Community Research and Development Information Service - CORDIS

Development and realization of a hydrophone streamer and positioning system

The hydrophone streamer itself has been designed and manufactured at the beginning of the project. Electronic equipment (amplifiers, filters, 48 channels) was required for processing the signals coming from the individual hydrophones before storage on-board the vessel. This equipment has been designed and manufactured in time for the 1st, 2nd and 3rd sea-trials. The positioning system is composed, on the one hand, of 3 transmitters (acoustical base, and the associated electronics) installed on the back of the tow-fish, and, on the other hand, of hardware and software required for processing the acoustical positioning signals. This equipment has been designed and manufactured in time for the 1st, 2nd and 3rd sea-trials.

A. Geometry:

1) Generalities: The geometry (structure and spacing of the hydrophones) has been defined in taking into account the various parameters of both the SPA (Steerable Parametric Array) and the water depths considered for the experiments. The array is composed of 3 parts with different spatial sampling:

i) Front part (16 m long) includes 9 hydrophones separated by 2 meters.

ii) Middle part (16 m long) includes 32 hydrophones separated by 0.5 meter.

iii) End part is identical to front part. This structure leads to a 48 meters array including 49 hydrophones. The 49-hydrophone signals will be digitised and stored on-board during the experiments.

2) Constant array offset thanks to SPA steering: The hydrophone array should be towed by the fish with variable offsets, but it is possible to maintain the offset constant by using the steering availability of the SPA. This steering will allow to make convergence of the bottom echoes onto the middle part of the hydrophone array whatever water depth.

3) Variation of grazing angle: In case of requirement of variation of grazing angle, front part and end part of the hydrophone array will be available for collecting echoes.

B. Acoustical data:

1) Characteristics of the hydrophones: The 49 hydrophones are identical:
-Wide bandwidth frequency response (10 Hz-40 kHz).
-Receiving sensitivity-171 dB (re 1V/Pa) with 20 dB preamplifier.
-Maximum working depth 300m. The 49 individual signals will be digitised and stored on board.

2) Obtaining a sampled array: The spatial sampling is optimised to provide a middle part being a properly sampled receiving array up to 1.5 kHz (wavelength about 1m, Nyquist criterion). The secondary frequencies of the SPA should be 3-6 kHz, so the middle part is not correctly sampled for this bandwidth. Nevertheless, some calculations have shown that ambiguous lobes will appear at 90 degrees (at 3 kHz) and at 30 and 90 degrees (at 6 kHz). The influence of these grating lobes should be reduced-using geometrical considerations- thanks to the SPA directivity; and by means of a time windowing processing.

C. Positioning data:

The positioning of the hydrophone array relative to the tow-fish will be achieved in real time on-board through a hybrid measurement:- short baseline acoustical positioning; depth measurement. The short baseline will be composed of 4 transmitters mounted on the tow-fish tail and several receivers which will be actually some of those of the hydrophone array. Several hydrophones will be used for both positioning and acoustical data. These hydrophones are not fixed in order to adjust their number and their position according to the requirements of navigation accuracy and real-time processing power. The position of each navigation hydrophone (relative to the transmitters on the tow-fish tail) will be processed using matched filtering and correlation. Furthermore, two depth sensors will be added to the end and front part of the hydrophone array. The depth information will be integrated into the hydrophone array positioning software. This should allow cancelling possible ambiguities.

D. Streamer array positioning:

The transmission/reception system used for the tests was the same than those used at sea. The transmission system is composed of 3 signals generators, followed by 3 linear power amplifiers. The source level has been decreased compared to sea trials, and is about 177 dB for each transmitter. The reception is achieved through a matched filtering stage (which can be bypassed), filters and amplifiers, and the echoes are digitised on a PC board. In addition to standard low frequency hydrophones, the array included high frequency hydrophones, which have been be used for acoustical positioning of the array. A special purpose device was developed which included two transmitters fixed on the ship and which transmitted suitable signals for the array positioning. The received high frequency signals have been processed in real time in order to provide with the angular displacement of the array in a horizontal plane. In addition to this device, several depth sensors provided with the immersion of the positioning hydrophones. The total system provided with an accurate estimation of the array distortions.

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French Naval Academy
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