1) The document investigates geophysical signatures from slip along rock discontinuities in a laboratory setting to identify precursors to shear failure. 2) It describes an experiment applying direct shear stress to gypsum and limestone samples with rough and smooth surfaces while recording ultrasonic wave propagation. 3) The results show that the location of maximum amplitude in the waveforms occurs earlier than the actual sample shear, indicating a measurable precursor to failure, whereas dominant frequency and peak arrival time occur at or after the time of shear.

1. Motivation and Objectives
Geophysical Waveform Frequency Attenuation as a Precursors to Rock Shear Failure
John Hinton1, Reza Hedayat2
Motivation
• The ability to predict shear failure in geomaterials has many potential applications.
Including geo-hazard risk mitigation and the ability to judge infrastructural integrity.
Experiments
• Form an ideal specimen of gypsum and Indiana Limestone using 36 Grit sand paper
to create a rough surface over half, leaving a smooth surface over the other half
• Apply direct shear stress on preformed rock sample
Geophysical Waveforms
Given a digital signal from each sensor, using MatLab:
1Student Researcher, Department of Geophysics, 2Faculty, Department of Civil and Environmental Engineering
CSM
Research Council
Objectives
• Investigate the geophysical signature from slip along rock discontinuities in a
laboratory setting
• Identify and interpret any precursors of failure
• Send acoustic pulses
through specimen at a
frequency of 1MHz, with a
delay of 10µs
• Record ultrasonic wave
propagation through
specimen during applied
stress, using a series of
recording sensors
Sample shears at wave
536
• Model and interpret peak amplitude and peak location of raw data
• Distribution models shows a peak in amplitude at wave 460
• Signal distribution stack shows peak arrival time in wave 516
Spectral Analysis
Dominant Frequency
Future Work
• Repeat experiment using fabricated switch-boxes, capable of recording with 40+ sensors
• Analyze and interpret Phase Spectrum
• 3 point bending test for fracture initiation and propagation in rocks
• Model peak amplitude and dominant frequency in the Fourier Domain using several
different windows and taper lengths
12P
5P
13P
10S 11P
6S
4S 9S 3S
7S
1S 2S 8S
Smooth Surface
Rough Surface
Spectral Analysis
• Amplitude spectrum shows peak at wave 456, earlier than raw data
• Amplitude spectrum shows peak frequency at wave 502, consistent with raw data
Collected Data
Conclusions
• The location of the maximum amplitude in the wave-set is earlier in wave number (and
therefore, in time) than the actual shear of the rock sample; this suggests that a precursor
is observable.
• Measuring the maximum amplitude location may be an earlier precursor than the peak
frequency or peak arrival time.
• Raw data shows P and S-wave arrivals of each transmitted wave