淺海環境中流速對高頻聲波傳播之研究

Abstract

本研究發展一考慮流動介質之聲線追蹤程式 (Current Ray-Tracing program) 以探討海流對高頻聲波傳播的影響。為符合高準確度之計算要求，本研究使用可變步階的高階 Runge-Kutta 演算法求解聲線微分方程。利用 Current Ray-Tracing 程式進行數值模擬研究探討等速流或等流切環境下之聲線傳播：結果顯示，在等速流的環境，聲線特性的擾動與流速有直接的關係。走時的擾動量主要受到沿聲線平均速度擾動量的影響，而走時的擾動量隨著角度的變化則受到聲線長度擾動量的影響。流速所造成之擾動量之數值並不大。在等流切環境，聲線會向流速較小的地方偏折。聲線偏折之垂直分量隨距離的變化與流切成反比。較平緩的聲線受到流切的影響較大。當流切大於聲速梯度，將對聲線路徑有顯著的影響。在地形隨距離呈小尺度變化的聲傳環境中，蒙地卡羅模擬顯示在較大的流速情形或較陡峭的聲線，其聲線之不可預測性較高。另，以等效聲速法模擬所造成的走時誤差主要來自與聲線長度與沿聲線平均速度的誤差。若使用 ESSP 法（將流速投影到波陣面法線方向上），其走時的誤差將降低至聲線長度的誤差。研究結果顯示，於具小尺度地形特徵之波導環境，高角度聲線之遠距離聲傳應使用正確的聲線追蹤方式進行模擬。

A ray-tracing program for a moving medium (Current Ray-Tracing program) is developed to study the effects of ocean current on high-frequency acoustic propagation. A higher-order Runge-Kutta integration algorithm with adjustable step size is implemented for high accuracy. Numerical simulations for the waveguide of constant current velocity or constant current shear agree well with the analytic solutions. For the case of constant current velocity, the average along-ray speed perturbation is the dominant factor in perturbed travel time. The angular variation of perturbed travel time is due to the perturbed ray length. The magnitude of all perturbed ray properties are very small. For the case of constant current shear, the range variation of vertical component of wavefront normal is inversely proportional to current shear and has less dependence of the wavefront normal angle. The gradual rays are more sensitive to the current shear. For the range-dependent waveguide of deterministic small-scale bathymetric structure, Monte Carlo simulations show that the small bottom slopes have a dramatic effect on ray paths: larger predictive uncertainty is observed for steeper rays or for the case of larger current magnitude. Modeling error of effective sound speed approaches on predicting travel time comes from errors in both ray length and average along-ray speed. When using the ESSP approach (projected current on the sound speed) the dominant error is the ray length prediction. The importance of correct inclusion of ocean current in the ray-tracing program for long-range acoustic propagation is shown using a realistic ocean environment with deterministic irregular bottom topography.