use of electrical resistivity methods for study of coal seams in parts of the jharia coalfield,...

Geoexploration, 17 (1979) 163-176 o Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

163

USE OF ELECTRICAL RESISTIVITY METHODS FOR STUDY OF COAL SEAMS IN PARTS OF THE JHARIA COALFIELD, INDIA

R.K. VERMA and N.C. BHUIN

Indian School of Mines, Dhanbad 826004 (India)

(Received February 6, 1978; accepted May 17, 1978)

ABSTRACT

Verma, R.K. and Bhuin, N.C., 1979. Use of electrical resistivity methods for study of coal seams in parts of the Jharia Coalfield, India. Geoexploration, 17: 163-176.

The response of several closely spaced coal seams located in the northeastern part of the Jharia Coalfield, and occurring within the Barakar Formation (of Gondwana age) has been studied using electrical resistivity methods employing Wenner, Two-electrode and Half- Schlumberger configurations. All the coal seams are characterised by high values of resistivity (up to 1140 Ohm-m) with respect to the background resistivity of the Barakar Forma- tion, which is about 300 Ohm-m. The profile using the Half-Schlumberger configuration shows sharp discontinuities on account of the presence of coal seams as compared to the Wenner profile. The Two-electrode profile does not give very useful results as the coal seams appear as resistivity lows rather than highs.

A number of vertical electrical soundings were also carried out over the coal seams along two profiles. The coal seams can be well picked up using this method.

INTRODUCTION

Electrical resistivity of rocks and minerals covers a wide range of magnitudes. The presence of a zone with anomalous resistivity perturbs the distribution nf the electrical current/potential lines in an otherwise homogeneous medium. Thus from the measurements of potentials on the surface it is possible to know something about the nature of subsurface layers. The electrical resistivity methods have been successfully applied to the study of different geological problems in several different ways (L&n, 1954; Van Nostrand and Cook, 1966; Hallof, 1966; Bhattacharya and Patra, 1968; Zohdy, 1969; Singh and Jha, 1972; Zohdy et al., 1973; Stanley et al., 1976). However, the response of coal seams on account of contrast in their electrical properties with respect to the surrounding medium has not yet been adequately studied.

The Jharia Coalfield, an important coalfield in India, extends over an area of 450 km2 between latitudes 23” 37’N and 23” 52’N, and longitudes 86” 06’E and 86” 30’E in the Dhanbad district of Bihar. It has many coal seams of different thicknesses. Electrical resistivity survevs have been used, for mapping a number of closely spaced coal seams in the northern part of this coalfield, using different electrode configurations. The results obtained are discussed in this paper.

164

GENERAL GEOLOGY OF THE JHARIA COALFIELD

The basin comprising the Jharia Coalfield is generally believed to have been formed by crustal subsidence during Gondwana times. The stratigraphic sequence in the basin is shown in Table I.

Crystalline gneisses of various types surround the Gondwana strata from all sides in the field. The sedimentary strata dip away from the gneissic contact everywhere in the field. The northern margin of the coalfield has a gentle arcu- ate shape and has a normal depositional contact with the metamorphic base- ment whereas the southern boundary of the coalfield constitutes a steeply dipping boundary fault. The Barakar Formation extends as a wide east-west belt in the northern part of the field. The rocks belonging to the Talchir Series are unfossiliferous sandstones and underlie the Barakar Formation, which consists of sandstones, conglomerates, shales, carbonaceous shales and coal seams. The coal seams are found only within the Barakar and the Raniganj formations.

SURVEY AREA AND LOCATION OF THE PROFILES

The profiles AA’ and BB’, along which the electrical resistivity surveys were carried out for mapping the coal seams are located in the northeastern part of the coalfield. The location map of the area is shown in Fig. 1 along with the geology of the area. The orientation of the profile AA’ is in NE-SW direction and its length is 930 m and that of BB’ is almost north-south while its length is 335 m.

The areas where the profiles AA ’ and BB’ are located are locally known as

TABLE I

Stratigraphic sequence of the study area (According to Fox, 1930)

Eocene--Cretaceous Dolerite and mica-peridotite

dykes and sills

Damuda System

Permian

Upper

Middle

Raniganj Series

(Upper coal

measures) Barren measures

Lower Barakar Series

(Lower coal measures).

Upper Carboniferous Talchir Series (Boulder bed)

Great unconformity

Lower

Gondwana

Division

Archean Metamorphic complex

(granites and gneisses)

165

/ 86022 86O23’

BARAKAR .

NtCHlNTPUR

6.4~6~0~~ R .s.

Fig. 1. Location of the profiles AA’ and BB’ and the geology of the area showing the posi- tions of the different coal seams. The figure also shows the location of the Schlumberger soundings.

the Chhotoboua area and the Gareria section, respectively. Both areas are close to the northern boundary of the Jharia Coalfield. The coal seams Nos. II-VIII are present within the Barakar Formation and are overlain by a layer of top soil of about 2.0 m thickness in the Chhotoboua area. The coal seams Nos. IX/X-XII are present within the Barakar Formation in the Gareria section. There are many collieries such as East Basuria, Basuria, Nichintpur, etc., in these areas, which are mining coal from some of the above mentioned coal seams. A number of quarries are present from which coal has already been ex- tracted partly or totally. Some of the quarries are full of water. A clear sequence of different formations can be seen within the quarries where there is no water. A small stream called Bansjor passes through this area. The profile AA’ was laid about 100 m away from this stream. The stream is creating a

166

lot of obstruction for the extraction of coal in this area. The Chhotoboua area is full of bushes and the topography is slightly undulating whereas the Gareria section is free from bushes and the topography is also more or less flat. Both areas are free from power lines and other disturbances.

DESCRIPTION OF THE COAL SEAMS ALONG PROFILES AA’ AND BB’

The east-west trending coal seams Nos. II, III, IV-B, IV-T, V-B, V-T, VI, VII and VIII across which the profile AA’ was chosen, form a part of the Barakar Formation in the Chhotoboua area. All the coal seams are exposed and their thicknesses vary from 1.17 m to 10.0 m. Coal seam No. II is the thickest (about 10.0 m). All the coal seams are dipping towards the south, the dip varying from 8” to 12”. On account of this, the exposed thicknesses of the coal seams are larger than their actual thicknesses.

The coal seams Nos. IX/X, XI and XII, across which profile BB’ was chosen. are exposed in the Gareria section. The thicknesses of these coal seams are about 8.0 m, 4.0 m and 1.8 m, respectively, and the strike is in east-west direction. All the coal seams are dipping towards the south, the dip varying from 2” to 7”. Coal seams Nos. XI and XII are very close to each other and are sep- arated by the Barakar Formation of about 4.5 m thickness.

FIELD INVESTIGATIONS ALONG THE PROFILES AA’ AND BB’

All the field observations were taken using a d.c. resistivity meter fabricated by the National Geophysical Research Institute, Hyderabad. Steel electrodes were used for driving a current into the ground while porous pots were used for potential electrodes. Resistivity profiling as well as sounding was carried out along the above mentioned profiles.

Resistivity profiling

Three different electrode configurations namely Wenner, Two-electrode and Half-Schlumberger, were used along both the profiles. The total electrode separation (L) was chosen to be 150 m for the Wenner and 50 m for both the Two-electrode and Half-Schlumberger configurations. The above mentioned L spacings were chosen as it is estimated that the depths of investigation are about O.llL for Wenner and 0.35L for Two-electrode (Roy, 1972), and for the Half-Schlumberger configuration it is about 0.25L (as nearly the same responses are obtained with the above spacing). The infinite electrodes for the Two-electrode and Half-Schlumberger configurations were kept at a distance of about 300 m in perpendicular directions. It was not possible to keep these electrodes at larger distances on account of the presence of dense bushes, quarries and power lines. Proper care was taken to keep the infinite electrodes undisturbed.

The first observations, for all the configurations were taken just over coal seam No. II for the profile AA’, whereas for profile BB’ the first observations

167

were taken just over coal seam Nos. IX/X. The resistivity anomalies were plotted, with apparent resistivity (pa) along the Y-axis and distance (R) along the X-axis.

Resistivity sounding

Six vertical electrical resistivity soundings (S, A to S6 A) were taken, using Schlumberger configurations, at different points along profile AA’ and two (S,B and S2B) along profile BB’. The locations of all the sounding are shown in Fig. 1. All soundings were taken parallel to the strike of the coal seams. Proper care was taken to prevent interference from self-potential, polarising effects, contact errors and other disturbances for the potential measurements in each sounding.

RESULTS AND INTERPRETATIONS

Pro files along AA ’

The results of the resistivity profiles using the Wenner and Two-electrode configurations are shown in Fig. 2 along with the geological section. The Wen- ner profile shows a large variation of resistivity anomaly ranging from 120 Ohm-m to 860 Ohm-m along this profile. All the coal seams are well reflected on the profile showing high values of resistivity. The relatively low resistivity values, of the order of 300 Ohm-m, are due to the presence of the intervening Barakar Formation. The resistivity values obtained using this configuration for the different coal seams vary from 350 Ohm-m to 860 Ohm-m. The width of the anomaly over the coal seams is seen to be large, apparently due to the relatively large exposed thickness in comparison to the actual thickness.

The resistivity anomaly obtained through the Two-electrode configuration, ranges from 10 Ohm-m to 850 Ohm-m along this profile. The coal seams are reflected on this profile showing low values of resistivity instead of high values. That is, all the coal seams behave as a “broad conductive zone”. This hap- pens when the total electrode separation (L), using the Two-electrode configuration, is greater than or equal to the thickness (t) of the body under investigation (Keller and Frischknecht, 1966, p. 186; Telford et al., 1976, p. 686). When L < T, the actual nature (conductive or resistive) of the requisite bodies will be reflected on the anomaly through this configuration. The order of resistivity values obtained using this configuration for the different coal seams varies from 166 Ohm-m to 360 Ohm-m. A resistivity value of about 300 Ohm- m is found for the intervening Barakar sandstone.

A peak resistivity value of about 830 Ohm-m is seen at a distance R = 340 m (R is the distance from origin of the profile) on the Wenner as well as the Two- electrode profiles. This high value may be due to the presence of basic intru- sives which are frequently found within the Barakar Formation. A zone of low resistivity of about 100 Ohm-m for the Wenner and 20 Ohm-m for the

!

-0

f t rr 0 a!

169

Two-electrode configurations is found at a distance R = 800 m to 930 m. This low zone is probably due to the presence of water-bearing weathered sandstones.

The results of resistivity profiles using the Half-Schlumberger configuration are shown in Fig. 3. The resistivity values show sharp variations ranging from 120 Ohm-m to 1140 Ohm-m along this profile. This profile gives better indi- cations regarding the thickness and presence of the coal seams. The widths of the anomaly over the coal seams are lesser in comparison to the widths obtained using the Wenner and Two-electrode configurations. The peak resistivity values for the different coal seams vary from 420 Ohm-m to 920 Ohm-m. A resistivity value of about 300 Ohm-m is observed over the intermediate Barakar Formation. A zone of relatively low resistivity (about 160 Ohm-m) between R-values of 800 m and 930 m is also seen along this profile.

Soundings along AA’

Six Schlumberger soundings (,!?,A to &A) were taken at different points along this profile. The purpose of taking these soundings was to determine the depth to the coal seams from the surface, the nature of resistivities of the different formations and finally to correlate the results with those obtained from the profiles.

All the sounding curves were interpreted by partial curve matching techniques (Bhattacharya and Patra, 1968). The results obtained are given in Fig. 4, 5, and 6. Some of the soundings such as &A, &A, &A and &A were taken very close to coal seams Nos. IV-B, IV-T, VI and VII, respectively. The end part of the sounding curves when interpreted, gives high values of resistivity, which indicates the presence of the above mentioned coal seams. The interpretation of the sounding curve SIA gives a thickness of about 20.8 m for the layer above coal seam No. II. This is because the elevation of the surface is high and this coal seam is not exposed near the surface. A vertical shift is observed on the sounding curve &A. The shift is most likely due to the signal comming from coal seam No. III which lies at a depth of about 25.0 m below the surface. The weathered layer, which was detected along the profiles is also reflected on the sounding curve S6A.

Profiles along BB’

Apparent resistivity curves for the three different electrode configurations along this profile are shown in Fig. 7. The values ranges from 280 Ohm-m to 790 Ohm-m for the Wenner configuration which has given a better response

Fig. 2. Results of resistivity profiles along AA’ using the Wenner and the Two-electrode configuration with L = 150 m and 50 m, respectively. Station interval for both the profiles is 5 m. The Wenner profile shows high resistivity values over the coal seams, whereas the Two-electrode profile shows the reverse nature.

170

I-

t

I- ”

I 8 *

WHO’ AllAllSIS38 lN3MVdd-f

0 I

171

420 nm - .

2! h2= 19.2 m, f2= 210 nm -

>- I-

h3= 0~: , p3= a40 nm _

5 -**X-

F 55 A 56 A Y :

h, = 1.0 m , r;= 175 n m h,=l.l m ,J =150 nm (L IO - h, = 3.0 m , f2 = 87.5 n m h2=2.2m,f2=75 nm 7

l- t h, = 4 875 n m

,” , P3=

h, = 6.8 m , f3 = 176 n m 5

2 h, = 6.6 m , f’, = 39 n m -

LL 4 3-

1 I I I!1111 I I I Illill h,= 0~ f5= 13oon m _ I I I111111 L III

I IO 100 1000

ELECTRODE SPACING, +N METERS -

Fig. 4. Result of Schlumberger soundings S,A, S,A and S,A taken along the profile AA’.

The end part of all the sounding curves gives high values of resistivity indicating the presence of the coal seams.

due to the coal seams than the Two-electrode and Half-Schlumberger configurations in which cases the ranges of resistivity variation were about 110 Ohm- m to 330 Ohm-m, and 150 Ohm-m to 830 Ohm-m, respectively. Coal seam No. IX/X is well reflected on all the three profiles but coal seam Nos. XI and XII are not well reflected on the profiles using the Two-electrode and the Half- Schlumberger configurations.

The results of the resistivity profiles over these coal seams can be summa- rised as follows.

(1) The resistivity value over coal seam No. IX/X is of the order of 425 Ohm-m using the Wenner configuration. The value decreases to 250-300 Ohm- m over the intervening Barakar Formation and rises again to a peak value of about 650-700 Ohm-m when the electrodes come close to coal seam Nos. XI and XII. The two coal seams are marked by sharp variation in the resistivity value.

(2) Using the Two-electrode configuration, the resistivity value is relatively, high over the coal seam No. IX/X and relatively low over the Barakar Forma-

Fig. 3. Results of resistivity profiles along AA’ using the Half-Schlumberger configuration with L = 50 m. Station interval is 5 m. Every coal seam has been reflected showing high values of resistivity.

172

5 ;

-x-x-x-

F!? s2 A

z (L IO h,=z.sm, f, = 59n m

h2=5,om, f2=472~m

h3= 0~ , &=3lonm

h,= 1.1 m, PI= 85 n m

h2=1.1 m, p2= 17 n m

h,= oc , f3= 85onm

I 1111111l 1 I I111111 I I111ll11 I IJ I IO 100 1000

ELECTRODE SPACING, y, IN METERS ----t

Fig. 5. Results of Schlumberger soundings S,A and S,A taken along the profile AA’. The shifting on the sounding curve S,A is attributable to the signal coming from coal seam No. III.

5 _

e

z-- I-

5

; hl= 1.1 m , 6 = 860 n m

!f VI 100

0.7 m, f2 = 34.4 n m

z h5= e , J’3-344onm

I-

zi

5 2 <

IO I IO 100 1000

ELECTRODE SPACING, +IN METERS -

173

tion. The reason for the reflection of the above coal seam as a resistive layer on this profile is the shallow dip. The dip is so small that the exposed thickness (t) is greater than the electrode separation (L). However, t < L in the case of coal seam Nos. XI and XII which respond as conductive bodies.

(3) Overall variation of resistivity values is found to be the highest using the Half-Schlumberger configuration.

(4) The fault (located at R = 270 m), which is indicated on the geological map of the area is also fairly well indicated, being characterised by sharp change in resistivity values.

(5) Resistivity of about 300 Ohm-m for the intervening Barakar Formation, which was also found along the profile AA’, is also observed along this profile. This confirms that the resistivity of the Barakar Formation is of this order of magnitude along the northern part of the Jharia Coalfield.

Soundings along BB’

The results of Schlumberger soundings SIB and S2B are shown in Fig.8. The curves were interpreted by partial curve matching techniques as mentioned earlier. From the results it is inferred that the thickness of the Barakar Forma- tion is about 3.0 m above coal seam No. IX/X at a distance R = 20 m where the sounding SIB was taken and the thickness is about 2.6 m above coal seam No. XI at a distance R = 195 m, where the sounding S2B was taken. The depths of coal seams at these locations were also observed to be the same along this profile. It is felt that as soon as the current lines penetrate the Barakar Forma- tion and touch the coal seams, the nature of the sounding curves changes re- sulting in high values of resistivity with the thickness of the second layer as almost infinite.

SUMMARY AND CONCLUSIONS

An attempts has been made to study the response of coal seams to electrical resistivity profilings and soundings using various configurations. It was observed that the coal seams in general are characterised by high values of resistivity. The differences in response of coal seams to different configurations can be explained as follows. The depth of investigation using the Wenner configuration is the least of all the three cases tried. Hence it is more sensitive to the presence of near-surface inhomogeneities which are well reflected using this configuration. In the case of the Two-electrode and the Half-Schlumberger configurations the depth of investigations being more, values of apparent resistivity are relatively high and they represent the effect of deeper layers better than those of shallow layers.

Fig. 6. Results of Schlumherger sounding S,A taken along the profile AA’. The end part of this sounding curve gives high value of resistivity indicating the presence of coal seam No. VI.

i II L

:

2;

5 r t -x--x--1(-

IO

h, = 3.0 m , J’, = 3s n m h, = 2.6 m , p; = 56 d. m

I- z

:

h,= 4 , p2= 315 A m h,= 0~ , p2 = 504-n. m

I 3 I I I ,IIIIl I II111111 I I, 11111l 1 1

I IO 100 1000

ELECTRODE SPACING , y,IN METERS +

Fig. 8. Results of Schlumberger soundings S,B and S,B taken along profile BB’. The interpretation of the end parts of the sounding curves gives high values of resistivity which indicates the presence of coal seams Nos. IX/X and XI, respectively.

Along the profile AA’ where a number of coal seams lie close to each other and dip at shallow angles, the Half-Schlumberger configuration has given good results as it has measured the gradient of potential instead of the potential difference. Hence in cases where a number of nearly horizontal discontinuities are present close to the surface, this configuration is likely to give better results as compared to others. Sometimes small fluctuations in resistivity values are observed while crossing the coal seams. These fluctuations appear to be due to crossing of the electrodes one by one across the coal seams. The extension of coal seams Nos. XI and XII, where the profile BB’ was chosen, has not been

marked geologically. However, their presence has been reflected on the resistivity profiles as well as on the sounding curves. Therefore it can be concluded that the electrical resistivity profilings as well as soundings can be suscessfully applied for mapping the coal seams having a reasonable thickness of overbur- den.

Fig. 7. Results of resistivity profiles along BB using the Wenner, Two-electrode and Half- Schlumberger configurations with L = 150 m, 50 m and 50 m, respectively. Station shifting is 5 m for all the profiles. Coal seam No. IX/X is well reflected on all the resistivity profiles showing high values of resistivity. Coal seams Nos. XI and XII are well reflected on the Wenner profile compared to the Two-electrode and Half-Schlumberger profiles. The presence of the faults at a distance of R = 270 m is indicated by sharp variation of resistivity on all the profiles.

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ACKNOWLEDGEMENTS

We are thankful to the Manager of East Basuria Colliery for providing ac- commodatian during the field survey. Thanks are due to Prof. B.B. Bhatta- charya for helpful discussions on the results of these investigations. Shri S.K. Gangopadhyay typed the manuscript and Shri B.N.P. Singh drafted the fig- ures.

REFERENCES

Bhattacharya, P.K. and Patra, H.P., 1968. Direct Current Geoelectrical Sounding. Elsevier, Company, Amsterdam/London/New York.

Fox, C.S., 1930. The Jharia Coalfield. Geol. Surv. India Mem., 56: 253. Hallof, P.G., 1966. The use of resistivity results of outline sedimentary rock types in Ire-

land. Mining Geophys., SEG, 1: 18-27. Keller, G.V. and Frischknecht, F.C., 1966. Electrical Methods in Geophysical Prospecting.

Pergamon Press, London, 183-187 pp. Lagn, O., 1954. Mapping nearly vertical discontinuities by earth resistivities. Geophysics,

19: 739-760. Roy, A., 1972. Depth of investigation in Wenner, Three-electrode and Dipole-Dipole d.c.

resistivity methods. Geophys. Prospect., 20: 329-340. Singh, J. and Jha, B.P., 1972, Resistivity profiltes over some dykes of Dhanhad. Geophys.

Prospect., 20: 130-141. Stanley, W.D., Jackson, D.B. and Zohdy, A.A.R., 1976. Deep electrical investigation in the

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bridge University Press, 686 pp. Van Nostrand, R.G. and Cook, K.L., 1966. Interpretation of resistivity data. U.S. Geol.

Surv. Prof. Pap., 499. Zohdy, A.A.R., 1969. The use of Schlumherger and equitorial soundings in ground water

investigations near El Paso, Texas. Geophysics, 34: 713-728. Zohdy, A.A.R., Anderson, L.A. and Muffler, L.J.P., 1973. Resistivity, self-potential and in-

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use of electrical resistivity methods for study of coal seams in parts of the jharia coalfield,...

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