Application of Visco-acoustic Full Waveform Inversion for Gas Cloud Imaging and Velocity Model Building

Abstract

Summary The earth exhibits physical properties that attenuate and disperse the seismic waves as they travel through the subsurface. A case study is presented that uses waveform inversion to invert for the quality factor Q on a marine dataset from offshore Malaysia. To illustrate the effects of attenuation and dispersion, we model data in acoustic and in visco acoustic models with and without low Q anomaly. The amplitude spectrum of the visco- acoustic data shows a drop in amplitude compared to the acoustic dataset. This reduction in amplitude is seen even at the low frequencies commonly used for waveform inversion and is greater in magnitude than the amplitude change due to the presence or absence of the Q anomaly. This illustrates that both amplitude and dispersion effects are measurable on the data. The relevance of visco-acoustic inversion is highlighted by comparing the results of inversion for acoustic, visco-acoustic with fixed Q and visco-acoustic with variable Q anomalies. All three waveform inversions improve the velocity model over the initial model. The visco-acoustic inversions are however an improvement over the acoustic only inversion, as illustrated by the better stack and flatter gathers. The Q anomaly retrieved by the inversion delimits the gas zone.