제주도 북서쪽 대륙붕퇴적물의 고해상도 음향 및 물리특성

Abstract

High-resilution seismic profiles and sediment sampling were cobducted to investigate the acoustic characteristics and physical properties of sediment deposit in the northwestern continental shelf of Jeju Island. Approximately (360) line-㎞ of chirp and sparker profiles was acquired. along with seismic profiling, (15) piston core samples were collected. High-resolution seismic profiles show the holocene mud deposit in cluding acoustic anomalies related to shallow gas and the complex sedimentary structure. The sedimentary sequence can be classified into three units. The uppermost Unit A displays aciustically transparent or indistinct layers with homogeneous seismic facies. Unit B is characterized by parallel reflections and ridge-and-swale topography. The lowermost Unit C. overlying the acoustic basement. forms confused inner reflectors and incision-fills. Four major echo types in the stydy area are classified as falst seafloor with sharp bottom echo (transgressive sediment sheets and relict sands). aoustically transparent echo (Holocene muds). mounded seafloor ( tidal ridges), and irregular echoes (channels).Core samples were anlyzed for sediment textures, physical properties (porosity, water content, bulk density, grain density and vane shear strength), and aciustic porperties (compressional wave velocity and attenuation). The measeured and calcuated results are; mean grain size 2.14~8.43 phi (average: 5.29 phi), porosity 32.8~72.6% (average: 56.8%), water content 15.8 ~ 50.5% (average: 34.8%), wet bulk density 1.5~2.5 g/㎤ (average: 1.8 g/㎤), grain density 2.5~3.1 g/㎤ (average: 2.7 g/㎤), compressional wave speed 1362~1666 ㎧ (average: 1514 ㎧). The areas of acoustically transparent echo units are composed of homogeneous and soft mud, partially including the gassy sediments, whereas the sandy sediments with shell fragments are dominant in the surface of last and eroded echo units.