THE EFFECT OF OCEAN FRONTS ON THE SIGNAL PERFORMANCE OF DEEP TOWED HYDROPHONE ARRAYS

The objective of this work is to study the effects of an ocean front on the acoustic performance of a towed hydrophone array where the source is on the cold side of the front and the receiving array is 200-300 nm away on the warm side. The Gulf Stream, typical of a strong ocean front, separates the cold slope waters from the warmer Sargasso Sea water. Sound propagating through this dynamic transition region exhibits different spatial characteristics at any point in the Sargasso Sea from sound not propagated through the front. The differences of interest are those due to the horizontal, range-dependent sound speed gradient presented by the front.;In an experiment at sea sources are placed on either side of the Gulf Stream Front at depths near the local deep sound channel axis. A source S1 located to the northwest of the front propagated 224 Hz continuous wave (CW) sound from the cold side of the front, through the front, and into the warm Sargasso Sea. The frontal location was determined by repeated expendable bathythermograph surveys. The second source S2 (230 Hz, CW) was placed well into the Sargasso Sea 239 km southeast of S1, free from frontal influence. Propagation through the front was at near normal incidence horizontally.;The 46 wavelength receiving array was towed at a depth of 1400 meters at a range of 390 km from S1 and at a depth of 830 meters at a range of 513 km from S1. These two receiver locations afford contrasting measurements of the acoustic field in regimes of different vertical arrival structure. In the deeper case, near the sound channel axis, the same array was suspended vertically to measure the vertical structure resulting from each source location.;Environmental data was obtained during the experiment for use in a deterministic, two-dimensional Corrected Parabolic Equation (CPE) propagation model. The modelling method is tested against measured acoustic results providing theoretical corroboration.;The spatial characteristics of sound arriving at directional, multi-element linear hydrophone array has direct impact upon the performance of that receiver. The Array Signal Gain (ASG) and the 3 dB response beamwidth are each affected. These performance criteria are measured and compared for the separate cases of propagation with and without a front.;Acoutic data recorded at sea is beamformed and studied to identify the statistical behavior of the ASG and 3 dB beam response width using both 20 minute and 5 hour averaging times. From the environmental data and known experiment geometry the measurement cases are computer simulated using the CPE generated field and measured tow parameters such as array depth, range from a given source, and array tilt.;It is shown that the multipath effect in cold to warm through-front propagation produces a measured 0.5 to 1.2 db improvement in the ASG and 12% decrease in the 3 db response beam width for the subject array when compared to propagation entirely on the warm side of the front. This improved array performance is closely duplicated by the CPE model thus verifying the ability of a 2-D model to accurately predict propagation through a strong front. The dominant effect is shown to originate from the vertical arrival structure incident on the tilted towed array and the front is associated with the causitive differences in this arrival angle structure. Further, for the situation of sources near the sound channel axis, the improvements in array performance are more pronounced as the receiving array depth increases toward the sound channel axis on the warm side of the front.