Seafloor Sediment Classification Using Nakagami Probability Density Function of Acoustic Backscattered Signals

해양 탐사에 있어서 해저퇴적물의 물성을 파악하는 것은 해양 연구의 기초 자료로써 활용되고 있다. 이러한 해저퇴적물의 물성을 파악하기 위해서는 시추를 통한 직접적인 방법이 있지만 경제적, 시간적 손실이 크고 공간적인 한계가 있다. 이에 음향 장비를 이용한 해저퇴적물 분류 연구가 활발히 진행 중에 있다. 본 논문에서는 해저 지형 조사 장비의 일종인 음향측심기에 의한 음향 신호의 나카가미(Nakagami) 분포를 분석하여 해저퇴적물의 특성을 분류하는 연구를 수행하였다. 나카가미 변수인 m 값의 변화에 따라 해저퇴적물의 물리적 특성이 달라지는 것을 확인하였고, 이는 해저퇴적물특성 연구의 기초자료로 활용되리라고 여겨진다. The physical properties of a seafloor sediment have been used as a basic data for the ocean survey. Conventional methods such as a coring, a drilling, and a grabbing have been used to explore the physical properties but these methods have a number of shortcomings as it is time consuming, expensive and spatially limited. To overcome these limitations, seafloor sediment classification using acoustic signals has been studied actively. In this paper, we obtained the backscattered signal from the seafloor sediment using an echo sounder which is one kind of seafloor topography equipment. Nakagami probability density function of the backscattered signals from the seafloor sediment was computed and a Nakagami parameter was compared with the physical properties of the seafloor sediment. We have confirmed that Nakagami parameter, m is correlated with the physical properties of a seafloor sediment. This study will be utilized as a basic data of the seafloor sediment research.22Nkc

Geochemical compositions of northeast equatorial Pacific sediments and implications for latitudinal position of the Intertropical Convergence Zone since late Miocene

A 570 cm-long sediment core retrieved from the northeast equatorial Pacific (9?570N and 131?420W, 5,080 m) were determined for elemental compositions in order to understand time-series changes in contribution of potential sedimentary end members since 10 Ma. Variations in chemical composition of bulk sediments are explained with temporal changes in dust provenance, formation rate of authigenic Fe-rich smectite, and most significantly in contribution of hydrothermal origin materials. Rare earth elements and other trace elements such as V, Ni, and Cu are mostly associated with hydrothermally-driven Mn and Fe phase. The inorganic silicate fractions younger than ?2 Ma are similar to Asian dust in composition whilethose older than ?2 Ma (to 10 Ma) show chemical affinity to South/Central American dust. Our findings suggest an increase in the supply of Asian dust to the site since 2 Ma likely due to southward shift in boreal winter-spring ITCZ range and/or aridification of central Asiambers since 10 Ma. Variations in chemical composition of bulk sediments are explained with temporal changes in dust provenance, formation rate of authigenic Fe-rich smectite, and most significantly in contribution of hydrothermal origin materials. Rare earth elements and other trace elements such as V, Ni, and Cu are mostly associated with hydrothermally-driven Mn and Fe phase. The inorganic silicate fractions younger than ?2 Ma are similar to Asian dust in composition whilethose older than ?2 Ma (to 10 Ma) show chemical affinity to South/Central American dust. Our findings suggest an increase in the supply of Asian dust to the site since 2 Ma likely due to southward shift in boreal winter-spring ITCZ range and/or aridification of central Asia1

Occurrence of near-seafloor gas hydrates and associated cold vents in the Ulleung Basin, East Sea

During the site survey cruise for proposed drill sites of the Ulleung Basin Gas Hydrate Expedition 01, nearseafloor gas hydrates were discovered in core sediments from both regions of basin plain (2066&#8211;2012 m water depth) and southern slope (898 m) of the Ulleung Basin. The gas hydrate-bearing cores were exclusively retrieved from high backscatter intensity areas in processed 13 kHz multi-beam data, implying high seafloor reflectivity. In high-resolution (2-7 kHz) sub-bottom profiles, the coring sites are also characterized by narrow (< about 500 m wide) acoustic blank zones reaching seafloor, where they have surface expressions of low-relief (< about 5 m high)mound. In the data from a 38 kHz split-beam echosounder, which was deployed for acoustic characterization of gas bubbles, there are no apparent gas flares associated with the blank zones. The recovered gas hydrates mainly consist of disseminated nodules or veins in clayey mud, which normally occur from 5&#8211;6 m below the seafloor to the maximum penetration depth (<8 m) of the cores. In some cases, they were associated with abundant scattered authigenic carbonate nodules. Compositional and structural analyses of selected gas hydrate samples revealed that they consist of structure I hydrates which contain more than 99% methane with carbon isotope values ranging from -64 to -80 per mil (PDB). The preliminary results of the site survey cruise collectively suggest that the near-seafloor gas hydrates are related to cold vents, where high seafloor reflectivity is caused by presence of gas hydrates and authigenic carbonates. Gas seeping activity in the cold vents appears to be currently dormant.1