Heather Schijns

A versatile facility for laboratory studies of viscoelastic and poroelastic behaviour of rocks

Novel laboratory equipment has been modified to allow both torsional and flexural oscillation measurements at sub-microstrain amplitudes, thereby providing seismic-frequency constraints on both the shear and compressional wave properties of cylindrical rock specimens within the linear regime. The new flexural mode capability has been tested on experimental assemblies containing fused silica control specimens. Close consistency between the experimental data and the results of numerical modelling with both finite-difference and finite-element methods demonstrates the viability of the new technique. The capability to perform such measurements under conditions of independently controlled confining and pore-fluid pressure, with emerging strategies for distinguishing between local (squirt) and global (specimen-wide) fluid flow, will have particular application to the study of frequency-dependent seismic properties expected of cracked and fluid-saturated rocks of the Earths upper crust.

Anisotropy of fractured mica-rich schist from Outokumpu, Finland: VSP measurements, laboratory measurements and theoretical model

A high resolution multi-azimuth multi-depth walk-away VSP is used to measure in-situ velocity anisotropy in the Outokumpu area of the Fennoscandian shield. The 2.5 km deep borehole used for the survey shows the area lithology to be primarily mica-rich schist, and the area is expected to demonstrate anisotropy as a result of lattice-preferred orientation of biotite and of aligned microcracks and fractures. Through the application of a τ-p transform to the walk-away VSP data set, qPand qS-wave phase velocities are calculated. A three dimensional velocity model is developed using the phase velocity measurements from the shallowest walk-away VSP where the receiver was at a depth of 1000 m. These velocities are modeled under the assumption that the schist can be represented as a fractured orthorhombic media, and that there is a single set of aligned fractures. A good fit is achieved between the model and experimental results, and the accuracy of the theoretical model is further investigated through comparison with known geology, borehole televiewer measurements of fractures and ultrasonic measurements on core samples.

High Resolution VSP in Outokumpu

In order to assist in determining detailed structure of bedrock nearby the Outokumpu deep drill hole and combine straight measurements done in deep drill hole to surface measurements, a zero-offset vertical seismic profile was acquired near the Outokumpu deep drill hole. In this new survey, 2 m depth increment was applied to obtain high resolution information. Using the first break picking, the interval P-wave velocities were determined and that match the sonics well. And from the stack result, it showed detailed information about sequence structure which also can be used for the interpretation of surface reflection seismic profiles.

Shear Modulus Dispersion in Cracked and Fluid‐Saturated Quartzites: Experimental Observations and Modeling

The work was funded through NSERC Discovery grants to D. R. S. and grant DP110101830 from the Australian Research Council to I. J. and D. R. S. H. S.’s visit to Canberra was funded by the NSERC Michael Smith Foreign Study Supplement.

Velocity dispersion measurements on micro-cracked and fluid-saturated rock

Seismic properties of saturated, cracked rock are expected to be strongly frequency dependent as a result of reversible fluid flow within the crack porosity at all scales caused by the oscillating stress induced by seismic waves. Laboratory measurements, typically made with frequencies on the order of MHz, must systematically overestimate in-situ seismic wave velocities that are typically measured with frequencies on the order of mHz-kHz, a range of frequencies applicable to earthquake teleseisms (< 10 Hz) through active exploration seismic and microseismic investigations (~10 to 300 Hz), mine seismology (~1 kHz) and finally geophysical logging (~ 10 kHz). Forced flexural and torsional oscillation of core samples in the laboratory allows measurement of seismic properties at lower frequencies (0.01-1 Hz) that are more directly comparable to typical in-situ frequencies. Here we describe progress in the development of flexural oscillation methods at the Australian National University (ANU) laboratory for use alongside the established torsional mode capability, as well as preliminary results from a thermally cracked synthetic sample of polycrystalline alumina and a thermally cracked core of Cape Sorell quartzite for comparison with ultrasonic measurements on the samples.

Seismic Anisotropy Measurements and Theoretical Model of Fractured Rock Using Multi- Depth Multi-Azimuth Walk-Away VSP from Outokumpu, Finland

A high resolution multi-azimuth multi-depth walk-away VSP is used to measure velocity anisotropy in the Outokumpu area of the Fennoscandian shield. The 2.5 km deep borehole used for the survey shows the area lithology to be primarily mica-rich schist, and the area is expected to demonstrate anisotropy as a result of lattice-preferred orientation of biotite and of aligned fractures. Through the application of a τ-p transform to the walk-away VSP data set, qPand qS-wave phase velocities are calculated. A three dimensional velocity model is developed using the phase velocity measurements from the shallowest walkaway VSP where the receiver was at a depth of 1000 m. These velocities are modeled under the assumption that the schist can be represented as a fractured orthorhombic media. A good fit is achieved between the model and experimental results, and the accuracy of the theoretical model is further investigated through comparison with known geology.