In acoustics, microbaroms, also known as the "voice of the sea", are a class of atmospheric infrasonic waves generated in marine storms by a non-linear interaction of ocean surface waves with the atmosphere. They typically have narrow-band, nearly sinusoidal waveforms with amplitudes up to a few microbars, and wave periods near 5 seconds (0.2 hertz). Due to low atmospheric absorption at these low frequencies, microbaroms can propagate thousands of kilometers in the atmosphere, and can be readily detected by widely separated instruments on the Earth's surface.
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Microbaroms are a significant noise source that can potentially interfere with the detection of infrasound from nuclear explosions that is a goal of the International Monitoring System organized under the Comprehensive Nuclear-Test-Ban Treaty (which has not entered into force). It is a particular problem for detecting low-yield tests in the one-kiloton range because the frequency spectra overlap.
History
Microbaroms were first described in 1939 by American seismologists Hugo Benioff and Beno Gutenberg at the California Institute of Technology at Pasadena, based on observations from an electromagnetic microbarograph, consisting of a wooden box with a low-frequency loudspeaker mounted on top. They noted their similarity to microseisms observed on seismographs, and correctly hypothesized that these signals were the result of low pressure systems in the Northeast Pacific Ocean. In 1945, Swiss geoscientist L. Saxer showed the first relationship of microbaroms with wave height in ocean storms and microbarom amplitudes. Eric S. Posmentier published his "theory of microbaroms" in 1967 based on the oscillations of the center of gravity of the air above the Ocean surface on which the standing waves appear, which fits well with observed data, including the doubling of the ocean wave frequency in the observed microbarom frequency.
Theory
Isolated traveling, ocean surface gravity waves radiate only evanescent acoustic waves, and don't generate microbaroms. Microbaroms are generated by nonlinear interactions of ocean surface waves traveling in nearly opposite directions with similar frequencies in the lee of a storm, which produce the required standing wave conditions, also known as the clapotis. When the ocean storm is a tropical cyclone, the microbaroms are not produced near the eye wall where wind speeds are greatest, but originate from the trailing edge of the storm where the storm generated waves interact with the ambient ocean swells.
Microbaroms may also be produced by standing waves created between two storms, or when an ocean swell is reflected at the shore. Waves with approximately 10-second periods are abundant in the open oceans, and correspond to the observed 0.2 Hz infrasonic spectral peak of microbaroms, because microbaroms exhibit frequencies twice that of the individual ocean waves. Studies have shown that the coupling produces propagating atmospheric waves only when non-linear terms are considered.
Microbaroms are a form of persistent low-level atmospheric infrasound, generally between 0.1 and 0.5 Hz, that may be detected as coherent energy bursts or as a continuous oscillation. When the plane wave arrivals from a microbarom source are analyzed from a phased array of closely spaced microbarographs, the source azimuth is found to point toward the low-pressure center of the originating storm. When the waves are received at multiple distant sites from the same source, triangulation can confirm the source is near the center of an ocean storm.
Microbaroms that propagate up to the lower thermosphere may be carried in an atmospheric waveguide, refracted back toward the surface from below 120 km and above 150 km altitudes, or dissipated at altitudes between 110 and 140 km. They may also be trapped near the surface in the lower troposphere by planetary boundary layer effects and surface winds, or they may by ducted in the stratosphere by upper level winds and returned to the surface through refraction, diffraction or scattering. These tropospheric and stratospheric ducts are only generated along the dominant wind directions, may vary by time of day and season, and will not return the sound rays to the ground when the upper winds are light.
The angle of incidence of the microbarom ray determines which of these propagation modes it experiences. Rays directed vertically toward the zenith are dissipated in the thermosphere, and are a significant source of heating in that layer of the upper atmosphere. At mid latitudes in typical summer conditions, rays between approximately 30 and 60 degrees from the vertical are reflected from altitudes above 125 km where the return signals are strongly attenuated first. Rays launched at shallower angles may be reflected from the upper stratosphere at approximately 45 km above the surface in mid latitudes, or from 60–70 km in low latitudes.
Atmospheric scientists have used these effects for inverse remote sensing of the upper atmosphere using microbaroms. Measuring the trace velocity of the reflected microbarom signal at the surface gives the propagation velocity at the reflection height, as long as the assumption that the speed of sound only varies along the vertical, and not over the horizontal, is valid. If the temperature at the reflection height can be estimated with sufficient precision, the speed of sound can be determined and subtracted from the trace velocity, giving the upper level wind speed. One advantage of this method is the ability to measure continuously – other methods that can only take instantaneous measurements may have their results distorted by short-term effects.
Additional atmospheric information can be deduced from microbarom amplitude if the source intensity is known. Microbaroms are produced by upward directed energy transmitted from the ocean surface through the atmosphere. The downward directed energy is transmitted through the ocean to the sea floor, where it is coupled to the Earth's crust and transmitted as microseisms with the same frequency spectrum. However, unlike microbaroms, where the near vertical rays are not returned to the surface, only the near vertical rays in the ocean are coupled to the sea floor. By monitoring the amplitude of received microseisms from the same source using seismographs, information on the source amplitude can be derived. Because the solid earth provides a fixed reference frame, the transit time of the microseisms from the source is constant, and this provides a control for the variable transit time of the microbaroms through the moving atmosphere.