Overpressure detection from seismic amplitude versus offset response: An application to gas-hydrates

Abstract

The seismic amplitude versus offset (AVO) analysis is an important tool for quantification of gas-hydrates and/or free-gas across a bottom simulating reflector (BSR). The high overpressure in the free-gas zone underlain by gas-hydrated sediments changes the seismic velocity and hence affects the AVO response appreciably. So the effect on AVO due to overpressure is to be evaluated before making quantitative assessment of gas-hydrates and free-gas across a BSR. Besides, knowledge of overpressure helps in planning the drilling process to avoid potential geo-hazard due to abnormally high pressures. Here we compute the AVO response of both P-P and P-S reflections from a BSR for possible detection of overpressure in the free-gas-bearing zone. The theoretical computation shows that high and negative AVO anomalies for P-P reflected waves indicate a high overpressure condition. Normal AVO trend for P-P reflected wave and relatively high maximum P-S reflection amplitude indicate the presence of overpressure.