Seismic Velocities and Elastic Moduli Correlations with Temperature in a Superhot and an Enhanced Geothermal System

This paper presents correlations derived by linear regression analysis between seismic velocities VP and VS and elastic moduli EP and ES with temperature in Los Humeros superhot (SHGS) and Acoculco enhanced (EGS) geothermal systems at depths down to 3 km below surface and temperatures up to 350 C. In Los Humeros, the seismic velocity models were derived from inversion of legacy active-seismic survey data acquired in 1998, as well as from passive seismic monitoring and ambient seismic noise interferometry carried out during 2017-19 by the GEMex consortium. In the Acoculco EGS, ambient seismic noise data were used. Steady state formation temperatures were re-evaluated during and after the end of the GEMex project using measurements provided by courtesy of Federal Electricity Commission of Mexico (CFE). The density data needed for the calculation of elastic moduli were provided by the GEMex consortium, as derived from inversion of regional and local gravity surveys. The analysis indicated that statistically significant correlations of seismic parameters to temperature exist in the vertical direction, namely exponential in Los Humeros superhot and logarithmic in Acoculco EGS, but that there is no correlation in the horizontal direction. This result suggests indirect relationship among the considered variables due to interdependence on other parameters, such as pressure, vapor saturation, etc. As the analysis was performed using only data obtained from sensing-at-surface methods, without direct geophysical calibration at depth, a distributed fiber-optic seismic and temperature sensing system at both surface and downhole is proposed for active-source and passive-seismic monitoring, as well as seismic-while-drilling by drill-bit source is considered for reverse vertical seismic profile (RVSP) recording whenever possible for future high-temperature geothermal applications.