Energetics of the deep ocean’s infrasonic sound field

Abstract

Simultaneous measurements of infrasonic (0.5–20 Hz) acoustic particle velocity and acoustic pressure made by the Marine Physical Laboratory’s set of freely drifting Swallow floats are analyzed in terms of the energetics of acoustic fields. Results from a recent deep-ocean deployment indicate that the midwater column’s acoustic potential and kinetic energy density spectra are equal above 1.7 Hz since, away from the ocean boundaries, the sound field is locally spatially homogeneous. Near the ocean bottom, the vertical spatial inhomogeneity is statistically significant between 0.6–1.4 Hz and 7–20 Hz. In the lower band, the pressure autospectrum decreases with increasing distance from the ocean bottom, whereas in the upper band, it increases due to the deep sound channel’s ability to trap acoustic energy at the higher infrasonic frequencies. For ship signals, the signal-to-noise ratio in the active intensity magnitude spectrum is 3–6 dB greater than in either of the two energy density spectra due to the vector nature of acoustic intensity. Although smaller than the net horizontal flux above a few hertz, a statistically significant net vertical flux density of energy occurs across the whole frequency band, from the ocean surface into the bottom. The direction of the net horizontal flux density for various discrete sources, e.g., a magnitude 4.1 earthquake, a blue whale, and commercial ships, is discussed. The net horizontal flux density of the background sound field between 5 and 12 Hz may have been determined by the surrounding ocean bottom topography in one experiment; its direction approximately coincides with the center of a topographic window. However, it also matches the heading toward a 4000-km-distant hurricane. A third possibility of an unknown, broadband source cannot be eliminated with the available data.