Energetics of the Deep Ocean's Infrasonic Sound Field

摘要

Simultaneous measurements of infrasonic (0.5-20 Hz) acoustic particle velocityand acoustic pressure made by a set of freely drifting Swallow floats are analyzed in terms of the energetics of acoustic fields. Results from recent deep-ocean deployment indicate that the midwater columns 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 7-20Hz. 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 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 surrounding ocean bottom topography in one experiment; its direction approximately coincides with the center of the topographic window. It also matches the healing toward a 4000-km-distant hurricane.