Experimental study on the relation between compressional wave velocity and physical properties of sandy sediments
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摘要:
砂土是主要的海底沉积物类型之一，明确砂质沉积物声学与物理性质的关系对海底底质和地层探测至关重要。本文利用超声探测仪和自制的试样制备测试装置，模拟制备不同沉积状态的砂土试样，同步开展超声测试和物理性质测试，探讨砂质沉积物声速测试方法及影响因素，揭示砂质沉积物压缩波速与物理参数的内在联系。试验结果和分析表明：换能器接触管壁的间接测量方法中，声波多路径传播可显著影响沉积物声速测量的准确性，而换能器接触试样的直接测量方法可避免这一影响；30 kHz至100 kHz的不同频率对压缩波速测量结果没有明显影响。砂质沉积物的压缩波速与密度、孔隙度、含水率有较好的相关性，相关系数分别为0.87、0.86、0.84，并且随密度的增大而增大，随孔隙度、含水量的增大而减小。砂质沉积物的压缩波速与中值粒径的相关系数小于0.6，对物质组成不敏感。另外，与声速相比，砂质沉积物的声阻抗与密度、孔隙度、含水量的相关性更高。砂质沉积物压缩波速对饱和度非常敏感，例如，饱和度从0.971增至0.994时，压缩波速从393.3 m·s−1急剧增大到748.5 m·s−1，需特别注意。
Abstract:Sand is one of the main type of submarine sediments. Figuring out the relation between acoustic and physical properties of sandy sediments is critical to seafloor and sub-bottom detection. In this paper, by using the ultrasonic detector and the self-developed sample preparation device, sand samples in different physical states are prepared to simulate different natural sedimentary conditions. Acoustic and physical properties are tested simultaneously, so as to reveal the effective measuring methods and its influence factors, and to explore the internal connection between the compressive wave velocity (CWV) and physical parameters of sandy sediments. Results and analysis indicate that the multipath propagation of sound wave can affect the measurement accuracy for the method with ultrasonic transducers touching the side wall of sediment container, while this effect can be avoided by measuring with a direct contact between transducers and sediment. No effects are found with different test frequencies among 30 kHz to 100 kHz. The CWV of sandy sediments, which shows good correlation with density, porosity and water content, with correlation coefficients 0.87, 0.86, and 0.84, respectively, increases with increasing density, while decreases with increasing porosity and water content. While the correlation coefficient between CWV and medium diameter is smaller than 0.6, which shows that the CWV of sandy sediments has no clear link to grading distribution. The correlation of acoustic impedance with bulk density, porosity and water content is bigger than that of CWV with them. In addition, special attention should be paid to the saturation of sediments because the CWV is very sensitive to saturation, for example, the CWV increases dramatically from 393.3 m·s−1 to 748.5 m·s−1 as the saturation increases from 0.971 to 0.994.
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Key words:
- submarine sediments /
- compression wave velocity /
- acoustic impedance /
- physical parameter
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表 1 直接测量法获得的砂质沉积物物理和声学参数值
Table 1. Physical and acoustic parameters of sandy sediment obtained by direct measurement
试验组别 中值粒径d50 /mm 密度ρ /g·cm−3 含水量ω /% 孔隙度n /% 饱和度Sr 压缩波速Vp /m·s−1 声阻抗Z /kg·m−2·s−1 FGI-1 0.246 2.033 23.005 38.329 0.992 616.9 1254.0 FGC-1 0.246 2.162 16.450 30.724 0.994 747.5 1616.3 FNI-1 0.230 1.930 29.002 44.175 0.982 444.2 857.1 FNC-1 0.230 2.047 22.301 37.547 0.994 506.6 1036.9 MGI-1 0.374 1.997 25.137 40.453 0.992 481.8 962.0 MGC-1 0.374 2.128 17.877 32.639 0.989 598.2 1272.7 MNI-1 0.460 1.943 28.030 43.373 0.981 405.6 788.0 MNC-1 0.460 2.001 24.230 39.898 0.978 426.4 853.3 CGI-1 0.973 2.079 20.366 35.551 0.989 552.8 1149.2 CGC-1 0.973 2.170 16.048 30.227 0.993 748.5 1624.1 CNI-1 0.587 1.918 30.102 44.991 0.986 393.3 754.4 CNC-1 0.587 2.017 22.970 38.797 0.971 438.0 883.6 
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表 2 间接测量法获得的砂质沉积物物理和声学参数值
Table 2. Physical and acoustic parameters of sandy sediment obtained by indirect measurement
试验组别 中值粒径d50 /mm 密度ρ /g·cm−3 含水量ω /% 孔隙度n /% 饱和度Sr 压缩波速Vp /m·s−1 声阻抗Z /kg·m−2·s−1 FGI-2 0.222 1.959 32.102 44.666 1.000 1488.5 2915.9 FGC-2 0.222 2.076 25.028 38.055 1.000 1531.0 3177.8 FNI-2 0.211 1.937 31.967 45.237 1.000 1511.2 2926.9 FNC-2 0.211 2.011 26.774 40.797 1.000 1521.1 3059.6 MGI-2 0.381 1.965 27.976 42.720 1.000 1454.4 2857.3 MGC-2 0.381 2.066 24.966 38.307 1.000 1469.8 3036.8 MNI-2 0.450 1.861 35.116 48.604 0.995 1514.1 2817.9 MNC-2 0.450 1.997 31.081 43.155 1.000 1544.3 3083.9 CGI-2 0.905 2.092 20.857 35.418 1.000 1488.0 3112.7 CGC-2 0.905 2.131 21.288 34.455 1.000 1510.6 3218.5 CNI-2 0.685 1.936 29.572 44.259 0.998 1525.1 2952.1 CNC-2 0.685 1.973 26.861 41.956 0.996 1542.0 3043.1
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表 3 不同工况下的间接法超声测试走时
Table 3. Travel time of indirect ultrasonic test under different working conditions
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表 4 砂质沉积物声学与物理参数的经验公式
Table 4. Empirical formula of acoustic and physical parameters of sandy sediment
物理力学参数 经验公式 R2 密度ρ /g·cm−3 Vp = 3729.2ρ2 − 13933ρ + 13415 0.87 Z = 8975.6ρ2 − 33447ρ + 31922 0.91 孔隙度n /% Vp = 1.1174n2 − 106.37n + 2934.1 0.86 Z = 2.6851n2 − 257.23n + 6926.9 0.90 含水量ω /% Vp = 1.6054ω2 − 96.921ω + 1876.5 0.84 Z = 3.8943ω2 − 236.37ω + 4382.9 0.88 中值粒径d50 /mm Vp = 1479.3d502− 1652.2d50 + 894.35 0.44 Z = 3453.1d502 − 3821.8d50 + 1921.6 0.42
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