Contents of January, 1998 - Vol. XIX No.1

A RE-LOOK INTO GRAVITY FIELD OF NORTHEAST INDIA
J.V. Rama Rao and R.M.C. Prasad

SEISMICITY OF NORTHEAST INDIA AND SURROUNDINGS - DEVELOPMENT OVER THE PAST 100 YEARS
J.R. Kayal

GROUND MOTION ESTIMATION CRITERIA FOR LARGE EARTHQUAKES IN THE URBAN AREAS OF NORTHEAST INDIA
Chandan K. Saikia and Paul G. Somerville

STOCHASTIC CHARACTERISATION OF COPPER MINERALISATION - A CASE STUDY
D.D. Sharma

MEHSANA HORST - TECTONICS AND ITS IMPORTANCE IN HYDROCARBON EXPLORATION, CAMBAY BASIN
M.C. Kandpal, N.K. Mahajan and B. Ghosh

FINITE DIFFERENCE EM MODELLING OF TWO-DIMENSIONAL BURIED CONDUCTORS IN AUDIO-FREQUENCY RANGE
K. Prabhakara Rao, K. Mallick and P.D. Saraf

HYDROGEOLOGICAL SURVEYS IN DROUGHT PRONE BARGARH DISTRICT, ORISSA, AIDED BY REMOTE SENSING STUDIES, GROUNDWATER EXPLORATION AND APPLICATION OF GIS TO DECIPHER GROUNDWATER POTENTIALS
S. Das, S.C. Behera, D.P. Pati and G. Prasad

 


A RE-LOOK INTO GRAVITY FIELD OF NORTHEAST INDIA

J.V. Rama Rao* and R.M.C. Prasad**

* Geophysics division, Geological Survey of India, Shillong
** Geophysics Division, Geological Survey of India, Hyderabad

Abstract

The gravity field of Northeast India is analysed with objectives of correlating with geology, inferring the crustal structure and tectonics and its relationship on seismicity and also to prepare crustal thickness map for the entire region.

Historically, this region is devastated several times by major earthquakes in recent past. By studying the areal distribution of the earthquakes with respect to inferred lineaments, four zones could be identified within the study area where the chances of major earthquakes are high. Twenty two lineaments have been identified with NW-SE, NE-SW, E-W and N-S trends. The gravity responses of different geological formations and role of lineaments in the disposition of lithology are studied. The relative variations of thickness of the crust are computed using Qureshy’s empirical relation. The variations are of the order of 33 Km over Shillong massif and Bengal Basin to more than 50 Km in the Himalayan region.Awarded the MBR Rao Gold Medal for best paper presented in the Seminar

Paper presented at the 23rd Annual Convention and Seminar on Exploration Geophysics during 18-21 November, 1997 and was adjudged as the best paper and was awarded the M.B.R. Rao Gold Medal

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SEISMICITY OF NORTHEAST INDIA AND SURROUNDINGS - DEVELOPMENT OVER THE PAST 100 YEARS

J.R. Kayal

Geological Survey of India, 4 Chowringhee Lane, Calcutta-16

Abstract

Northeast India is lying at the juncture of Himalayan Arc to the north and Burmese Arc to the east, and is one of the most active regions of the world. 18 large earthquakes (M 7.0) including two great earthquakes (M > 8.0) occurred in this region during the last 100 years. The 1897 great Shillong earthquake is the first instrumentally recorded earthquake in the country. Since 1964, with the inception of the WWSSN and increase of local networks, location quality of the epicentres are much improved. In the recent years, since early 1980s, several temporary microearthquake networks are run along with earthquake precursor investigations in selected areas of the region. Regional earthquake data (ISC and USGS), temporary close-spaced microearthquake (MEQ) network data and geophysical precursors of the felt/large earthquakes have given us some understanding of generating processes of the earthquakes in this region.

The seismicity in the main Himalayan seismic belt believed to be due to the collision tectonics between the Indian Plate of Eurasian Plate, and the earthquakes are generally correlated with the known regional thrusts, the Main Boundary Thrust (MBT) and Main Central Thrust (MCT). Distribution of epicentres is sparse, and the earthquakes are confined within a depth range 0-70 km. Thrust faulting of the earthquakes are reported. MEQ surveys in Arunachal Pradesh, in the Eastern Himalaya, however, revealed that the earthquakes are not correlatable with the regional thrusts. The earthquakes are rather generated by strike-slip mechanism; transverse tectonics are suggested in this part of the Himalaya. The earthquakes in the Burmese arc, on the other hand, are typical of subduction-tectonics. Intense activity is observed along the Indo-Burma ranges, and the earthquakes are as deep as 200 km. Normal, thrust and strike-slip faulting of earthquakes are observed. The seismological data reveal a subduction structure. The syntaxis zone, the meeting zone of the Himalayan Arc and Burmese Arc, is seismically active and produced the great Assam-Tibet earthquake (M:8.7) of 1950. The Shillong Plateau earthquakes are referred to the plate-boundary earthquakes which are produced by the stress accumulation due to the collision tectonics to the north and subduction tectonics to the east. The MEQ surveys in the Shillong Plateau and adjoining areas have revealed seismogenic faults and spatial variation of tectonic stress. Seismic tomography study, using the MEQ data, further confirms the fault zones at depth, and lateral variation in velocity structure. The great 1897 earthquake (M:8.7) occurred beneath this Plateau, and the high microearthquake activity indicates that the area is seismically very active. The activity in the Tripura folded belt is also referred to the plate-boundary earthquakes, the stress is transmitted mainly from the Burmese Arc. The low seismic activity in the Bengal Basin is interpreted to be due to a locked portion of the Indian plate below this basin. The E-W Dauki fault is possibly the surface expression of lateral segmentation of the Indian Plate, the Shillong Plateau of the north and the Bengal Basin to the South.

Continuous monitoring of deep resistivity, repeat microgravity measurements and microearthquakes surveys by temporary networks were useful to monitor precursor anomalies for felt/large earthquakes in this region. The deep resistivity survey shows that resistivity values may increase or decrease (40%-50%) or may show no change at all depending on the measurement direction, stress pattern, fracture/fault orientation and rock   formation. The repeat microgravity surveys indicate change in microgravity values (200-250 gal) before a felt/large earthquake in the region. The temporary MEQ network data indicates change in microseismicity level and velocity-ratio (V/V) before a felt/damaging earthquake in the area. Continuous monitoring of seismicity, deep resistivity and microgravity may be fruitful for prediction research of an impending large earthquake in this region.

 

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GROUND MOTION ESTIMATION CRITERIA FOR LARGE EARTHQUAKES IN THE URBAN AREAS OF NORTHEAST INDIA

Chandan K. Saikia and Paul G. Somerville

Woodward-Clyde Federal Services, 566 El Dorado St., Pasadena, CA 91101, USA

Abstract

Historical records show that the northeast region of India experienced several large earthquakes (M>6.0) during last two hundred years, including the June 12, 1897 and August 15, 1950 earthquakes (M³8.5) which caused widespread damage in the region. Considering the population growth and recent urban development in northeast India, the loss of lives, critical infrastructure, dwellings, dams, bridges and life-lines would be significant and perhaps more severe, if any of these earthquakes were to occur to-day. To reduce these losses, performance analysis of these structures under severe ground motions from large earthquakes becomes essential. Strong-motion recordings of large earthquakes in other geographical regions demonstrate that ground motions that are recorded at a site are frequently influenced by specific aspects of earthquake source, seismic wave propagation and local site conditions. Therefore, the choice of input motions to perform such analysis becomes crucial as conclusions can be biased when selected motions do not include the characteristics of the relevant earthquake source and site conditions. Ground motion time histories used for these analyses would ideally be recorded at sites from nearby earthquakes. Presently, there are no such recordings available for use. We need to operate strong-motion and broadband networks in the region to continually collect data for quantifying seismic parameters for seismic hazard estimation in the future. To compensate for the lack of nearby strong motion recordings from large earthquakes in northeast India, it is important that modern seismological methods be used to generate ground motion time histories for such events to include these effects. In this paper, we discuss methods for generating reliable ground motion time histories to develop design spectra for large earthquakes useful for performance analysis. We discuss their usefulness in the stress and deformation analysis for dams since dams fall into the category of critical resources for northeast India and are located close to the seismic sources.

Paper presented at the Workshop on The Great Shillong Earthquake - 1897 : A Centennial Retrospective during 18 November, 1997

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STOCHASTIC CHARACTERISATION OF COPPER MINERALISATION - A CASE STUDY

D.D. Sharma

National Geophysical Research Institute, Hyderabad

Abstract

Stochastic charachterisation of copper mineralisation including modelling aspects are discussed in this paper with reference to the mineralisation at Ingladhal, Karnataka. The exposed lode consisted of eleven horizontal levels (drives) driven from MRL 862 m to MRL 563 m in depth. Based on the auto-correlation function (acf), the partial auto-correlation function (pacf) and the Akaike criterion it is inferred that AR (1) model could be used to forecast copper prospects with a reasonable degree of confidence. As spectral representation may reveal useful information on hidden periodicities, the same has been carried out employing maximum entropy method. Significant spikes in the spectra in some of the levels indicated possible periodicity in mineralisation ranging approximately from 24m to 111m. The suggested model(s) would be highly useful to understand the charachteristics of copper mineralisation.

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MEHSANA HORST - TECTONICS AND ITS IMPORTANCE IN HYDROCARBON EXPLORATION, CAMBAY BASIN

M.C. Kandpal, N.K. Mahajan and B. Ghosh

RERIG, EBG, WRBC, Oil and Natural Gas Corporation Limited, Baroda

Abstract

The existence of Mehsana horst has played an important role in the exploration and exploitation of hydrocarbons in the northern part of the Cambay basin. The eastern flank of the Mehsana horst has proved to be rich in hydrocarbons with good reservoir facies, whereas the western flank has sporadic hydrocarbon occurrences (mainly in Bechrajee field) and is yet to be explored and exploited fully. The tectonic activity associated with the Mehsana horst, the global/local eustatic changes and the environment under which deposition took place during Late Eocene to Mid Oligocene are the main factors responsible for hydrocarbon accumulation in this part of the basin . Mehsana horst, trending in the NNW-SSE direction, had been an active and upcoming high till Late Eocene-Mid Oligocene period. Study of latest seismic and other Geo-scientific data have clearly revealed that the highest part of the horst at the technical basement level (Trappean level) was quite shallow (i.e. upto about 950 mts.). Due to the tectonic activity and global/local sea level changes, during Middle to Late Eocene, the horst got exposed and erosion had set in. Seismic lines in this part have clear evidences for the presence of erosional cycles, the carving of technical basement at places in the eastern side and the flattening of the top of the horst. The dumping of the weathered and eroded material from top of the horst in the eastern and western flanks is evident from the seismic data which might have created reservoir facies within Older Cambay shale / Olpad formations. These facies are represented by high amplitude seismic events.

During the deposition of Late Eocene & Mid Oligocene period, Mehsana Horst was exposed and the north-south flowing channels have skirted the exposed horst through lows (flanks) to create excellent reservoir facies in the eastern and western flanks of the horst. Together with the strati-structural entrapment conditions such as fault closures, wedgeout/ pinchout prospects and the existence of reservoir facies had created excellent ground for hydrocarbon accumulation in this part of the basin. This geological model has already been proved in the eastern flank of Mehsana horst, where number of oil fields like Lanwa, Balol, Santhal, Jotana etc. have been discovered. The western flank, which is analogous to the eastern flank, should also be prospective from hydrocarbon point of view which is proved by limited exploratory drilling.

Subsequently, the bald Mehsana horst, thus created was exposed, got weathered, and secondary porosity got generated within this product. This weathered top become relatively good reservoir with sporadic hydrocarbon occurrences. This thin weathered zone, about 30-40 m. thick, named as conglomerate zone or North Balol Pay, is known to have the hydrocarbon potential which is yet to be explored fully.

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FINITE DIFFERENCE EM MODELLING OF TWO-DIMENSIONAL BURIED CONDUCTORS IN AUDIO-FREQUENCY RANGE

K. Prabhakara Rao, K. Mallick and P.D. Saraf

National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007

Abstract

The electromagnetic method. is an effective tool in search of conducting ore bodies at shallower depths. However, the presence of host rocks and the overburden often modify or even mask the response of the target bodies.

Here an attempt is made (i) to study the response pattern of buried conductors and (ii) to analyse the effect of the host rocks. The finite difference technique is used to compute the H-polarisation apparent resistivities of two-dimensional geological structures. It is observed that in the presence of relatively conducting host rocks the detection of conductors at greater depths is not easy. An interesting phenomenon of resistivity inversion for deep-seated conductor is observed.

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HYDROGEOLOGICAL SURVEYS IN DROUGHT PRONE BARGARH DISTRICT, ORISSA, AIDED BY REMOTE SENSING STUDIES, GROUNDWATER EXPLORATION AND APPLICATION OF GIS TO DECIPHER GROUNDWATER POTENTIALS

S. Das, S.C. Behera, D.P. Pati and G. Prasad

Central Ground Water Board, Bhubaneswar

Abstract

The district of Bargarh,Orissa chronically suffers from droughts. An integrated approach of remote sensing studies, hydrogeological surveys and exploration was adopted to delineate both shallow and deep aquifers in the district and to assess their productivity characteristics, groundwater quality and groundwater potentiality. The district is underlain generally by the consolidated formations, consisting of Granite Gneisses,Khondalites, Charnockites with intrusives of Dolerites, Quartzites,Phyllite, Shale, Limestone and Dolomite. Buried Pediments, Intermotane valleys and fracture lineaments provide favouable locales for ground water storage. Ground water occurs under water table conditions in weathered residuum and under confined conditions in deeper fractured zones. The depth to water table values range from less than 1 m to 10 m or more below ground level.Specific capacity of dugwells varied from 0.56 to 142.1 LPM/M. The yield of exploratory wells was recorded upto 22 LPS depending upon its location under favourable topographic set up, closeness to fracture lineaments, thickness weathered residuum and number of fracturencountered in the borehole. The permeability of the weathered zone varies from negligible to 21 m/day which reduces sharply with depth.

GIS approach was adopted in ILWIS to integrate the collateral data like geomorphology,depth to water level, permeability of phreatic aquifers, soil characteristic in a relational data base to bring out an integrated thematic map delineating priority areas for groundwater development.

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