Recent Advances in Ocean-Bottom Cable Seismic Technology

Abstract

Summary The ocean-bottom cable (OBC) method of seismic data acquisition uses detectors of seismic reflections that are located and stationary on the ocean bottom: there are no streamers. As a result, the method can safely deal with obstacles like oil-field equipment platforms. OBC data, enhanced with numerous data acquisition and processing technologies, have a resolution which surpasses that achievable with today's towed streamer technology. These enhancements, as well as the geophysical reasons underlying the improved resolution are discussed below. Introduction Three technical questions most often asked about the Dual-Sensor OBC method are:Why should the method be used in relatively unobstructed areas where towed streamer data can be acquired?What is the quality of the geophone coupling with the method?What is being done to extend and improve the method? The answers to these questions are set forth in this paper tohelp define how this method fits into the geophysicist's tool kit for solving today's problems, Comparison Of Dual-Sensor OBC And Towed Streamer Geophysical Attributes The first of the most often asked questions about the Dual Sensor OBC method is "Why should the method be used in relatively unobstructed areas where towed streamer data can be acquired?" The following is addressed to that question. Acquisition Geometry Flexibility and Surface Consistency. The OBC method employs a stationary array of receiver stations on the ocean bottom and a marine vessel towing only a seismic energy source. The physical separation of the energy source from the recording spread provides the flexibility to record virtually any geometry, including those with shot linesorthogonal to receiver lines that provide a wide range of source-receiver azimuths. Such geometries, properly designed, avoid the illumination shadow zones created by wide-tow multiple streamer configurations (Beasley, 1995). They alsoproduce output traces with virtually all geologic dips properly imaged (Beasley, 1993). The OBC method's stationary receiver spread also yields a surface consistent recording geometry. Robust refraction and reflection statics algorithms can be applied to improve the processed data's bandwidth. The towed streamer data aredeficient in this regard due to streamer feathering. Recently, shot and receiver statics approaching 200 milliseconds weresuccessfully computed and removed from OBC data recorded near the mouth of the Mississippi River where the ocean bottom is covered with thick mud lumps (Carvill et. al, 1996). The area had previously been considered a no-record area using the towed streamer method. Consistency Of offsets And Azimuths In Cells. Streamer feathering has necessitated the concept of cell flexing in which traces from neighboring cells are "borrowed" to achieve a required population of source-receiver offsets. Again, the lack of receiver feathering with the OBC method eliminates the need for cell flexing and improves the resolution of the resulting migrated image. Receiver Location Accuracy. Each OBC receiver station's location is determined with range information from the first breaks recorded at that receiver from up to thirty nearby shot points. These computed locations are typically more accurate than those for streamer receivers that are located outside the acoustic ranging networks. The improved accuracy enhances the imaged results.