|Title||Synthesis of wavelet envelope in 2-D random media having power-law spectra: comparison with FD simulations|
|Publication Type||Journal Article|
|Year of Publication||2016|
|Authors||Sato, H, Fehler, M|
|Journal||Geophysical Journal International|
|Pagination||333 - 342|
The envelope broadening and the peak delay of the S-wavelet of a small earthquake with increasing travel distance are results of scattering by random velocity inhomogeneities in the earth medium. As a simple mathematical model, Sato proposed a new stochastic synthesis of the scalar wavelet envelope in 3-D von K´arm´an type random media when the centre wavenumber of the wavelet is in the power-law spectral range of the random velocity fluctuation. The essential idea is to split the random medium spectrum into two components using the centre wavenumber as a reference: the long-scale (low-wavenumber spectral) component produces the peak delay and the envelope broadening by multiple scattering around the forward direction; the short-scale (high-wavenumber spectral) component attenuates wave amplitude by wide angle scattering. The former is calculated by the Markov approximation based on the parabolic approximation and the latter is calculated by the Born approximation. Here, we extend the theory for the envelope synthesis of a wavelet in 2-D random media, which makes it easy to compare with finite difference (FD) simulation results. The synthetic wavelet envelope is analytically written by using the random medium parameters in the angular frequency domain. For the case that the power spectral density function of the random velocity fluctuation has a steep roll-off at large wavenumbers, the envelope broadening is small and frequency independent, and scattering attenuation is weak. For the case of a small roll-off, however, the envelope broadening is large and increases with frequency, and the scattering attenuation is strong and increases with frequency. As a preliminary study, we compare synthetic wavelet envelopes with the average of FD simulation wavelet envelopes in 50 synthesized random media, which are characterized by the RMS fractional velocity fluctuation ε = 0.05, correlation scale a = 5 km and the background wave velocity V0 = 4 km s−1. We use the radiation of a 2 Hz Ricker wavelet from a point source. For all the cases of von K´arm´an order κ = 0.1, 0.5 and 1, we find the synthetic wavelet envelopes are a good match to the characteristics of FD simulation wavelet envelopes in a time window starting from the onset through the maximum peak to the time when the amplitude decreases to half the peak amplitude.
|Short Title||Geophys. J. Int.|