For an approximately isotropic elastic solid,where is bulk modulus, is shear modulus and is mass density. Five relevant controls are:
- Mineral composition and texture: different elastic moduli and densities give different seismic velocities; compliant clay-rich material generally differs from crystalline igneous framework rock.
- Porosity, cracks and fractures: compliant voids reduce the effective moduli and usually reduce both speeds; aligned fractures produce seismic anisotropy.
- Effective confinement: increasing effective stress closes compliant cracks and tends to raise speeds. Higher pore fluid pressure at fixed total confinement can reopen or preserve cracks and lower the effective stiffness.
- Fluid saturation and phase: replacing gas by liquid commonly increases the saturated bulk modulus and ; fluids have no static shear rigidity, so changes through the frame, density and frequency-dependent interactions rather than a direct fluid shear modulus.
- Temperature and alteration: heating commonly reduces elastic moduli and may create thermal cracks; hydrothermal alteration can replace strong minerals by clay or other weaker phases. The sign depends on alteration products and crack sealing, so reservoir-specific measurements are needed.
These controls interact: a dense rock is not automatically faster if its moduli are also different.
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