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Hitting Rewind Button! Measure Ocean Bottom by Reversing Ocean Noise Back to its Origin

Dec 05, 2017     Email"> PrintText Size

Due to difficulties and high cost of direct measurement for the bottom parameters, geoacoustic inversion as an important kind of indirect methods has received considerable attention in underwater acoustics. 

Recently, researchers from the Institute of Acoustics (IOA) of the Chinese Academy of Sciences, University of Haifa, and University of Colorado Boulder present a passive geoacoustic inversion method, by using two hydrophones and creating time reversal of ocean noise, to measure the bottom parameters indirectly.

This passive acoustic inversion method, which combines noise interferometry (NI) and time reversal mirror (TRM) techniques, employs the ubiquitous ambient noise as a replacement of the designated probing signals.

There is no need for any source, so it is a conceptually simple and low-cost method.

For this passive method, a critical issue is Green’s function (GF) retrieval.

The GFs between two locations can be estimated by cross-correlating time series of ambient noise recorded at those locations.  The noise cross-correlation function (NCCF) is computed from concurrent measurements of ambient noise at two hydrophones. Through a TRM, the retrieved GF is time reversed and back propagated from one receiver, thus producing a focus in the vicinity of the other receiver.

Combined with the Hamilton sediment empirical relationship, the inverse problem is solved by searching over a suitable space of bottom parameter.

The displacements of main focus from the virtual sound source are caused by deviations of bottom parameters from their true values (Figure 1).

 

Figure 1. Displacement of main focus from the virtual sound source versus bottom sound speed. (Image by QIN Jixing) 

In contrast, a mismatch between the actual and assumed bottom parameters shifts and blurs the main focus, making the spurious additional foci more pronounced (Figure 2).

 

Figure 2. Time reversal of GF retrieved from noise cross-correlation function in the Shallow Water ’06 experiment. (Image by QIN Jixing) 

The results of this passive geoacoustic inversion are consistent with those evaluated independently by other means.

Their findings were published in IOP Science.

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(Editor: LI Yuan)

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