Blind channel estimation of underwater acoustic waveguide impulse responses using marine mammal vocalizations

Abstract

Some techniques for underwater passive acoustic localization make use of estimates for the direct and/or interface-reflected acoustic arrival times of an acoustic source at one or more underwater hydrophones.  This estimation task can be difficult for non-impulsive sound sources, such as humpback whales, due to later arrivals being masked by earlier ones.  In linear system analysis, the impulse response of a system is the system output when the input is an impulse (i.e., a short duration, large bandwidth signal).  The impulse response of a system contains information about the propagation environment, and expresses how a source signal interacts with the environment to yield the output waveform (e.g., that recorded by a hydrophone).  The recording of a relatively impulsive call (e.g., a sperm whale click) approximates the impulse response between the whale and hydrophone.  In recordings of such a call, arrival time estimation can be straightforward.  For non-impulsive calls, if the source waveform is known, the impulse response can be estimated through techniques such as deconvolution or cross-correlation.  The case of unknown, non-impulsive calls is particularly challenging.  However, given the wealth of information present in the impulse response, we seek to estimate impulse responses even in these adverse conditions.  Blind channel estimation (BCE) is the process of estimating the set of impulse responses between a single (frequently, unknown) source and multiple receivers, and can potentially help estimate direct and interface-reflected arrival times for non-impulsive marine mammal vocalizations since the impulse responses can be analyzed rather than the waveforms or spectra.  In this paper, we look at simulation and measured-data results using marine mammal vocalizations to estimate underwater acoustic impulse responses.