Observation and modeling of out-of-plane arrivals on near-bottom recorders at the Atlantis II Seamount Complex

This paper reports on measurements of out-of-plane arrivals from an airgun transect over the north slope of the Atlantis II Seamount Complex in the North Atlantic. The data were collected by a set of three near-bottom single-channel acoustic recorders. Time delay analysis is applied to assess the directionality of anomalous arrivals in the received time series that are interwoven between the in-plane bottom-surface and multiple bottom-surface reflected paths. Back propagation using estimated arrival angles in a three-dimensional ray-tracing model facilitates identification of bathymetric features associated with the arrivals. The responsible features were most commonly identified as local maxima that appear as steep-sided spires rising several hundred meters above the seafloor on the slope and near the base of Atlantis II. Arrival times for the predicted propagation paths are recreated by combining two-dimensional ray traces from the source to the bathymetric feature and from the bathymetric feature to the receiver to obtain modeled arrival times for comparison with observations. Multiple examples are analyzed, providing insight into likely contributors to the complex propagation in seamount environments. Forward modeling of the identified propagation paths using three-dimensional ray traces highlights challenges for ray-based acoustic modeling of propagation in the presence of highly variable bathymetry.