16 research outputs found
Geophysical imaging of the Luhoi geothermal field, Tanzania
No full textThe Luhoi prospect is a coastal basin located within Rufiji Trough along the passive continental margin of western Indian Ocean of Tanzania, a sector extending south of the termination of the eastern branch of the African Rift System. The structural pattern is dominated by tectonic features belonging to the WNW-ESE Tagalala Trend and to the NE-SW Selous Trend that have been active until recent times. The thermal manifestations are mostly located along a WNW-ESE direction flowingfollowing the Ruhoi River, in the south-western sector of the focal study area. The Wingoyongo Hill, located in the north-eastern sector of the focal area, forms a morphological high where emissions of H2S and bituminous staining were observed. Here, an old well intersected 800 m of quartz sandstone with minor intercalations of siltstone and claystone (Kipatimu Series, Lower Cretaceous). Magnetotelluric (MT), time-domain electromagnetic (TDEM) and gravimetric geophysical surveys were carried as part of a geoscientific study funded by the Ministry of Foreign Affairs of Iceland through Icelandic International Development Agency (ICEIDA) and the Nordic Development Fund (NDF). The goal of the survey was to advance the state of knowledge of the Luhoi geothermal prospect, in order to elaborate the conceptual model of the field. The survey area extends over a surface of approximately 75 km2, designed to include the Luhoi hot springs and the Wingoyongo fumaroles. The gravimetric data set is composed of 124 measurements collected on a regular grid at a nominal spatial sampling of 800 m, and 16 more to infer the regional field. The residual Bouguer anomaly map shows an elongated gravimetric high trending NE-SW with values up to 3 mGal, surrounded by gravimetric lows up to -2 mGal. Forward and inverse 2D/3D models image an asymmetrical horst like structure trending NE-SW. Both the thermal manifestations and the Wingoyongo Hill are aligned along the NW flank of the horst. Since the Wingoyongo well intersected sandstones for about 800 m, the horst like structure is interpreted as made by the Kipatimu sandstones. The two depressions bordering the horst like structure are filled with lower density materials, likely siltstones, claystones and/or mudstones, with an estimated maximum thickness of 1.1 km. The MT and TDEM data were acquired at 76 locations, with a nominal spatial sampling of 750 m. The static shift effect has been corrected by TDEM/MT phases joint inversion. MT impedances and tippers have been estimated by means of the remote reference technique with robust processing methods coupled with a coherence rejection scheme. Resistivity 3D inversion reveals two conductive anomalies coincident with the low-density sedimentary rocks bordering the horst structure. A clear updoming of resistivity marks the NW flank of the horst and it is interpreted as due to a combined effect of different alteration, lithology and fluid content and to reflect the main upflow of the geothermal system
Regional thermal anomalies derived from magnetic spectral analysis and 3D gravity inversion: Implications of potential geothermal sites in Tanzania
No full textTanzania is one of the several countries intersected by the East African Rift System (EARS) which is endowed by a geothermal potential that has been explored only to a limited extent. Here we present the first heat flux map over the region based on the Curie point depth (CPD) estimation from aeromagnetic data. We have estimated the base of magnetic sources as a proxy for the CPD from the radially average power spectra of the total magnetic field using the centroid and the de-fractal methods. Our results show that the CPDs range ca. 11 to 43 km and are comparable with the global CPD estimates but with more detail in our estimates. The heat flow has then been computed assuming a constant heat conductivity. In order to evaluate the results against crustal thickness, we have inverted the gravimetric regional field constrained by existing Moho depth from seismic receiver functions. Our analysis has revealed high heat flow values (over 100 mW/m2) along the EARS and at the Proterozoic collision boundaries that have been reactivated by the EARS. In general, the high heat flow anomalies coincide with the known surface geothermal manifestations and shallow Moho depth in the range ca. 30 to 35 km. A high heat flow anomaly is also found in the central part of the Tanzanian craton, likely related to the mantle plume imaged by seismic tomography. The most interesting areas for geothermal exploration in Tanzania, according to our results, are the EARS triple junction in the Rungwe volcanic province, the north Tanzania divergent zone and the areas of the Proterozoic collision boundaries reactivated by the EARS
New MT surveys and 3D resistivity imaging beneath the Ngozi-Rungwe volcanoes at the triple rift junction of the East African Rift System in SW Tanzania: Support for integrated interpretations of geothermal conceptual models
No full textThe geothermal system related to the Ngozi and Rungwe volcanoes, SW Tanzania, lies at the intersection of the west and east branches of the East African Rift System and has been investigated by many geoscientists for decades. Here we present a 3D electrical resistivity model based on 190 magnetotelluric resistivity soundings that have been integrated with geochemical and geological results to support the development of the geothermal resource conceptual model presented here. The model includes two separate reservoirs, a larger system located beneath the Rungwe volcano and a smaller chloride water reservoir located under the Ngozi caldera, which contains a neutral chloride hot spring with geothermometry >230°C. An extensive conductive clay cap with variable thickness extends along the 30 km long NW-SE trending Ngozi-Rungwe Fault Zone from the Kiejo area SE of the Rungwe volcano to the Ngozi caldera. The absence of geothermal surface manifestations directly over the inferred Rungwe upflow zone is consistent with effective sealing of the proposed underlying geothermal reservoir by the clay cap. The scarcity of thermal manifestations on the up-dip margins of the low-resistivity clay cap can be explained by coincidence of the base of the clay cap with impermeable Precambrian formations and by structural boundaries. This interpretation implies that the area with the highest geothermal resource potential is the Rungwe volcano where proposed drilling sites might intersect the proposed high-temperature reservoir
Imaging and modelling the subsurface structure of the Rungwe Volcanic Province in SW Tanzania with aeromagnetic data: An improved structural map to support geothermal exploration
No full textThe Rungwe Volcanic Province (RVP) in the East African Rift System, SW Tanzania, provides a unique opportunity to investigate geothermal resources in the context of particularly complex continental rifting processes. To support geothermal resource targeting in the RVP, we present a revised neotectonic structural map based on an interpretation of aeromagnetic data constrained by 2D-forward modelling of magnetic anomalies integrated with the distribution of previously reported faults, seismic epicentre locations, 3D magnetotelluric resistivity models and surface geothermal manifestations. Magnetic anomalies in the RVP, including the nationally prominent Mbeya anomaly, are related to the high magnetic susceptibility or remanent magnetism of Precambrian rocks and Cretaceous carbonatite intrusions buried in the rift under a varying thickness of non-magnetic sediments and volcanic rocks. Magnetic lineaments are related to structures controlling the geometry of the Precambrian rocks and concealed dikes and the thickness of the sediments and volcanics. The recent Ngozi and Rungwe trachyte volcanics have relatively low magnetic susceptibility comparable to the low susceptibility of the sediments in the rift basins. The revised neotectonic structural map shows prominent NW, NE and NS-trending magnetic lineaments that correlate with previously reported faults and alignments of seismic epicentres in the study area and with the regional trend of the rift segments. The NE-trending magnetic lineaments are consistent with interpretations of the current stress field in the RVP. The main volcanoes in the RVP, Ngozi, Rungwe and Kiejo (also known as Kyejo and Kieyo), are aligned with the NW-trending linear magnetic feature joining the Lupa and Livingstone rift border faults. This lineament is intersected and frequently displaced by the NE and NS-trending lineaments, suggesting that the NE to NS-striking structures are younger. The Rungwe and Ngozi volcanoes as well as numerous ‘‘monogenetic’’ eruption centres and the Mwakaleli caldera, which originated ca. 2 Ma ago (Ebinger et al., 1989) following a large explosive eruption emplacing widespread ignimbrite deposits, are located within a zone of low-intermediate magnitude magnetic features forming a basin-like structure surrounded by magnetic high anomalies of the Precambrian basement structures. We interpret the intersections between the NW-trending intra-rift faults and the NS and NE-trending faults as favourable locations for wells to target high permeability within the geothermal resource conceptual models previously constructed using 3D MT resistivity imaging integrated with supporting geoscientific data. The intersections provide a focus area for follow-up ground mapping of subtle features that may be associated with very recent fault movement
Regional thermal anomalies derived from magnetic spectral analysis and 3D gravity inversion: Implications for potential geothermal sites in Tanzania
No full textTanzania is one of the several countries intersected by the East African Rift System (EARS) endowed by a geothermal potential that has been explored only to a limited extent. Here we present the first heat flux map over the region based on the Curie point depth (CPD) estimation from aeromagnetic data. We have estimated the base of magnetic sources as a proxy for the CPD from the radially average power spectra of the total magnetic field using the centroid and the de-fractal methods. Our results show that the CPDs range ca. 11 to 43 km and are comparable to, but more detailed than global CPD estimates. The heat flow has then been computed assuming a constant thermal conductivity. In order to evaluate the results against crustal thickness, we have inverted the
gravimetric regional field data constrained by the existing Moho depth from seismic receiver functions. Our analysis has revealed high heat flow values (over 100 mW/m2) along the EARS and at the Proterozoic collision boundaries that have been reactivated by the EARS. In general, the high heat flow anomalies coincide with known surface geothermal manifestations and shallow Moho depth in the range ca. 30 to 35 km. A high heat flow anomaly is also found in the central part of the Tanzanian craton, likely related to the mantle plume imaged by seismic tomography. The most interesting areas for geothermal exploration in Tanzania, according to our results, are the EARS triple junction in the Rungwe volcanic province, the north Tanzania divergent zone and the areas of the Proterozoic collision boundaries reactivated by the EARS
Magnetic, gravimetric, TDEM and magnetotelluric joint interpretation at the Luhoi geothermal field, Tanzania
No full textWe show the interpretation of the gravimetric, magnetic, TDEM and magnetotelluric geophysical campaign carried out to better constrain the geothermal conceptual model of the Luhoi geothermal field in Tanzania. Geophysical modelling has imaged a NE trending horst 1 km wide and 5 km long formed by the denser Kipatimu (Lower Cretaceous) sandstones surrounded by the Upper Cretaceous Ruaruke claystones down-thrown up to 1 km by normal faulting. Ruaruke claystones show low resistivity (1-10 Ohm m) primarily due to its clay content, while the Kipatimu sandstones have generally higher resistivity (10-30 Ohm m) because of their prevalent electrolytic conduction
Regional thermal anomalies in Tanzania and improved geothermal conceptual models of the Ngozi and Rungwe prospects in SW Tanzania based on results from resistivity and potential field studies
No full textThis study investigates for the first time the heat flux distribution in Tanzania, the only East African country intersected by the eastern and western branches of the East African Rift System (EARS). The main goal of the project is to evaluate the geothermal potential of the region by integrating magnetotelluric, magnetic, and gravimetric data and models with existing petrophysical, seismological, volcanological, geochemical and structural information.
At the regional scale, heat flux in Tanzania has been determined from the radially averaged power spectra of the total magnetic field, interpreted as a proxy for the Curie Point Depth. These findings have been correlated to the Moho depth estimates from seismic receiver functions and 3D inversion of gravity data. The results indicate that high heat flow anomalies (exceeding 100 mW/m2) in Tanzania are situated along the EARS at the Proterozoic collision boundaries. Generally, these high heat flow anomalies align with known surface geothermal manifestations and shallow Moho depth (ranging from approximately 30 to 35 km). A high heat flow anomaly has also been detected in the central region of the Tanzanian craton, likely associated with the mantle plume imaged by seismic tomography. According to our findings, the most compelling areas for geothermal exploration in Tanzania are the EARS triple junction in the Rungwe Volcanic Province (RVP), the north Tanzania divergent zone in the eastern branch of the EARS, and the areas of the Proterozoic collision boundaries reactivated by the EARS.
At a more detailed level, the project focuses on the RVP, one of the most promising areas in Tanzania for prospective geothermal resources. High-resolution aeromagnetic data combined with known faults, seismic epicentres, and known surface geothermal manifestations, including thermal springs, hot ground areas, and 3D MT resistivity anomalies are employed to identify concealed structures likely to enhance the permeability of geothermal systems and to delineate structures and formations likely to demarcate the boundaries of geothermal systems within the RVP, particularly in the three most promising prospects: Ngozi (with geothermometry >230°C), Songwe (112°C), and Kiejo-Mbaka (140°C). The 3D resistivity model derived from the inversion of newly acquired and existing magnetotelluric data has been instrumental in updating the geothermal conceptual model of the Ngozi system and defining a new prospective high-temperature geothermal system beneath Rungwe volcano in the RVP.
The 3D MT resistivity inversion results indicate the presence of magmatically heated high-temperature geothermal systems with two discrete reservoirs, a small reservoir below Ngozi caldera and a larger reservoir beneath the Rungwe volcano. A magma chamber is imaged as a deep conductor situated approximately 6 km below the newly identified Ndaga ring structure located between the Ngozi and Rungwe volcanoes and is supported by several young monogenetic eruption centres, the Ngozi volcano to the north that developed into a caldera less than 1,000 years ago and the Rungwe volcano to the south that was formed about 10,000 years ago. The margin of the Ndaga ring structure is defined by the resistive phonolite domes formed approximately 9 million years ago, predominantly overlain by young volcanic lavas and/or pyroclastic materials. Modelling results that have identified the Rungwe geothermal prospect in addition to the previously interpreted Ngozi prospect have significantly expanded opportunities for discovering geothermal resources in the planned drilling campaign.
Structural investigations in the RVP utilizing magnetic data have revealed that the potential areas for geothermal resources in the RVP are located at the intersections between the NW-SE trending intra-rift faults with NE-SW, N-S, and E-W trending faults. These zones of likely enhanced permeability are bounded by generally low permeable Precambrian rocks to the west and east, respectively, which has significant implications for geothermal resource targeting and development in the RVP. As part of the efforts to bolster geothermal resource targeting in the RVP, an updated structural map has been developed based on aeromagnetic data interpretation constrained by 2D-forward modelling of magnetic anomalies, integrated with previously reported faults, seismic epicentres, 3D magnetotelluric resistivity models, and surface geothermal features.Hér er í fyrsta sinn lagt mat á varmaflæðið í Tansaníu, eina landinu í Austur-Afríku sem er skorið í sundur af bæði eystra og vestra rekbelti Austur-Afríku sigdalsins (EARS). Megin markmið þessa verkefnis er að meta umfang jarðhitans á svæðinu með því að samtúlka MT viðnámsmælingar, og segul- og þyngarmælingar ásamt tiltækum upplýsingum á sviði efnaog eðlisfræði bergsins, jarðskjálfta- og eldfjallarannsókna, jarðefnafræði og höggunar. Svæðisbundið varmaflæði í Tansaníu hefur verið metið út frá meðal aflrófi heildarsegulsviðsins, miðað við dýpi á Curie flötinn. Niðurstöður eru bornar saman við dýpi á Moho samkvæmt viðtökufalli jarðskjálfta og þrívíðri túlkun þyngdarmælinga. Niðurstöður benda til þess að í Tansaníu sé varmaflæði umtalsvert (meira en 100 mW/m2 ) í AusturAfríku sigdalnum við plötuskilin frá frumlífsöld. Almennt falla þessi háu frávik saman við þekkt jarðhitaummerki á yfirborði og lítið dýpi á Moho (um 30-35 km). Há frávik í varmaflæði sjást einnig miðsvæðis í meginlandskjarna Tansaníu og tengjast líklega möttulstróki sem kemur fram í túlkun skjálftamælinga. Samkvæmt okkar niðurstöðum eru áhugaverðustu svæðin til frekari jarðhitarannsókna í Tansaníu við þrípunktinn í AusturAfríku sigdalnum nærri Rungwe eldfjallasvæðinu (RVP), auk fráreksskila sigdalsins í norðurhluta landsins og svæðinu við plötuskilin frá frumlífsöld sem EARS hefur hreyft við á nýjan leik Hér er sjónum einkum beint að Rungwe eldfjallasvæðinu, einu álitlegasta jarðhitasvæði landsins. Niðurstöður flugsegulmælinga með hárri upplausn ásamt staðsetningu þekktra misgengja og jarðskjálfta, jarðhitaummerkja á yfirborði þar með talin goshverir og svæði með háum yfirborðshita auk viðnámsfrávika samkvæmt þrívíðri túlkun MT viðnámsmælinga voru nýtt til þess að kortleggja sprungur sem ekki sjást á yfirborði. Þær geta gefið vísbendingar um góða lekt í jarðhitakerfinu og afmarkað jarðhitakerfin á Rungwe svæðinu, einkum þau þrjú vænlegustu: Ngozi (hiti samkvæmt efnahitamælum >230℃), Songwe (112℃) og Kiejo-Mbaka (140℃). Viðnámslíkan samkvæmt þrívíðri túlkun nýrri og eldri MT mælinga gegndi lykilhlutverki við endurgerð hugmyndalíkans af Ngozi jarðhitakerfinu. Líkanið benti jafnframt til nýs háhitakerfis undir Rungwe eldstöðinni. Þrívíð túlkun MT mælinga bendir til þess að þar sé að finna háhita jarðhitakerfi, varmagjafinn sé kólnandi kvika. Þar séu tveir aðskildir jarðhitageymar, sá minni undir Ngozi öskjunni og sá stærri undir Rungwe eldstöðinni. Kvikuhólf kemur fram sem velleiðandi skrokkur á u.þ.b. 6 km dýpi neðan hinnar nýkortlögðu Ndaga hringlaga myndunar milli Ngozi og Rungwe eldstöðvanna. Tilgátan um kvikuhólf styðst jafnframt við það að þarna eru nokkur stök ung eldvörp, Ngozi eldstöðin fyrir norðan þróaðist í öskju fyrir minna en 1.000 árum síðan og fyrir sunnan er Rungwe eldstöðin sem myndaðist fyrir um 10.000 árum. Barmar Ndaga hringgangakerfisins takmarkast af háviðnáms fónólít-gúlum sem urðu til fyrir um 9 milljón árum. Eftir það hafa ung hraun og/eða gjóska lagst þar ofan á. Niðurstöður reiknilíkana hafa leitt líkur að tilvist Rungwe jarðhitakerfisins til viðbótar við Ngozi kerfið sem var þekkt fyrir. Þar með hafa aukist umtalsvert möguleikar á að finna jarðhita í fyrirhuguðu borverkefni. Sprungukortlagning RVP-svæðisins með segulmælingum bendir til þess að vænlegar jarðhita auðlindir á RVP-svæðinu séu að finna þar sem misgengi í sigdalnum með NV-SA stefnu skera misgengi með NA-SV, N-S og A-V stefnu. Þessi belti þar sem vænta má góðrar lektar afmarkast til vestur og til austurs af bergi frá forkambrium sem er almennt með lélega lekt og gefa þar af leiðandi góðar vísbendingar um jarðhita auðlindina á RVP-svæðinu og nýtingu hennar. Til þess að átta sig frekar á eiginleikum RVP-svæðisins var höggunarkortið betrumbætt með túlkun flugsegulmælinga og tvívíðri túlkun segulfrávika ásamt áður kortlögðum misgengjum, staðsetningum jarðskjálfta, þrívíðum viðnámslíkönum og jarðhita ummerkjum á yfirborði.Geothermal Training Program of Iceland (GRÓ-GTP
Assessment of the Kiejo-Mbaka geothermal field by three-dimensional geophysical modelling
No full textHigh-resolution magnetotelluric and gravity data have been collected over the Kiejo-Mbaka geothermal field, located along the NW–SE trending Mabka fault, in the Karonga Rift basin (East Africa Rift System). Such resolution allowed to reconstruct the field structure with unprecedented detail. Resistivity modelling has been obtained by three-dimensional finite-differences inversion of MT data, while density modelling has been accomplished by surface-oriented inversion of gravity data. Geophysical modelling has identified two sedimentary sub-basins separated by the Mbaka fault ridge, exposing the basement; these previously unknown sedimentary fills have a maximum thickness of ca. 1.5 km. The estimation of the clay cation exchange capacity (CEC) from magnetotellurics identifies a layer of low-temperature smectite alteration in the south-western sub-basin sediments, interpreted as a clay cap. The resulting updated conceptual model of the Kiejo-Mbaka geothermal system is therefore a fault-controlled system with lateral leakage into the sediments, expectably implying a larger reservoir volume than previously estimated
Three-dimensional geophysical modelling of Kiejo-Mbaka geothermal field, Tanzania
No full textThe Kiejo-Mbaka geothermal field is located close to the eastern margin of the Karonga Rift Basin and is part of the Rungwe volcanic province where the EARS splits up into its Western and Eastern branches in southern Tanzania. The area is characterised by a Precambrian gneiss metamorphic basement complex, outcropping along the NW-trending, SW-dipping Mbaka fault. Geothermal manifestations mainly consist of hot springs, flowing close to the Mbaka fault. An integrated geophysical survey was carried out over the Kiejo-Mbaka geothermal field by TGDC (Tanzania Geothermal Development Company), under the supervision of ELC-Electroconsult (Italy). The campaign included 76 Magnetotelluric (MT) and Transient Electromagnetic (TEM) soundings and 133 gravity measurements; a dense station grid allowed for a detailed geophysical 2D and 3D modelling. Two and 3D gravity modelling indicate that the positive residual Bouguer anomaly can be explained by a high density (3 g/cm3) body, constituting the gneiss basement, elongating NW-SE. NE and SW of it, lower density layers (2.5 g/cm3) are observable; the attitude of their bottoms is compatible with the Mbaka fault direction and the Livingstone fault trend (NNW). We found that 3D MT inversion was the only tool giving a reliable resistivity imaging in the Mbaka prospect. From the final 3D MT model, a very resistive body (>2000 Ohm m) deepening toward SE is visible; this body represent the gneiss basement, and the surfaces delimiting it are associated with the Mbaka fault and the Livingstone fault trend. Three conductive zones (less than 10 Ohm m) have been identified: two of them affect the Mbaka fault footwall, NE of the resistive basement, while another one is located beneath the plain, SE of it. This latter zone shows a thickness of about 1 km. It is apparent that the low-density regions well correspond with the high-conductivity zones imaged by the MT 3D inversion. The integrated geophysical interpretation then leads to two possible geological scenarios: these regions can be constituted by (post-rift) sediments (possibly affected by low-T geothermal alteration) or by intensively fractured and low-T altered basement; however, we stress that the possible geothermal alteration is not necessarily related to the present-day geothermal activity, and caution should be taken in result interpretation
Geophysical Exploration of the Kiejo-Mbaka Geothermal Field, Tanzania
No full textThe Kiejo-Mbaka geothermal prospect (Tanzania) lies along the eastern margin of the late Miocene - Pliocenic Karonga Rift Basin, and is part of the Rungwe volcanic province. The prospect is characterised by an uplifted and outcropping block of Precambrian Gneiss basement,
limited toward SW by the NW-trending, SW-dipping Mbaka Fault. The geothermal manifestations mainly consist of hot springs, occurring along this fault.
To get a detailed assessment of the prospect, an integrated geophysical survey has been carried out by TGDC (Tanzania Geothermal Development Company),
with the supervision of ELC-Electroconsult (Italy). A dense station grid was adopted, comprising 76 Magnetotelluric (MT) and Transient Electromagnetic
(TEM) soundings and 133 gravity measurements.
The 2/3D gravity modelling depicted the structure of the basement, primarily consistent with a tilted and uplifted block of the Karonga half-graben.
The high-density basement block (3000 kg/m3) is surrounded by lower density layers (2500 kg/m3), lying beneath the plain SW of the Mbaka fault and NE of it.
Magnetotelluric dimensional analysis indicated strong 3D conditions and yx-component phases often beyond 90°.
The MT 3D modelling identified a resistive body (>2000 Ohm m) deepening toward SE and representing the gneiss basement.
Two conductive layers (<10 Ohm m) are located on the ridge NE of the Mbaka fault, and another one (<5 Ohm m) lies
beneath the plain, showing a thickness of about 1 km.
We found a strict correlation between the low-density and the high-conductivity layers. This leads to hypothesise the presence of syn/post-rift
sediments possibly hosting a geothermal reservoir
