利用聲納反射係數探討海床沉積物特性; Using Acoustic Reflection Coefficient to Study Seafloor Sediment Features

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Title:	利用聲納反射係數探討海床沉積物特性;Using Acoustic Reflection Coefficient to Study Seafloor Sediment Features
Authors:	林智偉;Jhih-wei Lin
Contributors:	地球物理研究所
Keywords:	反射係數;底質剖面儀;sub-bottom profiler;reflection coefficient
Date:	2011-08-19
Issue Date:	2012-01-05 14:13:24 (UTC+8)
Abstract:	底質剖面儀為一種用來了解海床底下沉積物特性的高解析聲納系統，此種主動式的聲納系統可以記錄各地層間阻抗差所產生的反射訊號，繪製成地層的剖面。透過海床的回波計算出反射係數，是對底質作分類的重要依據，而此種方法相較於傳統的岩芯分析，節省了許多時間，也增加了更大空間探討的可能性。反射係數的異常主要是由於地表起伏和不同底質阻抗所造成。本研究區域位在台灣西南海域高屏峽谷與枋寮峽谷之間，主要的底質分佈以泥為主，以反射係數值為依據，也劃分出砂質沉積物區域，另外受到地表起伏的影響，也劃分了不均質海床沉積物造成反射係數雜亂分佈的區域，以及海床起伏程度較大導致聲波入射後以散射為主的區域。本研究探討了海床的粗糙度，大約是以0.4 m為分界，用來描述海床起伏所造成的損耗對反射係數值的影響程度，若是遇到反射係數值異常區，反射係數的標準差也會高於2 dB，顯示其值極不穩定。此區的可能高天然氣水合物賦存量，其游離氣特性傾向在水深五百公尺附近滲出，推測因此形成許多甲烷游離氣滲流構造及自生碳酸鹽礁，當底拖聲納經過滲流構造上方時，反射係數值有明顯降低的趨勢。探討表層沉積物反射係數值，粗糙面的散射影響很大。而散射程度跟欲探討的深度都和選用的聲納頻率直接相關，越想要探討表層，選用較高頻的聲納，但波長相對於表層的粗糙面易造成散射，若是使用低頻的聲納，可以避免掉散射造成的損耗，但所探討的反射係數值，為較厚的表層沉積物所貢獻。 A wide-band, frequency-modulated, subbottom profiling system (the chirp sonar) can produce artifact-free sediment profiles in real time . The function of a sediment profiler is to record echoes from the interfaces between sedimentary layers ; these layers correspond to breaks in acoustic impedance , generating reflections of the acoustic signal . The image is generated by calculating the envelope of each cross-correlationed acoustic return , mapping the amplitude of each sample in the envelope to a shade of gray and then stacking the arrays of shaded values side-by-side on a display . Underwater acoustic transducers are made with piezoelectric crystals . An electric field applied to these materials causes a deformation related to electrical excitation. These mechanical deformations in turn create acoustic waves. Since the transmitted Full Spectrum pulse is highly repeatable and its peak amplitude is precisely known, the sediment reflectivity values can be estimated from the peak pulse amplitude measurements of the bottom returns. The reflection coefficient anomaly is due to the acoustic impedance of sediments and the surface roughness . This study area is between the Kaoping Canyon and Fangliao Canyon. Mud volcanoes and gassy sediments have been identified from this marine geophysical survey, created by lateral tectonic compression with gas/fluid upward migration and rich dissociation of gas hydrate. Most of surface sediments are mud in this area, and the reflection coefficient is about -15(dB). Some very low reflection coefficient implies that the scattered field is predominant caused by the rough surface. The gas seeps distribute in water depth from five meters. When the deep-towed fish pass through seepage, the reflection coefficient is significantly decreased.
Appears in Collections:	[Graduate Institute of Geophysics] Electronic Thesis & Dissertation

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