Acoustics variability of air gun signals recorded at intermediate ranges within the Lau Basin

Abstract

During January–February 2009, an active‐source seismic survey was performed over the Eastern Lau Spreading Center in the Lau Back‐Arc Basin (21°S, 176°S). Acoustic signals generated by theR/V Langseth's 36‐gun pneumatic source array were recorded within the deep sound channel at offsets of 29–416 km. The local ocean acoustic environment is everywhere bottom limited, with seafloor depths within the study domain ranging from ∼1700–2800 m. Low‐frequency (4–125 Hz) sound levels are monitored using root‐mean‐square, energy‐flux‐density and zero‐to‐peak measurement techniques. From these field data, transmission loss is found to exceed the predictions of a geometric spherical spreading model. At similar ranges, arrival amplitudes vary by up to 20 dB and durations vary by a factor of three to six. The depth of the seafloor beneath the air gun source exhibits a positive correlation with arrival duration and a negative correlation with range‐corrected amplitude, explaining up to 30% of the observed variation in both parameters. The strength of this correlation, however, varies for stations lying at different azimuths, highlighting the importance of seafloor aspect and slope in the coupling of bottom‐interacting acoustic energy into the sound channel. Range‐dependent ray tracing shows that shots deployed over shallower seafloor are more likely to produce sound channel trapped signals that propagate with limited bottom interaction. This results in arrivals that are more impulsive, with shorter durations and higher amplitudes. Shots deployed in deeper water typically undergo a larger number of bounces and are characterized by more emergent, longer duration and smaller amplitude arrivals.