利用連續變頻聲納回聲影像及反射震測速度 分析來探討台灣東北海域沈積物之聲學特性

Abstract

本研究利用高解析度的連續變頻聲納來探討東北海域海床沈積物的回聲特徵分布、比對此區域內各岩心資料來建立東北海域海床沈積物特性及期分布狀況，並透過反射震測速度分析與震測剖面了解聲波在海床下沈積物中速度變化及其地質構造。 台灣東北海域陸棚區依海床上地形與聲納訊號穿透度的不同，共可定義出6種回聲類型，而此6種回聲類型又可歸納為三大類：平坦型、山丘型、不規則溝渠型。東海陸棚上大部分都是呈現平坦型的回聲特徵，靠近岸處則有海床呈規則波浪狀起伏的回聲特徵、呈現山丘狀的山丘型回聲特徵及不同規模大小凹槽的回聲特徵，一般皆被認為是由於潮汐作用所造成。由岩心資料對比結果顯示，陸棚上的沈積物類型以粗顆粒的貝屑、礁石、礫石及砂質沈積物為主。陸坡區與沖繩海槽回聲類型共可分四大類：清晰型、濃密型、拋物線型、不規則型。水深200~300公尺處仍以海床平坦的回聲特徵為主，水深500公尺以下則大多屬於清晰型的回聲特徵，沈積物主要為細顆粒沈積物。在棉花峽谷的部分則呈現極濃密的回聲特徵，亦以細顆粒沈積物為主。北棉花峽谷具有受崩移作用形成的梯型海床，為海底崩移後所造成的斷崖，而在地勢較陡區則呈現拋物線型的回聲特徵。在沖繩海槽底部海床大致平坦，此處沈積環境主要受濁流或遠洋沈積物影響。下部陸坡與沖繩海槽相接處，則呈現濁流造成的濁流岩的回聲特徵。 在反射震測速度分析方面，本研究分析了EW9509之三條分布於陸棚區的長支距震測剖面資料，建立這三條剖面的速度構造模式，以推導在陸棚區海床下沈積物的聲速變化情形。由此三線的速度剖面顯示：海床以下200公尺的聲速大致介於1500 m/s ~1900 m/s；海床下200公尺~800公尺聲速大致介於1900 m/s~2700 m/s；海床下800公尺~2000公尺聲速大致介於2700 m/s~3500 m/s。由於能夠進行速度分析的震測資料空間分布相當侷限，本研究另利用Hamilton(1980)的速度關係式求出最符合此三線求出之速度值的速度與時間關係曲線V(t)= 1510 + 1796.71× t – 720.114 × t2(t為單程走時，V(t)為t時的地層速度)，用以代表台灣東北海域陸棚區海床下沈積物的速度方程式。 根據研究區域內的岩心資料與連續變頻聲納地層回聲剖面的比對，便能瞭解不同沈積物所具有的聲學特性，粗顆粒的沈積物因具有較高的衰減係數故能量容易衰減，剖面上的訊號穿透度較差，反之，細顆粒沈積物的剖面則常呈現清楚且連續的次層反射訊號，訊號穿透度良好。由此，根據不同沈積物具有的聲學特性，我們便能透過回聲剖面描繪出整個區域內沈積物型態的分布情形。

This study uses high-resolution subbottom profiles (Chirp Sonar) data to map the distribution of seafloor echo types off northeastern Taiwan, core sample analyses results are used to correlate the sub-bottom echo characters to seafloor sediments. Multichannel seismic reflection data have also been used to derive sediment velocity structures and to provide seismic images for understanding the velocity variations beneath sea floor, and to know the geological structures in the study area. The echo characters observed on the East China Sea Shelf area are different from those observed in the East China Sea continental slope and Okinawa Trough area. Three types of echo character patterns (flat, mound and irregular gullied) and six sub-types are recognized on the East China Sea Shelf. Flat echo pattern is the dominant echo pattern in the shelf area, flat seafloor with regularly spaced wavy bedforms (echo type 1-4), mounded seafloor with smooth surface (echo type 2) and various-scale eroded seafloor (echo type 3-1 and 3-2) are observed in the offshore area. The developments of echo types in the shelf area are suggested to be influenced by tidal currents. In the area of East China Sea Slope and southern Okinawa Trough, four types of echo character patterns (distinct, indistinct, hyperbolic and irregular) and eight sub-types are identified. Distinct echoes with no or few subbottom echoes and nearly flat seafloor (echo type 1-a) are distributed in the upper East China Sea Slope, and distinct echoes with continuous and parallel internal reflectors (echo type 1-b) are distributed in the lower East China Sea Slope. Very prolonged bottom echoes occur in the Mien-Hua Canyon area, and the sediments are mostly clay there. The echo characters of stair-shaped seafloor, appear near the North Mien-Hua Canyon are suggested resulting from submarine landslides. The hyperbolic echo characters are presented at the upper East China Sea Slope and other steep slope areas. The seafloor in the southern Okinawa Trough is nearly flat, the sedimentary processes here are primarily influenced by turbidity current or pelagic sediments. The area between lower East China Sea Slope and west wall of southern Okinawa Trough presents echo characters of turbidites. To establish sediment velocity models, we use three large-offset multichannel seismic reflection lines on the East China Sea Shelf to derive sediment velocity structures. From the established velocity profiles, the velocity of the sediment from seafloor to 200 meters in depth is around 1500 m/s~1900 m/s, the sediment velocity from 200 meters to 800 meters deep is around 1900 m/s~2700 m/s, and the sediment velocity from 800 meters to 2000 meters deep is around 2700 m/s~3500 m/s. In order to derive a general velocity function for the shelf sediments, we use Hamilton’s velocity relation to fit all the seismic velocity values of the profiles, and obtained a velocity curve: V(t)= 1510 + 1796.71× t – 720.114 × t2(t: one way travel time, V(t):interval velocity at t). By correlating the sediment properties from core sample analyses with chirp sonar echo characters, we can understand acoustic properties of different sediments. Coarse-grained sediments can attenuate sound waves easily, and the sound penetration depth is shallow. On the contrary, the sound penetration depth of fine-grained sediments is deeper and the echo signal is clearer. Therefore, we are able to describe the distribution of sedimentary types in the study area from echo character pattern distribution map generated in the study.