Effects of variability associated with the Antarctic circumpolar current on sound propagation in the ocean

Abstract

S U M M A R YA series of small depth charges was detonated along a transect from New Zealand to Antarctica over a period of 3 days in late December of 2006. The hydroacoustic signals were recorded by a hydrophone deployed near the source and at a sparse network of permanent hydrophone stations operated by the International Monitoring System (IMS), at distances up to 9600 km. Our purpose was to determine how well signal characteristics could be predicted by the World Ocean Atlas 2005 (WOA05) climatological database for sources within the Antarctic circumpolar current (ACC). Waveforms were examined in the 1-100 Hz frequency band, and it was found that for clear transmission paths, the shot signals exceeded the noise only at frequencies above 20-30 Hz. Comparisons of signal spectra for recordings near the source and at the IMS stations show that transmission loss is nearly uniform as a function of frequency.Where recorded signal-to-noise ratios are high, observed and predicted traveltimes and signal dispersion agree to within 2 s under the assumption that propagation is adiabatic and follows a geodesic path. The deflection of the transmission path by abrupt spatial variations in sound speed at the northern ACC boundary is predicted to decrease traveltimes to the IMS stations by several seconds, depending on the path. Acoustic velocities within the ACC are predicted to vary monthly, hence the accuracy of source location estimates based only on arrival times at IMS stations depends on the monthly or seasonal database used to predict traveltimes and on whether we account for path deflection. However, estimates of source locations within the ACC that are based only on observed waveforms at IMS hydrophones are highly dependent on the configuration of the IMS network; a set of shots observed only at an IMS station in the Indian Ocean and another in the South Pacific was located to within 10 km in longitude, but was poorly constrained in latitude. Several sets of shots observed only at IMS hydrophones in the Indian Ocean were constrained to within 55 km in latitude but were poorly constrained in longitude.

Tracking Submarine Volcanic Activity at Monowai: Constraints From Long‐Range Hydroacoustic Measurements

“…We attribute the offset between mean back azimuths of volcanic episodes, averaging at 242.8 ± 0.3 and 243.4 ± 0.3° (two sigma standard deviation) for the 2003–2004 and 2014–2017 period, respectively, to the repositioning of the hydrophone sensors during the 2014 reinstallation of the H03 station. The systematic, counterclockwise deviation of 0.4 to 1° from the geodesic angle of arrival (243.8°) probably reflects a cumulative effect of uncertainty in sensor positioning (Nichols & Bradley, ), array geometry, and right‐lateral refraction of the acoustic signal along its great circle path following horizontal temperature gradients in the southern Pacific Ocean as well as between the hydrophone elements (de Groot‐Hedlin et al, ; Munk et al, ). Our estimates are in agreement with Evers et al () and Green et al (), who place the error inherent to IMS‐type triplet deployments at ≥0.4°.…”

Deep seafloor arrivals: An unexplained set of arrivals in long-range ocean acoustic propagation

“…Time fronts corresponding to weak surface-reflected bottom-reflected ͑SRBR͒ paths, which often attenuate very rapidly with range and are often unnecessary to successfully predict long-range, deep-water, propagation ͑de Groot- Hedlin et al, 2009;Heaney et al, 1991;Van Uffelen et al, 2009;Wage et al, 2003͒, can be seen as more horizontal time fronts at mid-water depths between 1076.5 and 1077.5 s and at shallow depths between 1078 and 1080 s.…”

Antarctic icebergs: A significant natural ocean sound source in the Southern Hemisphere

“…At a latitude higher than 60 S, the acoustic energy radiated near the surface propagates efficiently through the surface sound channel duct, crosses the Antarctic Convergence Zone, and then becomes trapped within the deep sound channel at midlatitudes [Chapp et al, 2005;de Groot-Hedlin et al, 2009]. Taking advantage of long baselines between OSU/NOAA EPP and IMS HA03N arrays in the Pacific of approximately 5600 km, iceberg locations ( Figure 1) were estimated from the acoustic time series using an iterative nonlinear least square method [Fox et al, 2001].…”