衛星影像應用於波浪能譜分析與近岸水深演算

Abstract

遙測技術的應用是由面的觀點來探討海面波場特性，其和傳統 式單點的波浪觀測相輔相成，有助全面了解波場在時間域與空間域 變化情形。本研究先行分析ERS-1(歐洲遙測衛星一號)之SAR(合成口 徑雷達)影像來推算波浪方向能譜並探討可能提高其解析度的各種分 析技巧；也利用SPOT衛星可見光影像分析花蓮海域近岸波場，藉以 瞭解影像上所代表的波場特性。衛星影像經由傅氏轉換、波譜分析 處理，獲致海浪波浪的波長、波向及波譜資訊。經與實測資料比對， 波長與波向誤差量不大，並無太大出入；波譜比對顯示主波數或主 頻甚為吻合，波譜形狀(波能分佈)也頗為一致，証實遙測分析海洋 波場方法之正確與可行。 遙測近岸波場的正確可行，進一步應用於近岸水深遙測演算， 採用波譜－水深法(Wave Spectrum Bathymetric Method，簡稱 WSB 法)。應用WSB法的分析台中港近岸海域不同時期與不同海況之兩幅 SPOT影像，分別由近岸海域波長及波向的改變推算出水深數值。WSB 法由波長變化推算近岸水深，其誤差量平均約為12%；由波向變化 推算水深誤差平均則約為28%，WSB法對高污染海域近岸水深資訊 獲取，值得進一步研究與應用。

The remote sensing of ocean wave field is based on the analysis of waves spatial distribution, which is complementary to the conventional method of observing the coastal waves by measuring the time series of surface elevation at one or a few limited points. Coupling with the conventional method, ocean surface waves can be analyzed soundly both in temporal and spatial domain. ERS-1 (The first European Remote Sensing Satellite) SAR (Synthetic Aperture Radar) and SPOT (Satellite Probatoire d' Observation de la Terre) images were used to analyze the wave field of Hwalien coastal water. The information of wavelength, wave direction, and wave spectrum were derived form wave images by way of Fast Fourier Transform (FFT). The difference between the image-derived wavelengths and those measured by the (directional) wave rider was not large, and also for the wave direction. Image spectra and in situ spectra correlated well with each other. Also, the wave energy distribution (spectral shape) and the spectral peaks were very similar. It proves that remotely sensing ocean surface waves is a feasible approach with acceptable accuracy. Following the spectrum analysis of wave image, the Wave Spectrum Bathymetric (WSB) method was developed and applied to estimate the coastal water depths. In cases study, the WSB method was used to analyzed two SPOT wave images, then to estimate the coastal water depths of Taichung Harbor. The coastal water depths were derived via change detection of wavelength and wave direction respectively. The average difference of coastal water depths derived from SPOT image and in situ data, was about 12% for wavelength change detection, and about 28% for wave direction change detection. The WSB method succeed in determining the coastal water depths. It takes advantages of remote sensing technology to obtain the synoptic bathymetry of coastal waters in safety, economically, frequently, and quickly. Also, the WSB method is very apt to determine the coastal water depth of high turbidity. It is worthy of developing soundly for the applications.