Low frequency scattering resonance and natural resonance in strong heterogeneity

Abstract:

Multiple scattering of wave in strong heterogeneity can cause resonance-like wave phenomena. For example, long period event in volcanic seismology and seismic coda in local earthquake. Collective behaviour in a many-body system is usually thought to be the source for generating the anomalies. However, the detail physical mechanism is not fully understood. Here I show by wave field modeling for microscopic bubble cloud model and 1D heterogeneity that the anomalies are related to low frequency scattering resonance in transient regime and natural resonance in steady state regime in a many-body system. Two kinds of resonances are a kind of wave coherent scattering enhancement or coupling oscillation phenomenon in strong small-scale heterogeneity. Their resonance frequencies are inversely proportional to heterogeneous scale and contrast and are much lower than the fundamental resonance frequencies of individual constituents in strong small-scale heterogeneity. The resonance frequencies will further shift toward lower frequency with random heterogeneous scale and velocity fluctuations. Low frequency scattering resonance exhibits the characteristics of localized wave in space and the shape of ocean wave in time. Two kinds of resonances are common wave phenomena in wave physics that includes mechanical, electromagnetic, and matter waves.