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Range estimates of whale signals recorded by triplets of hydrophones
  • Ronan Le Bras,
  • Peter Nielsen
Ronan Le Bras

Corresponding Author:[email protected]

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Peter Nielsen
Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization
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The International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) includes a hydro-acoustic network as one of the monitoring technologies. The underwater part of this network includes six stations each composed of two sets of three hydrophones or triplets, except for HA01 (Cape Leeuwin, Australia) which is composed of a single triplet. The hydro-acoustic network is now complete with the recent installation of the HA04 station located in the Southern Ocean island of Crozet (France). A large number of calls emanating from marine mammals are recorded by the hydrophones, and we present examples where the animals are sufficiently close to attempt a range estimate. We also present examples of scattered arrivals and related interpretations. One striking example of extremely accurate range estimation is obtained for a whale in the neighborhood of the Cape Leeuwin (Australia) HA01 IMS stations. The proximity to the station and in particular to hydrophone H01W2 was first hypothesized because a running cross-correlation computation showed that the apparent velocity of the source was very high and could not be explained by hydro-acoustic waves travelling within the SOFAR channel. Since the far-field, plane wave assumption does not apply anymore in this case, a grid search was implemented to locate the source of the signal with the added assumption that the source is close to the ocean surface. As a further confirmation of the proximity of the source to the hydrophones, and given the expectation that such a source would generate scattering from the ocean floor and from the free surface, reflections are observed and the travel time of the scattered waves confirm the position calculated from the grid search using the direct arrivals.