# P wave

It is also named primary (sonic) wave,
compression wave, longitudinal wave. Generally it is an elastic space wave.
During seismic events it is the fastest (and therefore primary) of the three
generated seismic waves (besides, **S wave** and superficial wave) with a
velocity *v _{p}* of 5.5 to
7.2 km/s in the Earth’s crust and 7.8 to 8.5 km/s in the Earth’s
mantle. Generally

*v*increases with density. It is called compression wave because particles move along (or longitudinally with) the direction of propagation leading to elastic compression and extension of a rock volume during an earth quake. In an isotropic medium the velocity of the P wave

_{p}*v*can be described by the following equation:

_{p}*v _{p} = 0.5*[{E*(1-ν)}/{ρ*(1-2ν)*(1+ν)}]*

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

# Poisson's ratio

The dimensionless *ν* describes the deformation behaviour of a
rock body in terms of the negative ratio between radial strain *ε _{33}* and axial strain

*ε*:

_{11}*ν = δ ε_{33}/δε_{11}._{}*

Usually the Poisson's ratio is calculated as the
ratio between the gradients of the load paths of the unload-load cycle for
both, the radial and the axial ε-σ_{1} curves of
a given uniaxial/triaxial test.

Generally *ν* ranges between 0.1
and 0.5. In rocks it rarely exceeds 0.3. Exceptions include weakly consolidated
sandstone and very soft claystone where *ν* can reach a value
of 0.45). Brittle rocks have a low Poisson’s ratio.

# Pore pressure

*P _{p}* is the pressure exerted by the brine within the pore space of a rock
which counteracts the applied (external) stress

*S*.

_{ij}# Porosity

The porosity of a specimen stands for the volume of all its voids (pores and open cracks) in relation to the total volume of the specimen. Using the bulk density ρ_{b} and the grain density ρ_{g}, the effective porosity φ_{eff} is calculated by the following equation:

*φ _{eff} = 1 – (ρ_{b}/ρ_{g})*

# News

23.08.2017 06:45

## Gesteinslabor at the Geothermal Congress 2017

The Geothermal Congress 2017 will take place in the BMW Welt (Munich) from 12-14/09.