| 摘要: | 東沙環礁國家公園位於遙遠的南海,擁有廣闊的環礁地形與底棲生物圈。近年來東沙環礁海草棲地受到劇烈的環境變化,大面積的海草消失棲地面積僅剩水沙類底棲型態。而調查水下環礁海洋資源，遙測影像具備低成本與高效率特色。光學衛星能夠長時間觀測研究區域，進而分析不同時間之棲地面積變化。
根據輻射傳播理論光譜資訊易受大氣影響及水體吸收,且太陽光輻射入水後能量隨著水深增加衰減。本研究運用紅外光波段校正各可見光波段，去除因海面不平均散射所造成眩光雜訊。大氣校正是依據理想輻射傳播理論進行暗像元去除法，另外針對研究區多雲氣候應用多尺度Retienx濾波器減少雲霧成份像元並強化邊緣。為減少多幅不同時影像輻射大氣影響和水深差異，運用白化-反白化相對校正使相同衛星影像有相同統計分佈。水深校正處理運用古典Lyzenga水深推導公式之底部反射變量，因穿透能力差異任意兩波段即可組成深度不變指數表達水下底部輻射強度資訊。
本研究影像分類只分析0到5公尺深度區域，對東沙環礁兩區的礁台深度不變指數進行最短距離分類法分成五類(海草,珊瑚,沙質,碎屑珊瑚,雲)。研究區(一)橫跨2014年夏天至2015夏天，約莫12公頃消失海草棲地僅存沙質棲地;而研究區(二)則是經歷整個2015夏天持續變遷至冬天海草棲地流失約3公頃。
目前推測東沙環礁受近年極端氣候影響，南太平洋年均海溫與1990年代上升許多東沙海溫異程現整體偏高。過去春季與東季頻繁的高溫海水使的東沙環礁海草及珊瑚等棲地生存被受挑戰。
;In the benthic environment, seagrass was considered as the ecological stabilization index of the marine habitat. Because of the low cost and frequent observations for multispectral satellite images, they are suitable to assess seagrass habitat change. To investigate bottom material directly from spectral information, sun glint, atmospheric and water depth correction are necessary.
In this study, the sun glint and water depth correction will be conducted first to remove the surface and bathymetric effects. Meanwhile, multi Retinex with color restoration method technique which integrate Homomorphic filter and white balance enhancement are implemented. After the processes, image dynamic range is improved which achieve atoll enhancement and cloudy pixel attenuation. In Order to minimize atmospheric effect between images, relative calibration Whitening and De Whitening method is also implemented among multi-temporal image by matching target image with reference image in spectrum statically. Then, water depth correction is applied by pairs band of habitat bottom reflectance.
A huge event of seagrass disappearance has been observed in summer 2014 and 2015. Study area 1 reduced about 12 km2 within three months also found study area 2 lose 3 km2 seagrass habitat. Since most habitat and atoll attenuation condition has been changed, depth invariant index from Lyzenga equation might need recent bathymetry information. In future work, we will try to analysis satellite sea surface temperature product relationship when occur anomaly sea water and extreme weather influence at Dongsha marine resource. . |
