High-Resolution, Deep-Tow Seismic Systems for Geotechnical/Geohazard Studies in Deep Water
Authors: Gettrust J.F. , Grossweiler Phil , Wood W.T. 
Published: October 2008

A high-resolution multichannel seismic system developed to study marine sediments in deep ocean basins has been adapted to study marine sediments in coastal waters. This technology allows investigators to obtain detailed imagery and compressional velocity estimates of the upper 1 km of sediments and is applicable to geohazard and geotechnical issues facing industry as they begin to work in deeper waters. An extension of this technology is proposed to directly generate shear waves in deep water; this application would further enhance our ability to resolve important parameters associated with sediment strength and fluid migration

Introduction

In the early 1980's the U. S. Navy developed technology to support the study of marine sediment properties in the deep ocean1. This technology was needed to provide detailed information about the geoacoustic properties that impacted the performance of Navy ASW systems. The instrument that was developed is an adaptation of conventional multichannel seismic technology; the source-frequency band was raised to resolve structure and physical properties within the upper 1 km of sediments. To achieve the geometry necessary to investigate sediments in areas where water-depths of interest range to over 5 km, a deep-tow system was required. The original system was lost at sea, but was replaced in 2002 with an upgraded system designed to overcome limitations with the original Deep Towed Acoustics/ Geophysics System (DTAGS).

Instrumentation

The Naval Research Laboratory (NRL) has developed a multichannel seismic system that can be used to study the detailed structure of the upper 1 km of sediments with no dependency on water depth. This system is designed around a Helmholtz resonator acoustic projector. This seismic source (Fig. 1) provides a source-waveform that is independent of tow depth (to ?6 km). Currently, the source produces a linear 250 ms swept-frequency waveform from 220Hz to 1kHz with a Sound Pressure Level (SPL) of 201 dB / 1 µPa @ 1 m. Not only can the source operate at any ocean depth, the source has proven to be highly repeatable; a single sample of the source waveform can be used to deconvolve the data for an entire experiment with negligible degradation in resulting wavelet. Another advantage of the deep-tow source configuration is that the source level can be significantly lower than a surfacetow source as spreading losses are significantly reduced.

Recently, concern about the impact of acoustic signals on mammals has caused the Navy to limit SPLs to 195 dB / 1 µPa @ 1 m; to accommodate this limitation while retaining performance, the system has recently been modified to support a 500 ms sweep at the lower SPL level.

While the Helmholtz resonator source has proven to be quite reliable, our experience with the original oil-filled (analog) hydrophone streamer was not encouraging. The pressure differentials along the array encountered during deployment and retrieval often resulted in saltwater intrusion. This commonly caused array failure after only two or three deployments.