Depth imaging with amplitude correction for localized absorption anomalies. A case study from the North-West Australian shelf

Abstract

Seismic amplitudes within target intervals are often affected by localized absorption anomalies in the overburden. In the North-West Australian shelf and in many other regions, the majority of such anomalies are caused by gas trapped in shallow sediments. We apply amplitude tomography to build a high resolution 3D absorption model and to take this effect into account in geological settings typical for the North-West Australian shelf. Two approaches have been developed recently to correct for absorption in depth imaging ? the first uses frequency independent models and the second based on Q- compensation with the linear frequency dependent assumption. We show how both techniques correct seismic amplitudes but their results are not frequency balanced. In order to achieve a better fit with the real seismic data, we propose and apply a mixed absorption model that combines frequency independent amplitude correction with the linear Q-compensation and reflects the presence of different effects responsible for seismic energy attenuation in real geological media. Prestack depth migration with this model corrects for the overburden effects and produces seismic data with spatially balanced amplitude and spectral characteristics.