IMPART OF PROCESSING ON THE AMPLITUDE VERSUS OFFSET RESPONSE OF A MARINE SEISMIC DATA SET

The analysis of prestack reflections may have a high diagnostic potent ial in the evaluation of the petrophysical characteristics of subsurfa ce targets. However, the recovery of reliable seismic responses, espec ially when examining amplitude versus offset (AVO) variations, is of t he utmost importance and is strictly dependent on the acquisition and processing steps. In order to evaluate the impact of different process ing sequences, we examine the AVO responses of three seismic events fr om a marine data set. Borehole data indicate that these events are rel ated to a lignitic sand, a gas sand and a cineritic bed. The AVO analy sis is focused mainly on the reflections from the gas sand. In particu lar, we compare the results of a standard processing sequence with res ults from a surface-consistent approach and with results from a proces sing sequence tailored to this specific case. A decreasing AVO trend o f the gas-sand reflection results from the analyses of data that have undergone the standard and the surface-consistent processing sequences . This contrasts with both theory and borehole information, which both predict an energy increase with offset. A detailed study shows that t he receiver array attenuation, neglected in previous studies, plays a major role in attenuating the far-offset reflections. Other propagatio n factors, such as offset-dependent geometrical spreading and Q absorp tion, produce only minor effects. Taking into account the above factor s, we apply a third processing sequence whose impact on the AVO trend is evaluated step by step and whose results are compared with the prev iously applied sequences. This new sequence leads to better agreement between the AVOs predicted from borehole data and those measured on su rface seismic data.