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http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/63929
標題: | 水聲層析法逆推流速場之研究 A Study of Current Field Inversion Using Ocean Acoustic Tomography |
作者: | Yen-Hsiang Chen 陳彥翔 |
指導教授: | 黃千芬 |
關鍵字: | 水聲層析法,傳遞走時差,流速估算,線性逆推方法,測站位置分佈,數值模擬, ocean acoustic tomography,differential travel time,current estimation,linear inverse theory,array geometry,numerical simulations, |
出版年 : | 2012 |
學位: | 碩士 |
摘要: | 本論文利用水聲層析法,精確量測兩聲學測站間聲波雙向傳播時間,逆推聲線所穿過的海洋水體的流速場與溫度場,獲得傳統定點觀測所難以量測到的大範圍海洋環境資料。本論文分為現場實驗與數值模擬研究方式進行。首先進行一維流場現場實驗,於基隆和平島八尺門水道兩側相距約五百公尺處,架設兩組使用 GPS 當做計時器的聲源/發射 (transceiver) 站,發射一組載波頻率為一萬赫茲、經 Μ 序列相 位調變後的信號。觀測期間正值大潮,共四小時。現場並有水平聲學 都卜勒流速剖面儀 (HADCP) 與溫鹽深儀 (CTD) 同步進行直接流速及溫度量測,以驗證本方法估算出來之流速與聲速。結果顯示,雙向聲波,走時法與直接量測結果變化趨勢一致。接著,以數值模擬方式,探討水聲層析法重建二維流場之效能。由給定一高斯波數功率譜,其相關長度投影至 x 與 y 方向為 5 公里,產生二維隨機流線函數。從合成流線函數計算所有聲線其聲波的去回傳遞走時差,當做線性逆推的輸入參數。由正則化方法來取得最適模型解,而正則參數則使用 L-cureve 方法當准據。探討不同測站分佈情形(圓形、南北向或是東西向)、測 站數 8 、10 以及12 組)以及測站位置對於逆推結果之影響分析。 研究結果顯示,在相同測站數量下,圓形分佈的測站有最佳的逆推結果;在所有情況底下, 10 組測站以圓形分佈在待側水體外圍,其傳遞資料含有各種方向的流速資訊,因此重建的流場與模擬流場最相似。 The study is to measure currents and sound speeds using ocean acoustic tomography (OAT). The thesis consists of a field experiment and numerical simulations. First, a reciprocal acoustic transmission experiment was conduced in Bachimen Narrows, which separates Hoping Island from Keelung during the spring tide period. Two acoustic tomography systems were situated on two sides of the water way, spaced with a distance of 500 m. Each station was equipped with a transceiver and GPS receiver. Reciprocal sound trans- mission experiments were successfully completed for 4 hours, using a carrier frequency of 10 kHz phase-modulated with the M sequence of 10th order. Range-averaged current and sound speeds, estimated from travel time data obtained reciprocally, show trends similar to the results of Horizontal Acoustic Doppler Current Profiler (HADCP) and the Conductivity-Temperature-Depth instrument (CTD) measurements. It is suggested that reciprocal sound transmission is applicable to current and sound speed measurements in the harbor sea. Second, a numerical simulation was used to investigate the performance of OAT method for 2-D current field reconstruction. An artificial two-dimensional random streamfunction was generated from the Gaussian wavenumber power spectrum with dominating correlation length of 5 km in both x and y directions. From the synthetic streamfunction, the differential travel times were computed between all pairs of tomographic stations as the input data to the inversion. Tikhonov regularization method is applied to solve the inverse problem with the regularization parameter determined by the L-curve criterion. Different number of stations (8, 10 and 12), various array geometries (circular, zonal, and meridional distributions) are used to reconstruct the cur- rent field, with/without the station position errors included. It is suggested that, for the same number of stations, the circular array geometry gives the best inversion results. Using this synthetic current field, the result from 10 stations located circularly on the periphery of the field performs the best of all the other cases. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/63929 |
全文授權: | 有償授權 |
顯示於系所單位: | 海洋研究所 |
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