福衛二號衛星多光譜影像推估南灣海域水深分佈

Abstract

對海洋研究者而言，海底地形的分佈是一項重要且必知的資訊，最常見的測深方式為使用船載測深儀，但是在船隻不易到達或深度太淺的沿岸區域因為有觸礁的危險，未能有實測的深度資料。而沿岸區域水深因受沉積與侵蝕影響，深度變化比例較大，而且該區域的各種用途使用頻率較高，所以如何能方便且安全得到沿岸水深資料也就更顯重要。 本文利用福衛二號衛星多光譜影像來推估台灣南端南灣海域中三個沿岸區域 (後壁湖、南灣及墾丁) 的水深分佈。光線進入水中後，剩餘的輻射值會隨著深度的增加而呈指數遞減，當輻射值碰到底部物質反射向上，衛星接收到經由底部物質反射向上的輻射值，接收到的水體輻射值的多寡與當地水深有密切的關係。因為沿岸地區波浪效應較明顯，波浪會增加各波段在海面的反射率，由於近紅外波段幾乎沒有來自水體的輻射，因此可以利用近紅外波段增加的輻射值估算因波浪增加的藍綠光。水體吸收綠光的能力較吸收藍光強，因此綠光進入水體後衰減的速度較藍光快，利用來自水體兩波段反射率的比值，比值的改變代表深度的變化。雖然不同的底部物質會改變藍綠波段底部反射率，但是改變的比例相近，而深度的改變同樣也會造成水體反射率的變化，且深度改變影響的程度遠大於底部物質的改變，兩個波段的水體衰減係數差值對不同類型的水體基本保持恆定。因此利用兩個波段水體反射率的比值來計算水深，可以減少因底部物質不同對推估結果的影響，所以本研究方法採用藍綠兩波段水體反射率的比值計算水深分佈。 利用三波段推算得到的水深大都較實測深度為淺，因為衛星接收到的輻射值主要來自於深度較淺的區域，這個遙測水深的方法可以應用在沒有實測水深資料的區域。因為水中的懸浮物質會改變藍綠波段間反射率的比值，所以，此方法適用水質清澈、水深小於 30 公尺的區域。

The bathymetry is important and necessary for oceanographer. The common method for measuring water depth uses ship-board echo sounder. For remote area that is not easily accessible by surveying boats and there is no in situ data. The bathymetry at costal area is affected by sediment and collapse and the percentage of change is large. The utility rate at costal area is higher than deep area, so it’s more important to get the bathymetry safe and convenience at costal area. We use the Formosat-2 multispectral image to calculate the bathymetry at Nanwan area, the bay at the southern tip of Taiwan. The radiance is exponential decay when the light penetrates into the water and reflects with bottom materials. The upward radiance is received by satellite and the radiance is relation to water depth. The wave amplitude is higher in costal region and it increases the surface reflectance towards the satellite. Because near infrared band has almost no reflection from the water body, and wave-induced reflectance is the same for all bands, any increase of near infrared radiance may be used to estimate the wave-contribution in blue and green band radiance. The water absorption is higher at green band than blue band so green light decays faster than blue light. The difference in radiance ratio of blue band to green band represents the changes in depth. A change in bottom albedo affects both bands similarly, but changes in depth affect the high absorption band more. Accordingly, the change in ratio because of depth is much greater than that caused by change in bottom albedo. The difference in attenuation coefficient between blue and green band is a constant with different water. We use the ratio to reduce effect on bottom albedo and then calculate depth. Most of the depth data calculated with three bands are shallower than in situ data. The reason is that most of the radiance received by satellite is radiated from shallowest part in a pixel. The application of this method can easily be extended to regions without in situ depth data. This method is suitable for the regions where depth is shallower than 30 m and water is clear, otherwise the attenuation of light varies with type and concentration of suspension material.