Cite this article as Löer, K., T. Toledo, G. Norini, X. Zhang, A. Curtis, and E. H. Saenger (2020). Imaging the Deep Structures of Los Humeros Geothermal Field, Mexico, Using Three-Component Seismic Noise Beamforming, Seismol. Res. Lett. 91, 3269–3277, doi: 10.1785/ 0220200022. Supplemental Material 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 fromwell 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. Introduction Los Humeros volcanic complex (LHVC; Fig. 1), located in the eastern part of the Trans-Mexican volcanic belt (TMVB), hosts a conventional geothermal field (Ferrari et al., 2012; GutiérrezNegrín, 2019). On-going hydrothermal activity makes the LHVC a favorable area for geothermal exploitation, and a geothermal power plant has been operating since the 1990s. The LHVC has been identified as an important natural laboratory for the development of general models of superhot geothermal systems (SHGSs) in volcanic calderas (e.g., Jolie et al., 2018). Although extensive geological field studies and well log analyses have provided many constraints on the near-surface geology of the caldera complex and conventional geothermal reservoir, conditions at depths greater than 2–3 km are largely unknown and currently being studied intensively (Jolie et al., 2018). It is assumed that superhot fluids could exist in the carbonate rock basement underlying the caldera (Jolie et al., 2018). These rocks might exhibit secondary permeability related to the damage zone of active resurgence faults and inherited pervasive basement structures (Lorenzo-Pulido, 2008; Rocha-López et al., 2010; Norini et al., 2015, 2019; Jolie et al., 2018). The maximum depth of these brittle structures is defined by the brittle-ductile (BD) transition zone, which thus plays an important role in geothermal exploration because upper crustal faults and fractures behave as hydraulic channels for the circulation of geothermal fluids (e.g., Ranalli and Rybach, 2005). In SHGSs that exhibit a positive thermal anomaly, the depth of the BD transition may differ from areas with a normal thermal gradient, as rocks become progressively more ductile with increasing temperature. Thus, a positive 1. Department of Civil and Environmental Engineering, Bochum University of Applied Sciences, Bochum, Germany; 2. Now at Department of Geology and Geophysics, University of Aberdeen, Aberdeen, United Kingdom; 3. German Research Centre for Geosciences GFZ, Section 4.8 Geoenergy, Section 2.2 Geophysical Deep Sounding, Potsdam, Germany; 4. Istituto di Geologia Ambientale e Geoingegneria, Consiglio Nazionale delle Ricerche, Area della Ricerca CNR—ARM3, Milan, Italy; 5. School of Geosciences, Grant Institute, University of Edinburgh, Edinburgh, United Kingdom; 6. Institut für Geophysik, ETH Zürich, Zürich, Switzerland; 7. Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Bochum, Germany; 8. Fraunhofer-Einrichtung für Energieinfrastruktur und Geothermie IEG, Bochum, Germany *Corresponding author: katrin.loeer@hs-bochum.de; katrin.loer@abdn.ac.uk © Seismological Society of America Volume 91 • Number 6 • November 2020 • www.srl-online.org Seismological Research Letters 3269 Downloaded from http://pubs.geoscienceworld.org/ssa/srl/article-pdf/91/6/3269/5176463/srl-2020022.1.pdf by University of Edinburgh user on 08 November 2020 thermal anomaly could potentially limit the volume of rocks in which secondary permeability may exist. We use three-component (3C) beamforming to extract structural information from ambient seismic noise. 3C beamforming is an array technique, which, like standard beamforming, not only estimates the dominant propagation direction and wavenumber of a recorded wavefield, but in addition determines the polarization of the wavefield by comparing phase shifts across different components (Riahi et al., 2013). As a result, different wave types can be distinguished and their propagation parameters analyzed separately. This allows us, for example, to estimate wavefield composition and surface-wave anisotropy, which is, however, beyond the scope of this study. Here, we consider fundamental mode Rayleigh waves only and extract dispersion curves from frequency–wavenumber (f-k) histograms; these are inverted for a shear-velocity depth profile using a reversible-jump Markov chain Monte Carlo (rj-McMC) algorithm. Although this algorithm is computationally expensive, it has the advantage of providing uncertainties for the velocity profile by finding the distribution of models that are consistent with data. 3C beamforming does not require impulsive (man-made or natural) seismic sources and is thus cheap, flexible, and applicable also in aseismic areas. Whereas cross-correlation-based ambient noise methods typically rely on month-long recordings, from beamforming, we extract stable dispersion curves from only 1 day of seismic noise data. Depending on the array geometry and seismic noise spectrum, the depth sensitivity of 3C beamforming can exceed that of other seismic methods by several kilometers, as we will show in this study. The analysis of four reflection seismic lines recorded across the LHVC, for example, provided 2D velocity maps and seismic sections down to 6 km at the most (Jousset, Ágústsson, et al., 2019). Ambient noise cross-correlation methods applied in the same area, but using a larger array, produce 3D tomographic Figure 1. (a) Simplified geological map of the Los Humeros volcanic complex (LHVC) and surrounding basement, on a shaded relief. The trace of the A-A′ geological cross section of panel (b) is shown. Triangles denote seismic station locations of the dense broadband (DB) network, circles denote geothermal wells. In the upper-right inset, the location of the LHVC within the TransMexican volcanic belt (TMVB) is indicated. (b) A–A′ schematic geological cross section showing the subsurface geometry of the main structures and stratigraphic units. Trace of the geological cross section is shown in panel (a). Modified from Norini et al. (2019). ENE, east-northeast; LH, Los Humeros caldera ring fault; LHh, inferred flexure plane of the Los Humeros trap-door caldera; LP, Los Potreros caldera ring fault; TF: thrust fault; RF, resurgence fault (red lines); WSW, west-southwest. The color version of this figure is available only in the electronic edition. 3270 Seismological Research Letters www.srl-online.org • Volume 91 • Number 6 • November 2020 Downloaded from http://pubs.geoscienceworld.org/ssa/srl/article-pdf/91/6/3269/5176463/srl-2020022.1.pdf by University of Edinburgh user on 08 November 2020 images down to a maximum of 10 km depth (Granados Chavarria et al., 2020; Martins et al., 2020). In a similar manner, a recent local earthquake tomography study provides information only of the upper 3–4 km (Toledo et al., 2020). We show that 3C beamforming provides information to greater than 10 km depth. In the following, we describe geology and available datasets, introduce both 3C beamforming and the rj-McMC inversion algorithm, and summarize our findings in Los Humeros and their implications for SHGSs, in general. Geology of LHVC The LHVC basement is composed of Mesozoic sedimentary rocks involved in the Late Cretaceous–Eocene compressive orogenic phase that generated the Mexican fold and thrust belt (sedimentary basement unit in Fig. 1) (Fitz-Díaz et al., 2017; references therein). The sedimentary basement rests above the Precambrian–Paleozoic crystalline basement of the Teziutlan Massif unit, made of greenschists, granodiorites, and granites (e.g., Suter, 1987; Suter et al., 1997; Ortuño-Arzate et al., 2003; Ángeles-Moreno, 2012; Fitz-Díaz et al., 2017) (Fig. 1a,b). Since the Eocene, the area underwent a limited extensional tectonic phase, associated with northeast-striking normal faults and the emplacement of Eocene–Miocene granite and granodiorite magmatic intrusions (Fig. 1a). The TMVB volcanic activity occurred from 10.5 to 1.55 Ma with the emplacement of fractured andesites, basaltic lava flows, and few volcaniclastic levels (old volcanic succession unit in Fig. 1) (e.g., Yanez and Garcia, 1982; Ferriz and Mahood, 1984; López-Hernández, 1995; Cedillo-Rodríguez, 1997; Carrasco-Núñez, Hernandez, et al., 2017; Carrasco-Núñez et al., 2018). Volcanic activity resumed ∼700 ka ago with the emplacement of the Pleistocene– Holocene LHVC (LHVC unit in Fig. 1) (e.g., Carrasco-Núñez, Hernandez, et al., 2017; Carrasco-Núñez et al., 2018). This volcanic complex represents a basaltic andesite–rhyolite system of two nested calderas, namely the outer Los Humeros caldera and the inner Los Potreros caldera (Carrasco-Núñez, Hernandez, et al., 2017; Calcagno et al., 2018) (Fig. 1a). The LHVC caldera stage occurred between ∼165 and ∼69 ka and consisted of two major ca

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

A passive seismic experiment using 25 broadband and 20 short‐period stations was conducted between September 2017 and September 2018 at Los Humeros geothermal field, an important natural laboratory for superhot geothermal systems in Mexico. From the recorded local seismicity, we derive a minimum 1‐D velocity model and obtain 3‐D Vp and Vp/Vs structures of Los Humeros. We improved the classical local earthquake tomography by using a postprocessing statistical approach. Several inversions were computed and averaged to reduce artifacts introduced by the model parametrization and to increase the resolution of the investigated region. Finally, the resulting Vp and Vp/Vs structures and associated seismicity were integrated with newly acquired geophysical and petrophysical data for comprehensive interpretation. The recorded seismicity is mainly grouped in three clusters, two of which seem directly related to exploitation activities. By combining new laboratory measurements and existing well data with our Vp model, we estimate possible geological unit boundaries. One large intrusion‐like body in the Vp model, together with neighboring high Vp/Vs anomalies, hints at a region of active resurgence or uplift due to the intrusion of new magma at the northern portion of the geothermal field. We interpret high Vp/Vs features as fluid bearing regions potentially favorable for further geothermal exploitation. Deep reaching permeable faults cutting the reservoir unit could explain fluid flow from a deeper local heat source in the area.

Local Earthquake Tomography at Los Humeros Geothermal Field (Mexico)

https://repository.tudelft.nl/islandora/object/uuid%3Af1527d06-c159-45e9-a028-25cbfc0ab1b1/datastream/OBJ/download

Source parameters of the 8 February 2016, Mw=4.2 Los Humeros earthquake by the inversion of InSAR-based ground deformation

The GEMex* project is a recently finalized European-Mexican collaboration that aimed to improve the understanding of two geothermal fields: Acoculco and Los Humeros Volcanic Complex . These sites are located in the Trans-Mexican Volcanic Belt, a region that hosts numerous active volcanoes and is favorable for geothermal exploitation. Currently, the &#160;Los Humeros Volcanic Complex is one of Mexico&#8217;s main geothermal systems with an installed capacity of ~95MW. Many studies have been performed at this site since the 70s highlighting several features and characteristics of the shallow subsurface. However a thorough knowledge of structures and behavior of the system at greater depths is still quite sparse. Hence one main objective of the GEMex project was to conduct several geological, geochemical, and geophysical experiments to investigate deeper structures for future development of local and regional geothermal resources.In this framework, for the period of one year (September 2017 to September 2018), a seismic array consisting of 45 seismic stations was set to record continuously at the Los Humeros Volcanic Complex. In this study we analyzed the continuous seismic records to detect the micro-seismicity mainly related to exploitation activities. After applying a recursive STA/LTA detection algorithm, we assembled and manually picked P- and S- phases of a catalog of about 500 local events. The detected events were mostly clustered around injection wells, with fewer events located close to known structures. We use the retrieved catalog to derive a new minimum 1D velocity model for the Los Humeros site. We then performed a joint inversion to obtain the 3D Vp and Vp/Vs structures of the geothermal field. A post-processing averaging of several inversions was also computed to increase resolution of the investigated region. In this study we will show the derived Vp and Vp/Vs models for the &#160;Los Humeros Volcanic Complex to emphasize various underground structures and potentially identify possible variations due to changes in temperature, fluid content, and rock porosity.&#160;*This project has received funding from the European Union&#8217;s Horizon 2020 research and innovation programme under grant agreement No. 727550 and the Mexican Energy Sustainability Fund CONACYT-SENER, project 2015-04-68074. We thank the Comisi&#243;n Federal de Electricidad (CFE) for kindly granting the access to the geothermal field for installation and maintenance of seismic stations.

Local Earthquake Tomography at the Los Humeros Geothermal Field in Mexico

Para realizar la tomografia sismica por atenuacion de ondas de coda (Qc - 1 ) en el campo geotermico de Los Humeros , Puebla , se han utilizado 95 sismos locales (Md≤3.6) con profundidades hasta 4.0 km, registrados en las estaciones de su red sismica, durante el periodo de diciembre 1997 a diciembre 2004. Se utilizo el modelo de retrodispersion simple, filtrados en cuatro rangos de frecuencias (2, 4, 6, y 8 Hz) y una ventana de 5 segundos. Para la representacion en 3D, se utilizo una aproximacion basada en elipsoides que representan dispersion de primer orden. Los resultados muestran que los valores de Qc para las frecuencias utilizadas tienen una dependenci a con la frecuencia de la forma: 06 . 0 86 . 0 12 24    f Qc , donde los valores bajos de Qc fueron observados en la zona de mayor actividad sismica y en la ubicacion de pozos inyectores y productores , mientras que los valores altos se observaron en la periferi a del campo geotermico. Asimismo, la distribucion de la atenuacion Qc - 1 en 3D y 2D muestra que las anomalias de alta atenuacion sismica se ubican en los extremos norte, sur y suroeste de la zona de explotacion actual , y a profundidades mayores a 2.5 km

/pdf/estudio-sismologico-del-campo-geotermico-de-los-humeros-1nqka055uw.pdf

Estudio sismológico del campo geotérmico de Los Humeros, Puebla, México. Parte II: Tomografía sísmica por atenuación a partir de ondas de coda (Qc - 1 ) de sismos locales

Extensive passive seismic monitoring was carried out between Sep. 17 and Sep. 18 over the Los Humeros (Mexico) geothermal field. This acquisition operation was conducted in the framework of the European H2020 project GEMex among different geophysical, geochemical and geological surveys. Seismic monitoring provided numerous data, whose processing is still on-going, to better characterize the underground structures and properties of the geothermal field. These results participate to the increase of our understanding of the local geothermal system. They can be utilized to propose new development areas, especially, to the north-west of the currently exploited zone, which showed temperatures greater than 380°C at ca. 2.5 km depth. For one year, a network of 45 shortand long-period seismometers was deployed in the surrounding of the Los Humeros geothermal field. The network layout was chosen to comply with several types of passive seismic processing methods: induced and natural seismicity characterization, travel-time tomography, ambient noise tomography, among others. We present here the results associated with the recorded seismicity. Besides several natural earthquakes in the region, induced earthquakes were regularly detected, at a rate higher than one event per day. Most of them were clustered in the vicinity of geothermal wells or known geological structures, at a depth between 1 and 3 km, consistent with the exploited reservoir interval.

One year of passive seismic monitoring of the Los Humeros (Mexico) geothermal field [in press]

In the framework of the international collaboration between Mexico and Europe for the development of geothermal energy (GEMex consortium), a seismic network of 45 seismic stations (25 broad-band and 20 short-period) was installed around the super-hot geothermal system of Los Humeros (Mexico) for more than one year. Los Humeros power plant is nested inside a quaternary caldera located in the eastern part of the Trans-Mexican Volcanic Belt that crosses the whole country from the Pacific coast to the Gulf of Mexico.

Angel Figueroa Soto

Papers

One year of passive seismic monitoring of the Los Humeros (Mexico) geothermal field [in press]

3D Anisotropic velocity model of the Los Humeros geothermal field, Mexico, using seismic ambient noise tomography.