An Automatic Method for Determination of Lg Arrival Times Using Wavelet Transforms

Abstract

The regional phase Lg is often used to estimate location and magnitude for sources closer than 1500 km.The complexity of Lg waveforms makes it difficult to consistently determine a unique Lg arrival time, thus affecting source location with a single station or array. This study tests an automatic method for timing Lg arrivals using wavelet transforms to decompose the Lg signal into its components localized both in time and scale.A Continuous Wavelet Transform (CWT) using a Daubechies order two (db2) wavelet is applied to 10 seconds of raw data containing the start of Lg. Initial positioning of the window is obtained using standard Lg travel time tables. The coefficients at scale 16 from the db2 decomposition are squared and the resulting time series is represented by an approximation of the 4'th level Discrete Wavelet Transform (DWT) using a Haar wavelet. A threshold detector is then applied to the resulting time series to determine the Lg arrival time. The method was tested using well located earthquakes and explosions from known mines ( m b < 4.0 ), recorded on the vertical components at TXAR (Lajitas, Texas) and PDAR (Pinedale, Wyoming) arrays. The Lg arrival time was automatically picked with a standard deviation of less than 1.5 seconds (less than 10 km 2 location error) for well known locations. Location errors are larger with the increase of distance and smaller with the increase in signal to noise ratio of events.

A wavelet-based method for simulation of two-dimensional elastic wave propagation

“…Anant & Dowla (1997) compared polarization information across a number of scales in determining P ‐phase arrival time, and used transverse compared to radial amplitude information at different scales in determining S ‐phase. Similar research was done by Tibuleac & Herrin (1999) in a study of Lg ‐phase arrivals using wavelets. Lilly & Park (1995) used multiwavelets to estimate the time‐varying spectral density matrix for three‐component seismic data.…”

On a Wavelet-Based Method for the Numerical Simulation of Wave Propagation

“…Applications of wavelets have been made for several aspects of seismic signal processing such as the determination of an onset time of specific phase's arrival [2,73], measurement of an anisotropy rate in certain regions [5], and estimation of a time varying spectral density matrix [52]. However, the applications of wavelets are not restricted to signal processing and have been extended to numerical analysis as well exploiting the adaptivity, compactness achievable in the wavelet domain.…”

Automatic Determination of Secondary Seismic Phase Arrival Times Using Wavelet Transforms

“…In their study, Tibuleac and Herrin (1999) also showed that, in the same frequency range, the amplitude difference between the Lg signal and the coda before Lg is much less after Fourier prefiltering than after wavelet prefiltering.…”

“…Different types of seismic phase-pickers, in time or frequency domain, have been previously developed (see Tibuleac and Herrin, 1999). Most of these techniques were developed to pick first arrivals, and none was demonstrated to be optimal for picking secondary phases.…”