Shallow Temperature Measurements at Juncalito, a

Shallow Temperature Measurements at Juncalito, a
Geothermal Prospect, Central Andes, Chile
Elías Lira*, Rodrigo Arcos, Jorge Clavero, Aldo Giavelli and Catalina Mayorga
Energía Andina S.A., Darío Urzúa 2165, Providencia, Santiago, Chile
*E-mail: elira@energiandina.cl
Abstract. The mapping of temperature variations at or
below the earth surface constitutes a key geothermal
exploration tool. In this work, the results of 25 days of
shallow temperature measurements are presented. First,
thirty-three temperature loggers were installed in an area of
approximately 25 km² for a period of eight days. The
purpose of this survey was to identify patterns of nearsurface lateral flow of thermal signatures associated with
known hot springs. The results of these surveys showed
relevant thermal anomalies not only associated with hot
springs but also on other sites without hydrothermal
discharge. A second survey of one-hundred temperature
measurements was performed for seventeen field working
days. The temperature measurements were obtained in an
area of approximately 37 km². In this survey, thermal
anomalies identified in the first survey were mapped at a
higher level of detail. These results are complemented with
Magnetotelluric and geochemical data, potentially
delineating zones of upflow and outflow of a geothermal
prospect.
volcanoes, dome-complexes and lava and ignimbrite fields
(Los Cuyanos-Sierra Nevada Volcanic Complex) (Clavero
et al., 1997, 1998), which is interpreted to be the heat
source of a geothermal system. The main structural feature
corresponds to a high angle N-S thrust system that
overrides volcaniclastic Permian-Triassic rocks (W) over
Oligo-Miocene volcanic and syntectectonic sedimentary
rocks (E), (Clavero et al., 1997, 1998).
Five zones of thermal springs have been recognized in the
northern, centre and southern parts of Juncalito area, the
Río Negro springs being the ones with higher temperatures
(up to 44°C) and larger flow (~20-30 l/s). A positive
compositional indicator of a thermal resource is the silica
concentration (130 to 160 mg/kg SiO2) of Río Negro
springs indicating quartz equilibration temperatures of 140
to 160°C (Mayorga, 2011).
3 Shallow Temperature Surveys
During March 10-17, 2011 thirty-three 1-meter deep
temperature measurements were recorded in an area of
aproximately 25 km² in the Pampa de Los Cuyanos, Río
Negro and Ojos de los Cuyanos stream zones (Fig. 1, white
triangles). One-hundred additional measurements were
made between May 8-24, 2011 in an area of 37 km² in the
same zones (Fig. 1, black triangles).
Keywords: Geothermal, shallow temperature, exploration
1 Introduction
For many decades, shallow temperature measurements
have been used as key geothermal exploration tool.
Subsurface temperature measurement at a depth of 1-meter
is an efficient method for mapping thermal anomalies with
a high level of detail. These thermal anomalies can be
associated with interesting geothermal features. However,
variations in near-surface temperatures could be caused by
a number of non-geothermal factors, including changes in
air temperature, soil moisture, near-surface groundwater
flow and thermal inertia. The effects of these variables
were qualitatively characterized during this survey.
The temperature measurements were corrected for
different non-geothermal factors. Changes in air
temperature and daily solar radiation cycle were corrected
by a simple moving average filter of the 12 and 24 hours
respectively, but in general, at a depth of 1-meter,
temperature variations induced by 24-hour solar radiation
cycle are almost completely damped out (Elachi, 1987).
The thermal inertia was considered in the duration of time
series: beginning with a one-day delay after installation for
the temperature to reach equilibrium.
2 Study Area Background
Surface temperatures during the first survey varied from -8
to 27°C compared with the temperatures at a depth of 1meter that varied from 8 to 33°C. During the second
survey, the surface temperatures varied from -5 to 9°C
whereas that the temperatures at a depth of 1-meter varied
between 4-42°C.
Juncalito area is located in northern Chile, in the southern
limit of the Chilean Altiplano, at an average altitude of
4100 m.a.s.l. The younger volcanism in the area is
associated with a Pleistocene-Holocene NW-SE volcanic
chain with a series of volcanic complexes, strato-
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Figure 1. Location of the temperature loggers. Thirty-three
temperature loggers were installed in an area of approximately 25
km² for a period of eight days (white triangles). In the second
survey, one-hundred temperature loggers were installed in an area
of approximately 37 km² (black triangles).
Figure 2. One meter deep temperature measurements during the
first survey. The temperatures varied between 8-33°C (blue to
red color scale). Background image is shaded topography.
4 Results and Discussion
Two main thermal anomalies were identified with
temperatures >15°C which greatly exceeded the
background temperature defined for the period and study
area (<9°C). The western margin thermal anomaly is
related to a hydrothermal discharge zone in the headwaters
of Ojos de los Cuyanos stream. The second thermal
anomaly of greater magnitude (~30°C) is related to Rio
Negro springs and is NE-SW aligned. A third thermal
anomaly of lower magnitude (~11°C) was identified near
the Azufrera de los Cuyanos volcano. This anomaly is not
related to known thermal springs and could be associated
with a deep heat source.
In the second survey, the thermal anomalies identified in
the first survey were mapped at a higher level of detail.
The thermal anomaly correlated to the headwaters of Ojos
de los Cuyanos stream is aligned in N-S direction and
suggests structural or lithological control. The thermal
anomaly correlated to Río Negro springs shows different
hydrothermal discharge zones. Thermal anomalies of lower
magnitude were identified in the Pampa de Los Cuyanos
and close to Azufrera de los Cuyanos and Juncalito
volcanoes. First, the thermal anomaly located in Pampa de
los Cuyanos could be associated with a thermal aquifer
that discharges in the Río Negro and Ojos de los Cuyanos
stream. Second, the thermal anomalies correlated to
Azufrera de los Cuyanos and Juncalito volcanoes may be
related to a deeper heat source.
Figure 3. One meter deep temperature measurements during the
second survey. The temperatures varied between 4-42°C (blue to
red color scale). Red border represents a very distinctive
conductivity anomaly defined by MT survey (<5 Ω.m).
Background image is shaded topography.
619
Lira, E. 2011. Estudio de Temperatura de suelo a 1 m de profundidad.
Proyecto Juncalito. Energía Andina S.A. (Unpublished), 31p.
Chile.
The thermal evidence detected shows a strong and direct
geographical relationship with a very distinctive
conductivity anomaly (<5 Ω.m, red border) suggesting the
presence of thermal fluids that discharge into Rio Negro
and Ojos de los Cuyanos stream (possible “outflow” zone).
This very distinctive conductivity anomaly defined by MT
survey, with an abrupt limit (structurally controlled) to the
west and a convex geometry, suggesting an “upflow” zone
of the potential geothermal system under the southwest
flank of Los Cuyanos volcanic Complex.
Mayorga, C. 2011. Geoquímica de Fluidos del área de Juncalito.
Proyecto Juncalito. Energía Andina S.A. (Unpublished), 26p.
Chile.
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