Ocean Bottom Cable Surveying Offshore West Africa: Operational And Technical Aspects
Authors: Sanders J.I. , Starr J.G. , Dale C.T. , King W.F. 
Published: October 2008

During 1993 three 3D surveys were conducted offshore West Africa utilizing the ocean bottom cable (OBC) method of marine seismic acquisition. These surveys are examined from an operational standpoint.

While streamer methods remain the most popular and economic method of marine acquisition in uncontested, open seas, the viability rapidly deteriorates in shallow water or congested areas. The ocean bottom cable method overcomes many of the shortcomings, but at the same time presents a wide variety of collection options which must be considered in survey planning.

Background:

Three Ocean Bottom Cable 3D surveys were recorded in the Cabinda Concession offshore Angola in 1993. The offshore Cabinda Province Concession Area is located in the western African Republic of Angola. Sonangol, the state oil company of Angola, holds a 41% interest in the Concession. Cabinda Gulf Oil Company Limited ("CABGOC"), an indirect wholly-owned subsidiary of Chevron holds a 39.2% interest in the Concession and is the Operator. Elf Petroleum (Angola) Limited holds a 10% interest and Agip Angola Ltd. holds the remaining 9.8%.

The Cabinda Concession Area, which is approximately 5,550 square km in size, is divided into three areas referred to as Areas A, B and C. All currently producing oil fields are located in Area A (see Figure 1), which constitutes approximately half the Cabinda Concession Area. It is predominantly a shallow water area of less than 250 ft water-depth, and contains all the producing oil field infrastructure in the concession. Production from B and C is scheduled for late 1994. Up until 1992 four 3D surveys and a grid of variable quality 2D seismic had been collected over Area A (see Figure 1). These surveys filled most of the need for development and exploration efforts in the 1980's but were found to be inadequate as geological studies became more detailed. In 1990 the consortium embarked on a systematic program of 3D data acquisition to cover the remaining areas of high potential for both exploration and development. The first survey covered the area around Kungulo field (see Figure 1), and was acquired with a single streamer and two shooting vessels. This configuration was adequate in the less densely obstructed areas. However, it was not possible to operate with streamers in the Malongo field areas. The need for complete 3D coverage resulted in the surveys described in this paper. During the planning stages for the project, Chevron personnel evaluated several options for recording seismic data including buoy telemetry and multi-streamer and source vessel operations. Of particular concern for any operation was the strong near surface northerly current which flows through the concession from the Zaire River (formerly Congo). Also, the water depths range from the surf zone out to in excess of 2ooft. The resulting receiver ghost could seriously impact data spectra from conventional hydrophone telemetry systems. Dual Sensor Ocean Bottom Cable crews had been successfully employed in the Gulf of Mexico in similar conditions by HGS for Chevron USA. This experience gave Chevron personnel confidence in the suitability of the technology for use in West Africa.