Analysis of Long-Range Transmission Loss in the West Pacific Ocean *

A long-range sound propagation experiment was conducted in the West Pacific Ocean in July 2013. Linear frequency-modulated signals with a frequency band of 260-360 Hz were transmitted by a transducer hung on a floating ship during the experiment and were received by a horizontal line array towed by another ship sailing away from the transducer. The maximum distance between the two ships was 1029 km. Signals were received at the distances 34-220 km, 612-635km and 926-1029km. Transmission loss versus distance between source and receiver was obtained and compared with the theoretical results predicted by the parabolic equation method program RAM. It is shown that RAM is adequate for estimating the transmission loss for distances up to 1029 km. When the water depth is larger than the surface conjugate depth, the ocean bottom rarely influences the transmission loss in the convergence zones. However, in the opposite situation, the ocean bottom contributes significantly to the transmission loss.