Design of Gravity Survey Network using Fractal Theory to Delineate Hydrocarbon bearing Jabera Structure, Vindhyan Basin, Central India

A gravity survey network was designed using fractal dimension analysis to delineate a domal structure (Jabera dome) reported in southeastern part of the Vindhyan basin, Central India. This area is also regarded as a `high risk-high reward' frontier area for hydrocarbon exploration in previous studies, hence our aim was to delineate shape and lateral extent of the reported domal structure. Based on the synthetic grid, designed using the concept of fractal dimension, gravity data is collected in Jabera-Damoh area of Vindhyan basin. The collected data is random, but the data density is significant, hence the data points are sorted in a way so that they are close to the synthetic grid points of given grid interval. After sorting the data, again the fractal dimension analysis using box counting method has been carried out to avoid the aliasing in the data due to interpolation and also to know the optimum number of data points sufficient for desired quality of Bouguer anomaly maps. Optimization of number of stations takes care of time and cost involved in the survey and the detectibility limit ensures that the data collected is good enough to resolve the target body under study. The fractal dimension analysis gives clue to select these parameters. It showed that it is always preferable to have well distributed station locations instead of clustering the observation points at some geologically known feature because clustering of data points below required station spacing is not going to add much information where as equally distributed observation points add the information. The study area lies in a difficult terrain of Vindhayn basin, hence according to the accessibility, fractal dimension analysis of the real data sorted approximately at regular grid intervals on 2,3, and 4 km has been done and using the concept of optimum gridding interval Bouguer anomaly maps of the region are prepared. The preliminary depth values of the major interfaces in the area were obtained from the 2D scaling spectral analysis of the data. Results of the scaling spectral method reveals that in the study area, there are three main depth interfaces at 5 km, 1.5 km and 0.8 km respectively which corresponds to the basement and lower Vindhyan interface, lower Vindhyan and Upper Vindhyan interface and upper most is the terrain clearance. For quantitative interpretation, we selected a profile across the target structure (reported as Jabera dome) and modeling of the gravity data acquired along the profile was carried out using Marquardt inversion approach. This profile is selected in order to estimate the tentative geological cross section across the conspicuous low gravity anomaly observed in the southern part of the study area. Deep Seismic Sounding (DSS) studies carried out by earlier workers indicated presence thick sediments in this part of the Vindhyan basin. The gravity anomaly drawn along this profile shows a typical anomaly pattern of a sedimentary basin faulted on its both margins. The modeling results show that the anomaly corresponds to a deep faulted basin in the crystalline basement in which the upper layer with density value of 2.46 g/cc corresponds to the upper Vindhyan rocks. This layer is underlain by a thick layer (1.0 to 6.5 km) of lower Vindhyan sediments. This layer which has gentle slope from NW to SE direction sits over the high density rocks comprising of Bijawar/Mahakoshal group.