The Over/Under Acquisition Test - Taking the Seismic to the Technical Limits

By

Ahmed Hagras¹, James Keggin¹, Hamed Shakhshir², Chris Koeninger², Sergio Grion²

(1) BP Egypt, Cairo, Egypt (2) WesternGeco, Cairo, Egypt

 Most of the 3D seismic surveys in the Nile Delta were acquired to address dual exploration targets: Pliocene and Pre-Messinian. Shallow Pliocene objectives require a high resolution image that is optimally achieved through shallow gun/streamer depths. In contrast, deep Pre-Messinian objective requires good penetration and signal/noise ratio that can be best obtained through deep gun/streamer depths. Therefore, conventional surveys with towing depths of 6-7m were conducted as a compromise between both objectives.

The Over/Under acquisition technique combines the data acquired using different cable depths and blends both high and low frequencies in a single image that could satisfy the requirements of both exploration objectives. Inspired by the results of the WesternGeco experiment in the Mediterranean, BP Egypt conducted an Over/Under acquisition test in the Nile Delta.

The vessel “Geco Diamond” recorded several vessel passes over an existing conventional 3D in the Western Nile Delta. The vessel was simultaneously towing four streamers of 8000m length at alternating depths of 7m and 11m at 50m separation. One data set of 4m streamer depth was generated from the combination of the Over/Under 7m & 11m data. Another set was generated from the combination of the new 11m and the old 7m data. Both sets were compared to the conventional data to assess the gain achieved at both high and low ends of the spectrum.

Comparisons and contrasts of the results of these tests could open new opportunities for acquiring superior shallow and deep data quality to reduce exploration risk in Nile Delta.