High-Resolution Deepwater Survey Sonar

Abstract

ABSTRACT As survey requirements moved into water depths greater than 2,500 meters, new technology was required to obtain high resolution geophysical data alone precise grid lines. This paper discusses a unique bottom referenced tow system that provides 100k Hz side scan and 3.5/7.O kHz sub bottom sonars; multiplexed over an 8,500 meter single coax tow cable. This design provides a stable towing platform that maintains a constant optimum height over the sea floor which requires minimum cable length 'alterations that affect the tow fish causing imperfections in the data. This system is meant to provide high quality, distortion-free mosiacs with uniform bottom and sub-bottom features recorded at a constant tow speed and height above the sea floor. Because the towing concept is bottom referenced, the topographic features are lost as the tow fish flies over a high relief area at a constant height off the sea floor. With enhanced electronic techniques, combined with a depth sensor in the fish, the topographic detail of the sea floor is recorded on the sub-bottom record; thus, the survey packageprovides the complete high resolution record of the true sea floor and sub-bottom features with resolution and quality never achieved before. Another important facet of this system is positioning of the tow fish. With 8,500 meters of cable out, it is imperative for record correlation to know where the tow fish is. A precise ultra-short baseline portioning system integrated with the tow fish telemeters the tow fish position back to the vessel. The position is then co-r-related to the vessel navigation system to allow position plotting along the survey grid. INTRODUCTION The offshore oil and gas industry is exploring for reservoirs in deep water exceeding 2,500 meters. As survey requirements move into these water depths, a new technology has been necessary to obtain high-resolution geophysical data of the sea floor. Improved techniques have been developed to acquire high-resolution, short-range side-scan sonar (100 kHz) and 3.5 or 7.0 kHz sub-bottom data in deep water environment. Extremely detailed records of the ocean floors geologic features have been gathered that compare to the data obtained in shallow water surveys. Developments include the design of the deep-tow vehicle and associated electronics, special towing requirements, and navigation for precise positioning of deep-tow system. Also, a multiplex system was developed to provide for dataacquisition and control of electronics over long cable lengths.(Figure 1) TECHNICAL REQUIREMENTS AND EQUIPMENT The acquisition of high-resolution data in deep water requires attention to numerous aspects not normally encountered in shallow water surveys. Shallow or surface towed acoustic sources suffer from losses due to signal attenuation and beam spreading. These losses increase in severity with greater water depths.