Center for Ocean Engineering, Massachusetts Institute of Technology, 77 Mass. Ave., Cambridge, MA, 02139, USA.
Sci Rep. 2020 Mar 11;10(1):4525. doi: 10.1038/s41598-020-61043-w.
Acoustics is the primary means of sensing and communication in the ocean for humans and many marine animals. Natural fluctuations in the ocean, however, degrade these abilities in ways that have been previously difficult to forecast. Here, we address this issue by predicting sensing and communication degradation in terms of acoustic attenuation, dispersion and temporal decorrelation at typical operational ranges and frequencies in continental-shelf environments. This is done with analytic expressions derived from first physical principles. The analytic expressions provide the statistics of the acoustic field after forward propagating through an ocean waveguide containing 3-D random inhomogeneities from the independent or combined effects of rough sea-surfaces, near-sea-surface air bubbles and internal waves. The formulation also includes Doppler effects caused by the inhomogeneities' random horizontal motion, enabling modeling and prediction over a wide range of environments and frequencies. Theoretical predictions are confirmed with available acoustic measurements in several continental-shelf environments using standard oceanographic measurements for environmental support. We quantify how the acoustic signals decorrelate over timescales determined by the underlying temporal coherence of ocean dynamic processes. Surface gravity waves and near-sea-surface air bubbles decorrelate acoustic signals over seconds or less, whereas internal waves affect acoustic coherence at timescales of several to tens of minutes. Doppler spread caused by the inhomogeneities' motion further reduces acoustic temporal coherence, and becomes important at the high frequencies necessary for communication and fine-scale sensing. We also show that surface gravity waves and bubbles in high sea states can cause increasingly significant attenuation as frequency increases. The typical durations of marine mammal vocalizations that carry over great distances are found to be consistent with the coherence timescales quantified here and so avoid random distortion of signal information even by incoherent reception.
声学是人类和许多海洋动物在海洋中感知和交流的主要手段。然而，海洋的自然波动以以前难以预测的方式降低了这些能力。在这里，我们通过预测在大陆架环境中的典型操作范围和频率下的声衰减、频散和时间去相关来解决这个问题。这是通过从第一物理原理推导出的解析表达式来完成的。这些解析表达式提供了在包含来自粗糙海面、近海面气泡和内波的独立或组合影响的三维随机非均匀性的海洋波导中向前传播后的声场的统计信息。该公式还包括由非均匀性的随机水平运动引起的多普勒效应，从而能够在广泛的环境和频率范围内进行建模和预测。使用标准海洋学测量来支持环境，通过在几个大陆架环境中进行的可用声学测量来验证理论预测。我们量化了在由海洋动力过程的基本时间相干性确定的时间尺度上，声信号如何去相关。表面重力波和近海面气泡在几秒钟或更短的时间内使声信号去相关，而内波则在数分钟到数十分钟的时间尺度上影响声相干性。非均匀性运动引起的多普勒展宽进一步降低了声学时间相干性，并且在用于通信和精细传感的高频下变得重要。我们还表明，高海况下的表面重力波和气泡会随着频率的增加而导致越来越显著的衰减。在远距离传播的海洋哺乳动物发声的典型持续时间被发现与这里量化的相干时间尺度一致，因此即使通过非相干接收也避免了信号信息的随机失真。
