Quality data at a reduced price

Abstract

A case history of an economical 3-D land seismic survey acquisition and processing is presented. The survey was acquired by Caltex Pacific Indonesia (CPI) over the Pedada field in Central Sumatra. The field has a complex structural and stratigrapbic history and the known oil accumulation is trapped in an upthrown block of a wrench fault system. The objective of the survey was to better define the complex structure, fault locations and stratigraphic variations.

This paper describes the survey design which used six different geometries: a group interval of 30 m, with shotpoint intervals of 15, 45 and 75 m inline and 30 and 60 m in the crossline directions. This yielded four different CDP bin sizes of 7.5 * 15, 15 * 15, 7.5 * 30 and 15 * 30 m (inline * crossline) corresponding to four areas of different structural dips. The area with small CDP bins correspond to steep dips in both inline and crossline directions, while the area with larger CDP bins correspond to gentle dips. This is a departure from the standard 3-D design which chooses for the whole survey one constant bin size corresponding to the steepest known structural dips. In this case, the traditional design would have dictated a constant bin sizes of 7.5 * 15 m for the entire survey. The variable bin sizes presented here resulted in a 57% acquisition savings or US$ 1,700,000.

The changing geometry produced several processing challenges. In particular, the processor had to decide how and where to apply Dip Move Out (DMO) and interpolation in the processing sequence. Extensive testing of processing tools and interpolation techniques on the most dense part of the survey, before completion of the acquisition, led to an efficient processing sequence that resulted in high quality data meeting all the user's objectives. The techniques and tools employed are shown with data examples