ACOUSTIC AND PHYSICAL PROPERTIES OF SURFACE SEDIMENTS IN NORTHERN TAIWAN STRAIT

Surface sediments in eleven piston cores and fourteen box cores collected from northern Taiwan strait are composed of clayey silt, sand-silt-clay, sandy silt, silty sand and sand which are discharged mainly from the Cho Shui River with minor distribution from other rivers in northern Taiwan. In northwestern Taiwan strait, the China Coast Longshore Current is a main agent for sediment trans­ port while the sandy sediments derived from Taiwan are characterized by high wet-bulk density and compressional wave velocity, but low porosity and sea water content, the silty sediments exhibit the reverse trends. The basic physical and acoustic properties are related to each other as follows. Vp=2860.449-72I.958M+ 147.548 M'-10.I57 M' (r=0.576037) Vp = 1562.913+5.070 S-0.I20 S'+O.ool S' (r=0.582375) M=-3.01I+0.I76 n (r=0.665624) p=2.452-0.026 W (r=0.744477) Vp : compresional wave velosity at 200 KHz (unit in m/sec) M: mean grain size (unit in 1') s: sand content (unit in %) n: porosity (unit in %) p: wet-bulk density (unit in gm/cc) W: sea water content (unit in %) Bulk modulus, shear wave velocity, Lame's constant and Young's modulus can be calculated from these basic physical and acoustic properties. The boundary between the sediments from northern Taiwan and those carried by the China Coast Longshore Current can be clearly identified by the cluster and factor analyses of these basic properties. The sediments from northern Taiwan are mainly transported to the northwest. The boundary between these two sources of sediments in northern Taiwan strait depends upon the discharges of the rivers on Taiwan island. The attenuation of the sediment does not show any clear relationship with other physical pro­ perties, except their grain sizes. The coarser the mean grain size reveals the higher attenuation. The attenuation within the surface sediment of the studied area is mainly affected by the scattering factor rather than the absorption of the sediment. The attenuation of 200 KHz compressional wave velocity is higher than 300 dB/m only in areas near a few estuaries, such as the She Tz river, Hou Lung river and Hsi Ro river (Cho Shui river).