Acoustic Tracking of Towed Bodies

Abstract

Listen

Translate article

I. ABSTRACT Results of tests are presented that show that a simple mechanically fixed mount ultra short-baseline transducer array can be successfully used to acoustically track deep and near-surface towed bodies. An in-water calibration procedure that reduces errors due to transducer array assembly system installation and sound velocity variations is also discussed. II. INTRODUCTION The use of ultra-short baseline acoustic systems is common in survey applications for tracking towed bodies. These applications include tracking of deep-towed seismic towfish and near-surface seismic streamers and airgun floats. A common requirement of these applications is accurate determination of a towed body position in geographical coordinates. This paper describes the results of two tests of an ultra-short baseline acoustic system featuring a fixed-mount transducer array1. The transducer array features wide-beam elements that constantly monitor the entire 360 degrees of the acoustic field. The array is most sensitive in the downward direction but it has sufficient sensitivity at near horizontal angles to perform well in towed body applications. The array's elements are arranged in a perpendicular pattern that enables the system to calculate the three dimensional position of an acoustic transponder. Near-surface tows were conducted in Puget Sound, Washington and deep tows were conducted in Hogs Fjord, Norway. Accuracy and range performance during these tests is presented and analyzed in this paper. This paper also describes the steps taken to achieve best accuracy. An in-water calibration procedure that will reduce errors due to transducer array assembly, system installation, and sound velocity variations is also presented. III. NEAR-SURFACE TESTS Near-surface tow tests were conducted in Puget Sound, Washington. The transponders used 22 and 24 kHz pulses of 1.33 ms duration. The source level was 190 dB//1 microPascal at 1meter. A towed beacon was tracked out to a maximum range of 365 meters at a depth of 13 meters in sea state 1 to 2 conditions. With the beacon lowered to 60 meters, a range of 1500 meters was achieved. Tow speeds ranged from 4 to 6 knots. Tilting the transducer array up 25 degrees so that its sensitive axis would point more closely at the transponder did not increase the achievable range. This result agrees with the general rule that in near horizontal acoustic transmission the achievable range is limited by environmental conditions (usually acoustic ray bending due to thermal variation). This environmental constraint was observed again when, in sea state 5 to 6 conditions, the transponder had to be lowered to 36 meters to achieve the 365 meter range achieved at 13 meter depth in calmer seas.