Unveiling a new deep water acoustic receiver and deep water acoustic source
Authors: Spychalski Stephen E. 
Published: October 2008

Abstract
High Tech, Inc has developed two new acoustic instruments with applications in the oil and gas industry as well as the naval research communities. The first instrument is a patented low-cost, deep-water acoustic receiver, which is fully operational to 6000 m water depths. The design is based on pressure compensated flexural discs; this results in no change in the sensitivity or frequency response of the hydrophone from the sea surface to 6000 m depth. The second instrument is a broad bandwidth, high output, deep-water acoustic source, which can be used at ocean depths to 6000 m. This source, which has been used successfully for over 20 years, provides highly-repeatable waveforms that are depth independent and span frequencies applicable to geotechnical and eohazard surveys.

Introduction
The U. S. Navy has, for many decades, required acoustic sources and receivers that can operate reliably to full ocean depths (?6000 ?). The U. S. Navy also needed acoustic sensors that would remain operational for long periods of time. Therefore, industry developed a number of acoustic receivers and sources that, although expensive, would meet the Navyâ??s requirements. Therefore, there exists a reservoir of knowledge about the technology required to design and produce deep-water acoustic receivers and transmitters1 that are reliable over periods of years and even decades.
As the oil and gas industry began to operate in deeper waters, the need for acoustic receivers and transmitters that could, operate in ever increasing depths became apparent. Fortunately, companies with experience in meeting the Navy equirements have the technology base to answer the needs of industry. While the cost of sensors has not been a major factor in the Defense Industry, it is a major factor in commercial applications. Therefore, the development of a low-cost hydrophone that is rugged and can operate in waters depths far beyond industries current needs is of great interest to the community. Acoustic sources that produce highly repeatable waveforms are of particular importance to the Navy. Therefore, a great vdeal of research has gone into developing transducers that can operate in deep-water, produce consistent source signatures, and will remain operational over long-periods of time. This knowledge base provided the technical impetus for the deepwater acoustic source. This transducer can be used to generate either continuous wave (CW) or swept frequency source signals depending on the needs of the operator. Moreover, it can do this with no noticeable change in source signature from changes in source depth. These attributes, required for Navy operations, translate well into industry needs for geotechnical/geohazard surveys.

The deep-water hydrophone
Fig. 1 demonstrates the construction of the deep-water hydrophone. The two flexural discs are attached at either end of a rigid cylinder. The cylinder has a pressure compensation orifice, which allows fluid contained in the outer pliable shield to flow (at a measured rate) into the inner cavity to compensate for static pressure changes as the unit descends or ascends within the water column.