Remote, real-time monitoring of cyclones with microseisms

Abstract:

Giving proper care to selecting microseisms from well isolated cyclones, these great oceanic storms can be monitored in real time by seismic recordings at stations 1200-4100 km distant from the cyclone's center. We treat ocean depths of 3.4-5.5 km. For the theoretically-computed microseism, which our procedure compares with the experimental data, we use a Green's-function approach in the frequency domain. Relating recorded displacement F and theoretical Green's function G, We have F(ω,r)=S(ω)G(ω,r) in which our only unknown is the generalized source function S(ω) and r is the distance to the center at any specific time. The basic result of this report is that the form of this function is A S_N(ω), where A is a real constant increasing with the strength of the cyclone and S_N(ω), is a positive real function of frequency, independent of cyclone-receiver separation and of cyclone strength. That is, for a given ocean basin, and a given receiver-region geology, at our current level of accuracy S_N(ω) is the same for all cyclone strengths and cyclone-receiver separations. Using the multimode approach, we've developed the numerical method for computing the Green's function for multilayered oceanic structures. For each of the 4 selected cyclones, the source functions for all locations along the path show a consistency which demonstrates that the recorded microseisms are radiated from the cyclone. The extracted source function exhibits spectra that are characteristic of ocean waves generated by cyclonic winds. With knowledge of distance between the source and receiver, cyclone A is therefore trivial to monitor in real time from remote recordings. At the current time, the cyclone's strength—generalized source function—must be related empirically to the cyclone's maximum wind speed, areal extent, and lateral velocity.