Improved Seismic Ground Truth from Sonic Logs


John P Castagna, James Gaiser, ARCO Oil and Gas Company




Sonic log reliability for seismic applications can be improved by processing digitally recorded sonic waveforms. Check shot calibrated "waveform-de- rived" sonic logs, obtained with long spacings when necessary, are an ade- quate and cost-effective means of seismic velocity control. Quality control displays are useful for error detection and correction. When a vertical seismic profile is available, the sonic log is an important complementary means of defining the seismic reflection character, Introduction Sonic logs are commonly inadequate for seismic applications such as synthet- ic seismogram construction, multiple identification, wavelet extraction, time-to-depth conversion, and interval velocity determination. Poor sonic log reliability is one factor accounting for the emergence of vertical seis- mic profiling (VSP) as a common procedure. Vertical seismic profiles often contribute no more to seismic exploration objectives than do high-quality, check shot calibrated sonic logs. Considering the price differential be- tween VSP and sonic velocity determination, it is worthwhile to take extra- ordinary measures to insure sonic log data quality over-and-above what is necessary for formation evaluation.


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