Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2013The interpretation of echoes collected by active remote-sensing systems, such as sonar
and radar, is often ambiguous due to the complexities in the scattering processes involving
the scatterers, the environment, and the sensing system. This thesis addresses
this challenge using a combination of laboratory and fi eld experiments, theoretical
modeling, and numerical simulations in the context of acoustic scattering by marine
organisms. The unifying themes of the thesis are 1) quantitative characterization of
the spectral, temporal, and statistical features derived from echoes collected using
both broadband and narrowband signals, and 2) the interpretation of echoes by establishing
explicit links between echo features and the sources of scattering through
physics principles. This physics-based approach is distinct from the subjective descriptions
and empirical methods employed in most conventional fisheries acoustic
studies. The fi rst part focuses on understanding the dominant backscattering mechanisms
of live squid as a function of orientation. The study provides the first broadband
backscattering laboratory data set from live squid at all angles of orientation, and conclusively
con firms the
fluidlike, weakly-scattering material properties of squid through
a series of detailed comparisons between data and predictions given by models derived
based on the distorted-wave Born approximation. In the second part, an exact
analytical narrowband model and a numerical broadband model are developed based
on physics principles to describe the probability density function of the amplitudes
of echo envelopes (echo pdf) of arbitrary aggregations of scatterers. The narrowband echo pdf model signi cantly outperforms the conventional mixture models in analyzing
simulated mixed assemblages. When applied to analyze fish echoes collected
in the ocean, the numerical density of sh estimated using the broadband echo pdf
model is comparable to the density estimated using echo integration methods. These
results demonstrate the power of the physics-based approach and give a rst-order
assessment of the performance of echo statistics methods in echo interpretation. The
new data, models, and approaches provided here are important for advancing the
eld of active acoustic observation of the ocean.Taiwan Merit Scholarship (NSC-095-SAF-I-564-021-TMS), Office of Naval Research
(ONR; grants N00014-10-1-0127, N00014-08-1-1162, N00014-07-1-1034), National
Science Foundation (NSF; grant OCE-0928801), Naval Oceanographic Offi ce
(grant N62306007-D9002), WHOI Ocean Life Institute, and the WHOI Academic
Programs O ffice funds