Nonlinear Interaction Of Ocean Waves Research Articles

Atmospheric infrasound generation by ocean waves in finite depth: unified theory and application to radiation patterns

SUMMARYBetween 0.1 and 0.5 Hz, infrasound signals recorded in the atmosphere are dominated by ocean-generated noise called microbaroms. Microbaroms propagate through the atmosphere over thousands of kilometres due to low absorption and efficient ducting between the ground and the stratopause. Different theoretical models have been developed to characterize the source of microbaroms, all based on the second-order nonlinear interaction of ocean waves. While early theories considered an infinite ocean depth and a source radiation depending on the acoustic wave elevation angle, other works have approximated the radiation pattern as a monopole, and found a considerable effect of the water depth. This paper reviews these models and extends the previous theories to the combined effects of both finite depth ocean and source directivity in both elevation and azimuth angles. It is found that the water depth has a negligible effect for the near-horizontally propagating acoustic waves that should dominate the measured microbarom records. Another important result is that the microbarom azimuthal variation can be highly directive locally, but it generally becomes isotropic when integrated over a realistic source region.

Modelling ocean ambient noise with finite ocean depth and comparisons with observations

The global International Monitoring System (IMS) network continuously detects coherent ambient infrasound noise between 0.1 and 0.5 Hz. This noise, referred to as microbaroms, is generated by the second order non-linear interaction of ocean waves, mostly during severe storms. A global and multi-year analyze of microbaroms highlights the strong influence of middle atmospheric conditions on the propagation. Various source models have been developed. Brekhovskikh et al. (1973) and Ardhuin and Herbers (2013) considered a source directivity effect in infinite depth ocean with radiative pressure depending on the wave elevation angle. Waxler and Gilbert (2006) and Waxler (2007) investigated the radiation of infrasound by ocean waves in finite depth ocean by monopolar sources. In this study, the combined effects of non-monopolar source and bathymetry on the radiation are addressed. Beyond theoretical issues, source modelisation and propagation through a realistic atmosphere are carried out. Comparing the predicted signals with the observed ones at all IMS stations shows good agreements for both directional and amplitude information. Building a global reference database of oceanic noise sources opens new perspectives for providing additional integrated constraints on middle atmosphere dynamics and disturbances.The global International Monitoring System (IMS) network continuously detects coherent ambient infrasound noise between 0.1 and 0.5 Hz. This noise, referred to as microbaroms, is generated by the second order non-linear interaction of ocean waves, mostly during severe storms. A global and multi-year analyze of microbaroms highlights the strong influence of middle atmospheric conditions on the propagation. Various source models have been developed. Brekhovskikh et al. (1973) and Ardhuin and Herbers (2013) considered a source directivity effect in infinite depth ocean with radiative pressure depending on the wave elevation angle. Waxler and Gilbert (2006) and Waxler (2007) investigated the radiation of infrasound by ocean waves in finite depth ocean by monopolar sources. In this study, the combined effects of non-monopolar source and bathymetry on the radiation are addressed. Beyond theoretical issues, source modelisation and propagation through a realistic atmosphere are carried out. Comparing the predicte...

Seismic Remote Sensing of Super Typhoon Lupit (2009) with Seismological Array Observation in NE China

The p-wave double-frequency (DF) microseisms generated by super typhoon Lupit (14–26 October 2009) over the western Pacific Ocean were detected by an on-land seismological array deployed in Northeastern China. We applied a frequency-domain beamforming method to investigate their source regions. Comparing with the best-track data and satellite observations, the located source regions of the p-wave DF microseisms, which corresponded to the strongest ocean wave–wave interactions, were found to be comparable to the typhoon centers in the microseismic frequency band of ~0.18–0.21 Hz. The p-wave DF microseisms were probably excited by the nonlinear interaction of ocean waves generated by the typhoon at different times, in good agreement with the Longuet–Higgins theory for the generation of DF microseisms. The localization deviation, which was ~120 km for typhoon Lupit in this study, might depend on the speed and direction of typhoon movement, the geometry of the seismological array, and the heterogeneity of the solid Earth structure. The p-wave DF microseisms generated in coastal source regions were also observed in the beamformer outputs, but with relatively lower dominant frequency band of ~0.14–0.16 Hz. These observations show that the p-wave DF microseisms generated near typhoon centers could be used as a seismic remote sensing proxy to locate and track typhoons over the oceans from under water in a near-real-time and continuous manner.

Spatial dependence of very low frequency wind-generated noise in the sound channel

Storm-driven non-linear interaction of ocean waves excites Very Low Frequency (VLF) (<5 Hz) sounds locally. As the surface wind speed increases, the slope of the VLF acoustic spectrum approaches approximately -20 dB/octave, corresponding to a similar behavior observed in the surface wave spectrum. The amplitude of the measured acoustic spectrum in this frequency region is strongly correlated with local surface wind speeds [J. Acoust. Soc. Am. 139(3), 1110 (2016)]. Due to the sound propagation conditions in the open ocean, it is expected that sound from distant storms will also provide a significant contribution to the measured sound field. The impact of distant meteorological activity on the local acoustic spectrum is examined by comparing the ambient noise recorded by the Comprehensive Nuclear-Test Ban Treaty Organization’s (CTBTO) hydro-acoustic monitoring system to global records of surface wind speed, remotely sensed via satellite.

How ocean waves rock the Earth: Two mechanisms explain microseisms with periods 3 to 300 s

AbstractMicroseismic activity, recorded everywhere on Earth, is largely due to ocean waves. Recent progress has clearly identified sources of microseisms in the most energetic band, with periods from 3 to 10 s. In contrast, the generation of longer‐period microseisms has been strongly debated. Two mechanisms have been proposed to explain seismic wave generation: a primary mechanism, by which ocean waves propagating over bottom slopes generate seismic waves, and a secondary mechanism which relies on the nonlinear interaction of ocean waves. Here we show that the primary mechanism explains the average power, frequency distribution, and most of the variability in signals recorded by vertical seismometers, for seismic periods ranging from 13 to 300 s. The secondary mechanism only explains seismic motions with periods shorter than 13 s. Our results build on a quantitative numerical model that gives access to time‐varying maps of seismic noise sources.

Excitation of microseisms: views from the normal-mode approach

SUMMARY Non-linear interaction of ocean waves is a widely accepted mechanism of microseism excitation for frequencies approximately between 0.05 and 0.5Hz. Longuet-Higgins published the most influential paper on this subject in 1950 and his main contributions can be summarized in two points; the first is on the double-frequency mechanism and the second is on the dependency of excitation on ocean depth. The two results were derived for different media, the first point for an incompressible liquid (ocean) and the second point for a compressible liquid layer over an elastic half-space. The two features naturally come out in the normal-mode formulae. We note, however, that the use of the Longuet-Higgins formula, that showed efficiency of excitation based on ocean depth, is not suitable for interpreting land-based observations because his formula was for displacement at ocean bottom. Reevaluation of this dependency for a land-based observation shows similar results for the depth of maximum excitation, but the sharpness of this peak is reduced significantly. Some recent observational studies tried to identify the source area of excitation by using this ocean-depth dependency, but such a result needs to be revised to a larger source region according to our results. We confirm that the maximum excitation of microseisms exists and occurs when the ocean depth is about 2.7km. It is thus not surprising to find an efficient pelagic source for microseisms, although existence of the wave‐wave interaction in the source region is essential.

Excitation of normal modes by non-linear interaction of ocean waves

SUMMARY Recent development in seismology has shown that seismic noise is a useful source of Earth structure study. Non-linear interaction of ocean waves is considered to be one of the important causes for seismic noise, although source locations and detailed mechanisms are not clear yet. In order to improve our understanding of this mechanism quantitatively, we derive a normal-mode excitation formula due to this effect. Longuet-Higgins' pressure formula naturally comes out from the vertical forcing term in the excitation coefficient. A novel aspect of our formula is the horizontal forcing term that becomes comparable to the vertical forcing term for frequencies below about 5 mHz (millihertz), especially in shallow oceans. This term is not likely to be important for microseisms because the main frequency range of microseisms is about 0.05–0.4 Hz but it may make significant contributions to the excitation of the continuous background oscillations (the hum) whose frequency range is 2–7 mHz.

A study of the relation between ocean storms and the Earth's hum

We previously showed that the Earth's “hum” is generated primarily in the northern oceans during the northern hemisphere winter and in the southern oceans during the summer. To gain further insight into the process that converts ocean storm energy into elastic energy through coupling of ocean waves with the seafloor, we here investigate a 4‐day‐long time window in the year 2000 that is free of large earthquakes but contains two large “hum” events. From a comparison of the time functions of two events and their relative arrival times at the two arrays in California and Japan, we infer that the generation of the “hum” events occurs close to shore and comprises three elements: (1) short‐period ocean waves interact nonlinearly to produce infragravity waves as the storm reaches the coast of North America; (2) infragravity waves interact with the seafloor locally to generate long‐period Rayleigh waves; and (3) some free infragravity wave energy radiates out into the open ocean, propagates across the north Pacific basin, and couples to the seafloor when it reaches distant coasts northeast of Japan. We also compare the yearly fluctuations in the amplitudes observed on the two arrays in the low‐frequency “hum” band (specifically at 240 s) and in the microseismic band (2–25 s). During the winter, strong correlation between the amplitude fluctuations in the “hum” and microseismic bands at BDSN is consistent with a common generation mechanism of both types of seismic noise from nonlinear interaction of ocean waves near the west coast of North America.