Foundation for Research & Technology
TNO Defensie en Veiligheid
|Source:||Papadakis, J.S.Bjorno, L., Proceedings of the 1st International Conference Underwater Acoustic Measurements: Technologies & Results , 28th June - 1st July 2005, Heraklion, Crete, Greece|
Physics · Geo-acoustic inversion · Ambient noise · Sub-bottom profiling
The angle and frequency dependent reflective properties of the seabed can be determined from beam-steered ambient noise measurements on a vertical array of hydrophones. From the up-to-down ratio the beam-smeared modulus square of the plane wave reflection coefficient is obtained. Geo-acoustic parameters can be inverted by matching the measured and modelled reflection loss assuming a certain number of sediment layers. The searching for the geo-acoustic parameters is done with fast global optimisation techniques such as differential evolution. An alternative approach is to Fourier transform the reflection coefficient producing the minimum phase impulse response, which explicitly shows layer boundaries. Spectral factorisation is applied to restore the lost phase of the reflection coefficient. This method offers the possibility of determining the number of layers and their order. Both methods were applied to reflection loss data extracted from ambient noise measurements on a drifting vertical array in the shallow waters in the Mediterranean. The results of both methods are compared showing good agreement between the two sets of sediment layering estimates. It is concluded that the methods offer a sub-bottom profiling technique without making additional sound where the array is sensing local seabed properties with a footprint of order the water depth.