Contents of April, 1998 - Vol. XIX No.2

SEISMIC VELOCITY FOR MATURITY ASSESSMENT: UPPER ASSAM BASIN, INDIA
R. K. Mallick, S. V. Raju and K. D. Gogoi

SEISMIC VELOCITIES FOR EFFECTIVE PLANNING OF EXPLORATORY WELL IN VIRGIN AREA - A CASE FROM NORTH BANK AREAS OF UPPER VOLTA
C. S. Singh, A. K. Khanna and S. N. Singh

PROCESSING AND INTERPRETATION OF GROUND MAGNETIC DATA ACQUIRED ALONG SILCHAR-IMPHAL-MOREH CORRIDOR (EASTERN CACHAR AND MANIPUR AREA)
D. Rai and V. Ramaswamy

APPLICATIONS OF MISFIT FUNCTION TOPOGRAPHY IN GEOPHYSICAL MODEL OPTIMIZATION
Saurabh K. Verma, Makku Pirttijarvi and Sven-Erik Hjelt

AN INTEGRATED CRUSTAL MODEL ALONG NAGAUR - JHALAWAR GEOTRANSECT
D. C. Mishra, Bijendra Singh, V. M. Tiwari, S. B. Gupta, N. Kameswara Rao and M. B. S. Vyaghreswar Rao

STUDY OF THE UTTARKASHI EARTHQUAKE IN TERMS OF RUPTURE MODEL AND ISOSEISMALS
A. Joshi

 


SEISMIC VELOCITY ANALYSIS FOR MATURITY ASSESSMENT: UPPER ASSAM BASIN, INDIA

R.K Mallick*, S.V. Raju* and K.D. Gogoi**

*Oil India Limited, Assam 786 602 India
**Department of petroleum technology, Dibrugarh University, Assam, India

Abstract

The Upper Assam foreland basin is an important onshore petroliferous region of India with reservoir rocks ranging in age from Paleocene to Miocene. Significant source rock intervals are found in the Upper Paleocene-Lower Eocene Sylhet Formation and Upper Eocene-Oligocene Barail Formation. In a previous study, the thermal maturity of source rocks obtained from measurement of vitrinite reflectance were compared with sonic log derived maturity. The relationship can be expressed mathematically, in simple terms. In The present study the method has been extended to thermal maturity evaluation from seismic velocity analysis. A good correlation has been obtained between maturity from seismic data and measured/calculated vitrinite reflectance values. This has enabled construction of thermal maturity map for the entire study area. The use of seismic velocity analysis is simple, reliable and does not entail any additional expenditure. It also has the advantage of providing preliminary estimates of source rock maturity in undrilled areas of the basin.

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SEISMIC VELOCITITIES FOR EFFECTIVE PLANNING OF EXPLORATORY WELL IN VIRGIN AREA - A CASE STUDY FROM NORTH BANK AREAS OF UPPER ASSAM

C. S. Singh, A. K. Khanna, S. N. Singh

GDAP Department, Oil India Limited, Duliajan, Assam - 786 602

Abstract

The continuing exploration venture of Oil India Limited (OIL) in the south bank of river Brahmaputra provided possible clue of hydrocarbon accumulation in north-bank of the river. In continuance to its extensive exploration programme, OIL acquired the PEL of about 4200 sq. km covering the north bank of river and subsequently collected about 1700 GLKM of 2D seismic data of different vintages from the erstwhile ONGC to assess the various exploration objectives in the area.

Based on the interpretation of these seismic data and other available geological information, OIL planned to drill a few exploratory wells in the area. Simultaneous to the above geoscientific work and planning for drilling, OIL deployed one of its 2D seismic crew to acquire data in the area during the year 1995-96. It was felt prudent to process these data on a high priority basis and review the exploration target prior to spudding the first well in the area.

The acquired data processed using Oil's in-house Landmark Seismic Data Processing system with Promax 6.1 processing software and the results were made available as expected. This paper discusses the result of this processing with specific emphasis on the advantage of interactive velocity analysis which provided reliable lead in guiding the well planning. It has been observed that the seismic velocity macro model estimated from the available drilling information in adjacent south bank wells and the geological correlation from the south bank area, needed considerable validation to meet the objectives. The generated seismic sections were converted to depth section using the products of interactive velocity analysis/manipulation and was a major input to the depth model.

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PROCESSING AND INTERPRETATION OF GROUND MAGNETIC DATA ACQUIRED ALONG SILCHAR-IMPHAL-MOREH CORRIDOR (EASTERN CACHAR AND MANIPUR AREA)

D. Rai and V. Ramaswamy

Geophysics Division, KDMIPE, ONGC, Dehra Dun - 248195.

Abstract

Magnetic data of Silchar-Imphal-Ukhrul and Palel-Moreh corridors (eastern Cachar and Manipur) have been processed and interpreted to bring out the basement configuration. The study reveals:

         thickness of sedimentary cover near Lakhipur (18 km east of Silchar) is about 6.5 km and it gradually becomes thinner towards Imphal,

         in the Manipur valley the maximum sedimentary thickness is about 4.0 km and

         in the eastern Manipur Hills the sedimentary thickness again increases and attains a value of about 6 km near Moreh at Indo-Burmese border where the ophiolite is exposed at the surface.

The basement configuration along Silchar-Imphal-Moreh corridor is arc type with convexity upwards. In brief, the study presents some new results on the subsurface geology which may be used in hydrocarbon exploration in the area.

Paper presented at the 23rd Annual Convention and Seminar on Exploration Geophysics during 18-21 November, 1997 and was Adjudged as one of the best papers and selected for citation

 

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APPLICATIONS OF MISFIT FUNCTION TOPOGRAPHY IN GEOPHYSICAL MODEL OPTIMIZATION

Saurabh K. Verma*, Markku Pirttijarvi** and Sven-Erik Hjelt**

*National Geophysical Research Institute, Hyderabad, India
** Department of Geosciences and Astronomy, University of Oulu, Oulu, Finland

Abstract

To proceed with any optimization technique, a measure of the distance between the observation and the response calculated from the model is required to be defined. This measure is called 'Observation function' or 'Misfit function' (MF, used in this paper). Several norms (l1, l2, ., lp, etc.) can be used to define this function. The behaviour of MF over a space described by a combination of model parameters can be displayed in the form of a MF topography (MFT). Construction of the MFT requires a thorough scanning of a vast model space.

The MFT maps can provide valuable clues on the correlation between the parameters defining the model space. For a dynamic (time-variant) system, such as a transient electromagnetic (TEM) method, the MFT is found to vary as a function of time. Thus it is possible to select suitable time-windows of observation which provide optimal resolution of the model parameters. Since various TEM systems employ different types of exciting pulses (sinusoidal, ramp, triangular, etc.) and recording time channels, the MFT's can also be exploited in comparing the performance of various TEM systems. Also, since MFT's are based on the scanning of a vast model space, in essence they represent a process similar to the grid search technique of optimization commonly used in finding non-linear parameters from geophysical data.

The above applications of the MFT are shown considering synthetic responses of i) a perfectly conducting half-plane (minimal) model in frequently domain and ii) a conducting finite plate model to commonly employed generic TEM systems. Both these models are assumed to be immersed in a non-conducting medium for the sake of simplicity. A field example from the nickel sulphide deposit, Mt. Keith South, Western Australia is also studied to demonstrate how the MFT maps can be used to obtain the model parameters.

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AN INTEGRATED CRUSTAL MODEL ALONG NAGAUR-JHALAWAR GEOTRANSECT

D. C. Mishra, Bijendra Singh, V. M. Tiwari, S. B. Gupta, N. Kameswara Rao and M. B. S. Vyagreswar Rao

*National Geophysical Research Institute, Hyderabad, India

Abstract

An integrated crustal density model along Nagaur - Jhalawar geotransect across the Aravalli fold belt is constructed based on the modelling of gravity data using the constraints from deep seismic reflection profiling results and near-surface geology. The observed gravity field along the transect shows "high" Bouger and Free-air anomalies over the fold belt and "lows" on its flanks over the Marwar and Vindhyan basins. Results of 2 dimensional gravity modelling indicate that the gravity high in the central part of the profile is partly due to a prismatic shaped high density body (3.09 g/cm3) in the lower crust extending from 18 Km upto 45 Km and partly due to the exposed high density metasediments of Delhi and Aravalli fold belt. The lows on the flanks have been attributed mainly to the presence of low density granites and sediments. The resultant crustal density model shows high density basement and lower crust underlying the adjoining Marwar basin in the west and Vindhyan basin on the east of fold belt. The large-scale thrusting of rocks along shear and fault zones from east to west might have resulted from continental collision during Proterozoic period.

 

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STUDY OF UTTARKASHI EARTHQUAKE IN TERMS OF RUPTURE MODEL AND ISOSEISMALS

A. Joshi

Lecturer, Department of Geophysics, Kurukshetra University, India

Abstract

 

Peak ground acceleration for Uttarkashi earthquake has been compiled by modelling rupture process. Field and simulated peak acceleration data and the isoseismal map prepared from synthetic and field data have been compared.

Modelling of the rupture plane is based on semi empirical method of Irikura (1986) which has been modified by Midorikawa (1993). Modelling of the rupture plane by this technique gives peak ground acceleration at the observation point.

After assuming the modelling parameters of rupture plane, peak acceleration using this approach was calculated at thirteen different stations that had recorded strong motion data of Uttarkashi earthquake of 20th Oct, 1991. A comparison of simulated and field peak acceleration less than 25% at six stations and less than 55% at seven other stations, thereby confirming the parameters of selected model and efficacy of the approach.

 

Hundred and sixty four different locations surrounding the rupture model for Uttarkashi earthquake were taken for simulation of peak ground acceleration. Peak acceleration at each location was converted into intensity on MMI scale by empirical relation between peak acceleration and maximum intensity on MMI. The comparison between the isoseismal maps based on synthetic and field data shows that the elongated axis of isoseismal map is dependent on the position of rupture plane and direction of rupture propagation from nuclear point.

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