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Remote Sensing Techniques as a Tool for Geothermal Exploration: the Case Study of Blawan Ijen, East Java
No full textAdvances in the Exploration of Geothermal Resources of the East Africa Rift System (EARS)
No full textThis work focuses on the geological, geophysical and geochemical exploration of the geothermal reservoirs located in the East Africa Rift System (EARS), with particular reference to the characterisation of some geothermal fields located in Ethiopia, Kenya, Tanzania and Malawi. Moreover, this study provides an updated overview of the procedures for the exploration of geothermal resources and can serve therefore as a best-practice guide for future endeavours. Field activities included geological surveys, geophysical investigations (gravity, electromagnetic and seismic measurements) and geochemical survey/analyses. Moreover, stratigraphic data and P&T logs were available at some explored geothermal prospects. An overview of the main investigated geothermal fields was given and three case studies were described in detail as representative examples of geothermal play types of EARS: (i) the Alalobeda field (Ethiopia), located in correspondence of the triple junction Read Sea-Aden Gulf-Main Ethiopian Rift and (ii) the Kiejo-Mbaka field (Tanzania), belonging to EARS’ western branch, both falling in the extensional domain play type, fault controlled or fault-leakage controlled; (iii) the Menengai field (Kenya), the second most important geothermal field in EARS, where a huge quantity of direct data from more than twenty drilled wells is available. The latter can be classified as convection-dominated magmatic play type.
Compared to geothermal fields of South-East Asia and Central America, the geothermal of EARS presents some peculiar characters and differences. The plutonic play-type (convection dominated), occurring in fore- or back-arc regions of fold-thrust belts along subduction zones, denotes a well-developed thick and continuous cap rock mainly formed by clay minerals. In the plutonic play of Menengai, the typical impermeable cap rock is practically missing. A “zonation” of the play types occurring in EARS can be recognized. The Western Branch is characterised by the presence of fault/fault-leakage controlled play types. In the Eastern Branch, geothermal plays are associated to active or quite recent volcanoes.
Due to the foregoing characters, a different approach should be followed in order to characterize properly the geothermal fields present in EARS. In a subduction context, geophysical results from electromagnetic investigations play a fundamental role in the exploration of potential geothermal reservoirs, as in such an environment they are often succesfully used to detect the occurrence of an impermeable cap rock overlying the reservoir (target zone). Therefore, if the resistivity structures inferred in EARS geothermal plays are simply associated with “standard” resistivity models of cap rock-reservoir formations, the inferred geophysical conceptual model may be grossly incorrect. Wherefore, an accurate and integrated interpretation of all the geoscientific data is essential. In this regard, a detailed structural survey is of primary importance especially in the fault-controlled plays, whereas its importance is often under-estimated in subduction realms. A high-resolution structural survey allows to define a detailed configuration of fractures and faults that may control the fluid upflow from the reservoir.
Concerning the application of geochemical methods, in EARS, typical approaches and models developed in the subduction geothermal systems should be re-evaluated. The high-temperature geothermal reservoirs of the Eastern branch (e.g., Olkaria and Menengai in Kenya, and Aluto-Langano in Ethiopia) host not only mature chloride waters, as the geothermal systems situated along subduction zones, but also mature bicarbonate-chloride and mature bicarbonate waters. In volcanic-magmatic regions, deep geothermal liquids are assumed to be produced through neutralization of initially acidic meteoric-magmatic aqueous solutions. The few available data for volcanic gases indicate that subduction zones volcanic gases are enriched in Cl relative to hot-spot and divergent-plate volcanic gases. Therefore, the comparatively small supply of Cl-bearing magmatic gas species (chiefly HCl) in the root of the Eastern EARS geothermal systems might be responsible for the comparatively low Cl contents of related geothermal liquids. The situation might be even more complicated in the western EARS, due to the absence of magmatic systems. Therefore, a more comprehensive approach to water classification is needed to distinguish mature waters from immature ones.
In view of the differences with the geothermal systems hosted in subduction zone environments, the future exploration and development of geothermal resources of EARS should thus consider that geothermal resources are rarely due to the presence of a hot magmatic source, but rather to the crustal thinning, which determines thermal anomalies of moderate intensity. Moreover, favorable thermal conditions are not always accompanied by an adequate hydrogeological setting, expecially when they occur in low permeability basalts. In these cases, it is essential to pay attention to the structural setting, in order to design the wells with the highest likelihood of intersecting permeable tectonic structures. Unlike in most Indonesian fields, where permeability tends to be widespread throughout the rock, in the EARS permeability appears in many cases limited to major faults. The planned program of drilling in several prospects of Ethiopia and Tanzania will make available further information improving the overall understanding of the geothermal characteristics of EARS
Cubic spline regularization applied to 1D magnetotelluric inverse modeling in geothermal areas
No full textElectrical resistivity interpretations in geothermal areas typically show a three domains geoelectric structure consisting in an upper
resistive horizon, an intermediate highly conductive layer (the 'cap-rock') and a fairly resistive zone, potentially hosting the
geothermal reservoir. The clay cap usually give rise to low electrical resistivity anomalies that represent attractive targets for
magnetotelluric exploration. Different algorithms are available to get 1D realistic pictures of the geoelectric structure from
magnetotelluric soundings, such as blocky/sharp boundaries or smoothed ('Occam') approaches. Here we introduce a new method
to model 1D resistivity profiles. In our approach, a cubic spline is assumed to be a realistic approximation of the true resistivitydepth
distribution. The advantages of the method are twofold. First, cubic splines need a much smaller number of model parameters
to be defined than the layered models used in smoothed inversion algorithms; this can be particularly useful in pseudo 2D/3D
inversions or for Monte Carlo approaches, where a large number of 1D models must be solved. Second, being the spline
intrinsically a smooth 1D resistivity depth profile, there is no need of solution regularization. An example of application of this
approach to MT single site data from the geothermal field of Menengai is presented, and a comparison with blocky and smoothed
models and the data from a deep exploratory well is made
Evaluation of geothermal resources in a hotspot realm: Mauritius Island (Indian Ocean)
No full textGeochemical and geothermal investigations were performed in the Mauritius Island, located along the
Seychelles-Mascarene Plateau, aimed at the preliminary assessment of possible geothermal resources.
The central part of the island may be the most suitable as characterized by the most recent volcanic
activity (0.03 Ma). Geochemical analyses of water samples collected from this area indicate no
mature water and the chemical features are ascribable to short-term water-rock interaction in shallow
hydrogeological circuits. A gradient borehole was drilled and thermal logs performed after complete
thermal equilibration to evaluate the thermal gradient in central part of the island. A value of 43 °C
km-1 was measured and a similar result was obtained by logging a deep well no longer used for water
extraction. The results point to a weak or null deep-seated thermal anomaly beneath Mauritius. This
might mean that the deep thermal processes (mantle plume) invoked to occur in the hotspot area do
not likely yield any particular thermal signature
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Integrated geophysical imaging of the Alalobeda geothermal field (Ethiopia)
No full textA geothermal exploration project was founded by the
Icelandic International Development Agency
(ICEIDA) and the Nordic Development Fund (NDF),
with the aim of assessing the geothermal resources of
Alalobeda (Ethiopia). The Alalobeda geothermal
system is located along the western margin of the
Tendaho Graben, where the NW tectonic structures of
the graben intersect the NNE trending Main Ethiopian
Rift (MER) lineaments yielding intense rock
fracturation. An integrated geophysical survey was
carried out during in 2014-15, over an area of about
140 km2. The geophysical campaign comprised 123
coincident magnetotelluric and time domain soundings
and 300 gravity stations. 2-D and 3-D gravimetric
models inferred two NW trending major normal faults
hidden by the graben sediment infill. Between them,
the basaltic bedrock shows a structure made by second
order horsts and grabens. The 3-D electrical resistivity
model reveals that the basalts cropping out and the
sediments covering the bedrock in the graben
depression show exceptionally low resistivities
(sometimes lower than 1 Ohm m). In this general
frame, three NNE trending fracture zones were
inferred by their enhanced conductivity in the 3D
model. The fracture zones are located in the shoulder
sector of the NW trending Tendaho Graben, where the
topography and the structural observations indicate the
presence of NNE trending MER normal faults. All the
three inferred fracture zones are put into evidence by
more resistive deeper anomaly, interpreted as hightemperature
alteration, and a shallower very
conductive layer, interpreted as a low-temperature
clay cap. The fracture zones do not continue towards
the plain and appear to be limited by the NW Tendaho
Graben main normal fault. The resistivity pattern
appears compatible with a main fluid path trending
NNE along the MER fracture zones while orthogonal
flow is likely driven by the intersecting NW Tendaho
faults
Three-dimensional geophysical modelling of the Alalobeda Geothermal Field.
No full textThe Alalobeda geothermal system is located in the Afar region (Northern Ethiopia), along the western margin of the Tendaho Graben. A multidisciplinary geoscientific study of the Alalobeda geothermal field, was funded by the Icelandic International Development Agency (ICEIDA) and the Nordic Development Fund (NDF), and carried out by the Geothermal Division of the Geological Survey of Ethiopia under the supervision of ELC-Electroconsult (Milan, Italy). In this paper, we show the results of the integrated geophysical survey, including 123 magnetotelluric and transient electromagnetic soundings and 300 gravity records. The high-density station coverage over a regular grid allowed the successful application of 3D MT and gravity inversion, producing detailed and realistic models of the resistivity and density distributions. The final 3D gravity model images of the bedrock surface hindered by the sediments reveal first-order normal faults controlling the graben main structures. Beneath the graben shoulder, a positive density contrast (+300 kg m-3) was modelled at depths of 1-2 km and interpreted as a propylitic alteration of the bedrock surface. The 3D resistivity model revealed three NNE-trending fracture zones beneath the shoulder of the Tendaho Graben, where topography and structural observations indicate the presence of NNE-trending MER normal faults. These fracture zones are characterised by a shallow conductive anomaly and a more resistive deeper anomaly. The shallow conductor is interpreted as a low-temperature clay cap, resulting from the fluid upflow coming from a deeper high-temperature resistive horizon. The fracture zones do not prosecute towards the plain and appear to be limited by the NW trending main normal faults of the Tendaho Grabe
Interpretation of Thermal Response Tests in Borehole Heat Exchangers Affected by Advection
No full textWe focus on the treatment of thermal response test data when both advection and short-period changes of surface temperature
occur. We used a moving line source model to simulate temperature-time signals under an advective thermal regime. The
subsurface thermal conductivity, the Darcy velocity and the borehole thermal resistance were inferred by means of an optimisation
procedure. In case of Darcy velocity lower than 10-7 m s-1, the underground thermal conductivity is comparable to that obtained by
means of the infinite line source model, which assumes a purely conductive thermal regime. The optimisation analysis was finally
applied to real thermal response test data. The temperature-time curves were filtered to remove the disturbing spectral components
associated with a non-optimal thermostatic behaviour of the apparatus. This produced reliable estimates of thermal and hydraulic
parameters. An independent method based on the analysis of temperature-depth logs was also used to validate the inferred
groundwater flow
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