Phased Array Analysis Incorporating the Continuous Wavelet Transform

Abstract:

Non-linear block thresholding of the continuous wavelet transform (CWT) of 2D phased array signals offer high time-resolution solutions for analyzing seismograms of local and regional seismic events. An initial denoising step on an array ensemble reveals those regions of the scale-time plane that contain high signal-to-noise arrivals. Individual seismic phase arrivals in ensemble, denoised seismograms can be partitioned using scale-time gating where CWT wave packets of an individual seismic phase on the scale-time plane for a reference array element are time-correlated with all other elements to find an optimum time shift for the phase across all elements. The seismic phase is then clipped out of the CWT of each array element using this optimal time shift for further analysis. The seismogram can be separated into component seismic waves for a detailed view of wave characteristics such as slowness and arrival azimuth using conventional frequency-wavenumber methods. However, the process can be taken further by using the CWT of each phase to construct high time-resolution signal beams over CWT scale. Local explosion data from the 2016 IRIS Wavefields community experiment in northern Oklahoma and the KSRS array in South Korea are used to demonstrate these techniques in separating surface wave modes and body waves and examining the scattering of regional phases. High resolution CWT processing in the 1 to 3 Hz band for northern Oklahoma reveals horizontal Rayleigh refraction and multipathing. Phase velocity measurements of component regional phases along with well-log data are used to construct a 1D velocity model under the IRIS experiment. Time lapse images of array beams for North Korean nuclear explosions show that phases in much of the regional seismogram within the scale band of 1 to 6s waiver in slowness and azimuth demonstrating significant horizontal refraction even though many phases within P, S, and Lg waveforms are highly directional, forming excellent beam patterns. Rayleigh waves in the scale band of 6 to 20s are highly coherent and directional over KSRS even with lapse times in the hundreds of seconds. The scientific challenge is to use these high time-resolution array images for characterizing the scattering properties of the crust and mantle in order to further characterize source properties.