Ultrasonic Imaging of the Onset and Growth of Fractures Within Partially Saturated Whitby Mudstone Using Coda Wave Decorrelation Inversion

Reuben Zotz-Wilson*, Lisanne A.N.R. Douma, Joel Sarout, Jeremie Dautriat, David Dewhurst, Auke Barnhoorn

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review

1 Citation (Scopus)
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Abstract

Using active ultrasonic source survey data, coda wave decorrelation (CWD) time-lapse imaging during the triaxial compression of Whitby Mudstone cores provides a 3-D description of the evolution and redistribution of inelastic strain concentrations. Acoustic emissions (AEs) monitoring is also performed between any two consecutive surveys. From these data, we investigate the impact of initial water saturation Sw on the onset, growth, and reactivation of inelastic deformation, compared to the postdeformation fracture network extracted from X-ray tomography scans. Our results indicate for the applied strain rate and degree of initial water saturation, and within the frequency range of our ultrasonic transducers (0.1 to 1 MHz), that inelastic strain localization and propagation in the Whitby Mudstone does not radiate AEs of sufficient magnitude to be detected above the average noise level. This is true for both the initial onset of inelasticity (strain localization) and during macroscopic failure. In contrast, the CWD results indicate the onset of what is interpreted as localized regions of inelastic strain at less than 50% of the peak differential stress the Whitby Mudstone can sustain. The seemingly aseismic nature of these clay-rich rocks suggests the gradual development of inelastic strain, from the microscopic diffuse damage, up until the macroscopic shear failure.

Original languageEnglish
Article numbere2020JB020042
Number of pages24
JournalJournal of Geophysical Research: Solid Earth
Volume125
Issue number7
DOIs
Publication statusPublished - 2020

Keywords

  • aseismic
  • coda wave decorrelation
  • imaging
  • inversion
  • mudstone
  • ultrasonic

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