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Reconciling sonic logs with check-shot surveys

Stretching synthetic seismograms

Harmony Geophysics, Houston, Texas, U.S.

Paul Lowrey

Loren & Associates, Houston, Texas, U.S.

Corresponding author: info@loreninc.com

The first 20% of the full text of this article appears below.

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:

• The sonics are contaminated—they measure properties of the damaged zone adjacent to the borehole, which do not represent the undisturbed rocks that the seismic and check shot measure; and they contain occasional severe mismeasurements and malfunctions. Contamination is particularly acute in clastic basins, where boreholes commonly wash out before logging.
  
• The check shots are contaminated—they cycle skip, misfire, and can be badly processed.
  
• The two measure different things—due to dispersion effects, borehole conditions, and all other field effects. (Velocities recorded by sonic logs should be higher than those recorded by check-shot surveys, or deduced from seismic data, because of dispersion effects; i.e., the sonic logs use higher-frequency sound pulses, which travel a few percent faster). The corrections necessary to convert sonic measurements to check-shot measurements are called SLED (sonic log environmental and dispersion) corrections herein.
  

Therefore, a three-stage correction process is required to remedy this three-stage problem: (1) purify the logs; (2) reject check shots . . . [Full Text of this Article]