The Digital Telemetry Implementation of the Dual Sensor, Ocean Bottom Cable Seismic System

Abstract

ABSTRACT The ocean bottom cable method of seismic reflection data acquisition has long been used in shallow water and in areas densely populated by obstacles, where towed streamer marine vessel access is rendered risky or impossible (Rigsby, et al, 1987). As production and drilling platform obstacles have moved to deeper and deeper water, the application of this method has moved with them. A seismic recording technique utilizing the Dual Sensorsm method (Barr, et al, 1989) became required to achieve temporal resolution. This method demands doubling of conventional channel capacity. A distributed digital telemetry system provides the solution. The implementation of the initial production prototype of that system is described. INTRODUCTION The location of the sensors on the ocean bottom causes each recorded reflection wavelet to be followed by a series of reverberations which are generated by the water column. As the water depth increases, the reverberation period increases proportionately, attended by a significant reduction in the ability of deconvolution methods to remove the reverberations during processing. In the frequency domain, this phenomenon manifests itself as peaks and troughs which are introduced into the amplitude spectrum. To eliminate this reverberation problem during the data acquisition and preprocessing stage, the Dual Sensor method was implemented. This method places a gimbal geophone unit with each hydrophone in the recording cable which is laying on the ocean floor. The signals from the geophone and hydrophone arrays are recorded on separate channels and they are combined during the initial stage of data processing. Prior to retrieving the ocean bottom cable for redeployment, the shooting vessel performs an additional pass. Firing the source directly over each set of geophone/hydrophone arrays creates an artificial "ghost". The resulting data provides the information necessary to combine the geophone and hydrophone signals optimally at each receiver location to cancel reverberation energy in the production reflection data. Since the introduction of the method, several opportunities have arisen to compare data recorded by this technique to conventional recording using a towed streamer (Barr, et al, 1990). In each case, the Dual Sensor data has proven to be of equal or higher quality. THE RECORDING SYSTEM The most recent enhancement of the Dual Sensor, ocean bottom cable system has been the development and deployment of a telemetry ocean bottom cable recording system. It consists of a new version of the MDS-18A recording system with line equipment completely redesigned for operation under water. The eight-channel remote units have been reshaped to a cylindrical steel housing with ocean bottom cable connectors at each end. This housing is designed for operation at depths up to 200 meters. The remote units are powered from a 200 VDC power source through conductors in the ocean bottom cable. The cable is approximately 2.54 centimeters in diameter and contains data and control transmission pairs, seismic signal pairs, power conductors and strength members.