南海沙丘地形之水下音傳研究

Abstract

台灣西南方-北南海陸波上，有許多因海洋內波所造成的特殊地形-沙丘。沙丘分布在160公尺至600公尺水深，發現其振幅最大可達16公尺，導致此海域的水下音傳會受到沙丘地形影響，而產生三維擾動效應。本文將使用拋物線方程式(parabolic equation)為基礎的數值模式進行模擬，數值模式可分為二維拋物線方程式(RAM)以及可計算三維拋物線方程式(FOR3D)的模式進行模擬計算，而模擬結果是否正確取決於許多因素，其中初始場佔有很大的影響力。根據過去的模擬結果發現RAM所使用的初使場(self starter)會出現過分初始化的現象而影響模擬結果，且為確保模擬結果正確需找出符合實驗聲源的初始條件，故本文將針對初始場的問題進行探討，並使用合適的初始場模擬2014年於此海域所獲得相關實驗資訊進行聲學模擬比對，分別以二維及三維傳播模式探討，分析南海沙丘對水下音傳所造成的三維擾動效應。

There are many underwater sand dunes caused by the internal wave located in the depth of 160 meters to 600 meters in northern South China Sea. The largest amplitude of these sand dunes is about 16m. Underwater acoustic transmission is affected by this topographic feature, and also has three dimensional effects. In this research the 2-D and 3-D underwater acoustic propagation models (RAM and FOR3D respectively), are used to simulate acoustic propagation in the region. There is starter issue to be resolved for the high-impedance, low-loss ocean bottom of the region. RAM with self-starter is overly initialized, thus the initial field is produced to match with the source beam-pattern. A tunable Gaussian starter is then used to avoid over initialization, which is a proper starter for the moored source. The acoustic field received at VLA emitted from the moored source is simulated with the proper starter. Results show temporal variations with respect to that of water column, and three dimensional effects over sand dunes.