investigation are presented to illustrate the vital role of
Geophysical mapping by seismic refraction technique for the actual
stage of mine construction and for future exploitation.
Introduction
The geological discontinuities in the roof rock mass above
underground coal mine workings play significant role in triggering
roof collapse and subsidence. It is necessary to design underground
mine workings in situation where practically no subsidence is
desired on the surface or at any level above the seam. Mining
selected for exploitation is determined mainly by the
characteristics of overlying geological deposits and the limits
imposed by the mining authority. In present day shallow U/G coal
mining practice in India follow the statutory requirement for
extending the mine workings to the up dip side where 15 meter hard
rock cover above the coal seam is present. DGMS recommends that
areas with hard cover less than 7.5 m. should not be developed.
Where hard cover is more than 7.5 m. but less than 15 m., width of
galleries may be restricted to 3.0 / 3.6m.Thus mine regulation
itself impose restrictions on development in coal mining owing to
the safety aspect of the mine. Violation of this recommendation
invites legal complications besides endangering the safety of the
mines. Therefore there is need to precise estimate and knowledge of
hard rock cover thickness over the coal seam workings in U/G mines.
The technique of seismic refraction survey was identified and
introduced and the efficacy
of this technique in the Indian coal mining environment has been
tested.
The seismic refraction method enables determination of seismic
velocity of subsurface layers with thickness which in turn can be
correlated with compaction of formation. The accurate determination
of seismic velocities of subsurface formation is important in
engineering consideration since the velocity is controlled by the
fundamental parameters of elastic strength and density and they
directly correlate with the material hardness and / or amount of
fracturing. In normal seismic refraction method only the
compressional wave velocities are determined. The measured
velocities may be evaluated qualitatively in terms of the degree of
weathering and fracturing of the known rock type, or an approximate
empirical relation with rock strength may be sought. The desired
formula is
Vp = [ {Y (1-)}/{ (1+ )(1-2 )}] , where Vp = Compressional
velocity, Y = Youngs modulus,
p = density of the material and = Poissons ratio.
Methodology
Seismic refraction survey was conducted in various coalfields of
coal India mainly in Kamptee, Raniganj & Karanpura coalfields
to measure compressional wave velocities as an aid in the
evaluation of compactness of the subsurface over shallow mine
workings. Data were acquired along various profile lines. The 24
channel refraction seismic data were acquired with a signal
enhancement seismograph. Geophone intervals of 10 m. and a 10 kg
sledgehammer / explosive source were used with impacts made at
various distances, offset and along the seismic profile. The
geophones located on straight line, relative elevations were
surveyed. The seismic data were stacked (for hammer source). For
each source point the data were recorded. The quality of the
seismic data was excellent and easily identifiable first breaks
were present. The refraction seismic data were processed and
interpreted using SIP set of computer programs from Rimrock
Geophysics Inc. Colarodo. The first arrivals on seismic records
were picked and corrected for elevation differences after fixing an
arbitrary datum. Time distance plot were then prepared and analyzed
using Intercept & Delay time techniques and thickness of
different subsurface layers were calculated depending upon
different seismic velocities which in turn gave idea regarding
compactness of formation. The inferred depths of different layers
are then plotted in the form of depth sections.
Case studies
1. Saoner Mine-III
The area under investigation of Saoner mine III lies in Kamptee
Coal fields on Nagpur Chhindwara Road at a distance of 40 Km. from
Nagpur. In order to delineate 15 m. hard cover over seam IV M, the
most potential of the five coal seams encountered in Kamptee coal
field, five Seismic profiles namely A-A, B-B, C-C, D-D and E-E were
laid down as shown in figure-1. Out of the five profiles E-E was
taken along the strike of the seam and rest of the four traverses
was taken along the dip across the incrop of the seam IV M.On the
basis of seismic refraction interpretation three distinct
subsurface layers were identified along the five profiles depending
upon velocity contrast. The top most inferred as weathered layer of
varying thickness 3 to 10.6 m. with velocity ranges from 340 to 495
m/s. The second layer considered as sub weathered varies in
thickness between 7.7 and 23.6 m. with velocity varying from 1100
to 1965 m/s. The bottom layer inferred as compact formation shows a
velocity variation from 2385 to 2600 m/s. The subsurface layers
along with coal seam have been depicted in figure 2 to 5. The
disposition of coal seam shown in the sections have been plotted
considering borehole data, spot level, formation dip and parting of
the seam. On different traverses where thickness of 15 m. hard
cover over roof of seam IV M is encountered were marked. Figure 1
shows a line L-L depicting 15 m. hard cover line which establishes
the boundary of the mine in up dip direction. The Sonic logging
conducted in a borehole falls in the study area and estimated
elastic velocity in consolidated
sandstone was found to be 2540 m/s at depth range 40 to 50 m.
from surface.
2.Sirka Underground mine
The area of investigation, Sirka lies at a distance of 10 Km
from Ramgarh town on Ramgarh-Giddi Road and is situated in the
eastern part of South Karanpura Coalfield surrounded by metamorphic
in the north and south. The Damodar river lies in the western and
southern part of the block. Objective of the seismic refraction
survey was to delineate 15 m. hard rock cover over Nakari seam,
occurring in the Sirka U/G mine (Panel B) for getting approval of
DGMS for depillaring the up dip side of the mine.
Seismic Refraction survey were carried out along the three
profile lines as shown in figure 6. Spot level of
Nakari seam at various locations from U/G mine plan helped to
arrive at the reduced level of top of Nakari seam in the area.
Three distinct subsurface layers were identified along all the
three profiles on the basis of velocity contrast calculated by
intercept / delay time technique of seismic refraction survey.
Layer 1 is weathered formation having velocity ranging from 400 to
500 m/s. with a thickness of 1.3 to 7.2 m. Second layer is
comprised of semi compact formation and has a velocity of 1450 to
1900 m/s. and of thickness 1.4 to 2.1 m. Layer three with a
velocity of 2000 to 2700 m/s. appears to be compact formation. Line
L-L drawn on the surface plan of panel B demarcates the 15 m. hard
cover over Nakari seam. The disposition of coal seam and various
subsurface layers have been shown in the depth sections along two
profile lines along dip ( figure 7 & 8).
3.Saoner Under ground mine 1
Seismic Refraction survey was carried out for the determination
of hard rock cover over seam IV(M) along the proposed panel at
Saoner U/G mine 1 for depillaring the up dip side of the mine. The
area of investigation falls in the Saoner phase 1 block in Nagpur
district of Maharashtra ( Topo sheet No. 55K/15). Seam IV(M) under
consideration is the most important seam in the block as its
thickness varies from 3.57 to 8.53 m. The investigation was carried
out in two adjacent palels E9 and E10 of Saoner U/G mine 1 ( figure
9). In all six seismic traverses were conducted along three profile
lines A-A, B-B & C-C each of 230 m. length. The Geophones were
kept at 10 m. interval. Reduced level were taken at every picket
station for making necessary correction to the travel times.
Conventional five point shooting was adopted for seismic refraction
survey. Traverse wise subsurface information and corresponding
depth sections are shown in figure 10 & 11.The analysis of
seismic refraction data shows that three sub surface layers could
be distinctly mapped based on their seismic velocity. Topmost
formation inferred as weathered layer has a variation in velocity
ranging from 520 to 745 m/s and its thickness from 1.9 to 8.4 m.
The seismic velocity and thickness of the second layer deciphered
as sub weathered layer varies from 1800 to 2100 m/s and from 12.4
to 29.5 m. respectively. The seismic velocity of the bottom layer
inferred as hard formation varies from 2350 to 2700 m/s. Line L-L
drawn on the surface plan demarcates the 15 m. hard cover over seam
IV(M).
ConclusionsThis technique has found increasing use for shallow
subsurface exploration for engineering sites. The result obtained
by applying this method is found to be encouraging. It can be
concluded that seismic refraction survey can be applied with
confidence for subsurface characterization over shallow coal mine
workings and should therefore help to optimize drilling programme.
The seismic velocities determined by this method will help in
guiding the rock strength. Project experience with seismic
refraction methodology demonstrates that the target depth needs to
be favorable and the required length of the traverse to acquire
deep images is often limited. The depth of investigation is based
on source to receiver distances, the overall length of the receiver
array, and surrounding ambient noise. Based on 10 m. geophone
receiver spacing, typically survey depths average appx. 70 to 80 m.
for 24 channel arrays. The method is therefore usually most
effective for mine subsidence application in U/G mine as well as
studying rip ability characteristics.
References
Dobrin,M.B.; Introduction to geophysical prospecting, 1985
Redpath,B.B., Seismic Refraction Exploration for Engineering
site investigation, Explosive Excavation Research Laboratory,
Livermore, California,1973.
Rimrock, Seismic Refraction Interpretation Program, Geometrics,
Geophysics Inc., USA.
Handbook of Engineering Geophysics, Volume1: Seismic, Bison
Instruments Inc, Minneapolis, Minnesota, USA.
Report on Seismic Refraction Survey for delineation of hard rock
cover over coal seam-IV(M) at Saoner Mine-III, Kamptee coalfield,
CMPDI, Jan,1999.
Report on Seismic Refraction Survey for delineation of hard rock
cover over Nakari seam in Sirka colliery, South Karanpura
Coalfield, CMPDI, March1997.-IV(M) at Saoner Mine-III, Kamptee
coalfield, CMPDI, Jan,1999.
Report on Seismic Refraction Survey for delineation of hard rock
cover over coal seam-IV(M) at Saoner Mine-I, Kamptee coalfield,
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