Measurements of ambient noise and sperm whale vocalizations in the northern Gulf of Mexico using near bottom hydrophones

Abstract

The Littoral Acoustic Demonstration Center (LADC) consisting of the University of Southern Mississippi (USM), the University of New Orleans (UNO), and the Naval Research Laboratory at Stennis Space Center (NRL-SSC), with guidance and technical assistance from the Naval Oceanographic Office (NAVOCEANO), was formed to do ambient noise and marine mammal acoustic measurement and analysis. Three Environmental Acoustic Recording System (EARS) buoys, designed and produced by NAVOCEANO, were deployed by LADC in the northern Gulf of Mexico (GoM) in the summer of 2001. These bottom-moored omni-directional hydrophone recording systems were modified by NAVOCEANO to sample almost 12 kHz, so that the vocalizations of sperm whales could be recorded. The Sperm Whale Acoustic Monitoring Program (SWAMP) was conducted during that summer by the Minerals Management Service and its collaborators. The EARS buoys recorded during the entire 36 days of SWAMP from 17 July through 21 August of 2001. The EARS buoy hydrophones, 50m above the bottom, were placed on a downslope line, ending at the largest concentration of sperm whale sightings in the northern GoM, in 600m, 800m, and 1000m water depths. The moorings were instrumented with self-recording environmental sensors to obtain time series data of temperature, conductivity, and pressure at specified depths spanning the water column. Four cruises were made to deploy and recover the buoys and to collect a suite of environmental measurements, including CTD and XBT casts and a chirp sonar survey for bottom properties to support propagation modeling. In between the first and second cruises, Tropical Storm Barry moved through the area and changes in the oceanographic properties were observed. Each EARS buoy recorded a bandwidth of 5859 Hz for 36 days. These data clearly reveals sperm whale vocalizations, passing ships, and seismic airguns. Marine mammal vocalizations and airgun signatures have been isolated and are being analyzed. Spectral levels for ten minute averages of ambient noise on four different days show moderate shipping levels except during the passage of the tropical storm. A plateau in the noise spectrum from 200 to 1000Hz on one day is due to the presence of sperm whales. Spectrograms show sperm whale clicks and creaks and the seismic airgun signal is very clear.