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TIMODAZ – Characterisation of Rock Mass Crack Damage
Using Ultrasonic Surveys
Juan M. Reyes-Montes 1 , William S. Pettitt 1 , Jonathan R. Haycox 1 , Jennifer R. Andrews 1 ,
R. Paul Young 2
Summary
1 Applied Seismology Consultants, UK
2 University of Toronto, Canada
This paper presents a summary of two different approaches to interpret the evolution of
crack density in a rock mass induced by stress changes associated with excavation and heating-cooling
processes. These techniques were initially developed as part of the OMNIBUS
project, part funded by the EC under the 5 th Euratom framework and are being further extended
to study the impact of thermal-induced stress changes on the host rock as part of the
TIMODAZ project (in the 6 th Euratom research framework programme).
Two approaches have been investigated. First, full waveform ultrasonic data from laboratory
compression tests and in-situ experiments were correlated with a suite of tests in finite difference
models (Haycox and Pettitt, 2004)[1] with variable fracture density, size and fluid
content. Second, a non-interactive crack effective medium theory allows the derivation of
velocity anisotropy and splitting of elastic waves from modelled crack density, aspect ratio
and fabric orientation for moderately jointed samples (e.g. Schubnel et al., 2006)[2]. For the
purpose of this investigation, the isotropic case is considered, for which the method was
used to invert elastic wave velocities to infer the evolution of crack density and aspect ratio.
1. Introduction
Disposal in deep underground geological formations is one of the likely options to dispose of medium
and high level radioactive waste. A key element in the evaluation of the long-term safety of an
underground disposal site is the study and control of the Damaged Zone (DZ) surrounding the infrastructure.
The evolution of the DZ is affected among other factors by excavation induced stress
changes, pore pressures and the heating-cooling cycle of the waste.
Ultrasonic monitoring provides a unique means to non-destructively remotely monitor the evolution
of fractures and stress disturbance around the DZ. In particular, the monitoring of changes in waveform
attenuation and propagation velocity can be used as an estimate of the evolution of crack damage.
In this paper, two different approaches are presented that invert the modelled effect of a fracture
network on the transmission of elastic waves so as to interpret the evolution of rock mass properties
related to its capability to transport fluids.
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