Maguer A, Bovio E, Fox WL, Schmidt H
NATO SACLANT Undersea Research Centre, La Spezia, Italy.
J Acoust Soc Am. 2000 Sep;108(3 Pt 1):987-96. doi: 10.1121/1.1285953.
Understanding the basic physics of sound penetration into ocean sediments is essential for the design of sonar systems that can detect, localize, classify, and identify buried objects. In this regard the sound speed of the sediment is a crucial parameter as the ratio of sound speed at the water-sediment interface determines the critical angle. Sediment sound speed is typically measured from core samples using high frequency (100's of kHz) pulsed travel time measurements. Earlier experimental work on subcritical penetration into sandy sediments has suggested that the effective sound speed in the 2-20 kHz range is significantly lower than the core measurement results. Simulations using Biot theory for propagation in porous media confirmed that sandy sediments may be highly dispersive in the range 1-100 kHz for the type of sand in which the experiments were performed. Here it is shown that a direct and robust estimate of the critical angle, and therefore the sediment sound speed, at the lower frequencies can be achieved by analyzing the grazing angle dependence of the phase delays observed on a buried array. A parametric source with secondary frequencies in the 2-16 kHz range was directed toward a sandy bottom similar to the one investigated in the earlier study. An array of 14 hydrophones was used to measure penetrated field. The critical angle was estimated by analyzing the variations of signal arrival times versus frequency, burial depth, and grazing angle. Matching the results with classical transmission theory yielded a sound speed estimate in the sand of 1626 m/s in the frequency range 2-5 kHz, again significantly lower the 1720 m/s estimated from the cores at 200 kHz. However, as described here, this dispersion is consistent with the predictions of the Biot theory for this type of sand.
理解声音穿透海洋沉积物的基本物理原理对于设计能够探测、定位、分类和识别埋藏物体的声纳系统至关重要。在这方面，沉积物的声速是一个关键参数，因为水 - 沉积物界面处的声速比决定了临界角。沉积物声速通常使用高频（数百千赫兹）脉冲传播时间测量从岩芯样本中测得。早期关于亚临界穿透砂质沉积物的实验工作表明，在2 - 20千赫兹范围内的有效声速明显低于岩芯测量结果。使用比奥理论对多孔介质中传播进行的模拟证实，对于进行实验所用的那种沙子，砂质沉积物在1 - 100千赫兹范围内可能具有高度色散性。本文表明，通过分析在埋藏阵列上观察到的相位延迟的掠射角依赖性，可以直接且可靠地估计较低频率下的临界角，进而估计沉积物声速。一个二次频率在2 - 16千赫兹范围内的参量源被指向一个类似于早期研究中所调查的砂质底部。使用一个由14个水听器组成的阵列来测量穿透场。通过分析信号到达时间随频率、埋藏深度和掠射角的变化来估计临界角。将结果与经典传输理论相匹配，得出在2 - 5千赫兹频率范围内沙子中的声速估计值为1626米/秒，再次明显低于在200千赫兹时从岩芯估计的1720米/秒。然而，如本文所述，这种色散与比奥理论对这种类型沙子的预测是一致的。
