In-situ geoacoustic measurement in the northern continental shelf of South China Sea and characteristics of mid-frequency sound speed
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摘要: 本文首先简要介绍了基于高频微振动贯入技术的底质声学原位测量系统，该系统由机械液压、声学换能器、声波发射采集、总体控制和辅助测量等单元组成，可以用于海底沉积物的中频声速和声衰减系数的原位测量。2025年4月，该系统搭载“向阳红01”号科考船在我国南海北部陆架开展了9个站的底质声学原位测量，其中8个站的最大贯入深度超过3 m，频率覆盖范围1.6 kHz～10.0 kHz。计算表明，9个站的声速特征差别明显：第一组3个站的声速比在1.01～1.03之间变化，第二组4个站的声速比明显小于1.0，介于0.97～0.98之间，第三组的两个站的声速比介于前面两组之间，声速比在1.0附近。与同步获得的沉积物柱状样对比发现，声速特征与沉积物的物理性质参数有高度的相关性，当含砂量较高和含水率较低时沉积物的声速大于近底海水的声速，当含砂量较低和含水率较高时沉积物的声速则低于近底海水的声速，含砂量和含水率可能是决定沉积物声速是大于还是小于近底海水声速的主要因素。9个站中，粒径较大（$ {\varPhi } $=4.6～4.7）的沉积物中声速频散约为2%，而粒径较小（${\varPhi } $=5.8～5.9）的沉积物中频散不到1%。另外，利用Hamilton模型预测的各站的声速比原位测量结果高出3.5%～8%，并且预测值均大于近底海水的声速。Abstract: This paper briefly introduces the in-situ geoacoustic measurement system (SAS), which is based on high-frequency micro-vibration penetration technology. The system is composed of mechanical-hydraulic units, acoustic transducers, an acoustic emission and acquisition unit, an overall control unit, and auxiliary measurement units. It is designed for measuring the mid-frequency sound speed and attenuation coefficient of seafloor sediments. In April 2025, the system was deployed aboard the R/V "Xiangyanghong 01" to conduct in-situ acoustic measurements at nine stations in the northern continental shelf of South China Sea. At eight of these stations, the maximum penetration depth exceeded 3 meters, covering a frequency range of 1.6 kHz -10.0 kHz. The calculated sound speed shows distinct differences among the nine stations: the sound speed ratio at the first group of three stations varies between 1.01 and 1.03; at the second group of four stations, the ratio is significantly less than 1.0, ranging between 0.97 and 0.98; and at the third group of two stations, the ratio lies between the previous two groups, around 1.0. Comparison with synchronously obtained sediment core samples reveals a high correlation between sound velocity characteristics and the physical property parameters of sediments. When the sand content is high and the water content is low, the sound velocity in sediments exceeds that of near-bottom seawater; when the sand content is low and the water content is high, the sound velocity in sediments is lower than that of near-bottom seawater. Sand content and water content may be the primary factors determining whether sediment sound velocity is greater or less than that of near-bottom seawater. Across the nine stations, sound speed dispersion is approximately 2% in coarser sandy sediments ($ \mathit{\Phi }= $4.6～4.7), but less than 1% in finer silty sediments ($ \mathit{\Phi }= $5.8～5.9). Furthermore, the sound speed ratios predicted using the Hamilton formulas for each station are 3.5% to 8% higher than the in-situ measurement results, with all predicted values exceeding the sound speed of the near the seafloor seawater.
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图 2 SAS测量原理图和总体设计图
右上角小插图代表系统的几何关系和测量原理。其中，发射换能器安装在扩展臂上，扩展臂顶端距探杆轴线2 m；接收换能器位于探杆的下端，距探杆顶端3 m。红色虚线代表声线传播路径，蓝色点划线代表海底，扩展臂位于海底之下表示系统沉入海底沉积物中。
Fig. 2 SAS’s schematic diagram of measurement principle and general assembly drawing
The schematic of geometry in the upper right corner represents the time-of-flight measurement principle of the system. Specifically, the transducers are mounted on an extended arm, 2 m from the axis of the probe; the receiver is mounted at the lower end of the probe, 3 m from the top of the probe. The red dashed line indicates the sound ray path, and the blue dash-dot line represents the seafloor. The extended arm being below seafloor indicates that the system sinks into the soft sediment.
图 3 SA101和SA247站海底沉积物中和近底海水中接收信号波形图。其中，2 kHz和4 kHz来自低频声源，6.3 kHz和8 kHz来自中频声源。红色代表在沉积物中测得的声信号，蓝色代表在近底海水中测得的声信号。声信号幅值进行了归一化处理。
Fig. 3 Received signals waveform diagram obtained in sediment and seawater near the seafloor at station SA101 and SA247. Specifically, 2 kHz and 4 kHz data are from the low-frequency transducer, 6.3 kHz and 8 kHz data are from the mid-frequency transducer. Red represents sound signals measured in sediments, blue represents sound signals measured in near the seafloor seawater. Signal amplitudes are normalized with reference to one.
图 4 SA101等站海底沉积物/近底海水的声速比
横轴代表测量声波的频率，单位kHz；纵轴代表沉积物的声速与近底海水的声速之比。
Fig. 4 Sound speed ratios of the sediments to the near-seafloor seawater at nine stations, including the SA101.
The horizontal axis represents the frequency of the sound waves in kHz; the vertical axis represents the sound speed ratio of the sediments and seawater near the seafloor.
图 5 SA101、SA90和SA188站海底沉积物/近底海水的声速比
横轴代表测量声波的频率，单位kHz；纵轴代表沉积物的声速与近底海水的声速之比。
Fig. 5 Sound speed ratios of the sediments to the near-seafloor seawater at SA101, SA90, and SA188 stations.
The horizontal axis represents the frequency of the sound waves in kHz; the vertical axis represents the sound speed ratio of the sediments and seawater near the seafloor.
图 6 SA233、SA220、SA298-1和SA247站海底沉积物/近底海水的声速比
横轴代表测量声波的频率，单位kHz；纵轴代表沉积物的声速与近底海水的声速之比。
Fig. 6 Sound speed ratios of the sediments to the near-seafloor seawater at SA233, SA220, SA298-1 and SA247 stations.
The horizontal axis represents the frequency of the sound waves in kHz; the vertical axis represents the sound speed ratio of the sediments and seawater near the seafloor.
图 7 SA275和 SA126站海底沉积物/近底海水的声速比
横轴代表测量声波的频率，单位kHz；纵轴代表沉积物的声速与近底海水的声速之比。
Fig. 7 Sound speed ratios of the sediments to the near-seafloor seawater at SA275 and SA126 stations.
The horizontal axis represents the frequency of the sound waves in kHz; the vertical axis represents the sound speed ratio of the sediments and seawater near the seafloor.
表 1 SA101等测站相关信息
Tab. 1 Related information of SA101 and other stations
站位
序号站位
名称最大贯入
深度/m近底海水
声速（m/s）水深/m 1 SA101 3.30 1524.5 103 2 SA90 3.25 1523.9 101 3 SA188 2.54 1517.3 116 4 SA233 3.31 1515.9 174 5 SA220 3.35 1515.2 149 6 SA298-1 3.28 1501.0 336 7 SA247 3.29 1516.6 135 8 SA275 3.24 1515.5 294 9 SA126 3.30 1516.8 183 
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表 2 三类发射换能器的发射频率
Tab. 2 The transmitting frequencies of three types of transducers.
低频换能器 (kHz) 1 1.6 2 2.5 3.15 4 5 中频换能器 (kHz) 3.15 4 5 6.3 7 8 9 10 12.5 高频换能器 (kHz) 6.3 7 8 9 10 12.5 16 20
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表 3 SA101等站沉积物柱状样的主要物理性质参数
Tab. 3 Main physical property parameters of sediment core samples from SA101 and other stations.
站位 湿密度*（g/cm³） 孔隙度*（%） 含水率*（%） 砂*（%） 粉砂*（%） 黏土*（%） 平均粒径*（Φ） 沉积物类型 SA101 1.83 0.51 40.5 44.7 46.8 8.5 4.60 砂质粉砂为主 SA90 1.87 0.49 48.3 56.4 27.0 16.6 4.73 粉砂质砂为主 SA188 1.79 0.54 43.7 35.7 44.7 19.6 5.08 砂质粉砂夹粉砂质砂 SA233 1.72 0.60 59.6 19.2 67.8 13.0 5.57 砂质粉砂为主 SA220 1.66 0.62 60.7 13.7 72.4 13.9 5.88 黏土质粉砂和砂质粉砂 SA298-1 1.67 0.62 60.1 14.6 69.4 15.7 5.87 砂质粉砂为主 SA247 1.70 0.62 68.6 14.9 71.3 13.8 5.81 砂质粉砂为主 SA275 1.83 0.52 43.2 39.1 50.6 10.3 5.50 砂质粉砂为主 SA126 1.69 0.60 57.9 9.9 76.4 13.8 5.64 粉砂为主 注：*代表贯入深度之上各层沉积物相应物理参数的平均值，例如湿密度*代表某站贯入深度之上各层沉积物湿密度的平均值；平均粒径采用Krumbein（1934）提出的以2为底的对数来表征，即$ \mathit{\Phi }=-{\log }_{2} d $, $ d $是沉积物颗粒的直径，单位为mm。
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表 4 利用Hamilton模型得到的9个站的沉积物声速比预测值与本次调查实测值
Tab. 4 Predicted sound speed ratio of the sediments by Hamilton formulas and measured sound speed ratio of this survey at 9 stations
站位 SA101 SA90 SA188 SA233 SA220 SA298-1 SA247 SA275 SA126 本次调查实测值 1.026 1.026 1.015 0.968 0.976 0.975 0.975 1.004 0.992 密度公式 1.062 1.076 1.050 1.025 1.016 1.018 1.025 1.062 1.023 孔隙度公式 1.066 1.083 1.054 1.027 1.022 1.022 1.020 1.062 1.027 粒径公式 1.077 1.074 1.068 1.056 1.048 1.058 1.049 1.066 1.053
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