Estimating fin whale distribution from ambient noise spectra using Bayesian inversion

TitleEstimating fin whale distribution from ambient noise spectra using Bayesian inversion
Publication TypeThesis
Year of Publication2015
AuthorsMenze, S
Refereed DesignationRefereed
SupervisorsSchrum, C
NERSC SupervisorsSagen, Hanne,, Hobæk, H.,
Secondary TitleMaster in Marine Ecosystems and Climate
Number of Pages117
Date Published06/2015
PublisherGeophysical Institute, University of Bergen
Thesis TypeMaster
Thesis NumberNA
Keywordsacoustic monitoring, ambient noice, marine mamals
Passive acoustic monitoring is increasingly used to study the distribution and migra-
tion of marine mammals. Marine mammal vocalizations are transient sounds, but the
combined sound energy of a population continuously repeating a vocalization, adds up
to a quasi-continuous chorus. Marine mammal choruses can be identified as peaks in
ocean ambient noise spectra. In the North Atlantic, the fin whale chorus is commonly
observed as peak at 20 Hz. This thesis proposes a method to estimate the distribution
of vocalizing fin whales based on a set of fin whale chorus recordings. This is an ex-
tremely under-determined inverse problem. The method is based on Bayesian inverse
theory and uses simulated annealing to estimate the most likely distribution of sound
sources (vocalizing whales) on a geodesic grid. This includes calculating a transmission
loss matrix connecting all grid nodes and recorders, using an arbitrary sound propagation
model. Two models were successfully implemented: geometrical spreading and the ray
trace model BELLHOP. The inversion method was tested under different scenarios. The
results indicated that an imprecise transmission loss matrix is tolerated by the inversion
method. The accuracy of the method depended mainly on the number and distribution of
recorders. For the Norwegian sea, simulations showed that fin whale chorus inversion is
possible using as few as 12 recorders between Iceland and Svalbard. An inversion based
on data from published fin whale chorus observations indicated realistic winter distribu-
tion patterns. Existing methods to study marine mammal distribution are often confined
to the summer months and a limited area. Future application of the proposed method
admits automatic year-round monitoring of marine mammal distribution on a basin-wide.
Author Address