應用時間反轉聲層析技術進行海流量測：與交互相關法之比較研究

Abstract

本研究旨在實現一套結合時間反轉法與沿岸聲層析之聲納系統。沿岸聲層析是透過基站間的聲波走時，以反推出海水中的溫度場與流場等物理特性。為提升系統對多重路徑干擾的抑制能力，本研究採用最大長度序列訊號，並透過相位調變方式與 17.857 kHz 載波合成後發射，接收端則進行解調與都卜勒匹配濾波器處理，以獲得相關函數並推算傳播時間。此外，本研究應用時間反轉法，利用聲波通道的響應特性，使因反射、繞射與折射等多徑效應所擴散的訊號，能在時間反轉後重新聚焦於原始發射點。 本文建立了一套具備時間反轉發送與接收功能之沿岸聲層析聲納系統，整合訊號記錄、反轉處理與再傳輸配置。為驗證時間反轉法推算傳播時間差的可行性，設計了基站間正反向同步傳播實驗，並以傳統交叉相關法所得時間差為參考，與時間反轉法的結果進行誤差比對與準確性評估。然而，聲波在水中傳播常受海底反射、繞射與地形起伏等多重路徑效應影響，使訊號經由不同路徑抵達接收端，導致中心頻率偏移與波形失真，影響走時估算精度。特別是當系統部署於移動載具與固定基站之間時，相對運動產生的都卜勒效應將加劇干擾，進一步影響濾波結果之可靠性。為此，本研究設計兩項實海域實驗：其一為固定基站實驗，用以驗證時間反轉法於靜態條件下的穩定性，其二為移動載具與固定基站實驗，針對載具運動所產生的都卜勒效應進行補償與驗證。兩項實海域實驗驗證時間反轉推算傳播時間差的結果符合預期，時間反轉技術可作為交叉相關法的替代方案，有效簡化其在傳播時間差估算上的處理流程。為未來海洋環境監測提供可靠的技術基礎。

This study develops a sonar system integrating time reversal and coastal acoustic tomography. Estimates ocean temperature and current fields by measuring acoustic travel times between stations. To mitigate multipath interference, a maximum length sequence modulated onto a 17.857 kHz carrier is transmitted. The receiver applies demodulation and Doppler matched filtering to obtain correlation functions and estimate travel times. Additionally, this study applies time reversal to exploit the channel’s impulse response, allowing signals scattered by reflection, diffraction, and refraction to refocus at the original source location after time reversal processing, thus improving resolution and focusing capability. A sonar system with time reversal transmission and reception is implemented, combining signal recording, reversal processing, and retransmission. To validate time reversal for estimating travel time differences, bidirectional transmission experiments are conducted and compared with results from conventional cross-correlation. Multipath effects and Doppler effects from relative motion between a moving vehicle and a fixed station can degrade estimation accuracy. Therefore, two sea experiments are performed: a fixed station test for stability and a moving vehicle test for validating Doppler compensation. The results demonstrate that the time-reversal technique yields accurate travel time estimates. As an effective alternative to the conventional cross-correlation method, it significantly simplifies the processing required for differential travel time estimation, thereby offering a reliable and efficient foundation for future ocean monitoring applications.