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Residual strength

The resistance of a rock (or mineral compound) to slide on a fracture plane established during failure (see uniaxial compressive strength). The value is determined at the end of an unconfined compressive strength test or triaxial test by allowing the test plug to slide on the already established fracture plane until equilibrium is reached. As for UCS the residual strength is best determined by a series of single stage tests or by one multistage tests, where he results from different confining pressure levels are used to calculate residual strength at unconfined conditions.

S wave

It is also known as shear wave, secondary wave, transversal wave. During a seismic event S waves lead to particle movement perpendicular to the direction of wave propagation, consequently, a rock body is sheared transversally. Since fluids cannot be sheared they occur only in solids. From the lack of shear waves in the Earth’s outer core we know that it consists of molten iron (whose rotational motion probably is responsible for the Earth’s magnetic field). S waves travel slower than P waves (therefore they are named secondary waves), but faster than surface waves. Within an isotropic medium the velocity of S waves vs is given by:

vs =0.5*[E/{2*ρ*(1-ν)}]

with E as Young’s modulus, ν as the Poisson’s ratio and ρ as density.

S waves polarize into two orthogonal components of differing velocities (so-called shear wave splitting): a horizontal SH wave and a vertical SV wave due to the alignment of fluid-filled pore space. Shear wave splitting is regarded as a measure in earthquake prediction and as a tool for modelling crude oil recovery (e.g. Crampin & Gao 2006). The velocity difference between SH and SV gives information about the stress state in the crust and the probability of seismic failure or wellbore stability.

Shear modulus

G describes the relation between an applied shear stress S13 to the resulting shear strain ε13 and is expressed as:

 G = 0.5(S13/ε13) (Zoback 2007).

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