Abstract:
The acoustic properties of marine sediments have a direct effect on the
propagation of sound in the ocean. In the frequency range of interest (50 - 500 Hz)
the sediment can be modelled as a fluid. Assuming horizontal stratification of the
ocean bottom, the acoustic parameters of interest are the compressional wave
speed, the compressional wave attenuation and density as a function of depth.
An inverse method based on a perturbation technique is presented in this
thesis for the determination of these parameters. A monochromatic source
experiment is proposed because of the desirability of such an experiment for
determining the acoustic properties of an anelastic medium. The input information
is the plane wave reflection coerricent as a function of the angle of incidence at a
fixed frequency. A nonlinear integral equation relating the variations of these
acoustic properties from a known reference value to the plane wave reflection
coefficient is derived. This is then linearised using the Born approximation. The
region of validity of the Born approximation is derived and based on this the
optimum angular aperture for the input data is obtained.
The linearised integral equation is a Fredholm integral equation of the first
kind. An acceptable stable solution of the integral equation is obtained by imposing
a priori constraints on the solution. The inversion method is tested using synthetic
data and inversions are carried out for various examples of the attenuation
coefficient profile and the sound speed profile. The results obtained with noise free
data show good agreement between the true profiles and the reconstructed profiles.
The resolution obtainable with the data set is studied using the resolving power
theory of Backus and Gilbert and the inversion method is shown to provide
adequate resolution. The effect of additive noise in data is examined and inversions
performed with noisy data yielded stable acceptable results.
