Directions in ocean-bottom surveying

Abstract

The benefits of data acquired at the ocean bottom have repeatedly been demonstrated in various areas and under a wide range of conditions. Significant successes have for instance been booked over obstructed fields, in structurally complex areas, and in areas with shallow gas charging. Other promising fields of application, such as direct hydrocarbon indication (DHI) from combined S- and P- information, are maturing. The added value in these applications stems partly from the more favorable geometries of ocean-bottom system (OBS) surveying, such as wider ranges of offset and azimuth, and partly from the ability to record true vector fields of both P- and S-waves. In spite of the advantages, the share of OBS data in the seismic market has remained relatively small. Apart from the successes in areas with shallow gas charging, the potential of S-wave data has been left largely unexploited. The majority of the recent OBS campaigns aimed at the acquisition of superior P-wave data. Key obstacles to the more general application of OBS are the high cost of acquisition and the suboptimal quality of the data, particularly of the shear-wave component. Optimal data quality can be warranted by planting the sensors onto the seabed, so as to establish firm coupling to the subsurface. Unfortunately, planting of ocean-bottom sensors is currently time-consuming, and increases the survey duration and costs. There is thus a need for systems and methods that offer both high efficiency and consistent data quality. This paper first reviews the historic developments of OBS, follows with an analysis of the relevant quality and efficiency issues, and finishes with possible directions and solutions.