Constrained AVO inversion using genetic algorithm and FDR-PSO method and the estimation of gas-hydrate and free-gas in Mahanadi offshore Basin, India

Abstract

This work deals with the estimation of P-wave velocity (VP), S-wave velocity (VS) and density (ρ) from constrained AVO inversion using genetic algorithm and FDR-PSO method, followed by rock physical modeling for saturation of gas-hydrate and free-gas along a seismic line in the Mahanadi basin, India. The VP and thickness obtained from reflection traveltime inversion are used in constrained AVO inversion for the estimation of Vs and density ρ. The VP and VS of hydrate bearing sediments are estimated as 1602 m/s and 406 m/s, and 1610 m/s and 408 m/s using genetic algorithm and FDR-PSO method respectively. Presence of free-gas is indicated by a lowering of VP below the BSR lying at 246 m below sea floor, and an increasing trend in negative reflection coefficients. Comparable elastic parameters within minimum error limits of 0.5 %, obtained by both the methods, show the reliable estimation. Corresponding to the Poisson ratio of 0.4653, the saturation of gas-hydrate is obtained as 4 % using the three-phase Wood equation. The free-gas is estimated as 1% based on Gassmann equation.