JULY-OCT, 2002

Contents of July-October, 2002- Vol. XXIII No.3,4

Dewashish Kumar

CRUSTAL STRUCTURE ALONG THREE GEOPHYSICAL TRANSECTS IN THE CENTRAL PART OF THE BAY OF BENGAL
M. Subrahmanyam, N. Vasudeva Rao and T.K.S Prakasa Rao

SCANNING THROUGH THE SEISMIC ANOMALIES IN SEARCH OF OIL TRAPS
–A CASE STUDY FROM UPPER ASSAM OIL FIELDS

S.N.Singh and V.K.Sibal

Oil India Limited, Duliajan, Assam.

Abstract

Detection of seismic anomalies and the subsequent exploitation of these anomalies have always provided front end leads in oil exploration. The fact that the seismic signatures are a form of analytical signal, the seismic characteristics depicted by subsurface features allows the dissection of overall responses into constituent contributors. This has pursued the geophysicist to unravel the embedded information from the seismic signals through a wide variety of attribute studies. The attribute analysis has been quite revealing when used systematically and with caution.

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VALIDATION OF ANOMALY FROM THE SEISMIC DATA: A CASE STUDY FROM SAURASTRA OFFSHORE AREA

B.N.Singh, S.N.Singh

Oil India Limited, Duliajan, Assam

Abstract

Non uniqueness of geophysical anomalies have always been a stumbling block in geophysical interpretations. The observed seismic responses often lead to different explanations where as the real expected solution is unique. In the context of oil exploration, it has been difficult, at times, in distinguishing the seismic anomalies caused by major structural features e.g salt domes, shale diapir, igneous intrusions, basement horst etc., owing to the similarities of the response manifestation on seismic section. This has led to pitfalls in seismic data interpretation and have necessitated studies in convincingly inferring the nature of causative bodies, as well as have made the integrated studies imperative in the industry.

During the exploration venture pursed by Oil India Limited (OIL) in saurashtra Offshore area, a few similar cases were encountered where the uniqueness in interpretation and thus an unanimous conclusion on the nature of the causative body could not be ascertained. In an attempt to explain the anomalies, a study was carried out by the authors with preclusion that these seismic anomalies are worth investigating from hydrocardon exploration point of the view. Special and advanced processing such as Wavelet processing, PSDM (Pre Stack Depth Migration)etc. have been carried out to enact the anomalies and the environment better. The result of gravity and magnetic observation have been interpreted and integrated to validate the different possibilities.

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GEOPHYSICAL SURVEY FOR DELINEATION OF SEEPAGE ZONES AT DYKE NO.1
OF DHAROI PROJECT, GUJARAT

R.S. Ramteke

Chief Research Officer, Central Water and Power Research Station, Khadakwasla, Pune - 411024

Abstract

The Dyke No.1 of Dharoi Project, Gujarat was constructed on a stream which was also bunded up earlier on the downstream of the dyke known as old babsar Tank. Excessive seepage was noticed at downstream toe of the dyke which continued even after the construction of loading berm and partly grouting the foundation through the bottom of the cut off trench (COT). A geophysical survey, comprising seismic refraction and Electrical resistivity soundings and profiling was carried out to delineate the zones that are prone to excessive seepage so that suitable remedial measures could be adopted. Three underwater seismic profiles and one continous land profile were taken parallel to the dyke axis. Four electrical resistivity soundings and two profilings were also conducted. The results of seismic survey indicated the presence of low velocity zones below the bed level of COT. These zones were also identified by the electrical resistivity results and it appears to be continuous from the downstream toe of dyke to the Babsar tank. It was inferred that these weak zones in the bedrock may be responsible for the observed seepage.

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ENVIRONMENTAL GEOPHYSICAL STUDY FOR GROUND WATER IN COASTAL BELT OF DIGHA, WEST BENGAL

D.K Saha, P.Chakraborty, K. Choudhury and B.S.R Murthy

Geological Survey of India, 27, Jawaharlal Nehru Road, Kolkata - 700 016

Abstract

Integrated geophysical surveys comprising Electrical resistivity and Seismic refraction methods have been employed in the coastal belt of Digha for investigating the nature and status of sub-surface saline water contamination, if any, and suggest the remedial measures to arrest possible saline water intrusion into the aquifer. Geophysical survays have delineated the different sub-surface geological formations like dune sand, top sandy soil, saline sand and saline clay. It is also inferred from geophysical interpretation that the thickness of near surface saline zone decreases away from the shore. Fortunately for Digha, clay layers present at different shallow sub-surface levels, which have probable extensions under the sea, have acted as barriers against any large scale saline water intrusion in the deep aquifer. Clay formations are predominant upto a depth of about 60m in the area below which aquifer zone has been demarcated. Ground water can be effectively exploited from a depth of 90m - 120m in the area.

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CHARACTERIZATION OF GROUNDWATER FLOW REGIME IN A CRYSTALLINE ROCK THROUGH FRACTURE NETWORK PROGRAM

Dewashish Kumar

Indo-French Centre for Groundwater Research, NGRI, Hyderabad - 500 007 email: dew_kumar@yahoo.co.uk

Abstract

Characterization of groundwater flow regime was carried out in typical crystalline rocks of Dhanbad in the Saraidhela area of northern India. It is a hard rock terrain and consists of fractured felspathic gneiss and quartzite. Three sets of fracture were studied and a conceptual model of aquifer was established using NETWORK program.

Out of three sets, Set I and Set III are the major conduits for groundwater flow. The cardinal aquifer parameter like porosity, permeability, velocity and flow rate are obtained. The velocity calculated for Set I and Set III are 1.5207 * 10-09 m/s and 1.5193 * 10-09 m/s respectively for a head difference of 0.001 m. The porosity is less than 1% in both the sets and the permeability for the two sets are 1.4 * 1004 m/day and 1.0 * 1004 m/day respectively.

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CRUSTAL STRUCTURE ALONG THREE GEOPHYSICAL TRANSECTS IN THE CENTRAL PART OF THE BAY OF BENGAL

M. Subrahmanyam, N. Vasudeva Rao and T.K.S Prakasa Rao

Department of Geophysics, Andhra University, Visakhapatnam - 530 003.

Abstract

The Bay of Bengal lithosphere is known to have been experiencing compressive stresses in a N-S direction while subduction forces along the Andaman Island Arc-trench system impart an E-W tensile regime. Subsidence due to sediment loading aided by the influence of horizontal and vertical forces may be deforming the Bay of Bengal lithosphere. Majumdar and Biswas (1995) explained the widespread diffuse earthquake activity and the N-S compressive fault plane solutions in the Bay of Bengal as due to intraplate deformation. An attempt has been made to investigate the crust-mantle structure along E-W, NW-SE and N-S sections in the central part of the Bay of Bengal from the free-air gravity and total intensity magnetic data. In the case of E-W and NW-SE profiles the crustal thickness is found to be different on either side of 850E ridge. The crust appears to be thin east of the 850 ridge compared to the crust on the western side. The 850E ridge rocks are found to be strongly magnetized. A near N-S section in the vicinity of 870 longitude revealed that the sediment thickness increases from about 100N towards North and a corresponding deepening of the Moho was observed. Effect of the 850 E ridge and 860 E fracture zone between 50N and 100 N was found to be pronounced because the transect is aligned along a flank of the ridge and crosses the fracture zone.