Joint pressure and temperature effects on seismic properties of gneisses
and amphibolite

Pressure and temperature change simultaneously in the Earth’s crust from
surface to depth. Joint pressure and temperature changes influence many
different physical properties. There are many studies on samples at
elevated pressure, where the influence of open cracks, fractures, voids
and pores have been studied. Applying confining pressure has a direct
influence on crack closure, and this influence on dynamic properties
(density and elastic modulus, bulk, shear and young’s) of rocks above
200 MPa is assumed linear with the linear increase in wave speed. This
is because it is generally assumed that most cracks are closed above 200
MPa, which in nature would correspond to a depth of ~7-8
km. However, from the KTB deep drilling well in Germany, it is known
that fluid-filled fractures and pores can remain open until 8 to 9 km
depth. Applying temperature can affect the dynamic properties of rock by
thermal expansion, possibly reopening cracks that were closed at
pressures >200 MPa, and thermally expanding grains. This
influence is also assumed to be linear at a temperature below partial
melting, and in the absence of phase transitions. A similar effect has
been observed by a number of research groups during laboratory
experiments and calculating seismic velocity results under 600 MPa
confining pressure and 600^{o}C temperature. In this
work, an effort has been made to mathematically investigate the
influence of temperature and pressure on the seismic properties
(velocity of pressure and shear waves, density and Poisson’s ratio) of
crystalline rocks, measured during laboratory experiments. Elastic wave
speeds, moduli and density are increasing as a function of pressure and
decreasing as a function of temperature. However, these pressure and
temperature-related changes are shown to be nonlinear from room
conditions up to 600^{o}C and 600 MPa. In this
presentation, we focus on non-linear changes mainly in the high-pressure
portion of the velocity as a function of pressure (>200
MPa). When confining pressure is applied, measured P- and S- waves show
an increase in velocity and decrease in anisotropy. However, the effect
of temperature on measured P- and S- waves show a decrease in velocity
and increases in anisotropy. These changes are not very different from
linear, but it is not possible to fit velocity as a function of pressure
or temperature with linear mathematical functions. The implications of
non-linear relationships between pressure, temperature and elastic wave
speeds are discussed in this presentation.