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Improved static corrections over sand dunes for seismic line NC151-V532, western Libya.

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Libyan Petroleum Institute
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Abstract

Concession NC151 is located in the northwestern part of the Murzuq Basin (Libya), which is covered by linear sand dunes that reach heights of ~100 m above the gravel plain. The sand dunes cause large increases in the travel times of reflected events in seismic data. In recent years, the conventional method used to calculate field static corrections has been to interpolate the near-surface velocity structure between upholes. Results are often unsatisfactory on lines that cross the dunes because the reflection events contain false structures that correlate with sand dune topography. These structural artifacts are caused by residual static errors which are too large for automatic statics programs to correct during processing. An alternative method of calculating field statics is to pick the first breaks on the Vibroseis field records, calculate the delay times at each station, and use the delay times directly as the field statics after applying a linear adjustment to match them to the upholes. This simple version of the refraction method does not require surface velocity-depth models to be produced, and gives much better results than the conventional field static method. Since this method does not require any near-surface velocity-depth model because it works in time, it can be used in areas with variable LVL or HVL (e.g., sand dunes or permafrost). The maximum power autostatics method was chosen from several residual statics methods that are available in the ProMAX package. Two iterations of maximum power autostatics and velocity analysis followed by non-surface-consistent trim statics improved the final stacked section.

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