The Upper Assam foreland basin is an
important onshore petroliferous region of
The continuing exploration venture of
Oil India Limited (OIL) in the south bank of river
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.
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:
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,
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 2½ 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
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.