Antarctic Exploration Using A Non-Constant Diameter Solid Towed Hydrophone Array
Authors: Pearce R.E. , Marschall R.A. , Stinson D.L. 
Published: October 2008

Recent marine seismic surveys have been conducted in the Antarctic using non-constant diameter solid towed hidrophone arrays. These compact, robust M hydrophone arrays Were well-suited to Antarctic exploration where (1) the Levels of ambient noise could range from Knudsen Sea State 4 down to below Sea State Zero, (2) array repair would be Extraordinarily difficult and (3) oil-filled array leakage would Be particularly damaging to this unique and pristine environment.

INTRODUCTION

In both marine seismic exploration and sonar towed hydrophone arrays, there is an increasing demand fix longer array lengths and smaller array diameters. The trend toward decreasing diameters has been mostly driven by the need to reduce handling and weight problems brought on by longer array lengths and the increased use of multiple arrays. Oil-filled hose towed hydrophone arrays are handicapped by high self-noise levels, cable-oil handling problems, and susceptibility to damage causing environmental pollution by cable-oil spills. To avoid the problems associated with large, bulky oil-filled hose arrays, solid arrays of very small average diameter are being designed and built for geotechnical surveying, geophysical exploration and long range surveillance sonar applications.

ARRAY HYDRODYNAMICS

Since turbulent boundary layer (TBL) noise decays the hyperbolic cosine of the distance between the turbulent boundaries layer and the receiver, decreased array diameters dramatically increase yhis noise component in constant diameter arrays at any given flow velocity. Under optimum conditions, oil-filled hose seismic arrays have a noise floor mostly caused by TBL noise aliasing through grating lobes of the group beam patterns [1]. This TBL â??noise barrierâ?? can be overcome by using an array of non-constant diameter. In contrast to a conventional hydrophone array where all the sensor elements are in the same constant diameter hose, these arrays place each hydrophone within its own hydrodynamic housing, specially shaped to maintain laminar fluid flow along most of its length. Although the shape of the hydrodynamic housing was derived from computational fluid dynamic simulations, the resulting profile resembles the shape of a trout when viewed from above. Hence the name Trout array was coined for towed arrays of this genre. Full implementation of this revolutionary design further calls for the housings or Trout to be of varying flexural rigidity, i.e., relatively hard-headed and soft-tailed, to maximize transverse stability [2]. The hydrodynamic acoustic features of the individual modules are shown in Figure 1. These modules are connected in a sequential array by a cable as thin as possible, consistent with necessary electrical and mechanical requirements. The performance of the arrays are enhanced as the diameter of the electromechanical cable between the modules is decreased. Ceramic cylindrical shell hydrophones, while robust, are heavy, making balancing thin array difficult. Light weight ceramic bender hydrophones are somewhat fragile and not suitable for certain applications likely to be subject to shocks. Piezoelectrically active plastic hydrophones, such as those of polyvinylidene fluoride (PVDF), are both robust and lightweight, using cylhdricsl hydrophones that permit the ekctromachanicsl cable To pass-through them allows for an axially symmetric construction providing high levels of acceleration canceling and immunity to extensional vibrationally-induced noises [3-6].