Constraints on fracture distribution in the Los Humeros geothermal field from beamforming of ambient seismic noise

Abstract. Faults and fractures are crucial parameters for geothermal systems as they provide secondary permeability allowing fluids to circulate and heat up in the subsurface. In this study, we use an ambient seismic noise technique referred to as three-component (3C) beamforming to detect and characterize faults and fractures at a geothermal field in Mexico. We perform 3C beamforming on ambient noise data collected at the Los Humeros Geothermal Field (LHGF) in Mexico. The LHGF is situated in a complicated geological area, part of a volcanic complex with an active tectonic fault system. Although the LHGF has been exploited for geothermal resources for over 3 decades, the field has yet to be explored at depths greater than 3 km. Consequently, it is currently unknown how deep faults and fractures permeate, and the LHGF has yet to be exploited to its full capacity. Three-component beamforming extracts the polarizations, azimuths and phase velocities of coherent waves as a function of frequency, providing a detailed characterization of the seismic wavefield. In this study, 3C beamforming of ambient seismic noise is used to determine surface wave velocities as a function of depth and propagation direction. Anisotropic velocities are assumed to relate to the presence of faults giving an indication of the maximum depth of permeability, a vital parameter for fluid circulation and heat flow throughout a geothermal field. Three-component beamforming was used to determine if the complex surface fracture system permeates deeper than is currently known. Our results show that anisotropy of seismic velocities does not decline significantly with depth, suggesting that faults and fractures, and hence permeability, persist below 3 km. Moreover, estimates of fast and slow directions, with respect to surface wave velocities, are used to determine the orientation of faults with depth. The north-east (NE) and north–north-west (NNW) orientation of the fast direction corresponds to the orientation of the Arroyo Grande and Maxtaloya–Los Humeros Fault swarms, respectively. NE and NNW orientations of anisotropy align with other major faults within the LHGF at depths permeating to 6 km.
