DELINEATION OF GEOMETRY AND SPATIAL DISTRIBUTION OF BASIC BODIES USING MAGNETIC AND RESISTIVITY METHODS IN SOHAGPUR COAL FIELD AREA, MADHYA PRADESH

The state of Madhya Pradesh in Central India is known to bear ‘A’ grade coal seams within Gondwana basin. An alluvium-covered area to the north-east of Shahdol was considered to be prospective for the exploration of coal. A number of basic intrusive are encountered in the area and these appear to have played an important role for enhancement of the rank of coal seams. Delineation of the geometry and spatial distribution of these basic bodies has therefore become necessary for exploration of high rank coal seams. Magnetic and resistivity surveys were mainly carried out. Geologically, the alluvium covered area was known to be composed of rocks of different ages such as Precambrian, Gondwana Triassic, Cretaceous (trap) and older alluvium in succession. Gondwana basins are formed over basement depressions or in the downthrown side of the faulted Precambrian. Gondwana sediments lying over such basement sub-basins are prospective areas for the exploration of coal. The magnetic map was vitiated through the presence of high amplitude and small wavelength anomalies due to a thick blanket of basic bodies lying above the Gondwana sediments. Fluctuations in magnetic responses are observed at a few locations which may be due to the presence of basement faults? Low intensity but distinct anomaly patterns are observed in the south-western part of the area possibly indicating subsurface basic intrusive. 2D modeling of the magnetic data has effectively brought out the basement depth varying between 680 m to 1460 m an increasing trend from west to east. Selected resistivity soundings confirm that the overlying high resistivity layer (107-390 Ohm-m) and of thickness (0.7-386.3 m) may possibly indicate the geometry of basic bodies from the surface to depth of 547.5 m. The basement could not be picked up.


Introduction
The Gondwana sediments have been deposited in several known paleo-channels (faulted basins) over the Precambrian rocks in India. These sediments are known to be coal-bearing and coal being mined from these basins in the states of West Bengal, Bihar, Madhya Pradesh and Maharashtra. A large lava flow of basalt covered the Gondwana and Triassic sediments during the Cretaceous period which is known as the Deccan trap. Subsequently, Tertiary rocks and older alluvium covered this flow at many places. The area was then subjected to tectonic disturbances which resulted in the formation of faulted basins. The Sohagpur coal field area covered by this survey falls within such a region on the western part of Rewa basin (Figure 1). The Sohagpur coal field extends over a length of about 103 km in E-W direction with width of about 43 km in the N-S direction in the Shahdol district of Madhya Pradesh (MP). The Bouguer gravity map of the Sohagpur area indicates an undulating nature (a number of lows and highs) of the basement topography (Joga Rao et al. 1984) which is covered by a variable thickness of lower Gondwana sediments. Most of these lows lie along the central part of the Sohagpur coal field, north of Bamhani-Chilpa fault. The high appears to be due to compounding effects of subsurface intrusive and shallow basement rocks.
The area of high rank coal is mainly bounded between the Bamhani-Chilpa fault to the south and the Pali to the north. A lot of geological studies have been carried out which showed that there is Http://www.granthaalayah.com ©International Journal of Research -GRANTHAALAYAH [148] a change in coal seams development on either side of the Bamhani-Chilpa fault. Since the occurrence of basic intrusive (low angle dykes) appear to have played an important role for enhancement of rank of the coal seams (Venkatappyya et al. 1968), it was felt necessary to have an idea about the geometry and spatial distributions of these basic bodies with the help of geophysical surveys by magnetic and limited resistivity methods.

Geology
Sohagpur coal field is a part of large sediment filled Rewa Gondwana basin within the drainage area of Son River (Ghosh et al. 1996). The Precambrian basement rocks are not exposed in the vicinity of the coal field, which crop out as inliers within the Talchir formation in other areas. The Talchir strata are found to the south of the coal field and continue beneath the coal measures. The overlying Barakar formation contains the regionally distributed economic coal beds of Sohagpur coal field. Barren measures (sandstone and siltstone, a chloritic matrix) strata are overlain by Raniganj beds. Raniganj coals are associated with interlaminated and rippled fine grained sandstone and shale. Raniganj strata are, in turn, overlain by Late Permian and Early Triassic Pali rocks which are represented by a sequence of very coarse grained to pebbly sub-arkosic to arkosic sandstones, interspersed with thin shale bands. Lameta strata occur along the fringes of trap covered hills in the south-west part of Sohagpur coal field. They rest unconformable over the Gondwana strata ( Figure 2). Gondwana strata are traversed by a large number of basic dykes and sills. Extraneous heat source from the igneous bodies probably attributed to the rank enhancement of the coal seams. The geological formation in the area, their sequence, nature of occurrence in the coal field is given below:  Thickness  Age  Alluvium/soil  0-30  Recent and Sub-recent  Sills, dykes and flows of Deccan trap  Upper Cretaceous  Lameta beds  60-100  Upper Cretaceous  ------------------------------------------Unconformity----------------------------------------

Structure
The major structural features of this area are sets of sub-parallel east-west trending strike fault which have variously affected the Gondwana sediments. The most important one of these eastwest faults have been described as the Bamhani-Chilpa fault which runs through the central part of Sohagpur coal field dividing the area into a shallow uplifted southern block and a northern down thrown segment. This fault has been conduit for intrusion of dykes and sills of Deccan trap affinity (Figure 2), a suite of iron rich continental dolerite basalt type. The coalfield underwent at least two major tectonic movements after the coalification process was completed. The first was probably of post Triassic age when the southerly dipping extensional fault came into existence. The second tectonic movement probably coincides with the effusion of Deccan trap, when most of the major faults were re-activated.

Layout of Field Operations
Magnetic survey was carried out employing a Scintrex make Vertical Force (VF) Fluxgate Magnetometer (MF-2, reading accuracy 1 to 10 nT) along N-S profile at 1 km interval (Figure 1) from the western part of the area. The observations were taken at 200 m apart along each traverse and reduced to the base station. The absolute value (VF) was 24500 nT observed at base station. The base of magnetic profiles and zero contours in magnetic contour map corresponds to this value. The observed magnetic data have been corrected for diurnal and normal corrections. All observations have been taken with N-S orientations and as such heading error was negligible.
A Scintrex TSQ-3 time domain resistivity unit with digital stack readout was used for field surveys. The grounded or the current sending electrodes were made of steel stakes buried in soil pits. RDC-10 receiver of Scintrex make has been used for measuring the potential difference between two non-polarisable electrodes (copper dipped in saturated copper sulphate solution). The deep resistivity soundings were taken at suitable locations in an attempt to determine depths to the basement rock and to obtain depths, character and thicknesses of the different layers overlying the basement.

Magnetic Survey
The magnetic (VF) data were plotted along 29 traverses ( Figure 3) show distinct geological correlation. Lithological boundaries are clearly reflected. Raniganj formation reveals low amplitude (100 nT to 500 nT), wide ranging magnetic anomalies whereas exposed low angle basic bodies register high amplitude and small wavelength magnetic anomalies comprising "highs and lows" due to dipole nature. High amplitude anomalies are due to shallow/exposed dykes. The low amplitude anomalies observed over Raniganj formation is mostly due to disposition of basement at depth as Raniganj (sedimentary) formation is of non-magnetic character. The high amplitude, small wavelength anomalies observed in the southern as well as northern part of the surveyed area are directly over basic bodies which are exposed on the surface or at shallow depth ( Figure 3). Apart from these three major anomaly zones, one broad anomaly zone of less amplitude has been observed in the southern part of the area along Trav-5, south of Jarwahi and continues eastward with southern trend and passes through Chainpa, south of Maiki, Nabalpur upto area south-east of village Hari representing the disposition of basic bodies along this zone which may be exposed or be at shallow depth.

Magnetic Anomaly Map
The vertical magnetic intensity anomalies are contoured and presented in Figure 4. The anomaly zones described in Figure 3 are clearly reflected in   The interpreted thickness of Gondwana sediments from spectral analysis matches with that obtained from 2D basement mapping, based on the magnetic modeling. In the present area of investigation, fourteen Schlumberger soundings with a maximum current electrode separation (AB) of 2.4 km have been carried out. The length of the current and potential dipoles are increased keeping MN<1/5 of AB, where AB is current dipole length, MN is potential dipole length.

Deep Resistivity Sounding
It may be noted that there is an overlapping resistivity range for the Deccan trap and Gondwana sediments, which makes it difficult to mark the interface between them. Demarcation of the Precambrian morphology, however, has been possible by the resistivity survey results, because of high contrast between the Deccan trap/Gondwana sediments and the Precambrian rocks. The sounding technique used is the Schlumberger system which is relatively insensitive to surficial changes in electrical properties and provides sounding curve of much better quality in heterogeneous terrain than does either the dipole or Wenner techniques (Kunetz, 1966   The observed sounding curves are of KHA and HKA type, and in general, four or five subsurface formations are interpreted in the study area using "Orellana and Mooney" Master curves (Orellana and Mooney, 1966). The initial model parameters were used as input data to arrive at refined and accurate layer parameters using inversion techniques involving filter coefficients (Koefoed, 1976). A standard inverse modeling program 'Resist' is then used to get the model parameters. The details of soundings in the area are also given in Figure 7.

Geo-Electric Section
The interpreted vertical resistivity distribution and inferred lithology are shown for easy comprehension in Figure 7. The higher resistivity basic body is inferred from surface to a depth of 547.5 m. The basic body is characterized by resistivity ranging 107-390 Ohm-m and thickness varies from 0.7-386.3 m. At greater depths, basic body inhomogeneity has also been interpreted. High resistivity basement could not be picked up by resistivity survey.

Conclusions
The systematic magnetic survey carried out along N-S traverse in Sohagpur coal field area has brought prominent magnetic anomaly zones which indicate excellent correlation with surface geology. The area under investigation is dominantly occupied by Raniganj formation associated with intrusion of basic bodies which are well reflected by magnetic data. Lateral disposition of basic body is well depicted along magnetic anomaly zones. Three sets of structural discontinuity are interpreted in the magnetic (VF) contour map which may represent the change in the disposition of basic body in the area. As the Gondwana formation is of generally non-magnetic in character, the contour pattern in the area is mainly governed by basic intrusive. Magnetic anomalies of low amplitude over Raniganj formation reflect the disposition of Archaean basement in the area. 2D magnetic modeling and spectral analysis of vertical magnetic field in the central part of the area has brought out the thickness of the Gondwana sediments that is underlain by basement rock. The vertical electrical sounding observed in the area at specified location has brought out lithological variations in term of resistivity distribution. The high resistivity layer (107-390 Ohmm) may possibly indicate the geometry of basic bodies right from the surface to depth of 547.5 m. The basement depth could not be picked up.

Acknowledgements
The Director General of the Geological Survey of India is gratefully acknowledged for his kind permission to publish this paper.  Figure 2 Geological map of Sohagpur coal field, Madhya Pradesh. Figure 3 Magnetic (VF) anomaly along north-south traverses with geology. Figure 4 Magnetic (VF) contour map of Sohagpur coal field, district Shahdol, Madhya Pradesh. Figure 5 Magnetic (VF) profile along AO-OB. Lower boundary of Gondwana sediments inferred from 2D modeling and spectral analysis are also shown. Figure 6 Schlumberger resistivity sounding in Sohagpur coal field area, district Shahdol, Madhya Pradesh. Figure 7 Interpreted vertical resistivity distribution along with lithology.