Ocean sound levels in the northeast Pacific recorded from an autonomous underwater glider

Abstract

Ocean gliders are a quiet and efficient mobile autonomous platform for passive acoustic monitoring and oceanographic measurements in remote marine environments. During July 20-August 6 2012, we used a Teledyne Webb Research Slocum G2 glider equipped with a hydrophone logging system to record ocean sound along a 458 km north to south traverse of the outer continental shelf break along the U.S. Pacific Northwest coast. Glider derived recordings yielded a unique perspective on the variation of ambient sound with depth, where natural wind generated surface processes were identified as a dominant acoustic contributor to spectral levels in the region. Near and far-field vessel radiated noise were also found to add significant energy to ambient conditions. Spatially distributed measurements of ambient sound levels recorded from the glider were consistent with long-term spectral estimates from fixed station, deep ocean hydrophone array measurements during the 1990-2000's in the region. Ocean sound level measurements captured by a mobile glider are shown to be an effective and valuable asset for describing ocean surface wind conditions and characterizing spatial and temporal changes in the underwater acoustic environment over a broad regional scale.

Fig 1. Map of the north-south glider mission down the Oregon coast.

Green path shows the position of the glider. The black triangle marks the position of buoy 46089 used for wind speeds and AIS ship tracks are shown as grey lines surrounding the glider path. Circles indicate positions for vocal encounters where Pacific white-sided dolphins (blue) and humpback whales (magenta) where observed in the acoustic records. Bathymetry map was compiled and generated by Susan Merle (NOAA-PMEL/ Oregon State University) using openly accessible bathymetry data from the NOAA National Centers for Environmental Information (NCEI) and Marine Geoscience Data System (MGDS).

Fig 2. Two-dimensional cross sectional views of environmental data collected along the glider flight path.

Water temperature (upper), salinity (mid), and calculated sound velocity (lower) measurements show the 2-D spatial structure of water column properties along the continental shelf break to 650 m depth.

Fig 3. Glider spectrograms during the mission and glider dive profiles on July 30, 2012.

a) A spectrogram calculated from 4 second hanning windows with no overlap showing the spectral energy content recorded during the 18 day glider mission duration. Broadband periods of elevated acoustic energy levels associated with higher wind speeds are observed over 1–3 day periods throughout the mission. b) A 15 minute period showing airgun explosion signals (10–1000 Hz) recorded by the glider near the beginning of the mission on July 22, 2012. c) A 1 day long spectrogram from July 30 showing the acoustic influence of a nearby vessel appearing as discrete bands in energy up to 3 kHz. The glider’s dive profiles throughout the day are also shown.

Fig 4. Spectral probability distributions (SPD) of acoustic energy in 3 depth intervals.

SPDs calculated from 4 second hann windows, no overlap, as recorded by the glider at 3 distinct depth intervals; shallow (10–200 m); mid (200–400 m); and deep (400–650 m). Median (50^th percentile) values shown as black lines and 99^th and 1^st percentiles shown as white lines.

Fig 5. Noise level frequency band averages comparing ship and wind-dependent processes during the glider mission.

a) Frequency band averaged (1–1.05 kHz) glider sound levels (thick line) plotted with wind speeds measured at buoy 46089 (thin line) and ASCAT (diamond-dash) satellite daily wind speed averages at 0.25° scale following the glider. b) Frequency band averaged (50–100 Hz) glider sound levels (thick line) and AIS ship track lengths within a 50 km radius of the daily median glider position (circle-dash). Averaged noise levels are smoothed with a 5 point median filter followed by a 6 hour low pass hann filter.

Fig 6. A linear least squares regression fit (R² = 0.57, p << 0.05) of frequency band averaged (1–1.05 kHz) median noise levels recorded by the glider as a function of wind speeds measured at buoy 46089.

Median noise levels are calculated from 10 cm/s bins and measurements from all depths are combined. 95% confidence intervals are shown as dashed lines.

Fig 7. RAM propagation model output for ship noise generated from two vessel positions with respect to the glider (f = 100 Hz).

(upper) Propagation loss from a range of 80 km SW of the glider traveling upslope from deeper water to the continental shelf break near 1000 m depth (lower) Energy loss from a range of 80 km to the NE of the glider propagating down the continental shelf toward the glider position. Sound source originated at 5 m depth.

Fig 8. 2-D scatter maps showing ambient conditions along the continental shelf break from the glider’s north to south progression.

The (a) wind dependent frequency band and (b) ship noise band. Noise levels are calculated from 10 minute averages and the data during the mission along the glider track is shown on the upper x-axis. Note the change in energy scale between the upper and lower figures.

Fig 9. Spectrograms of bioacoustic signals observed along the glider mission.

(a) 7 second long spectrogram of the low frequency component of commonly observed Pacific white-sided dolphin whistles (1–4.4 kHz); (b) an 11 second long spectrogram of low frequency (10–1500 Hz) humpback whale vocalizations recorded by the glider.

Fig 10. A comparison of median (50^th percentile) glider and fixed station spectral levels.

Glider measurements from 3 depth intervals during the 2012 mission plotted against median spectra from a fixed, seafloor hydrophone system recorded from 1994–2007 near the same area in the northeast Pacific [18]. Values for APL/UW spectra taken from Andrew et al.’s, Fig 4 System h.