Shallow water mid-frequency ambient noise observations off southern California

The shallow water environment is challenging for geoacoustic inversions due to site-specific complex bathymetry and sub-bottom characteristics as well as time-evolving oceanography. Shallow water mid-frequency (0.5–10 kHz) ambient noise and source tow transmission observations are presented that were collected with a 2D array (512 elements arranged in 8 vertical staves of 64-elements each all half-wavelength spaced at 6 kHz). The data were collected in August-September 2021 WSW of the Ports of Los Angeles and Long Beach and NW of Santa Catalina Island. The water depth at the array site was 311 m with the bathymetry gradually becoming deeper to the north and was ∼600–900 m deep in the east-west region for ships transiting in/out of LA/Long Beach. The mid-frequency observations included tonal source tow transmissions for characterizing propagation in the region, the radiated signatures of ships transiting the area, and the relatively quiet periods between. The 2D array geometry facilitates decomposing the acoustic field in azimuth as well as elevation. The long-term intent is to implement azimuthally and range-dependent geoacoustic inversions for the seafloor properties in this region as well as to compare shipping as sources of opportunity to the tonal transmissions.