Imaging the Deep Structures of Los Humeros Geothermal Field, Mexico, Using Three-Component Seismic Noise Beamforming

Abstract

We present a 1D shear-velocity model for Los Humeros geothermal field (Mexico) obtained from three-component beamforming of ambient seismic noise, imaging for the first time the bottom of the sedimentary basement ∼5 km below the volcanic caldera, as well as the brittle-ductile transition at ∼10 km depth. Rayleigh-wave dispersion curves are extracted from ambient seismic noise measurements and inverted using a Markov chain Monte Carlo scheme. The resulting probability density function provides the shear-velocity distribution down to 15 km depth, hence, much deeper than other techniques applied in the area. In the upper 4 km, our model conforms to a profile from local seismicity analysis and matches geological structure inferred from well logs, which validates the methodology. Complementing information from well logs and outcrops at the near surface, discontinuities in the seismic profile can be linked to geological transitions allowing us to infer structural information of the deeper subsurface. By constraining the extent of rocks with brittle behavior and permeability conditions at greater depths, our results are of paramount importance for the future exploitation of the reservoir and provide a basis for the geological and thermodynamic modeling of active superhot geothermal systems, in general.

Local Earthquake Tomography at Los Humeros Geothermal Field (Mexico)

“…The selected minimum 1-D velocity model ( Figure 5) shows not only a good agreement with alternative studies on the region (Lermo et al, 2008;Löer et al, 2020) but also one possible geological boundary observed in retrieved well data (Norini et al, 2019). A prominent discontinuity is seen at ∼−1.2 km depth in Figure 5 for both Vp and Vs models.…”

“…The seismicity presents a systematic shift toward ∼0.5–0.8 km greater depths and some improvements in clustering after the 1‐D inversion. Two velocity models proposed by Lermo et al (2008) ( P wave) and Löer et al (2020) ( S wave) are marked in green and magenta, respectively, in Figure 5a. The Vp model proposed by Lermo et al (2008) was derived using a seismic reflection profile and reaches an approximate depth of −0.5 km, below which a default value of 5.18 km/s is assigned.…”

“…Finally, we manually select the model with the lowest misfit value (black lines in Figures 4a and 4b) that best coincides with the main trend of most frequent velocity values (red lines in Figures 4c and 4d) as the minimum 1-D velocity model. (Lermo et al, 2008;Löer et al, 2020), (b) the resulting Vp/Vs ratio, and (c) the earthquake distribution over depth after the 1-D inversion. Solid lines indicate the depth intervals with best sensitivity for each model.…”

“…The Vp model proposed by Lermo et al (2008) was derived using a seismic reflection profile and reaches an approximate depth of −0.5 km, below which a default value of 5.18 km/s is assigned. The Vs model derived by Löer et al (2020) was obtained using three‐component ambient noise beamforming and is most sensitive in the interval between −0.5 and 10 km depths. Notice the good correlation between the derived minimum 1‐D Vp model and the model obtained by Lermo et al (2008), especially between −2.0 and −0.5 km.…”

Local earthquake tomography at the Los Humeros geothermal field, Mexico

“…The seismicity presents a systematic shift toward ∼0.5-0.8 km greater depths and some improvements in clustering after the 1-D inversion. Two velocity models proposed by Lermo et al (2008) (P wave) and Löer et al (2020) (S wave) are marked in green and magenta, respectively, in Figure 5a. The Vp model proposed by Lermo et al (2008) was derived using a seismic reflection profile and reaches an approximate depth of −0.5 km, below which a default value of 5.18 km/s is assigned.…”