Stress Measurements Using Oriented Core

Stress measurements using oriented core are classified as destressing restressing techniques. These techniques involve completely decoupling a volume of rock from the stress field, then reloading the rock volume back to its original stressed condition (Villaescusa et al., 2003b). The intention is to return a rock core volume to its in-situ state. The method discussed here has been called the WASM AE stress measurement technique (Villaescusa et al., 2002). It is a technique that utilizes a completely decoupled volume of rock from exploration core that is reloaded to its original stress state by reference to one indirect parameter, the acoustic emission event count.

Basically, the method involves a sequence of six steps:

1. An oriented sample volume, usually common oriented exploration core (termed here the main core), is isolated from a rock mass.
2. The main core is transported to a rock mechanics laboratory and resampled by a number of smaller subcores that are taken at certain orientations relative to the axis of the main core.
3. The oriented subcores are precision ground for rightness and flatness, then fitted with suitable acoustic emission sensors.
4. Each subcore is tested under monotonically increasing uniaxial load (stress). The acoustic sensors measure the event count rate attributed to the deformation, dislocation, and propagation of preexisting cracks and the initiation of new cracks, as the stress isincreased.
5. The applied stress versus the count rate is approximately bilinear with the change of relationship indicated by a demonstrable increase in noise count rate at a certain stress level (Figure 4.60). This transition point is taken to indicate the largest contemporary stress experienced by the subcore in the direction of the subcore axis.
6. The stress measurements for the oriented subcores are used in conjunction with their orientations relative to the original oriented core to determine the largest contemporary stress field experienced by the main core (Figure 4.61). Provided the rock specimen has been selected from an area previously in equilibrium with gravitational loading and tectonics (Windsor et al., 2006, 2007), this is the maximum previous stress to which a particular rock mass has been subjected by its environment.

This section presents the scalar characteristics (i.e., the stress magnitudes alone) from approximately 240 WASM AE rock stress tensor determinations obtained from different geological and geodynamic regimes from different continents and compares them to results compiled in an Earth Rock Stress Tensor Database (ERSTD) (Windsor, 2009). The data comprise results from techniques that attempt to measure, without a priori assumption, the complete rock stress tensor (e.g., it does not include results obtained from the hydraulic fracturing technique). The data are presented as reported, without prejudice or censorship.

The distributions of the vertical stress, the principal normal stresses, and the maximum shear stress with depth in the upper 3 km of Earth’s crust from the WASM AE data set and from the ERSTD are shown in Figures 4.61 through 4.63, respectively. Figure 4.62 indicates that both data sets are distributed about a theoretical linear relationship for vertical stress given by σv= zγr where z is the overburden depth and γr is the unit weight of rock, which is set here at 27 kN/m3. The WASM AE data appear to fit better with this relation than the ERSTD.

The distribution of principal normal stresses (σ1, σ2, and σ3) with depth given in Figure 4.63 shows a low frequency of tensor measurement below 1.5 km, with scatter increasing with depth. It indicates slight nonlinearity of the WASM AE data set and greater nonlinearity of the ERSTD. Note that the ERSTD is influenced at depth by a greater frequency of deeper- and lowerstress magnitudes measured around South African mine sites. Figure 4.64 shows the distribution of the maximum shear stress from WASM AE and from the ERSTD. Both data sets show nonlinearity and considerable scatter with depth, which is thought to be linked to the variability in the shear strength of Earth’s crust and its ability to sustain shear stresses.

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