Ocean bottom seismic instrumentation

Abstract

During the past several years the quality of seismic data obtained in the deep oceans has improved markedly due to the development of better instrument packages and recording systems. These improvements take advantage of improved understanding of the seismic coupling problem, the effects of ocean currents, and Stoneley waves. The coupling problem arises from the inability of an ocean bottom seismometer (OBS) to move with the ocean bottom in response to seismic waves when the bottom is soft. The transfer function between the motion of the bottom and the OBS is good at very low frequencies and poor at high frequencies, the corner depending on the softness of the bottom and the mechanical characteristics of the OBS. The buoyancy of the OBS also has a pronounced effect on the response. These coupling effects are not observed on hydrophones. Several solutions to the above problems are being proposed and tested. First, since many of the problems with collecting high quality seismic data on the ocean floor are related to the ocean bottom interface, one solution is to remove the sensors from the interface and place them in drill holes under the ocean floor. This operation is considerably more expensive than emplacing OBS's but preliminary results suggest that the increase in data quality is significant. Solutions to the coupling problem include sensor burial, increasing the surface area in contact with the ocean bottom, and making sensor packages neutrally buoyant in the sediments. Current noise problems are greatly improved by decreasing the OBS cross section to the water. Stoneley wave generated noise is truly seismic and can be discriminated against by using multiple sensors or hydrophones that are much less sensitive to these signals.