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弹性沉积层海底的低掠射角反射存在奇异性极大值的频率特征, 其特征对浅海远程声传播会产生显著的影响. 针对中国南海东沙海域的一次海底与波导联合测量到的频率间隔小的海底共振与声虹吸现象. 通过分析弹性沉积层海底的低掠射角反射特征, 理论推导了沉积层与剪切波的共振频率表达式, 并分析了海底反射特征对远程声传播的影响. 结果表明: 在弹性沉积层海底模型下, 受剪切波调制的小掠射角反射特征会引起指定频率的剪切波在沉积层发生共振, 从而导致水中传播的声能被沉积层禁锢而出现声虹吸效应. 进一步根据海底剪切波共振频率相关参数的敏感性及耦合性的分析结果, 提出了一种结合海底与波导观测信息的地声参数反演策略用于获取实验海域的底质参数, 反演结果验证了弹性沉积层海底模型对水体中声虹吸效应的作用机制.The low-grazing-angle reflection on elastic sediment seabed exhibits abnormally enhanced frequency characteristics, which significantly influences long-range sound propagation in shallow water. To study the influence of elastic sedimentary layer seabed environment on long-range sound propagation in shallow waters, we conduct a joint measurement of seabed and waveguide sound propagation in the Dongsha area of the South China Sea. The measurements show for the first time that the seabed resonance and the sound siphon effect occur simultaneously. Notably, this effect is different from the sound siphon effect observed in low-sound-speed seabed environments, as it exhibits smaller frequency intervals. By analyzing the low-grazing-angle reflection characteristics of the elastic seabed, we develop a theoretical model for the resonance frequencies of shear waves in elastic sediment layers under small grazing angles and investigate their influence on long-range sound propagation. The results indicate that under an elastic seabed model, the low-grazing-angle reflection modulated by shear waves induces resonance at specific frequencies within the sediment layer. This trap acoustic energy in the seabed, leading to the sound siphon effect. Furthermore, we analyze the sensitivity and coupling of key parameters related to the resonance frequency of shear wave. According to these findings, we develop an inversion strategy that integrates seabed and waveguide observations to estimate geo-acoustic parameters of the experimental area. The inversion results validate the mechanism by which the elastic seabed model contributes to the sound siphon effect in the water column.
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图 1 实验环境及示意图 (a)声传播实验区域地形及接收设备的位置; (b)海底与波导声传播同步测量实验示意图
Fig. 1. Experimental environment and schematic diagram: (a) Topography of sound propagation experiment area and location of receiver; (b) schematic diagram of synchronous measurement experiment of sound propagation in seabed and waveguide.
图 2 邻近海域多道地震剖面反演结果
Fig. 2. Seabed profile inversion using multichannel seismic data in adjacent areas.
图 3 海底OBS测量的振速信号 (a)声源与OBS距离16.46 km的归一化频谱, 水平质点振速的径向(顶部)和横向(中部)分量, 垂直质点振速分量(底部); (b)距离OBS 2.7—24.8 km范围内15个爆炸声源信号的平均PSD
Fig. 3. Particle velocity signal measured by OBS: (a) Normalized spectrum of a sound source and OBS at a distance of 16.46 km, normalized spectra of the radial (top) and cross-range (middle) components of the horizontal particle velocity, as well as vertical particle velocity (bottom); (b) the power spectral density averages 15 explosion sound source signals within the 2.7–24.8 km range from the OBS.
图 4 VLA测量的爆炸声信号 (a)声源与VLA距离19.2—50.0 km范围的PSD, 其中每个距离的PSD级采用VLA所有阵元数据的平均结果; (b)声源与VLA距离20.8 km的PSD
Fig. 4. Explosion sound signal measured by VLA: (a) PSD within the range of 19.2–50.0 km from the sound source and VLA, where the PSD of each distance is the average result of all array metadata of the VLA; (b) PSD with a distance of 20.8 km between the sound source and the VLA.
图 5 三个不同距离声源的20—150 Hz频段的PSD在37 Hz存在能量凹陷
Fig. 5. The PSD of the 20~150 Hz frequency band has an energy depression at 37 Hz.
图 7 海底反射损失与频率的关系, 其中反射损失是利用Porter[26]的BOUNCE程序计算获得 (a)低声速液态沉积层海底的地声参数见表1; (b)弹性沉积层海底的地声参数如表2; (c)高声速液态沉积海底采用了表2中除了剪切波参数以外的所有参数
Fig. 7. Relationship between seabed reflection loss and frequency. Porter’s BOUNCE program calculates the reflection loss: (a) The geo-acoustic parameters of the seabed of low-velocity liquid sediments are shown in Table 1; (b) geo-acoustic parameters of elastic sedimentary seabed are shown in Table 2; (c) all the parameters except the shear wave parameters in Table 2 are used in the high-velocity liquid deposition seabed.
图 8 弹性海底低掠射角反射与剪切波共振的频率关系. 绿色、蓝色和黑色曲线分别表示掠射角为2°, 5°和10°的反射损失曲线; 红色圆圈是沉积层剪切波共振频率((8)式)
Fig. 8. Frequency relationship between low grazing angle reflection and shear wave resonance in elastic seabed. The green, blue, and black curves represent the reflection loss curves for grazing angles of 2°, 5°, and 10°, respectively; the red circle is the shear wave resonance frequency (Eq. (8)).
图 9 传播损失的频率特征, 其中声源深度为50 m, 接收深度为80 m (a)低声速液态沉积层海底; (b)弹性沉积层海底; (c)高声速液态海底
Fig. 9. Frequency characteristics of propagation loss, where the sound source depth is 50 m, and the receiving depth is 80 m: (a) Low-velocity liquid sediments seabed; (b) the elastic sedimentary seabed; (c) the high-velocity liquid seabed.
图 10 剪切波波共振频率小掠射角反射损失特征的敏感性分析 (a)底部反射损失与剪切波衰减的关系, 同一环境取低掠射角($\theta < \arccos \left( {{{{c_1}} \mathord{\left/ {\vphantom {{{c_1}} {{c_{{\text{p2}}}}}}} \right. } {{c_{{\text{p2}}}}}}} \right)$)的反射损失极大值; (b)共振频率263.1 Hz对剪切波衰减的敏感性; (c)共振频率263.1 Hz对基底剪切波衰减的敏感性; (d)共振频率263.1 Hz对纵波衰减的敏感性
Fig. 10. Sensitivity analysis of small grazing angle reflection loss characteristics of shear wave resonance frequency: (a) Relationship between bottom reflection loss and shear wave attenuation, and the maximum reflection loss of low grazing angle ($\theta < $$ \arccos \left( {{{{c_1}} \mathord{\left/ {\vphantom {{{c_1}} {{c_{{\text{p2}}}}}}} \right. } {{c_{{\text{p2}}}}}}} \right)$) is taken in the same environment; (b) sensitivity of the resonance frequency 263.1 Hz to the shear wave attenuation: (c) sensitivity of the resonance frequency 263.1 Hz to the base shear wave attenuation; (d) sensitivity of the resonance frequency 263.1 Hz to the longitudinal wave attenuation.
图 11 剪切波共振频率闭式表达式的耦合关系
Fig. 11. Coupling relationship of shear wave resonance frequency closed-form expression.
图 12 (a) OBS接收的振速信号水平分量和垂直分量的时域波形, 该声源距离OBS 19.82 km; (b)利用复声强法计算的0—20 km声源的俯仰角
Fig. 12. (a) Time domain waveforms of the horizontal and vertical components of the particle velocity signal received by the OBS. The sound source is 19.82 km away from the OBS; (b) the elevation angle of 0–20 km sound source is calculated by using the complex sound intensity method.
图 14 实际波导环境的弹性沉积层海底参数敏感性分析, 红色虚线为真值
Fig. 14. Sensitivity analysis of seabed parameters of elastic sediment layer in actual waveguide environment, Red dotted line is true value.
图 15 根据反演结果计算的50—250 Hz频段传播损失与实验测量得到的传播损失结果对比 (a)声源与VLA的距离20.8 km; (b)声源与VLA的距离24.99 km
Fig. 15. Propagation loss in the 50–250 Hz band calculated from the inversion results is compared with the experimental measurement: (a) Distance from a sound source to VLA is 20.8 km; (b) from the sound source to VLA is 24.99 km.
表 1 低声速沉积层海底的声学参数
Table 1. Acoustic parameters of low sound velocity sediment seabed.
海底声学参数 压缩波声速/${\text{(m}} {\cdot} {{\text{s}}^{{{ - 1}}}})$ 层厚/${\text{m}}$ 密度/${\text{(g}} {\cdot} {\text{c}}{{\text{m}}^{{{ - 3}}}})$ 压缩波衰减/${\text{(dB}} {\cdot} {\lambda ^{{{ - 1}}}})$ 水介质层 1499 100 1.0 0 沉积层 1465 15 1.6 0.1 基底 1650 — 1.9 0.2 
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表 2 弹性沉积层海底参数
Table 2. Acoustic parameters of elastic sediment seabed.
声学参数 压缩波声速/${\text{(m}} {\cdot} {{\text{s}}^{{{ - 1}}}})$ 剪切波声速/${\text{(m}} {\cdot} {{\text{s}}^{{{ - 1}}}})$ 层厚/${\text{m}}$ 密度/${\text{(g}} {\cdot} {\text{c}}{{\text{m}}^{{{ - 3}}}})$ 压缩波衰减/${\text{(dB}} {\cdot} {\lambda ^{{{ - 1}}}})$ 剪切波衰减/${\text{(dB}} {\cdot} {\lambda ^{{{ - 1}}}})$ 水层 1499 — 100 1.0 0 — 沉积层 1600 700 15 1.6 0.1 0.1 基底 2800 1600 — 2.2 0.2 0.2
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表 3 弹性沉积层海底地声参数反演结果
Table 3. Inversion results of seafloor geoacoustic parameters of the elastic sedimentary layer.
层 反演参数 寻优区间 反演结果 沉积层 ${c_{{\text{p2}}}}{\text{/(m}} {\cdot} {{\text{s}}^{{{ - 1}}}})$ 1550—1800 1640.7 ${c_{{\text{s2}}}}{\text{/(m}} {\cdot} {{\text{s}}^{{{ - 1}}}})$ 500—800 681.0 $h{\text{/m}}$ 2—40 14.5 ${\rho _{2}}{\text{/(g}} {\cdot} {\text{c}}{{\text{m}}^{{{ - 3}}}})$ 1.55—2.1 1.65 ${\alpha _{{\text{p2}}}}{\text{/(dB}} {\cdot} {\lambda ^{{{ - 1}}}})$ 0.05—0.5 0.14 ${\alpha _{{\text{s2}}}}{\text{/(dB}} {\cdot} {\lambda ^{{{ - 1}}}})$ 0.05—0.5 0.11 基底 ${c_{{\text{p3}}}}{\text{/(m}} {\cdot} {{\text{s}}^{{{ - 1}}}})$ 1850—3000 2476 ${c_{{\text{s3}}}}{\text{/(m}} {\cdot} {{\text{s}}^{{{ - 1}}}})$ 1510—1800 1757.6 ${\rho _3}{\text{/(g}} {\cdot} {\text{c}}{{\text{m}}^{{{ - 3}}}})$ 2.15—4.0 3.5 ${\alpha _{{\text{p3}}}}{\text{/(dB}} {\cdot} {\lambda ^{{{ - 1}}}})$ 0.1—0.6 0.22 ${\alpha _{{\text{s3}}}}{\text{/(dB}} {\cdot} {\lambda ^{{{ - 1}}}})$ 0.1—0.6 0.29
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