Ocean Bottom Scanning Sonar - A New Tool For Underwater Reconnaissance
Authors: Ransone M.A. , Fischer R.C. 
Published: December 2007

Determining the extent of damage of offshore drilling platforms is a major concern of petroleum companies. The development of the "Ocean Bottom Scanning Sonar" (OBSS) by the Westinghouse Underseas Division has brought about a system which lends itself favorably to a new acoustic survey technique. This system has proven its value in several recent survey efforts. In one, a downed aircraft in Lake Michigan was located. Another was a damage appraisal to two drilling platforms off the Louisiana Coast. This paper discusses the acoustic device design. Its specifications, and performance in actual survey efforts. The basic system philosophy is to insonify a narrow strip of ocean bottom and to record the reflected acoustic energy on a precision graphic recorder. Records produced by this device give knowledge of the bottom contours and an object's size and shape. Resolution obtained is In the order of 1 ft with a range of 400 ft. and 6 ft with a range of 2500 ft. Acoustic energy is generated and received by electronics carried in an underwater vehicle. The vehicle is towed at 2 to 5 kt 30 ft off the bottom for short-range. high-resolution search, and at 180 ft off the bottom for long-range search efforts. The equipment is operated by three men and is easily installed aboard a surveying boat that has adequate stern space, At present Western Geophysical is planning to search for possible oil fields using OBSS as a part of their survey services to various oil companies.

Introduction

Underwater objects have generally been observed by high-resolution optical methods, low-resolution sonic methods. and the human eye. This latter method, while the best system from many points of view, has such limitations that generally it is reserved for very specific, short-term tasks. if it can be used at all. Ocean Bottom Scanning Sonar, a precision, side-looking, narrow beam, sonar, fills in a large part of the gap between high resolution optical sensors and conventional precision echo founders. This gap, both in resolution and range. has been narrowed to the point that objects may be observed and inspected acoustically with resolutions of less than 1 ft at ranges of several hundred feet. with only a slight penalty in resolution, this mapping width can be widened to 1250 ft on a side. Resolution is 1 ft for a mapping width of 400 ft and 6 ft for a mapping width of 2500 ft. The equipment mobility and installation ease makes the OBSS an excellent tool for surveying the ocean bottom.

STATEMENT OF THEORY AND DEFINITIONS

The Ocean Bottom Scanning Sonar technique is to insonify a portion of the bottom with pulsed acoustic energy and to display a voltage proportional to the reflecting strength of the bottom. The smaller the area insonified by one pulse, the more useful the device becomes to detect small bottom irregularities and small objects lying on the bottom.

The principles behind the OBSS system became functional when a method was obtained to focus acoustic energy along a narrow strip of ocean bottom. Thus, when the projector transducer is pulsed, acoustic energy impinges on the ocean bottom and is reflected back to a receiving transducer, or hydrophone, from a narrow strip of the ocean bottom. The reflected energy (called reverberation) is a series of acoustic pulses whose amplitude and spacing are dependent upon the roughness of the bottom. The reverberation duration or she system range is a function of the vertical beamwidth and the lowest level of reflected energy that can be sensed by the hydrophone. The transducer design is a state-of-the-art technique for producing focused acoustic energy on a narrow strip of ocean bottom, and is a proprietary design of Westinghouse.

DESCRIPTION OF EQUIPMENT AND PROCESSES USED

The transducer must be moved with respect to the bottom to insonify successive bottom strips.