本論文之研究目的在於探討海面所引起之環境噪聲在具不均勻海床聲道中之傳播。在傳統海洋波導環境中之模擬上，大多假設海床聲學性質，包括密度與聲速分佈均為常數。然而從文獻中顯示~(如~Hamilton~cite{Hamilton})~，海床的聲學性質乃呈現不均勻漸增變化，因此，為了儘可能真實的模擬海床之聲學性質，本論文乃以不均勻海床聲道對於海面所引起之環境噪聲之影響為探討的主題。

為了探討環境噪聲的模式，本論文的研究方法乃依據~Kuperman~及~Ingenito~cite{KWFI}~所發展的環境噪聲模式，並考量海床不均勻漸增的聲學性質變化。為了求解聲學變化性質，吾人利用~Robins~cite{Robin1993}~的模式來加以分析。~Kuperman~及~Ingenito~所發展的噪聲模式是計算在分層的海洋環境中表面噪聲之空間特性，其假設聲源為隨機且連續，分佈於接近海面的無窮平面上之單點聲源，唯該模式並未探討沈積層裡的真實變化情形，而~Robins~的探討重點在於考量兼具密度及聲速變化之沈積層上，平面波入射所造成的反射。本研究之理論基礎即結合上述兩模式，藉以探討不均勻海床聲道對於海面所引起之環境噪聲之影響。

本研究結果顯示，聲速呈不均勻漸增變化的海床底質，由於其聲速緩慢的變化，因此降低了界面的阻抗，以致其反射係數遠較聲速呈常數變化的劇烈震盪緩和許多。在波導環境中，海床不均勻變化對波數譜~(wavenumber spectrum)~所造成的影響主要在於連續譜的範圍內。整體而言，波數譜受制於波導環境系統整體變化的影響，因此難以對單一模態作個別分析。

另一方面，由噪聲強度的結果可以看出，海面聲源所產生之噪聲，並不因海床聲學性質不均勻變化有而顯著的改變，主要的原因是本文的操作頻率均為低頻，因此海床聲學性質的改變並不會造成很大的差異性。從水平相關性的結果可以得知，噪聲場的相關長度，隨噪聲源的相關長度增加而增加；垂直相關則不一定，噪聲場的垂直相關長度，隨噪聲源的相關長度增加而無明顯的變化。

In the traditional analysis of acoustic wave propagation in an ocean waveguide, it's generally assumed that acoustic properties, including density and sound speed profile at seabed are taken to be constant. However, recent experimental data provided by Hamilton~(1980)~ have shown that the sediment layer in the seabed experiences a transitional change in which the density and the sound speed vary continuously from one value at the top to another at the bottom of the layer. The objective of this study is to investigate the surface-generated ambient noise in an isovelocity waveguide with a non-homogeneous fluid sediment layer.

The noise model was first proposed by Kuperman and Ingenito~(1980) in the study of surface-generated ambient noise using normal mode approach, and the model proposed by Robins (1993) in the study of the sediment layer change in which the density and the sound speed vary continuously. It is demonstrated that the noise intensity may be affected by the stratification mainly through the continuous spectrum, in that the continuous spectrum is equally important as the normal modes in the present analysis.

The continuous variation of the sediment layer reduces the contrast of the interface, which in turn affects the wavenumber spectrum, particularly in the continuous spectrum region. The results show that the horizontal correlation length of the noise field increases as that of the noise random sourse increase, but the vertical correlation length of the noise field decreases as that of the noise random sourse increase.

第一章 引言 1
1.1 研究動機與目的‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥1
1.2 文獻回顧‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥3
1.3 研究方法‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥4

第二章 理論 6
2.1 簡介‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥6
2.2 聲波方程式‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥6
2.2.1 水層中之聲波方程式‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥6
2.2.2 沈積層的聲波方程式‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥8
2.3 聲波方程式的解‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥9
2.3.1 水層中聲波方程式的解‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥9
2.3.2 沈積層內聲波方程式的解‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥11
2.4 噪聲場所產生的空間相關函數‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥14

第三章 聲波在沈積層上的反射 17
3.1 各層之波數積分表示式‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥17
3.2 線性系統‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥19
3.3 結果與討論‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥20

第四章 環境噪聲在具不均勻海床聲道中之傳播 29
4.1 修改後的Pekeris waveguide‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥29
4.1.1 推導過程‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥31
4.1.2 波數譜分析‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥32
4.1.3 與OASES的環境模擬比對‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥35
4.2 真實狀況的波導環境‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥39
4.2.1 線性系統‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥39
4.2.2 波數譜分析‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥40
4.3 噪聲場強度分析‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥45
4.3.1 噪音強度‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥48
4.3.2 空間相關性‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥49

第五章 結論與建議 54
5.1 結論‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥54
5.2 建議‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥‥55

附錄A 式(2.51)~ 式(2.55)的推導過程 58

附錄B Hankel function的積分式 60

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