Inversion of gravity anomaly due to a contact (fault) and its application for graben tectonics across Godavari basin

摘要

An inversion technique for gravity anomaly due to a contact (fault) using all the observation points along a profile is developed. Based on least-square inversion, the increments in different parameters are estimated and iterated to obtain the final model, Marquardt factor (lambda) is used for quick convergence to the actual solution. This technique is tested on synthetic data for faults with different inclinations and with errors upto 20-30%, In most of the cases the computed values are within 5% of actual value, However, the computation is least sensitive to the inclination of the fault, and therefore its initial value is chosen very carefully to obtain a realistic model, A Bouguer anomaly profile across the Godavari basin, which shows a central 'low' corresponding to the Gondwana sediments and 'highs' along adjoining shoulders representing a typical graben structure, is chosen for application of the above technique. Removing a suitable regional, the Bouguer anomaly due to the central Gondwana basin is separated, and then fitted with the computed values using two different models: (i) a basin model of density contrast -0.35 g/cc, and (ii) two contacts (faults) on either side of the basin with the same density contrasts. The characteristics of the contacts derived from the second model, including the maximum thickness of sediments, match quite well with those obtained from the first model and also with other known geological and geophysical information from the region. Both the contacts along the basin margin with adjoining basement rocks show deep inclination of 70-80 degrees, and the maximum thickness of sediments is approximately 5 km for a density contrast of - 0.35 g/cc. The nature of gravity anomaly along northern contact clearly indicates a faulted contact, while the southern side contact may be faulted only in specific sections. [References: 9]