An environmental assessment in the Strait of Sicily: measurement and analysis techniques for determining bottom and oceanographic properties

Abstract

Executive Summary:In October 1997, the En Verse 97 shallow water acoustic experiments were jointly conducted by SACLANT Centre, TNO-FEL and DERA off the coast of Sicily, Italy. The primary goal of the experiments was to test rapid environmental assessment (REA) techniques for shallow water. Most of the oceanographic REA methods discussed here are well developed and emphasis in this report is on determining the sea-bed properties through inversion of measured acoustic data. To develop an operational system, the REA techniques should be capable of identifying sea-bed characteristics over large shallow water area §. In principle, using a towed source measured on receivers at a fixed location, range dependent bottom properties along a selected track could be estimated. The research presented here describes the first steps in developing such a technique. The En Verse 97 sound sources transmitted over a broad-band of frequencies (90-600 Hz) and the signals were measured on a vertical array of hydrophones. The acoustic data were continuously collected as the range between source and receiving array varied from 0.5-6 km. An extensive seismic survey was conducted along the track providing supporting information about the layered structure of the bottom as well as layer sound speeds. The oceanic conditions were assessed using current meters, satellite remote sensing, wave height measurements and casts for determining conductivity and temperature as a function of water depth. Geo-acoustic inversion results taken at different source/receiver ranges show sea-bed properties consistent with the range dependent features observed in the seismic survey data. Some of the issues addressed in this report are: the feasibility of towed sound source geo-acoustic inversions, applying the method to determine range dependent sea-bed properties, and the dependence of the inversions on frequency band and changing water volume. In October 1997, the EnVerse 97 shallow water acoustic experiments were jointly conducted by SACLANT Centre, TNO-FEL and DERA off the coast of Sicily, Italy. The primary goal of the experiments was to determine the seabed properties through inversion of acoustic data. Using a towed source, the inversion method is tested at different source/receiver separations in an area with a range-dependent bottom. The sources transmitted over a broad-band of frequencies (90-600 Hz) and the signals were measured on a vertical array of hydrophones. The acoustic data were continuously collected as the range between source and receiving array varied from 0.5-6 km. An extensive seismic survey was conducted along the track providing supporting information about the layered structure of the bottom as well as layer sound speeds. The oceanic conditions were assessed using current meters, satellite remote sensing, wave height measurements and casts for determining conductivity and temperature as a function of water depth. Geo-acoustic inversion results taken at different source/receiver ranges show sea-bed properties consistent with the ran...

Depth-shifting of shallow water guide source observations

“…[3][4][5] Alternatively, geoacoustic inversion methods can be used to estimate more accurate seabed parameters, which are typically the source of greatest environmental uncertainty. Inversions can be done using either transmissions of known wave forms from calibrated sources 6,7 or from sources of opportunity such as loud merchant ships. 7 Other authors have suggested the use of guide sources for mitigating environmental uncertainty.…”

<title>Tracking targets using matched field observations</title>

“…The community has developed many techniques for data association (i.e., finding which measurement is derived from which target) and for tracking maneuvering targets. [2][3][4] On the other hand, on MFP appears to have focused on signal processing 10,19 : the focus is, not unreasonably, on detecting the target. Treatments of combined tracking/MFP systems are not common, but most concentrate on signal processing, with the idea that a "track" is really a sequence or track-segment of detections that make sense from dynamics considerations.…”

Range-dependent geoacoustic inversion of vertical line array data using matched beam processing

“…4,5 On the contrary, a VLA configuration is relatively stable and capable of receiving sound fields from all directions and can probe large areas around the VLA with a towed source or multiple sources. [6][7][8][9][10][11] Although there is some computational complexity in propagation modeling, VLA configurations are still effective in range-dependent inversion.…”

“…The MF inversion has been successfully applied to estimate bottom characteristics in various shallow water environments. [1][2][3][4][5][6][7][8][9][10][11][12] Actual shallow water environments usually have range variability in seabed properties as well as in bathymetry or ocean sound speed. However, in conventional geoacoustic inversions using a vertical line array ͑VLA͒ and a single source, the geoacoustic properties have been typically assumed to be range independent.…”

“…To prevent such degradation in inversion modeling accuracy, research has been conducted on rangedependent geoacoustic inversion. [4][5][6][7][8][9][10][11][12] Towed horizontal line array ͑HLA͒ schemes are one such range-dependent inversion method. The towed HLA system has several advantages over a VLA, such as mobility and manageability, and provides simplicity of the inversion modeling based on assuming range independency in local regions between the source and receiver.…”