Wave Measurements Using Upward - Looking Sonar for Continental Shelf Applications

This paper was selected for presentation by the OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference or its officers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Abstract An upward looking sonar instrument, the Ice Profiling Sonar (IPS), has been developed, and successfully used for obtaining time series measurements of ice keel depths over the continental shelves of the Arctic in support of scientific research. Since the winter of 1996/97, it has been extensively used off the coast of Sakhalin Island, Russian Federation, in support of offshore oil and gas research aimed at characterizing the seasonal sea-ice regime. The IPS instrument has now been modified to extend its capabilities to provide accurate measurement of ocean waves. The instrument uses a high frequency acoustic transducer (420 kHz). It has a very narrow conical beam (2 degrees width at-3 dB) to minimize the spatial smoothing of surface waves across the sonar footprint. With reduced power consumption, and an expanded internal storage capacity of 64 Mbytes (flash EPROM), the new instrument is capable of continuous measurements of wave amplitude at a sampling rate of 1 Hz over deployments of up to nine months duration. During March to April 1998, an evaluation of the performance of the modified IPS instrument, through intercomparison with a wave-rider buoy, was conducted in open ocean conditions off the Pacific west coast. Instantaneous wave heights of up to 11.5 m were measured. The results indicate good agreement between the measurements obtained from the IPS instrument and those measured using the wave-rider buoy. The subsea IPS instrument is very well suited to wave measurement in areas where sea ice or shipping are hazards to surface installations. Introduction Measurement of waves at continental shelf depths has been addressed with a variety of ocean instrumentation (Stewart 1). For some applications, such as where information …