Study on effects of complex bathymetry on the deep acoustic field in Western Pacific Ocean

Abstract Seafloor bathymetry has an important effect on acoustic propagation. In a deep water experiment in the Western Pacific in 2020, acoustic field in the reliable acoustic path (RAP) zone, the shadow zone, and the refracted zones were measured with shallow sources and deep receivers. Two important phenomena were observed. First, in the first shadow zone, the signal level (S) changing with source range presents a “W” shape: it sharply decreases by 19 dB after the source just pass the RAP boundary and then increases by 11 dB immediately, which produces the first S trough; after remaining stable for a range interval of 29 km, it drops again by 12 dB and then soars by 16 dB, which produces the second S trough. Secondly, the range spans of refracted zones that expected to be 29 km narrows down to only 13 km. Based on the precisely measured bathymetry and the range-dependent acoustic model-parabolic equation (RAM-PE), the S distribution were reproduced and its characteristics were explained. It was found that rather than the large-scale bathymetry change (e.g, large seamount or break shelf), it is the local bathymetry change that have significant effects on the S distribution.