Application of a seismic network to baleen whale call detection and localization in the Panama basin-a Bryde's whale example.

Baleen whales use sounds of various characteristics for different tasks and interactions. This study focuses on recordings from the Costa Rica Rift, in the Eastern Tropical Pacific Ocean, made by 25 ocean-bottom seismographs and a vertical array of 12 hydrophones between January and February 2015. The whale calls observed are of two kinds: more commonly, repetitive 4-5 s-long signals separated into two frequency bands centered at ∼20 and ∼36 Hz; less commonly, a series of ∼0.5 to 1.0 s-long, lower amplitude signals with frequencies between 80 and 160 Hz. These characteristics are similar to calls attributed to Bryde's whales which are occasionally sighted in this region. In this study, the repetitive calls are detected using both the short-term average/long-term average approach and a network empirical subspace detector. In total, 188 and 1891 calls are obtained for each method, demonstrating the value of the subspace detector for highly similar signals. These signals are first localized using a non-linear grid search algorithm and then further relocalized using the double-difference technique. The high-resolution localizations reveal the presence of at least seven whales during the recording period, often crossing the instrument network from southwest to northeast.

Detecting changes at the leading edge of an interface between oceanic water layers.

Many physical phenomena in the ocean involve interactions between water masses of different temperatures and salinities at boundaries. Of particular interest is the characterisation of finescale structure at the marginal interaction zones of these boundaries, where the structure is either destroyed by mixing or formed by stratification. Using high-resolution seismic reflection imaging, we present observations of temporal changes at the leading edge of an interface between sub-thermocline layers in the Panama Basin. By studying time-lapse images of a seismic reflector between two water boundaries with subtle differences, we provide empirical constraints on how stratified layers evolve. The leading edge of this reflector, which is characterised by a gradual lateral decrease in vertical temperature contrast ([Formula: see text]), increases in length over ~3 days coupled with an increase in [Formula: see text]. A critical mixing state, in which turbulent diffusion is gradually replaced by double-diffusion as the dominant mixing process, is thus revealed.