Deep-Towed Seismic System Design For Operation At Depths Up To 6000m
Authors: Fagot Martin G. , Moss George J. , Milburn Darrell A. , Gholson Norman H. 
Published: October 2008

The deep-towed seismic system is a measurement tool being developed for marine scientists interested in the geological, geophysical, and geoacoustic properties of the deep ocean (6000 m) sea floor and its upper subbottom structure (500 m). Based on acoustic performance predictions, it was found that the deep-towed system design concept will meet the scientific requirements. Analytical techniques described in this paper are used to compare themeasurement performance of a deep-towed seismic system with one towed near the water surface, in deep water. It was found that towing the system near the sea floor produces significant advantages in geoacoustical parameter determination and spatial resolution.

Design and analysis studies have also beenperformed to determine hardware requirements for an experimental system which will be used to test and eva1~ate a newly developed Helmholtz resonator sound source. The at-sea test planned for the experimental system will also provide information necessary for the design of a final system. In this paper, hardware and the major parameters affecting both the experimental and final system designs are reviewed. Also, the detailed characteristics of the experimental system and the general projected characteristics of the final system are discussed.

Introduction

The need to improve fleet system performance prediction has increased with the advent of more sophisticated Navy sonar systems. Definitive models of the ocean floor and subbottom as transmission media which refract, diffract, and dissipate, as well as reflect, acoustic energy are critical to this need. A multi-channel streamer and low frequency sound source system, as depicted in Figure 1, towed near the bottom in the deep ocean, can provide the high resolution geoacoustic parameter inputs required for these models. 1

The system concept, towed streamer and low frequency sound source, is similar to that employed by the marine oil exploration community with one primary, and distinct, exception. The deep-towedsystem will operate a few hundred meters off the bottom in ocean depths up to 6000 m; by contrast, oil exploration systems operate near the ocean surface in relatively shallow water. In addition, the deep-towed system will be designed to precisely measure the upper 500 meters of subbottom structure. The oil exploration systems are designed to penetrate many thousands of meters, at the expense of resolution in the upper sediment section.

Performance prediction studies for the deeptowed system show a significant improvement in geoacoustic parameter determination and spatial resolution over conventional surface-towed systems operating in this deep-water environment. The portion of the studies presented here deals with the measurement of the interval velocity of thinlayers and the reduction of the acoustic footprint. A review of these two areas will show the substantialimprovement that can be obtained with a deeptowed system.

The system hardware development is employing a technical approach formulated to minimize risk.