Study Of The In-situ Acoustic Measurement Technique And Geoacoustic Properties Of Marine Sediments

The acoustic parameters of seafloor sediments are the basic data of research on ocean acoustic field environment. The acoustic properties of marine sediments are indispensable to analyze acoustic propagation loss of seafloor and build geoacoustic model. The research can provide basic materials and key techniques for acoustic classification of marine sediments, seafloor acoustic field evaluation, and marine acoustic survey. This paper consists of 2 parts: firstly, the research on in-situ acoustic measurement of marine sediment can develop a towing in-situ measurement system; secondly, analyze the geoacoustic properties of seafloor sediments from Qingdao offshore and the southern South China Sea, on the micro level like physical properties and on the macro level like sedimentary environment. This paper developed the measurement system and built the acoustic parameter prediction equation of Qingdao offshore and geoacoustic model of southern South China Sea, which can provide the effective methods and basic data for further acoustic properties research.This paper was supported by the Ocean Special Public Sector Research ProjectResearch of marine sediments in-situ acoustic measurement, and developed the towing in-situ measurement system, depending on the position type measurement system, which is the result of National 863 Plan ―the in situ acoustic measurement technique of marine sediments‖. The paper mainly introduced the structure of measurement system, the technical index, the test in laboratory flume and in Qingdao offshore. With this in-situ measurement technique, the acoustic parameters of seafloor sediments were measured from 45 positions in Qingdao offshore. Based on the good correlation between acoustic parameters(sound speed and attenuation) and physical parameters(particle size and composition content), the empirical equations of sound speed – grain size, sound speed – sand content, sound speed – clay content, and sound speed – sand-mud ratio were built by this paper, which could provide effective methods for the prediction of sound speed. This paper also analyzed the relationship between the distribution characteristics of sound speed and sedimentary environment of this study sea area.Compared with the northern South China Sea, which was the research focus field of geology and underwater acoustic, the study on acoustic properties of marine sediments from southern South China Sea was still relatively lack. Southern South China Sea, with a typical oceanographic, sedimentary and underwater acoustic environment, also with important military value, is significant to carry out systemic study on the acoustic properties of marine sediments. Acoustic parameters and physical parameters of 21 sediment cores have been measured, and the distribution of sound speeds was obtained, the relationship between geoacoustic properties and sedimentary environment was analyzed. Comparing with the sediments of slope and trough, the sediments of continental shelf are characterized by coarser mean grain size, lower porosity and higher wet density. Sediments in the trough are characterized by fine grain size, high water content, high porosity, low density and low sound speed. Physical parameters of sediments in the slope are lower than corresponding properties in the continental shelf and closer to corresponding properties in the trough. On the micro level, the acoustic properties were controlled by porosity, mineral composition, microstructure, grain size. On the macro level, the acoustic properties were close to sedimentary environment, such as hydrodynamic conditions and sediment source.The comparison results of 3 sediment cores positions from shelf, slope and trough indicate there were obvious differences among the vertical variation properties of 3 positions, and 3 vertical variation patterns of sound speed could be obtained. The vertical increasing model is applicable to the area with deep water, weak hydrodynamic conditions and strong compaction and consolidation effects, such as trough valley. The vertical invariant model is applicable to the area with deep water, rapid sedimentation rate and weak sedimentation reworking effects, such as continental slope. The complicated vertical variation model is applicable to the coastal zones with shallow water, strong hydrodynamic conditions, complex sediments and strong reworking effects(erosion, transport, etc.), such as offshore shelf. According to the vertical variation characteristics of sound speed and stratified structure of sediments, the sound speed structure models of sediments from trough, slope and shelf were built. These models consist of trough- stratified model, trough- unstratified model, slope- stratified model, slope- unstratified model, shelf- stratified model, shelfunstratified model.In order to analyze the application of speed calculation model, the sediment sound speed of the southern South China Sea were calculated by using Wood equation, Buckingham model, and Biot-Stoll model respectively. The results show that the calculated sound speeds with theory model have same change trend with the measured sound speed. The comparison between distribution properties of measured sound speed and calculated sound speed indicate that the distribution of measured sound speed was more complex and with more obvious local changes. By contrast, Biot-Stoll theory, which focuses on the porosity of sediments, was applicable to the coarse sediments from continental shelf. With both fluid and elastic dual characters, Buckingham theory was more applicable to the fine grain sediments from continental slope and trough.