Seismo-Acoustic signatures of temperature and pressure dependence of geothermal
reservoir rocks, two samples of volcanic rocks (Iceland), have been observed using
laboratory experiment measured at in-situ conditions. The analysis of the
seismo-acoustic signatures has been employed using the acoustic waveform recorded
during laboratory experiment in terms of their P-wave velocities, instantaneous
frequency, and spectral energy. The acoustic wave propagation was measured at a
constant confining and pore pressure, but different temperature (Tmax 250 °C). After
several temperature cycles, pore pressure was slowly reduced at a constant
temperature of 150°C by pumping pore-fluid out of the sample, until the liquid–steam
phase transition was reached. Only when pore-fluid was pumped out beyond this point
the pore pressure continued to drop. During the experiment the P and converted wave
velocities decrease with the temperature rise. For two couples of temperature cycles
it seems most likely that no hysteresis phenomenon of P wave velocities was observed.
During the liquid–steam phase transition experiment stage, almost no P wave velocity
change was observed. In addition to the velocity measurement the instantaneous
frequency and its spectral energy have been computed. In accordance to the decrease
of velocities the instantaneous frequency and spectral energy of the seismic waveform
have shown systematic change with the temperature change. In this analysis, it can be
shown that, not only the (confining and pore) pressure, the temperature change has
substantial and systematic signatures on the seismic properties of the rocks.

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