Those properties that serve to describe the ability of a material to withstand stress without undergoing permanent deformation. All solid substances, including rocks, follow **Hooke's law**, that is, the proportionality relation between strain (or deformation) and stress (or force per unit area). The stress-strain ratio in simple linear compression or expansion is **Young's modulus of elasticity** (E),

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where F/A is the force per unit area or stress, and ΔL/L is the strain or elongation or shortening per unit length under the application of expansion or compression.

The stress-strain ratio under hydrostatic compression or expansion is the **bulk modulus of elasticity** (K),

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where ΔV/V is the volume expansion or shrinkage per unit volume under the application of expansion or compression.

The stress-strain ratio in shearing, or application of a force, tangential to the surface displaced, is the **rigidity** or **shear modulus of elasticity** (µ).

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where F/A is the shearing stress and ΔL/L is the shearing strain or deformation without change in bulk volume.

**Poisson's ratio** (s) is a measure of the geometric change of shape

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where W stands for width. It is always comprised between 0 and 1/2, its theoretical value being 1/4 for elastic bodies. The above-mentioned properties are responsible for the propagation of sound or [[acoustic wave|acoustic waves]] through rocks.

Two types of body waves are propagated through elastic media:\\
(a) longitudinal or [[compression wave|compressional waves]] wherein the back and forth oscillations of particles are in the direction of propagation, their velocities being given by

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where r<sub>b</sub> is bulk density of rock.\\
(b) Transverse or [[shear wave|shear waves]] wherein the back and forth oscillations of particles are in a direction perpendicular to the direction of propagation, their velocities being given by

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The longitudinal waves always arrive before the transverse waves, and to the present time, the former only have been used extensively in well logging. (From Pirson.)