Geothermics 71 (2018) 200–211
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Geothermics
journal homepage: www.elsevier.com/locate/geothermics
The Los Humeros (Mexico) geothermal field model deduced from new
geophysical and geological data
MARK
⁎
Jorge Arzatea, Fernando Corbo-Camargob, , Gerardo Carrascoa, Javier Hernándezc,
Vsevolod Yutsisd
a
Centro de Geociencias, National University of Mexico (UNAM), Blvd. Juriquilla #3001, Querétaro, 76230, Mexico
CONACYT–Centro de Geociencias, National University of Mexico (UNAM), Blvd. Juriquilla #3001, Querétaro, 76230, Mexico
c
Posgrado en Ciencias de la Tierra, Centro de Geociencias, National University of Mexico (UNAM), Blvd. Juriquilla #3001, Querétaro, 76230, Mexico
d
Instituto Potosino de Investigación Científica y Tecnológica (IPICYT), Camino a la presa San José #2055, Col. Lomas 4ª sección, 78216, San Luis Potosí, Mexico
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Los Humeros geothermal field
Magnetotelluric soundings
Propylitic alteration
The Los Humeros volcanic complex, a 21 × 15 km diameter caldera edifice nesting volcanic domes and a
complex faulting structure, is located at the eastern edge of the Trans-mexican volcanic belt (TMVB). It is a
young edifice (< 0.5 Ma) that hosts one of the five main geothermal fields of Mexico with still an important
energy production (∼65 kW installed capacity). Being one of the more studied producing fields, the geothermal
system of the caldera is largely unknown at depths greater than ∼2.4 km, which is the approximate penetration
range of the available geothermal wells. Here we present the results of a geophysical survey in Los Humeros
caldera and surroundings with the aim to provide further insight on the physical characteristics of the geothermal system at depths greater than 2.4 km. The survey comprised 70 broadband magnetotelluric (MT)
soundings distributed within and in the periphery of the caldera edifice of which we present here three EW
profiles. We also accomplished a mesh of 718 accurately leveled gravity stations. These data sets were complemented with 13 TDEM soundings for static shift control, as well as with the aero-magnetic digital chart of the
area (#E14-3, SGM, 2004). The MT data analysis yielded an average electric azimuth of N23W for the central
profile where the field production is concentrated, which is quite consistent with the mapped NW-SE geological
structure. However, at individual frequency ranges the strike follows the local faulting structures, most of which
are apparently controlled by the deeper crater structure. At the production zone, the conductivity model reveals
the existence of an eastward dipping resistive body, which follows the isotherms registered at the wells. The
inclined conductivity interface above it seems to play an important role in controlling the heat and fluid flow
towards shallow depths. Petrographic studies of well samples provide evidence of mineralogical assemblages
that suggest magnetite-metasomatose hydrothermal alteration. The production zone coincides with maximum
gravity and magnetic gradients, at the western edge of the well-defined circular crater anomaly. At this point the
central MT section (profile 1) shows the shallower depth (2–3 km) to the relatively high resistivity
(∼400–500 ohm-m) and magnetized intrusive-like body. The interpreted geophysical and surface geological
data backed by well data support a reservoir and plume model structure consisting of a resistive propylitic core
that feds the geothermal field through fractures and deep- seated faults. Surface conductors associated with
stratified mineralization produced from lixiviation of geothermal fluids are well differentiated from deeper
conductors. The better preserved northern sector of the Humeros caldera yields an anomalous deep conductor at
depths of 6–7 km below sea level (profile 5) as well as at the central production sector (profile 1) whereas the
anomalous conductivity zone at the southern profile (profile 6) could be as shallow as 5 km below sea level.
These anomalous conductivity zones are expected to be associated to the primary energy source of the Los
Humeros geothermal system in the form of partial fusion or hypercritical trapped fluids within the upper crust,
conditions that have been concluded to prevail along cordilleras of the American continent (e.g. Hyndman,
2017).
⁎
Corresponding author.
E-mail address: fercorbo@geociencias.unam.mx (F. Corbo-Camargo).
http://dx.doi.org/10.1016/j.geothermics.2017.09.009
Received 7 June 2017; Received in revised form 17 September 2017; Accepted 20 September 2017
0375-6505/ © 2017 Elsevier Ltd. All rights reserved.
Geothermics 71 (2018) 200–211
J. Arzate et al.
1. Introduction
(Arellano et al., 2003; Gutiérrez-Negrín and Izquierdo-Montalvo,
2010). Except for an early phase of prospection (Mena and GonzálezMorán, 1978; Flores-Luna et al., 1978; Álvarez, 1978; Arredondo, 1987;
Campos and Garduño, 1987), the geophysical studies focused in the
physical modelling of the geothermal system as a whole are practically
absent or are too regional (Campos-Enríquez et al., 2005). Until now,
the conceptual models of the caldera geothermal field of Humeros have
been deduced from geochemical and temperature data (e.g. CastilloRomán et al., 1991; Aragón et al., 2000; Verma et al., 2011) or from two
dimensional modelling with drilled wells within the production zone
(García et al., 2000), however both models are incompatible to each
other. Deep probing geophysical studies (seismic or electromagnetic
geophysical techniques) focused in the geological modeling of the caldera structure, heat source distribution, and general configuration of
the reservoir was not available until now. Although induced seismicity
studies provide very useful information (Ponce and Rodríguez, 1977;
Lermo et al., 2008; Rodriguez et al., 2012; Urban and Lermo, 2013)
they are not enough in themselves for reservoir modeling. Only recently
low intensity seismicity is being applied to constrain the low-velocity
heat source distribution of the geothermal system (Lermo J., Pers.
comm.).
An alternative deep probing geophysical technique becoming
widely used in geothermal prospecting is the broad-band
Being one of the oldest producing geothermal fields in Mexico, Los
Humeros geothermal field (Fig. 1) is well known from several perspectives. The large quantity of drilled wells (∼50) has provided
abundant sampling for geological, lithological and geochemical analysis mainly in the neighborhood of the production field. Isotopic, fluid
inclusion, gas geochemistry, and geochemical modeling publications
are available since the early 90 s (Barragán et al., 1991; GonzálezPartida et al., 1991; Martinez and Alibert, 1994). These and further
studies have allowed to understand better the behavior of the geothermal reservoir. They include geologic mapping (Pérez-Reynoso,
1978; De la Cruz, 1983; Yáñez and García, 1980; Ferriz and Mahood,
1984; Carrasco-Núñez et al., 2017) subsurface geology (i.e. Viggiano
and Robles, 1988; Cedillo, 2000; Cedillo, 1997; Viggiano and FloresArmenta, 2008; Lorenzo-Pulido, 2008; Gutiérrez-Negrín and IzquierdoMontalvo, 2010; Carrasco-Núñez et al., 2017), hydrogeological
(Cedillo, 1997, 2000) petrological studies (Verma, 1983; Ferriz and
Mahood, 1987; Verma, 2000), not to mention the vulcanological
(Carrasco-Núñez and Branney, 2005; Wilcox, 2011; Carrasco-Núñez
et al., 2012; Dávila-Harris and Carrasco-Núñez, 2014), structural published work of the caldera (Garduño et al., 1985; López-Hernández,
1995; Norini et al., 2015), and the geothermal conceptual model
Fig. 1. The Trans-Mexican-Volcanic-Belt (TMBV) extents from the
western coast in the Pacific to the eastern end at the Gulf of Mexico
(a). The star at the eastern end of the TMVB shows the location of the
Los Humeros caldera (LHVC) geothermal field (b). Los Humeros
geothermal field (red circle) is located at the northern boundary of the
Serdán-Oriental basin, marked with dashed lines (c). (For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
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2. Geology of Los Humeros
magnetotelluric (MT) sounding method. Not only has the advantage of
being a natural field technique, which avoids the need for large energy
sources to run the ground measurements, but also and equally important is that interpretation techniques and equipment have improved
considerably in recent years and are readily available. Many studies can
be found in the international literature (e.g., Didana et al., 2017; Heise
et al., 2016; Amatyakul et al., 2015; Nimalsiri et al., 2015; Kalscheuer
et al., 2015; Bertrand et al., 2015; Piña-Varas et al., 2014; Oskooi and
Darijani, 2014; Ooskoi and Manzella, 2011; Árnason et al., 2010;
Cumming and Mackie, 2010; Newman et al., 2008; Oskooi et al., 2005;
Wannamaker et al., 2004; Volpi et al., 2003; Wannamaker et al., 2002;
Cumming et al., 2000; Pellerin et al., 1996; Johnston et al., 1992;
Wright et al., 1985), giving evidence of successful applications of
magnetotelluric (MT) surveys for modeling geothermal systems taking
advantage of the tensor character of the ground impedance and the
redundancy of data. The MT surveys have been extensively used in
geothermal prospecting in the world, because of their penetration depth
and good lateral resolution capable of distinguishing different lithologies or geological media. The presence of water, the presence of faults,
different types of mineralogical and hydrothermal alteration by the
presence of clays, temperature changes, can also be deduced when
constraint from the conductivity structure that results from the MT data
modeling. Also, the MT method can resolve the low resistivity, low
permeability hydrothermal smectite clay cap that acts as the trap over
high temperature geothermal reservoirs in both sedimentary and igneous environments (e.g. Ussher et al., 2000; Kumar et al., 2010).
However, such clays also inhibit the formation of permeable open space
at the top and margins of reservoirs, even where intersected by fractures, thus the understanding of the geometry of the high conductivity
clay alteration is important for the geothermal well targeting stage
(Davatzes and Hickman, 2009). Because MT is typically the most costeffective method of imaging resistivity, the MT method is often the
default geophysics method chosen for geothermal exploration
(Cumming and Mackie, 2010). In this work, we present a new geophysical model of the Los Humeros geothermal system based upon the
interpretation of three EW profiles comprising 36 broad band magnetotelluric soundings constrained by the Bouguer anomaly and the “reduced-to-pole” total magnetic anomaly from aeromagnetic data, these
correlated with the surface geology and structural data available.
2.1. Regional geology
Los Humeros geothermal field is located at the eastern part of the
Trans Mexican Volcanic Belt (TMVB) forming the northern boundary of
the Serdán-Oriental basin (Fig. 1), which is characterized by a bimodal,
monogenetic volcanism comprising mostly isolated small cinder and
scoria cones, large rhyolitic domes and both basaltic and rhyolitic
maars. The oldest rocks in the area form the so-called Teziutlán Massif,
a crystalline complex made of metamorphic and intrusive rocks, including green schists, granodiorites and granites of Paleozoic age
(207–181 Ma; Yáñez and García, 1980) that form the regional basement
(Quezadas-Flores, 1961; Viniegra, 1965). These rocks are unconformably covered by ≤3000-m-thick, highly deformed Mesozoic
sedimentary basement, dominated by limestone and minor silt and
shale, which are out cropping in a few windows within the Serdán
Oriental basin and are part of the subsurface underlying a thick andesitic formation beneath Los Humeros caldera. Isolated Tertiary intrusions (of 14.5 and 31 Ma; Yáñez and García, 1980) of granodiorite and
syenite crop out in the surrounding areas, whereas a voluminous Miocene andesitic volcanism (8.9–11 Ma; Carrasco-Núñez et al., 1997;
Gómez-Tuena and Carrasco-Núñez,2000) associated with the Cerro
Grande volcanic complex is outcropping to the west of the basin. This
volcanism correlates with the Cuyoaco Andesite formation (LópezHernández, 1995) and the Alseseca Andesite (Yáñez and García, 1980),
dated at 10.5 Ma. A voluminous andesitic volcanism that characterize
the subsurface of Los Humeros (1.6–2.6 Ma; Carrasco-Núñez et al.,
2017), has been correlated with that reported by Yáñez and García
(1980) to the north and northeast of Los Humeros known as the Teziutlán Andesite, dated at 3.5 and 5 Ma and at 1.55 Ma by Ferriz and
Mahood (1984).
2.2. Surface geology
Prior to the formation of Los Humeros caldera, some isolated
rhyolitic lava flows dated at 0.46 Ma were emplaced in areas surrounding Los Humeros caldera (Ferriz and Mahood, 1984). A revised
version of the geologic map (Carrasco-Núñez et al., 2017) is shown in
Fig. 2, which update the previous maps made by Pérez-Reynoso (1978),
Fig. 2. Surface geology of the volcanic deposits of Los Humeros caldera defined by the circular pattern of faults (after Carrasco-Núñez et al., 2017). The evolution of Los Humeros caldera
involves two major caldera-forming events, which are responsible of the main structural features that characterize the composed caldera complex. The most recent volcanism of Los
Humeros comprises both effusive and explosive eruptions. Most andesitic and basaltic lavas are related to ring-fractures as young as 6.7 kyr in the center and to the east and south of the
caldera (Dávila-Harris and Carrasco-Núñez, 2014). Continuous black lines indicate the location of the interpreted magnetotelluric profiles, and doted lines indicate the location of gravity
station tracks.
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assemblages including epidote, chlorite, calcite, pyrite, quartz and notably magnetite deeper than 930 m below the surface. Secondary
magnetization hence could be of importance along some paths of mineralization and has also been reported by Prol-Ledesma and Browne
(1989) in this zone. These mineral associations are found filling fractures, vesicles, and replacing primary minerals such as pyroxene and
plagioclase. In addition, the mineralogical ensemble that includes the
presence of epidote as a mineral geo-thermometer reflect conditions of
moderate to high temperatures, ranging from > 180 °C to 340 °C, as
have been proposed in other geothermal fields (Reyes, 1990; Reyes,
1998; Lagat, 2007). The abundance and distribution of propylitic alteration minerals within of Los Humeros field vary with depth and from
one well to another, but particularly appear below the central area of
the production geothermal field. For example in the well H-8 propylitic
alteration occurs from 1830 to 630 masl; in the well H-43 at
1980–1093 masl; in the well H-10 from 1710 to 700 masl; in the well H19 from 1670 masl to the bottom of the well (958 masl); in the well H20 from 1910 masl to the bottom of the well (430 masl); in the well H42 from 1650 m to the bottom of the well (690 masl), and in the well H55 from 1160 masl to the bottom of the well (550 masl). On the other
hand, clay minerals have been identified towards the shallower parts of
the geothermal field. According to Prol-Ledesma (1990) and Izquierdo
(1993) these clay minerals are kaolinite, montmorillonite and illite,
which indicate a mineralogical ensemble of argillic alteration and the
presence of chlorite may indicate magmatic fluid origin.
De la Cruz (1983), Yáñez and García (1980), Ferriz and Mahood (1984).
The evolution of Los Humeros caldera involves two major calderaforming events, which are responsible of the main structural features
that characterize the caldera complex. The 21 × 15 km oldest caldera
(Los Humeros) was formed at 460 ky (Ferriz and Mahood, 1984) by
super-eruptions involving about 115 km3 (Ferriz and Mahood, 1984)
producing the rhyolite Xaltipan ignimbrite. The second caldera-forming
eruption produced a 9–10 km structure (Los Potreros caldera) at
100–140 km3 (DRE) (Ferriz and Mahood, 1984; Carrasco-Núñez and
Branney, 2005) of a double compositionally zoned rhyodacitic and
andesitic Zaragoza ignimbrite (Carrasco-Núñez and Branney, 2005)
with a volume of 12 km3 (Ferriz and Mahood, 1984) to 15 km3
(Carrasco-Núñez and Branney, 2005). Many other eruptive events occurred between these major episodes (∼360 ky–140 ky) including the
emplacement of high-silica rhyolite domes, followed by a series of
plinian eruptions that produced ca. 10 km3 (Dense-rock equivalent) of
rhyodacitic pumice fallout (Faby Tuff; Ferriz and Mahood 1984), which
were dispersed to the southeast (Ferriz and Mahood, 1984; Wilcox,
2011). During the late Pleistocene (60 ky), rhyodacitic and andesitic
lavas erupted, followed by explosive eruptions, producing several
eruptive units such as the Xoxoctic Tuff, erupting 0.6 km 3 of a dacitic
pumice fall deposit at ∼50 ky and pyroclastic flows and breccia deposits named as the Llano Tuff (Ferriz and Mahood, 1984; Wilcox,
2011), around 28 ky (Rojas, 2016). The most recent volcanism of Los
Humeros comprises both effusive and explosive eruptions. Most andesitic and basaltic lavas are related to ring-fractures previously dated at
40 and 20 ky (Ferriz and Mahood, 1984) but because their large errors
they are not reliable. Contemporaneous with that activity, combined
explosive activity produced the Cuicuiltic tuff, forming a rhythmic alternation of trachyandesitic and basaltic fall layers, which have been
recently associated with different vents located both in the center and
to the east and south of the caldera at 6.7 ky (Dávila-Harris and
Carrasco-Núñez, 2014). The final activity is associated with trachyandesitic and/or olivine-bearing basaltic lava flows erupted inside the
caldera and on the southern flank, as well as dacitic-rhyodacitic lavas
(Arenas) near the SW caldera rim. The subsurface geology of the geothermal field was revised by Carrasco-Núñez et al. (2017). They proposed nine different lithostratigraphic units that can be grouped into 4
main groups: regional meta-sedimentary basement, pre-caldera andesitic volcanism, caldera volcanism and post-caldera activity.
3. Gravity and magnetic images
The gravity data was collected following road tracks (Fig. 2) mainly
within the larger Humeros caldera, accomplishing a total of 720 accurately leveled gravity stations. The data were corrected by altitude and
topography using local and extended digital elevation models referred
to the WGS-84 positioning model. The complete Bouguer anomaly map
of the caldera was obtained from the interpolated, topography-corrected data set, and the residual (Fig. 3a) estimated from subtraction of
a second order fitted regional field. The reduced to pole magnetic
anomaly map of the caldera was also generated (Fig. 3b) using data
from the total field aeromagnetic digital charts (1:250,000) of the
Mexican Geological Service (SGM: http://www.sgm.gob.mx/). The
flight altitude of the magnetometer and the separation of fly lines were
300 m and 1000 m respectively, corresponding to a 2004 SGM acquisition campaign. The anomalous gravity field obtained after corrections
varies from −11 to +12 mGal, with a prominent almost circular
gravity minimum. Similarly, the reduced to the pole magnetic map
covering nearly the same area, show the spatial variation of magnetic
anomaly that ranges from −350 to +400 nT, with a composed
2.3. Evidence of mineral assemblages
Based upon the petrographic study of samples from wells of Los
Humeros geothermal field, it has been observed evidence of propylitic
hydrothermal alteration in the wall-rock with mineralogical
Fig. 3. The residual map of the complete Bouguer
anomaly chart (−18 to +8 mGal total amplitude) of
Los Humeros caldera (a) and Reduced to the Pole
magnetic anomaly (−280 to 430 nT total scale)
chart of nearly the same area. Blue color indicates
minimum values of density (left) and magnetic susceptibility (right), and red tones indicates maximum
values. Dotted semicircular white lines mark the
crater rim of the composed Humeros-Potreros calderas (Carrasco-Núñez et al., 2017). Narrow white
continuous lines indicate mapped faults within the
geothermal field, many induced by ground subsidence. Dashed yellow lines mark interpreted trend
of structures that control the geothermal output of
the system. Black continuous W-E lines indicate the
position of the interpreted magnetotelluric profiles.
The triangular shape formed by yellow dashed lines
in the magnetic anomaly map delineate the extension of the central magnetic high within the caldera,
interpreted as secondary magnetization due to mineralized ensembles containing magnetite, as result of propylitic alteration. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of
this article.)
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possible explanation is in terms of dyke-like magnetized intrusives, but
this would imply an associated high-density anomaly that is not observed in the gravity residual. A more probable explanation for the
observed magnetic anomaly is the existence of secondary magnetization
induced by the magnetite arrays accumulated along fluids discharge
faulted paths as result of widespread propylitization in the upper part of
the reservoir.
magnetic high fairly centered in the caldera rim.
When compared with the mapped structure of the Los Humeros
caldera obtained from surface geology, both the gravity minimum and
the magnetic high correlates well with the geometrical center of Los
Humeros caldera rim (Fig. 3). However, the location of the mapped
faults and fractures within Los Humeros caldera edifice, which are
spatially correlated with the geothermal field production zone (small
dotted ellipse, Fig. 3), lie coincidently at the maximum gradients of
both, gravity and magnetic anomalies in the western border of the
caldera, particularly true for the gravity residual. The southern limit of
the structural caldera edifice poorly correlates with the gravity anomaly
in this zone. It continues and extends southwards beyond the main Los
Humeros crater structure, where the younger volcanic Los Potreros
caldera system had its main activity (Carrasco-Núñez et al., 2008). The
lower relative density of the dominant pyroclastic material filling the
large and smaller caldera structures explain the gravity minimum observed within the respective volcanic rims. The upward continuation of
the complete Bouguer anomaly yields estimated thickness of the volcano-sedimentary crater filling of about 3 km, which is set when the
minimum vanishes (Zecúa, 2017).
In turn, the reduced-to-pole magnetic anomaly of Los Humeros
caldera, deduced from the total field aero-magnetic chart of the zone
(E14-3, SGM, 2004), delineates quite clearly the NW-SE general trend
of the regional tectonic structure (dotted yellow lines), as well as the
mapped Los Humeros crater rim (dotted white line, Fig. 3b). The major
magnetic intensity lies broadly at the geometrical center of the large
caldera and have an approximate triangular shape at its base (dotted
yellow lines). One of the sides of the triangular magnetic high coincides
with the main fault system at the western edge of the caldera structural
rim, that coincides with the production zone (dotted ellipse), which
appears to change the azimuth within the geothermal field (Fig. 3b).
However, as the flight line separation of the aero-magnetic survey was
1000 m at a height of 300 m, it is probable that the observed anomaly is
composed by individual magnetic dipoles not resolved by the data. A
4. The magnetotelluric data and geoelectric strike
The magnetotelluric (MT) data were collected in the frequency
range of 104–10−3 Hz, along 3 profiles that were approximately perpendicular to the surface trace of the main fault system, oriented approximately NS (Fig. 4), along which most of the geothermal and injection wells are concentrated The MT profile 1 (P1) consists of 15
broad-band MT stations, the MT profile 5 (P5) 13 MT stations and the
MT profile 6 (P6) 8 MT stations. We acquired the data with 4 instruments running synchronously to allow for remote reference processing
at those sites close to the geothermal plant, with each site registering
continuously during periods of 20–24 h. The acquired time series of the
measured electromagnetic (EM) fields (ex, ey, hx, hx, hz) were processed
using standard fast Fourier transforms (FFT; e.g., Simpson and Bahr,
2005) and robust cascade decimation (Wight and Bostick, 1980) algorithms. The resulting impedances were converted to resistivity and
phase shift field curves that provided the basis for the 2D modeling.
Previous to the inversion process, the dimensionality and thus, the
electric strike of the data set were evaluated using the phase-based
distortion tensor analysis of Caldwell et al. (2004). The phase method
approach (Bahr, 1991; Caldwell et al., 2004; Martí et al., 2004) is less
sensitive to galvanic distortion effects such as static shift, and provides
more stable estimations of the regional strike. Yet, a set of 13 TDEM
soundings were acquired distributed within the caldera to correct the
static shift possible affecting the resistivity curves. The weighted
average electric strike (Fig. 4) obtained for profile P1 was N23W, for
Fig. 4. Individual MT sounding electric strike azimuths (red arrows) for the three analyzed profiles,
obtained from the average of the 40 measured frequencies of each sounding, defining average structural trends. To the left of the figure the rose diagrams show the strike dispersion along each profile.
The weighted average electric strike obtained for
profile P1 was N23W, for profile P6 was roughly NS.
Sites of profile P5, were rotated at two different
strike angles; N25W for the western 2/3 of the profile, and N45E for the eastern sites (25, 36, 27 and
28). (For interpretation of the references to colour in
this figure legend, the reader is referred to the web
version of this article.)
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the data inversion was performed using the raw apparent resistivity and
phase curves, adding a 10% error floor for the resistivity values and 5%
for the phases. Fig. 5a shows the trade-off between the MT data fit
(RMS) and the resistivity model smoothness controlled by the regularization parameter tau (τ). In the 2D NLCG inversion algorithm, the
regularization parameter controls the overall smoothing of the model
(Tikhonov and Arsenin, 1977). As the value of tau increases, the resistivity model becomes smoother. The corner of the L-curve (Türkoglu
et al., 2015) defines the optimum value of tau for a given MT data set
and provides a compromise between fitting error RMS and the MT
roughness of the resistivity model. The results yield an optimum tau
value for the profile P1 is 5, for profile P5 is 10, and for profile P6 is 3
(Fig. 5a). We use these smoothing parameters and 100 iterations for the
data inversion, the latter fixed by the trade-off between number of
iterations and the decrease of RMS errors (Fig. 5b). The resistivity
sections obtained are shown in Fig. 5c, where blue and purple tones
represent resistive zones, and red and yellow colors are associated with
high conductivity bodies. Small residuals obtained from the comparison
of the raw TE and TM resistivity and phase curves with the calculated
data from the resistivity models indicate a good fitting of the field data
(Fig. 6).
profile P5 was roughly NS, and for profile P6 the estimated average
strike angle was N25W. Although the variability of the strike along the
profiles suggests a more complex than 2D structure, for modeling
purposes we used these weighted strike values.
To carry out the 2D inversion the MT soundings were projected onto
the defined profile traces, and subsequently were rotated to the regional
strike defined for each profile. The most widely used strategy for 2D
inversion, is to rotate mathematically all the MT stations of the profile
to the electric strike, so that the main electric field direction (Ex) is
aligned perpendicular to the profile direction. To model the data, the
soundings of profiles P1 and P6 were rotated to the average strike
N23W and SN respectively. Sites of profile P5, were rotated at two
different strike angles; the western sites were rotated to the strike
N25W while the eastern sites (25, 36, 27 and 28) were rotated to an
angle of N45E. The 2D algorithm of Rodi and Mackie (2001) was used
for the simultaneous inversion of both the apparent resistivity and
phase and for both polarization modes (TE and TM). The final model
was obtained using a generic mesh of 80 ohm-m semi space resistivity.
Subsequently, we vary the density of the mesh to increase the number
of cells to obtain more detail in areas of interest, where conductivity
anomalies appeared in the first place. To avoid data loss by smoothing,
Fig. 5. (a) The optimum value for inversion parameter τ for a given MT data set is obtained from the corner of the L-curve, which provides a compromise between fitting RMS errors and
the roughness or smoothness of the resistivity model. (b) The optimum τ value for profiles P1, P5, and P6 are 3, 5, and 10 respectively. (c) 2D inversions were obtained with 100 iterations
to reduce at minimum values the RMS errors. Blue and purple tones represent resistive zones starting above ∼250 ohm-m, and green and yellow-red tones are associated with conductive
(> 100 ohm-m) and high conductivity (> 15 ohm-m) zones respectively. Conductive layering is observed above 3 km depth, and deeper bellow 5–8 km from the surfaces depending on
profile location. Vertical lines represent depths reached by drilled wells. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this
article.)
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Fig. 6. Observed and calculated apparent resistivity (upper two sections) and observed and calculated phase curves (lower two sections) as a function of frequency for profiles 5 (left), 1
(center), and 6 for TE (a) and TM (b) polarization modes. The main features of the measured data are reproduced by the calculated data from the inverted model, particularly for the TM
mode.
profile P5 reveals a quite symmetric basin on top of the resistive altered
body, the southern P6 profile outlines the Los Potreros crater within the
Los Humeros caldera compose structure. The location of these profiles
can account for the observed differences, suggesting a highly deformed
caldera structure toward the southern Los Potreros caldera, and a quite
symmetric crater basin of around 3 km maximum depth to its top in the
north of the Los Humeros caldera crater.
At the production zone across the P1 MT profile, the conductivity
model suggests the existence of an eastward dipping resistive body
which follows the 250 °C isotherm available from instrumented wells.
The inclined conductivity interface seems to play an important role in
controlling the heat and fluid flow towards shallow depths. At the
western edge of the calderic edifice along the central MT section the
high resistivity dyke-like body shows the shallower depth observed.
Petrographic studies of well samples provide evidence of mineralogical
assemblages that reflect a propylitic association at several wells. The
interpreted geophysical and surface geological data backed by well
samples support a propylitic-type geothermal system, which is fed
through the interfaces and faults, and pouring through the high resistivity altered body.
4.1. Resistivity sections along the MT profiles
On top of the resulting resistivity models the interpreted faults dips
and depths are plotted (dashed lines in Fig. 7) as well as the location of
drilled wells available (vertical continuous lines). Some of the faults
have surface expression but several others have not and therefore are
inferred based upon resistivity discontinuities in the model profiles. All
over the studied zone, the surface conductors to depths above ∼1 km
above sea level are associated to stratified mineralization produced
from lixiviation of geothermal fluids generally known as the reservoir
seal. These anomalous conductors are often associated to geothermal
systems and are due to altered geological units that are borne out from
the precipitation of minerals forming electrically conducting soils such
as smectite clay layering and others.
The interpreted resistivity sections across the caldera edifice show a
contrasting electrical behavior of the resistive (∼1000 ohm-m) body
located at varying depth of ∼1 and 2 km from the surface (Fig. 7). The
central MT profile in particular (P1), which crosses the main production
zone of Los Humeros geothermal field shows the resistive body being
closer to the surface than at the adjacent profiles. While the northern
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Geothermics 71 (2018) 200–211
J. Arzate et al.
Fig. 7. Resulting resistivity sections across the caldera edifice (lower images), which show a contrasting electrical behavior. Topography and complete Bouguer anomaly residual are plot
in the upper part of the figure above each one of the MT resistivity sections. Dashed thick grey lines follow the interpreted crater basin of the composed Humeros caldera yielding
maximum depths of ∼3 km. The central MT profile (P1), which crosses the main production zone of the geothermal field shows the resistive body being closer to the surface than at the
adjacent profiles. The 250 °C isotherm (red lines) follows the dipping interface of the resistive body, which tend to shallow depths towards the western flank of the crater structure and
deepens towards the caldera center. While the northern profile P5 reveals a symmetric basin outlined by a resistive basement, the southern P6 profile describes a quite irregular resistive
basement as consequence of the overlapped crater structure of the southern Los Potreros caldera. Dashed black lines represent surface constrained continuations or assumed faults.
Continuous thin vertical lines are drilled wells projected onto the MT sections. Yellow stars represents all the seismicity in the caldera from 1997 to 2015 associated to the field
exploitation (Lermo et al., 2016), which is concentrated only in the profile P1 around the main production zone of the geothermal field. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
when it is cut (e.g. H5).
The overlying Mesozoic basement sedimentary rocks (in green), are
found unaltered at the west of the crater but are vastly altered below
the production field. The alteration degree of the basement in this area
avoids the identification of resistivity contrasts to identify stratigraphy
such as to the west of P1. However, our profiles do not extend enough
to the East to be able to identify the basement. Yellow triangles in
Fig. 8, with intermediate resistivities (150–250 ohm-m) assumed to
have low permeability, are interpreted as geochemical seals formed by
precipitation of non-conducting minerals at temperatures above
250–300 °C. We hypothesize that the presence of such seals at specific
locations, drive the geothermal fluids through the weakened interfaces
and maximum effort axis of the altered intrusion. Thus, the alteration
mainly occurs at the contact surfaces throughout which pressured
thermal fluids make their way to shallow depths (arrows in Fig. 8).
Upwards pointing arrows in the figure indicates proposed flowing paths
across the three analyzed profiles according to the evidence of drilled
geothermal wells (red vertical lines), with depths reaching ∼3 km
below the surface. Based on the location within the production field, we
drew as continuous black lines high permeability faults whereas the
dashed thin lines are interpreted as having lower permeability. The
dashed thick lines on the sections represent the surface trace of Los
Humeros fault, which has an electrical azimuth of ∼N25W and generally speaking has the major hydrothermal potential at some sectors.
Yellow stars represent local seismicity for one-year period (Lermo et al.,
2016) induced by pumping in or out pressurized fluids from wells,
The better preserved northern sector of the Los Humeros caldera
edifice yields an anomalous conductor at depths of 6–7 km below sea
level (P6), whereas at the central production sector the anomalous
conductivity zone could be as shallow as 5 km (P1) below sea level.
These anomalous deeper conductivity zones are regarded as the primary energy source of the Los Humeros and Los Potreros geothermal
system in the form of partial fusion and/or hypercritical trapped fluids
within the upper crust. The southern profile, which is regarded highly
influenced by the more recent Los Potreros eruption event suggest a
slightly shallow deeper conductor at ∼5–6 km below sea level in the
eastern margin of the profile.
4.2. Conceptual geothermal model
The conceptual 2D models of the Los Humeros field were obtained
subdividing the resistivity sections according to the resistivity ranges as
shown in Fig. 8. The models were constrained with surface structural
geology, information from wells that included isotherms, and by the
gravity and magnetic data charts outlying the lateral limits. The deeper
conductivity anomalies in red (∼20 ohm-m) at the bottom of the profiles are interpreted as the heat source of the geothermal system,
namely a lower crustal trap of recycled mantellic fluids and partial
fusion, differing from the magmatic chamber model in the way they are
emplaced. The heat source is surrounded by a relatively resistive
(∼500 ohm-m) faulted upper crustal body matrix (in grey) that is cut
by several wells at shallow depths (∼1.5 km) showing low permeability
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J. Arzate et al.
Fig. 8. (a) Conceptual 2D models of the Los Humeros geothermal system. Subdivisions have been drawn according the resistivity ranges of the geology constrained magnetotelluric
models. The red sectors mark zones of anomalous conductivity (< 50 ohm-m) associated to the heat source of the system. The grey blocks, with high relative resistivity values
(> 500 ohm-m) are interpreted as widespread propylitic altered intrusive, magnetized and highly fractured at the top (darker blue tone). The conductive crater filling (1–25 ohm-m),
marked with a pastel tone, groups volcanoclastic and mineralized altered geothermal products, including the stratified clay sealing layering within the caldera crater. The resistive
(> 400 ohm-m) light brown top unit generally map the distribution of unaltered volcanic products. The green blocks to the western side of the sections represent a varying conductivity
intruded limestone basement prevailing before the caldera formation. The yellow triangles, of intermediate resistivities values (150–250 ohm-m), represent zones of precipitation with
low permeability acting as seals for the geothermal flow. Red and orange arrows indicate proposed flowing paths of upwards migrating mineralized fluids along structural contacts. Red
lines represent production wells of the geothermal field reaching ∼3 km below the surface. Continuous black lines are interpreted as high permeability faults whereas the dashed thin
lines are interpreted as low permeability faults or sealed contacts. The dashed thick line represents the main fault Los Humeros, present in the three profiles, that has an electrical azimuth
of ∼N25W and generally speaking has the major hydrothermal potential. Yellow stars in P1 represent local seismicity (Lermo et al., 2016) associated to the field exploitation, which is
concentrated around the main production zone. The white arrows pointing east indicate regional groundwater flow according to various authors. Question marks indicate unknown
thickness of magnetized layer. (b) Location of the interpreted MT profiles on top of the surface geology. (c) Legend of the interpreted resistivity model configuration. (For interpretation of
the references to colour in this figure legend, the reader is referred to the web version of this article.)
with its top lying at ∼5 km from the surface, estimated from geochemistry balances and thermal modeling of well samples. From a
geophysical perspective, Campos-Enriquez and Garduño-Monroy
(1987) also proposed two separated heat sources associated to the
geothermal system, with depths deduced from basic 1D modeling of MT
data at depths of 10 and 20 km for Los Humeros caldera and southwest
Los Potreros caldera respectively. In a subsequent paper CamposEnríquez et al. (2005) proposed a funnel type of caldera model assuming magma differentiation took place between 3.5 and 4.5 km of
depth, but did not mention the deeper sources in the new context. From
the geochemical analysis of well samples Prol-Ledesma (1998) found
that in a decade of exploitation, the Humeros geothermal field inflow of
high temperature-highly saline fluids, superheated vapor and gases
from deeper depths have increased in the discharged fluids of the field,
being an indication of the increase the presence of HCl, CO2 and H2S,
which is also an evidence of the lower crustal depths origin of the
geothermal fluids (Arellano et al., 2003).
Still, the model we obtained is not consistent with a shallow magmatic chamber model in the classical sense, or with a stratified magmatic differentiation structure at depth as presently accepted (Verma,
which distributes along a narrow volume. The horizontally pointing
white arrows indicate the recharge direction of the surface aquifer
system topping the reservoir, which according to some authors (e.g.
Prol-Ledesma, 1998) its contribution has been decreasing in proportion
to deeper fluids previous apportion. The uppermost part of the electrical images defines quite clearly the ∼3 km deep bottom of the crater
structure at different latitudes and the surrounding and upper top rock
units. The crater filling is formed by a conductive (1–25 ohm-m), low
density package of pyroclastic and geothermal alteration products
(pastel tone), and the surface resistive (> 300 ohm-m) unaltered
younger volcanic products (light brown).
5. Model comparisons
One of the first conceptual models of the Humeros caldera geothermal system was described by Robin (1982) in terms of the petrologic evolution, proposing that the heat source of the caldera geothermal system was a shallow magmatic chamber. Verma (1985),
suggested that the heat source of both, the Los Potreros and Los Humeros calderas was a stratified magma chamber with two conduits,
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Acknowledgements
1985; Arellano et al., 2003; Campos-Enríquez et al., 2005). Instead, the
base of our conceptual model consists of a large, fractured propylitic
intrusive body of average crustal density and relatively high resistivity
(∼500 ohm-m), presumed having high calorific capacity and being
heated from bellow. The mineral precipitation along the lower crustal
fault systems, have formed dyke-like zones of mineral ensembles along
fluid paths that allow lower crustal fluids to migrate upwards. According to geochemical data (Prol-Ledesma, 1998) discharge fluids of
the Los Humeros geothermal field are a mix of near surface and lower
crustal depth fluids, with an increasing proportion of the later as the
geothermal field ages. This lead us to propose the deeper conductors
that prevail in our MT resistivity images as to correspond with the
primary source of fluids and heat of the geothermal system in the form
of hyper critical trapped fluids and partial fusion. We estimate a depth
to the top of the hotter zones bellow the geothermal field at 6–7 km and
5 km respectively for the central and southern areas of the caldera
edifice.
Funding was provided by project PO-5 “Innovative application of
modern techniques for geothermal exploration by the integration of
geological, geochemical and geophysical methods, study case of Los
Humeros volcanic field”, which forms part of the CeMie-GEO
Consortium No. 2007032 funded by Fondo de Sustentabilidad SENERCONACYT. Partial funding comes from PAPIIT grant No. 106314. We
want to thank the Geothermoelectric Project Management of the
Electricity Federal Commission (CFE), and particularly to Magaly Flores
and Cecilia Lorenzo for the facilities to develop this project.
J. Arzate wishes to acknowledge the use of equipment and resources
in this work, obtained from Conacyt #221487 and Papiit # IN116816
research projects. We also would like to thank to the many graduate
and practicing students from UNAM and other institutions that participated along several field campaigns of data acquisition: Diana
Martínez Hernández, Jorge López Alvis, Isela Zecua Alejo, Alberto
Gutiérrez, César Daniel Castro Soto, Andrés Arzate Monterrubio, Pablo
Arzate Monterrubio, Raúl Sánchez García, Anaid Fragoso Irineo, Rafael
Felix Maldonado, David Ernesto Torres, Daniel Arturo Velázquez,
Valeria Anahí Luévanoa and Leidy Itzel Evangelista. Finally, many
thanks to Roberto Molina and two anonymous reviewers that helped us
to improve the article content.
6. Conclusions
The acquisition and processing of the magnetotelluric data followed
accurate standard procedures to obtain the best possible conductivity
models in the sense of data quality fitting but also in the sense of inversion parameter decisions, which may alter drastically the final results. The obtained resistivity models were correlated on the ground
with detailed surface geology, and the residuals of the complete
Bouguer anomaly and reduced-to-pole total magnetic anomaly map of
the crater structure and surroundings. The gravity residual image reveals a circular minimum anomaly shifted to the north of the Humeros
caldera crater defining a quite preserved caldera crater. The gravity
minimum extents to the southwestern portion of the large rim of the
composed calderas, suggesting the continued breccia and sedimentary
filling of the Los Potreros caldera. The magnetic high observed at the
center of the composed caldera rim, is probably composed by at least
three independent magnetic dipoles responding to different magnetized
bodies of low density, which implies secondary magnetization. We
hypothesize, although is not modeled at this point, that the magnetic
high defines above Curie point magnetic ensembles, which are related
to widespread propylitic alteration induced by multipath dehydration
trajectories. At some wells, magnetite has been detected as shallow as
900 m from the surface that gives support to this argument. Higher
gravity and magnetic gradients in the west of the caldera edifice,
soundly correlate with Los Humeros main fault system, where most of
the productive wells are located, suggesting in principle that the contact
interface are the more promising geothermal targets as long as there is
good permeability.
Based on the above and the resistivity models we obtained a model
that consists of a large, fractured propylitic intrusive body of average
crustal density and relatively high resistivity (∼400–500 ohm-m) being
heated from bellow. The fluid and heat flux reach surficial depths
through deep seated faults that reach middle crustal depths. This is
supported by geochemical evidences of mineralized water from magmatic origin. Deeper conductivity anomalies observed in the resistivity
sections are interpreted as the primary heat source of the geothermal
system in the form of hyper critical trapped fluids and partial fusion.
Hyndman (2017) has well documented the common place of trapped
crustal fluids across all American continent. We estimate a depth to the
top of the hot plumes below the geothermal reservoir at 6 to 7 km (P1),
and 5 km (P6) respectively for the central and southern areas of the
Humeros and Potreros calderas respectively.
Despite the results advanced in this study, there remains more detailed analysis to be done such as the tridimensional inversion of the MT
data, which is currently in process, however there are still limitations
that we are solving related to the quantity of data the computers can
handle, which badly influence the resolution of the obtained models.
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