The Ocean Bottom Cable: A Modern Marine Multicomponent Seismic System

Abstract

Abstract Although 3D seismic began onshore, it quickly spread to the offshore environment. The earliest marine 3D surveys were performed with streamers and the newly developed compass techniques for positioning. Shallow water 3D surveys soon followed with utilizing some novel permutations of conventional onshore technology. Initially, "bay cable" systems were employed, leveraging what had become very successful and efficient 2D shallow water systems. When the need to go into congested areas of deeper water developed, new challenges emerged. First of all, 24-hour operations were required to economically meet the challenges of 3D coverage. This was addressed in 1984 with larger vessels, two shifts of personnel, and living quarters on each of the functioning vessels. Secondly, deeper water operations were challenged by the cancellation of signals due to water layer reverberations. Experiments with vertical hydrophone cables proved successful, but were ruled impractical (A practical and efficient employment of the vertical hydrophone cable has since emerged). It was obvious that at least two measurements were required - geophones and hydrophones were chosen as a practical method. Thus the industry was introduced to the dual sensor. The early systems utilized up to 10 miles of analog cable and TI DFS V recording. This was quickly superceded by distributed systems accommodating the doubling of channel requirements. Beginning in the early 1990's the addition of horizontal geophones illustrated the potential of faithfully recording the total vector content of the seismic wave field. The additional requirement of doubling the channel capacity of the instruments and cables was met. The service industry is now in a very heated race to field systems to meet these challenges. Ocean bottom cable (OBC) is at the forefront of that race. Introduction Marine multicomponent seismic systems are a natural development from the earliest production offshore seismic systems. Multicomponent normally means the detection and exploitation of P-waves (compressional) and s-waves (shear). P-waves are those whose particle motions oscillates in the direction of propagation, while s-waves have a particle oscillation orthogonal to the propagation. The most common marine seismic systems utilize an array of streamers towed behind a vessel that also tows the seismic source. The streamers are populated with a relatively dense array of hydrophones or pressure detectors. These detect the seismic energy in the form of pressure changes near the oceans' surface. In order to detect particle motion particle velocity sensors must be placed on the seafloor. Seismic waves, at least those containing information about subsurface structure and lithology are described by vectors travelling through a three dimensional solid. Conventional marine exploration using pressure sensors are recording only scalars and thus have an inadequately parameterized problem. Thomsen (1998) posed this rather eloquently in terms of cost/benefit ratio. Historical Footnotes Modern ocean bottom cable owes its current status and configuration to many developments over the last century. Although this paper is limited in scope, the reader is referred to an excellent presentation on the historical aspects of OBC, which was presented by Fred Barr at the 1995 SEG workshop on ocean bottom seismic. This presentation was preserved on videotape and is available for viewing.