Measurement of Sound Propagation, Down Slope to a Bottom-Limited Sound Channel

Signal transmission loss and spatial coherence data for source-receiver separations between 100 and 250 km were acquired in the Gulf of Mexico with a calibrated seismic measurement system (400 m deep), a towed projector (100 m deep) which emitted 67 and 173 Hz tones, and a moored Webb sound source at 988 m depth driven at 175 Hz. Environmental data such as the range dependent bathymetry and sound velocity profiles were measured. The 67 Hz data showed a persistent sound transmission loss of approximately 90 dB whereas the 173 Hz showed several pronounced loss minima between 100-90 dB. Slope enhancements were found to be on the order 2-4 dB @ 67 Hz and 6 dB 0 173 Hz when compared to flat bottom calculations. Pair-wise coherence data show the combined effects of multipath interference and signal-to-noise ratio. Estimates of signal coherence length from the coherent summation of streamer hydrophones yield coherence lengths between 70 and 300 meters at a frequency of 173 Hz. Fast asymptotic coherent and normal mode transmission loss calculations produced results consistent with measured data for the deep flat portion of the measurement tracks when measured geo-acoustic profiles or related bottom loss curves w.re used. The implicit finite difference parabolic equation calculations were consistent with range-averaged data for the flat portion of the track as well as on the slope. These results show that if proper qualitative description of the sub-bottom velocity profiles are used, then computations with either a parabolic equation or normal mode technique are consistent with experimental results.