Acoustic impedance logs computed from seismic traces

Abstract

Acoustic impedance, the product of seismic velocity and density, is a basic physical property of rocks. Seismic traces are converted into pseudoreflection‐coefficient time series by appropriate initial processing, then into acoustic impedance by the inversion of the time series. Such pseudologs are roughly equivalent to logs recorded in wells drilled at every seismic trace location. They yield important information concerning the nature of the rock and variations in lithology. To obtain the best quality pseudologs, careful initial processing is necessary: true‐amplitude recovery, appropriate deconvolution, common‐depth‐point (CDP) stack, wave‐shaping, wave‐equation migration, and amplitude scaling. The low frequencies from moveout velocity information are inserted. Both the short‐period information computed from reflection amplitudes and the long‐period trend computed from reflection moveout are displayed on acoustic impedance logs. Possible causes of pseudolog distortions are inaccuracies of amplitude recovery and scaling, imperfection of deconvolution and migration, and difficulties of calibrating the pseudolog to an acoustic log derived from well logs. Such calibration increases the precision; facies variations observed in well logs can be extrapoled to large distances from the wells, leading to a more accurate estimation of hydrocarbon reserves.