Acoustic Resonances and the Determination of the Seafloor Properties:

Theory and Experiment

A0015 Margarete S. Fokina Institute of Applied Physics, Russian Academy of Science

A0016 Vladimir N. Fokin Institute of Applied Physics, Russian Academy of Science

A0017 Viktor V. Kurin Department of Radiophysics, Nizhny Novgorod State University

A0018 Leonid M. Kustov Department of Radiophysics, Nizhny Novgorod State University

This paper deals with the problem of seabed identification by acoustic

means. It has recently been shown [A. Nagle, H.Uberall and K-B.Yoo.

Inverse Problems 1 (1985) pp.99-100] that thephysical properties of a

layered ocean floor may be determined from an analysis of the

resonances in frequency, or in the angle of incidence, which appeared

in bottom-reflected acoustic signals. The characteristics of

resonances (position, amplitude and half-width) were used for recovery

of significant bottom parameter such as acoustic impedance,

attenuation coefficient and sediment thickness. The resonance

formalism previously developed by H.Uberall was extended for case of

elastic sediment layer overlying elastic half-space was used for

determination of bottom parameters. The resonance positions and

widths are explicitly obtained by real equations in term of given

material quantities. Characteristic equations for explicit

determination of the position of each individual resonance

contribution to the reflection coefficient are derived. Analytical

expressions for the angular and frequency resonance positions and

half-width are found. The exact expression for reflection coefficient

was transformed to a Breit-Wigner form. The dependences of the

resonance position, half-width of resonances and it's amplitude from

seabed parameters were investigated in the planes of two independent

bottom parameters. On the base of these investigations a method for

determination of significant bottom parameter was proposed. For its

validation it was decided to process the data from a tank experiment

in which all the parameters were controlled and measured with high

precision. Experimental results obtained on a small-scale sediment

model and results of numerical simulation were used for the resonance

method validation. This study was performed in the framework of the

RFBR project (No 00-05-64956).