Pole STIC, ENSTA Bretagne (Université Européenne de Bretagne), 2 rue Francois Verny, 29806 Brest Cedex 9, France.
J Acoust Soc Am. 2011 Jul;130(1):61-71. doi: 10.1121/1.3592230.
Acoustic propagation in shallow water is characterized by a set of depth-dependent modes, the modal depth functions, which propagate in range according to their horizontal wavenumbers. For inversion purposes, modal depth function estimation in shallow water is an issue when the environment is not known. Classical methods that provide blind mode estimation rely on the singular value decomposition of the received field at different frequencies over a vertical array of transducers. These methods require that the vertical array spans the full water column. This is obviously a strong limitation for the application of such methods in an operational context. To overcome these shortcomings, this study proposes to replace the spatial diversity constraint by a frequency diversity condition, and thus considers the case of a field emanating from an impulsive source. Indeed, because of the discrete nature of the wavenumber spectrum and due to their dispersive behavior, the modes are separated in the time-frequency domain. This phenomenon enables the design of a modal filtering scheme for signals received on a single receiver. In the case of a vertical receiver array, the modal contributions can be isolated for each receiver even when using a partial water column spanning array. This method thus eliminates the receiving constraints of classical methods of modal depth function estimation, although it imposes the use of an impulsive source. The developed algorithm is benchmarked on numerical simulations and validated on laboratory experimental data recorded in an ultrasonic waveguide. Practical applications are also discussed.
浅水中的声波传播具有一组与深度相关的模式，即模态深度函数，它们根据水平波数在距离上传播。对于反演目的，当环境未知时，浅水中的模态深度函数估计是一个问题。提供盲目模式估计的经典方法依赖于在垂直传感器阵列上不同频率下接收场的奇异值分解。这些方法要求垂直阵列跨越整个水柱。这显然是此类方法在实际应用中的一个很大的局限性。为了克服这些缺点，本研究提出用频率分集条件代替空间分集约束，从而考虑了脉冲源辐射场的情况。事实上，由于波数谱的离散性和它们的频散特性，模态在时频域中是分开的。这种现象使得能够为单个接收器接收到的信号设计一种模态滤波方案。在垂直接收器阵列的情况下，即使使用部分跨越水柱的阵列，也可以为每个接收器隔离模态贡献。因此，尽管需要使用脉冲源，但该方法消除了经典模态深度函数估计方法的接收约束。所开发的算法在数值模拟中进行了基准测试，并在超声导波中记录的实验室实验数据上进行了验证。还讨论了实际应用。
