本論文主要的研究目的在於探討點聲源入射不均勻沉積層與彈性底床之反射機制。延續自Robins有關平面聲波入射平滑海床反射問題之研究，將沉積層中密度視為廣義指數函數變化，聲速為三種不同連續型態變化，分別為常數、$k^2$-linear、inverse-square 變化，並且考慮底層彈性效應，最後將平面聲波改為全向性點聲源，探討海床上之反射機制。其中，根據Hamilton先前的研究指出，最符合實際海床性質的為廣義指數密度函數搭配inverse-square聲速變化，其餘兩種聲速變化也各有其應用環境，因此本研究除探討各種分佈之特性外，會將重點放在最符合實際海洋環境的模型上。而本研究之前期研究已在第五屆水下技術研討會發表，主要探討為沉積層中密度對點聲源反射場之影響。

本研究中，聲源採用全向性點聲源，而此聲源波譜中包含所有波數域，平面聲波僅為眾多波數域中一分量，故點聲源將不同於平面聲波，其反射場將有更多干擾現象。而反射場之強弱與變化亦將會受制於不同參數，如聲源頻、底層阻抗、密度及聲速變化梯度等。本研究將經由海床與底層之邊界條件，建構出一套線性系統，進而求解點聲源反射聲場之解析解，並改變聲場中各種參變數，探討其對反射場的影響。本研究所得之結果將利用OASES加以驗，並得知沉積層的不均勻性質僅對反射場當中某一區域有所影響，因此該區域成為地聲性質判讀的重要依據。至於底層彈性效應能否激發，則必須視頻率與沉積層厚度搭配而定。而這些結果將可提供海床聲學一套更完整、更符合真實環境之研究模式。

本研究除了求解海床性質之解析解，探討各種參變數對海床上反射聲場的影響外，更期望能藉由本研究的探討，建立與實際海上作業接近的模型，並找出影響反射聲場較為關鍵的條件，進而將本研究成果應用於地聲反算等問題上。

Acoustic point-source reflection from an unperturbed seabed with a continuously varying density and sound speed profile in a fluid-like sediment layer overlying an elastic basement is considered in this paper. This fundamental problem has illuminated many interesting phenomena about the effects of sediment stratification on acoustic wave interactions with the seabeds.

The analysis employs the analytical solutions for the Helmholtz equation in the sediment layer, in which the density and the sound speed are, respectively, described by three kinds of geological models. The results have demonstrated that the effects of sediment stratifacation are confined in a region in range, where the reflection field is dominated by the reflected ray bundles containing ray components with the grazing angles which are neither too steep nor too shallow.

The effects of many other factors, such as frequency, sediment thickness, basement elasticity, on the reflection fields are investigated, in particular, the interface wave component may become important in the region close to the seabed, if the sediment thickness is relatively thin in comparison with the acoustic wave length.

第一章 緒論
1.1 研究主題與研究動機
1.2 文獻回顧
1.3 研究方法
1.4 論文範疇
第二章 理論模式
2.1 簡介
2.2 聲波方程式
2.3 聲波方程式之解析解
2.4 邊界條件
2.5 結語
第三章 基準驗證
3.1 OASES聲學模組簡介
3.2 不同層化性質之結果驗證
3.3 底層彈性體之結果驗證
3.4 結語
第四章 反射場分析
4.1 各項參數影響性分析
4.2 底層彈性效應分析
4.3 結語
第五章 結論與建議
5.1 結論
5.2 建議
附錄A 平面聲波自具有彈性底層與連續層化沉積層上粗糙海床反射問題之研究
附錄B 點聲源自非均勻海床上反射問題之研究 (I)

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