Reconciling sonic logs with check-shot surveys: Stretching synthetic seismograms

Abstract

Every seismic interpretation project needs to begin with an attempt to tie seismic reflectors to geologic units via synthetic seismograms. However, the synthetics normally do not tie character-wise to the seismic, and require stretching (occasionally squeezing). Check-shot surveys are frequently used in an effort to determine the amount of stretch necessary. However, the time-depth relationships from check shots usually differ by alarming amounts from the time-depth relationships from sonics. There is usually no discernible pattern to the mismatches. The interpreter knows that both check shots and compressional acoustic (sonic) logs are subject to error, so there is no way to know which to trust. Since no interpretation can ever be more credible than the well-seismic tie it is based on, this problem is worthy of considerable attention. The reasons that check shots do not tie sonics are many, but may be grouped into three categories: Therefore, a three-stage correction process is required to remedy this three-stage problem: (1) purify the logs; (2) reject check shots …