O.C. Rodríguez and S.M. Jesus orodrig@ualg.pt , sjesus@ualg.pt
SiPLAB-FCT, Universidade do Algarve
8005-139 Faro, Portugal
Comments: download pdf file
Ref.: Proc. of ICCES' 2004,
(ISBN:0-9657001-6-X), p.1747-1752, Madeira (Portugal), June 2004.
Abstract : Travel time based inversion is a technique of
Ocean
Acoustic Tomography, which allows to estimate a field of sound speed
perturbation, c, by inverting a system of linear equations,
relating travel time and c, through the so-called observation
matrix. Such matrix can be calculated by representing the waveguide as
a layered system, with c being estimated at each layer. For
practical purposes, the parameterization of c on a basis of
orthogonal func-tions can be more advantageous. For instance, the
inversion developed in introduces an expansion of c on a
basis of two-dimensional plane waves, whose amplitudes are estimated in
order to map the temporal evolution of the environmental field in a
complex bathymetry, through the usage of several sources and a
significant amount of receivers. Theoretical modes can be used also as
a basis for the estimation of c(r, z). For a reduced amount of
receivers the vertical structure of the field can be expanded on the
modes, while the range dependence can be constrained using horizontal
plane waves. Such choice of basis significantly reduces the number of
parameters to be estimated, but its reliability depends on plane wave
propa-gation along range, which is only a particular case. Furthermore,
travel time inversion requires the synchronization between the emitted
and received signals, so the instant of emission can be taken as the
origin of the time axis for the arrivals. Otherwise, the technique has
to be modified in order to develop the inversion. In the discussion
presented in relative-time inversion is developed, by expanding c
on a basis of empirical modes, and optimizing the match between
observed and modeled arrivals within the search space of mode
amplitudes. Although the validity of such approach depends on the
uniqueness of the solution, the corresponding estimates of
depth-averaged tempera-tures provided a good agreement with independent
observations. The approach discussed in this paper reconsiders the
parameterization of c on a basis of theoretical modes, without
imposing particular analytical constraints to their amplitudes, except
that their variation is sufficiently smooth along range. Thus, wave
propagation is not restricted to plane waves. Further, the system of
equations is rewritten in order to relate modal amplitudes to relative
arrivals. The performance of this approach is discussed using
environmental and acoustic data from the INTIMATE 96 experiment.
ACKNOWLEDGMENT: this work was partially supported by INTIMATE, ATOMS and NUACE projects from FCT - Portugal.