應用移動船聲層析於綠島尾渦流之模擬研究

Abstract

本研究測試移動船聲層析法測站間之幾何分佈對臺灣綠島尾渦流之流場重建影響。聲層析之組合陣列由六個聲波發收器所組成，這些聲波發收器之佈放方式可以是固定於海床上（稱為固定式測站）或是被船拖曳（移動式測站）。本研究使用海洋數值模式（MITgcm）模擬的綠島尾渦流流場，於8 X 8平方公里範圍內進行不同陣列組合之聲層析數值模擬研究：六個固定式測站、五個固定式測站配合一個移動式測站，並考慮各組陣列其固定式測站於空間中不同之幾何分佈，除了六個固定式測站之組合外，移動式測站均沿著聲層析範圍的周邊移動。 為了評估不同陣列配置對於三維流場重建之影響，本研究先於二維海洋環境進行模擬測試，再將二維模擬結果的最佳陣列配置延伸至三維海洋環境。在進行聲層析模擬研究前，先利用不同均方根長度的等向高斯函數進行一系列的解析測試以評估模式在不同陣列的解析度。相比於傳統全為固定式陣列之結果，使用移動船聲層析所重建之流場殘差從13%降至6%。於三維模擬中，為了增加測站所能接收之特徵聲線之數量，拖曳式聲波發收器佈放之深度取決於測站點之水深；海洋環境分割為七層，每層厚度為四百公尺。從三維移動船聲層析模擬之模擬結果得知，最佳之重建流場在第三層。由移動船聲層析所重建流場之殘差為10%，優於傳統陣列的17%。本模擬研究之結果可提供未來實海域實驗測站配置之參考。

This study examines different configurations of acoustic array using moving ship tomography (MST) for mapping the Kuroshio-induced current wakes in the lee of Green Island southeast of Taiwan. The acoustic tomographic array consists of six acoustic transceivers; each transceiver can be either moored at one position or towed by a moving ship. Synthetic tomographic experiments using the ocean current output from Massachusetts Institute of Technology General-Circulation-Model are performed in a region of 8X8 km2 for two configurations: six moored transceivers and five moored transceivers with one ship-towed transceiver. Different spatial configurations of the moored transceivers are considered. Except for the configurations of six moored transceivers, the ship moves around the periphery of the study region. To evaluate the effectiveness of different array configurations for map- ping the three-dimensional (3D) current wakes, the potential configurations are selected from those spatial configurations considered in the two-dimensional (2D) ocean environment. Prior to conducting the 2D/3D tomographic reconstructions, isotropic Gaussian functions with different RMS lengths are used to assess the resolution of each array configuration. Compared with the traditional tomographic arrays of only moored transceivers, the current reconstructions using the MST show a reduction in residual error from 13% to 6%. Both MST arrays and the six moored transceivers are examined for the 3D inversions. To increase the number of acoustic eigenrays between the transceivers, the depth of the transceiver is selected according to the bathymetry. The water column is discretized into seven layers with a thickness of 400 m. From the 3D results, the optimum reconstruction is observed in the 3rd layer, and the reconstruction error using the MST is reduced to 10% compared with 17% observed using the traditional tomographic array. This simulation study can provide the design of the tomographic array for the field measurements near Green Island.