Analysis of Local Explosion Waveforms for 1D Crustal Structure Using Interactive Non-Linear Block Thresholding and Phased Array Methods

Date: 4/24/2019

Time: 04:30 PM

Room: Cascade I

The 2016 IRIS Wavefields experiment geometry allows for an interesting variety of seismological techniques for inferring local crustal structure from a dataset of 12 well-timed and positioned explosions near the array. Contracted by the Air Force Research Laboratory, explosions ranged from 2000 lbs to 250 lbs charge size with four explosions at each of three locations 15, 35, and 67 km from the center of the IRIS experiment. Subarrays of the 381 three-component elements are used to accurately determine wave phase velocities and azimuths for a variety of distinct, local wave phases and the relatively broad bandwidth of the 5Hz nodal sensors produces waveforms rich in frequency-dependent wave propagation information. Noise is suppressed using a non-linear block thresholding scheme on the continuous wavelet transform of the data. In addition, scale-time windowing before inverse transforming cleanly separates the fundamental mode Rayleigh wave from the first higher mode, allowing for independent measures of both group and phase velocity as a function of frequency for each mode. Removing the modes from the data via scale-time windowing exposes local P and S phases for better analysis with the phased subarrays. Observations of relative P and S wave travel times, phase velocities, and Rayleigh mode dispersion curves are combined with a nearby acoustic well log of the Paleozoic sediments and shallow crystalline basement to produce a 1D starting model for full waveform inversion. Use of phased array and wavelet transform thresholding techniques produces many constraints on the structure model that are essential for understanding the source excitation of local and regional seismic phases from small explosions.

Presenting Author: Charles A. Langston

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