【作者】 潘国富；

【导师】 曹景忠； 马在田；

【作者基本信息】 同济大学 ， 海洋地质学， 2003， 博士

【摘要】 海底浅部沉积物声学特性直接影响海洋声学环境，影响对海底面、海底浅表层沉积物及其下伏地质体和人工构筑物的声波探测效果。同时，通过声学特性进而了解沉积物其它特性，为海底声学分类、沉积学研究、海洋土工学研究和海底工程环境条件评价等提供基础资料。南海北部海域，海洋、地质乃至水声环境均非常典型，一直为诸学科的研究热点海区。而且在军事上又十分重要。对其海底沉积物声学特性进行研究具有重要意义。论文综合应用沉积物取样测试、钻孔测井和海底声学剖面探测等方面资料，对南海北部海底浅部沉积物声学特性进行了系统研究。采集、测试分析了南海北部不同海区近百个站位海底沉积物物理性质和密度、声速等声学性质。特别是应用“弯曲元”剪切波换能器和共振柱首次获得陆架和陆坡区数个沉积物样品的剪切波速度、压缩波和剪切波衰减的可信数据，二种测量方法测得的剪切波速度具很好的可比性，填补了中国海区这方面的空白。根据实测和收集到的数百组数据的全区和分区的统计分析，沉积物声速、密度等声学性质与孔隙度、平均粒径及含水量等之间具有较好的相关性，但相关程度各不相同，且各海区间有差异，其中又以与孔隙度相关性最好。可用孔隙度较有效地预报声速，讨论了可用于预报的经验公式。统计分析同时表明，沉积物声阻抗变化主要决定于密度，并获得了本区从密度预报声阻抗的经验关系。基于实测和预报的数值，研究了本区海底表层沉积物密度、现场声速、声速比、声阻抗比分布。它们呈有规律的区域变化，并与水深、沉积物类型等有密切关系。主要表现为近岸河口海湾区细颗粒沉积物为高含水量、高孔隙度、低密度、低声速，且有声速比小于1的低声速海底出现；外陆架残留沉积物则主要以低含水量、低孔隙度、高密度、高声速、高声速比为特征；而中、下大陆坡和深海盆的细颗粒、半深海一深海沉积物又以高含水量、高孔隙度、低密度和低声速为特征，且一般呈声速比小于1的低声速海底，但与同为细颗粒的近岸沉积物有显著差异：海底沉积物声学性质的上述平面分布特征主要是沉积环境和沉积作用过程影响的结果。根据典型站位柱状岩芯声学性质测试数据分析，不同沉积地貌单元间沉积物声学性质的垂向变化存在明显差异。从陆架到陆坡、深海盆，声速垂向梯度有规律地从大变小，站位间垂向变化的差异性也明显变小。对钻井资料研究表明，沉积物密度、声速、声阻抗等声学性质的垂向变化主要受古沉积环境及沉积后沉积物被改造、埋藏后压实与固结等作用过程的影响。对海进海退变化频繁的陆架区，数十米深度范围内的声学性质垂向变化宏观上主要受海平面升降等沉积环境变化的影响，密度、声速、声阻抗均与CaCO₃含量具负相关性；而沉积环境相对稳定的中、下陆坡和深海盆区，沉积物压实作用对声学性质影响加大，密度、声速、声阻抗与CaCO₃含量出现一定的正相关性。声学性质的垂向变化一定程度上反映了古环境的变化。综合海底声学地层剖面和钻孔、取样资料，根据获得的海底浅部地层结构剖面，分析了本区从珠江口近海到深海盆剖面的海底浅部沉积地层结构和各层的声反射特征，研究了空间展布变化和各层沉积物声学性质。陆架区由于频繁的海进海退变化的影响，海底呈明显的分层结构，但层厚度、声反射特征和沉积物声学特性等在平面和垂向上均出现很大变化。陆坡和深海盆区，很多海底声反射界面可是浊流、CaCO₃含量变化等原因造成，平静沉积区的反射结构以平行、亚平行为主，厚度随下伏地形起伏而变化，而滑坡等沉积物搬运作用强烈区，则反射结构以杂乱为主，也可呈无反射的“透明体”，层厚度变化显著，特别是在陆坡的陡坡区。分类研究了区内浅地层剖面测量揭示的海底最上部沉积层的回声反射特征及其区域分布，分析了九类特征区的沉积学特征及沉积物声学性质，反映出海底回声反射特征与一定的沉积环境、沉积作用过程和海底沉积物声学性质相联系。对分区海底沉积地层剖面结构和回声反射特征的研究可为海底地声建模提供十分重要的基础资料。更多还原

【Abstract】 Acoustic characteristics of seabed sediments are basic to underwater acoustic environment and to explorations of sea bottom, surficial and sub-bottom sediments and underlying geologic bodies and man-made obstructions. Meanwhile, they can be used to infer other properties of seabed such as physical-mechanical properties, which provides important information for seafloor acoustic classifications, sedimentation, marine geotechnics, submarine engineering condition evaluations, etc. Northern South China Sea, with a typical oceanographic, geological and underwater acoustic environment, is an area of general interest. It is significant to carry out systemic research on the acoustic characteristics of its seabed sediments based on comprehensive analysis of sediment sampling and testing, borehole logging and sub-bottom acoustic profiling data.Seabed sediment samples from up to 100m stations in the Northern South China Sea were recovered and tested for physical and acoustic properties such as （saturated bulk） density and sound velocity. Particularly, several samples were measured for shear wave velocity and attenuation coefficients of compressional and shear wave by bender transducers and resonant column techniques and credible results were gained for the first time in this area.The statistical correlation between seabed sediment acoustic and physical properties shows that both sound velocity and density has a significant correlation with porosity, mean grain size and water content, but with different correlation coefficients for different property parameters and physiographic provinces. Porosity is the parameter showing the highest significant correlation with sound velocity and density, and can be used to predict sound velocity with appropriate equations discussed. A detailed examination also reveals that variations in acoustic impedance are almost entirely controlled by changes in density and equations predicting impedance from density are presented.On the basis of measured and predicted acoustic property data, the distributions of density, in situ sound velocity, sound velocity ratio （ratio of the seafloor sediment sound velocity to the water sound velocity overlying the seafloor） and acoustic impedance ratio of the seafloor sediments were mapped. These properties show regional change from the coastal area to deep sea basin and directly related with water depth, sediment types and other factors. Fine sediments in inshore areas such as estuaries are characterized by high water content, high porosity, low density and low sound velocity, with low velocity seafloor （sound velocity ratio<1） in some locations. Seafloor sediments on the outer shelf, mainly relict sediments, show low water content, low porosity, high density and high velocity and velocity ratio. Fine hemi-pelagic and pelagic sediments, located from middle-lower slope to deep sea basin, are high water content, high porosity, low density and low velocity, generally showing low velocity seafloor, but different from fine sediments of inshore areas. It is turned out that acoustic property distributions of seafloor sediments are mainly controlled by depo-environment and sedimentation processes.Based upon data analysis of core acoustic properties, there are obvious differences between different provinces over the property variation configuration with depth below seafloor. Both sound velocity gradient and difference between cores become smaller from continental shelf to slope and deep sea basin. According to borehole sample analysis, it was concluded that vertical variations of all density, velocity and impedance of seabed sediments are influenced mainly by paleo-environment as well as remoulding and compaction after deposition. On the continental shelf, with frequent transgressions and regressions, vertical variations of sediment acoustic properties within teens of meters blow seafloor are controlled mainly by sea-level change. All density, velocity and impedance share a negative correlation with CaCO₃ content. On the other hand, on the middle-lower slope and in deep sea basin, with a comparative stable depo-environment, compaction processes have a greater influence on sediment acoustical properties. All density, sound velocity and impedance share a positive correlation with CaCO₃ content. Variation of sediment acoustic properties with depth below seafloor contains within it a paleo-environment signal.A longitudinal sub-bottom structure profile is presented from Pearl River Mouth to deep sea basin in a approximate southeastern direction based upon acoustic profiling, borehole drilling and coring data. Reflection configuration, sequence distribution and sediment acoustic properties within each sequence were studied in detail. Sub-bottom on the continental shelf shares a good layering structure due to frequent transgressions and regressions, but all sequence thickness, reflection pattern and sediment acoustical characters show a great change. In the slope and deep sea basin areas, many sub-bottom reflectors caused by turbidity current deposits and CaCO₃ content fluctuations. Quite pelagic sedimentation is characterized by parallel-subparallel reflection configuration and changeable sequence thickness with underlying topography. In the area with strong mass-wasting processes such as slumping and sliding, reflection pattern appears chaos, distorted or reflection-free and its sequence thickness varies greatly, particularly on the steep slope. Furthermore a study on seabed echo-character and its distribution revealed by sub-bottom profiling was carried out. Nine echo-character types, together with their sedimentation and sediment acoustic properties, were mapped and analyzed. It is turned out that echo-character can be related to depo-environment, sedimentation processes and sediment acoustic properties. Research on sub-bottom structure profile and echo-character is of great value in seabed geoacoustic modeling更多还原