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SITAR Report Summary

Project ID: EVK3-CT-2001-00047
Funded under: FP5-EESD
Country: Norway

FARIM- a software code for joint estimation of seabed impedance and roughness

Geoacoustic inversion of monostatic data from the main sea trial of the SITAR project has been carried out by NTNU. The method used, named FARIM - Frequency Analysis based Roughness and Impedance estimation Method, estimates the roughness and impedance of the sea bottom from calibrated sonar data. In presence of roughness at the bottom, energy is scattered incoherently away from the receiver. A closed form expression for the coherent part of return with RMS roughness height as parameter is obtained from Kirchhoff scattering theory. The underlying assumption is that roughness is a stochastic variable with a Gaussian probability density. The observed shift in the received signal spectrum, represented by the centre of gravity frequency, towards lower frequencies is then compared to the theoretical shift and an estimate for roughness is obtained. Calculating the bottom impedance simply from the ratio of received to transmitted energy generally results in an underestimation of impedance because of incoherent scattering. The estimated roughness is used to calculate an improved estimate of the bottom impedance.

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O.S. Bragstads plass 2B
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