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The structural outline of the Main Ethiopian Rift-southern Afar transition region. Important features shown include the axial Wonji Fault Belt, regional lineaments, and the locations of the major volcanic centers. The inset map illustrates the regional context and the locations of the detailed, larger portion of this figure. Figure 4 is also outlined. Modified from Chernet et al. 1998. Colour version of this figure is available in electronic edition only.
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The Aluto-Langano geothermal field is located in the central southern portion of Ethiopia within the Ethiopian Rift Valley. The gravity of the area was surveyed in an attempt to delineate the subsurface structure and to better understand the relationship between the geothermal systems and the subsurface structure. The gravity data were analyzed usi...
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... Main Ethiopian Rift (MER) extends some 400 km NNE from 5°N to 9°N (see Fig. 1). The MER has an average width of 70 km and is bounded by large normal faults with displacements as much as of 1000 m (Di Paola 1972). The Wonji Fault Belt (WFB) was formed by tectonic fragmentation of the rift floor (Fig. 3). It was active since the early Quaternary (Mohr 1967), and it has a system of dextral en-echelon displacements. The Aluto volcanic complex is a Quaternary central volcano located along the WFB in (Fig. 4) (Gianelli and Teklemariam 1993). The oldest eruption of the Aluto volcano is dated at 155 000 ± 8000 years and is characterized by a ...
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... famous Wonji Fault Belt is clearly identified in Figs. 11e and 12. It is shifted slightly to the east compared to the location defined in geological studies (see Figs. 3 and 4). These results indicate that the Aluto-Langano geothermal field is located in a highly fractured ...
Context 3
... Main Ethiopian Rift (MER) extends some 400 km NNE from 5°N to 9°N (see Fig. 1). The MER has an average width of 70 km and is bounded by large normal faults with displacements as much as of 1000 m (Di Paola 1972). The Wonji Fault Belt (WFB) was formed by tectonic fragmentation of the rift floor (Fig. 3). It was active since the early Quaternary (Mohr 1967), and it has a system of dextral en-echelon displacements. The Aluto volcanic complex is a Quaternary central volcano located along the WFB in (Fig. 4) (Gianelli and Teklemariam 1993). The oldest eruption of the Aluto volcano is dated at 155 000 ± 8000 years and is characterized by a ...
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We propose an algorithm to automatically locate the spatial position of anomalies in potential field images, which can be used to estimate the depth and width of causative sources. The magnetic anomaly is firstly enhanced using an edge detection filter based on a simple transformation (the Signum transform) which retains only the signs of the anoma...
Citations
... In relation (2), the first-order vertical derivative of potential field data is given as ∂P ∂z . However, HGA and ASA filters perform unsatisfactorily in highlighting the buried source edges (Saibi et al., 2012;Prasad et al., 2022;Alvandi and Ardestani, 2023). Numerous normalized filters have been proposed, incorporating the ratio or combination of directional derivatives derived from potential field data, aimed at simultaneously capturing all boundaries of shallow-level and deep-seated buried sources. ...
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... The compilation results produce gravity data with a resolution of 230m/px, which is close to the resolution of ground measurements in several volcanoes. For example, in geothermal Kenya with a spacing of 650 m between points [13], Aluto-Langano, Ethiopia, with a spacing of 0.5 -7 km [14], Kinigi, Rwanda with 0.8 -1.6 km [15], Beppu geothermal Japan with a spacing of 250 m and 2 km [16]. Therefore the GGM+ data is considered effective for fault mapping and geological structure in the Jaboi volcanic area. ...
Geothermal is a very expensive investment industry. Therefore, it is necessary to map a geological structure in the sub-surface, i.e., faults, and rock formations that control volcanic hydrothermal systems to reduce investment risk in the exploitation of geothermal. On the other hand, the hydrothermal system aims for flow paths connecting reservoir wells for fluid production. The Jaboi Volcano, with an estimated 80 MWe located on Weh Island, Indonesia, has been planned by the government to develop electrical energy, where the excess energy will be exported to Banda Aceh via undersea cables. We use global gravity model plus (GGM+) in a resolution of ~230m/px for mapping the geological structure of Jaboi volcano. Based on GGM+ data analysis, the Bouguer anomaly data shows low gravity values in volcanic areas, namely 46 – 69 mGal. These data only represent rock density values with low density in geothermal areas. We also calculate the residual anomaly from the Bouguer data using the high-pass-filtering technique, wherein the volcanic area, several high-gravity anomalies (1 – 1.4 mGal) correspond to the Leumomate fault in the direction of NW-SE. The same pattern is also obtained in the area with a suspected Ceunohot fault in the SW – NE direction. This research demonstrates the optimization of gravity satellite that free access to be used in mapping geological structures in geothermal Jaboi. Finally, we conclude that GGM+ data is a very efficient and cost-effective technique to detect geological structures around the Jaboi volcano, which developing countries can use as a preliminary study for evaluating and exploring geothermal energy
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Mapping the boundaries of subsurface geological structures constitutes a fundamental aspect of interpreting gravity data. Various methods relying on gravity derivatives have been introduced to delineate these boundaries. However, these approaches are not without their limitations, often leading to the generation of false edges or resulting in divergent outcomes. This article introduces a novel method that utilizes the power law approach and balanced total horizontal derivative for mapping gravity data edges. To assess the robustness of the proposed technique, we conducted tests on model examples and applied it to a real case study in northwest Vietnam. The results demonstrate that our method can accurately and clearly map edges in comparison to existing methodologies, while also preventing the generation of spurious edges in the output maps.
... One of the major applications of the magnetic interpretation is to outline geologic boundaries such as faults and contacts. Many different techniques have been introduced to extract the boundaries of the sources from magnetic anomalies (Ekinci et al., 2014;Saibi et al., 2012Saibi et al., , 2019. Cordell (1979), Fedi (2002), Cella et al. (2009), Yuan and Geng (2014), and Kha et al. (2018) used the horizontal and vertical gradients to locate the source edges. ...
The analytic signal (AS1) has been widely used for mapping geological features from magnetic data. Some enhanced versions of the AS1 have been introduced to improve its effectiveness. Here, we compare the performance of these filters and introduce a filter obtained from the tilt angle and the directional analytic signals. For better performance on low latitudes, the directional analytic signals are obtained from the vertical integral of the magnetic anomaly rather than its magnetic potential. The proposed filter shows an apparent performance improvement in mapping the edges of magnetic sources, as shown with synthetic models and real example from the Olympic Peninsula, USA. The great advantage of the proposed technique is that the obtained result has lower influence from the magnetization vector direction than the AS 1 and other related techniques. The findings demonstrate that the present method not only outlines the boundaries more precisely and clearly, but also reveals the presence of several structures obscured by nonmagnetic sediment in the study area, which are not determined by other methods.
... The disadvantage of these techniques is that they perform poorly in localizing the deep boundaries (Saibi et al. 2012;. Many normalized techniques have been introduced as a function of the ratio of derivatives to map all boundaries of shallow and deep structures. ...
Improving the horizontal boundaries of subsurface geological structures is one of the main objectives in interpreting potential fields. To solve this problem, a number of different algorithms have been introduced based on the derivatives of the field. However, these algorithms have some drawbacks, e.g., the determined edges do not match the actual boundaries. Here, we present a new algorithm based on the gradient amplitude and its derivatives which yields more precise and clear boundaries. The robustness of the proposed technique is illustrated using theoretical examples and a real example from Kon Tum province, Vietnam. Our results show that the proposed technique can produce results with better resolution and minimizes the artifacts in the pseudo-boundary map.
... The WAFZ is another fault, intersecting the Aluto, along which no wells have been drilled. mote sensing (e.g., Birhanu et al., 2018;Albino and Biggs, 2021), and geophysical methods such as passive seismics (e.g., Nowacki et al., 2018;Wilks et al., 2020), gravity (e.g., Saibi et al., 2012), magnetic mapping (e.g., Mulugeta et al., 2021), and electrical resistivity sounding (e.g., Samrock et al., 2015;Hochstein et al., 2017;Cherkose and Mizunaga, 2018;Samrock et al., 2021). A detailed review of the studies conducted at Aluto is beyond the scope of this study. ...
Volcano-hosted high-temperature geothermal reservoirs are powerful resources for green electricity generation. In regions where such resources are available, geothermal energy often provides a large share of a country's total power generation capacity. Sustainable geothermal energy utilization depends on the successful siting of geothermal wells, which in turn depends on prior geophysical subsurface imaging and reservoir characterization. Electromagnetic resistivity imaging methods have proven to be a key tool for characterizing magma-driven geothermal systems because resistivity is sensitive to the presence of melt and clays that form through hydrothermal alteration. Special emphasis is often given to the "clay cap," which forms on top of hydrothermal reservoirs along the flow paths of convecting geothermal fluids. As an example, the Aluto-Langano volcanic geothermal field in Ethiopia is covered with 178 densely spaced magnetotelluric (MT) stations. The 3D electrical conductivity model derived from the MT data images the magma body that acts as a heat source of the geothermal system, controlling geothermal convection and formation of alteration zones (commonly referred to as clay cap) atop the geothermal reservoir. Detailed 3D imaging of the clay cap topography can provide direct insight into hydrothermal flow patterns and help identify potential "upflow" zones. At Aluto all productive geothermal wells are drilled into zones of clay cap thinning and updoming, which is indicative of underlying hydrothermal upflow zones. In contrast, nonproductive wells are drilled into zones of clay cap thickening and lowering, which is an indicator for underlying "outflow" zones and cooling. This observation is linked to fundamental characteristics of volcano-hosted systems and can likely be adapted to other geothermal fields where sufficiently detailed MT surveys are available. Therefore, high-resolution 3D electromagnetic imaging of hydro-thermal alteration products (clay caps) can be used to infer the hydrothermal flow patterns in geothermal reservoirs and contribute to derisking geothermal drilling projects.
... The WAFZ is another fault, intersecting the Aluto, along which no wells have been drilled. mote sensing (e.g., Birhanu et al., 2018;Albino and Biggs, 2021), and geophysical methods such as passive seismics (e.g., Nowacki et al., 2018;Wilks et al., 2020), gravity (e.g., Saibi et al., 2012), magnetic mapping (e.g., Mulugeta et al., 2021), and electrical resistivity sounding (e.g., Samrock et al., 2015;Hochstein et al., 2017;Cherkose and Mizunaga, 2018;Samrock et al., 2021). A detailed review of the studies conducted at Aluto is beyond the scope of this study. ...
Volcano-hosted high-temperature geothermal reservoirs are powerful resources for green electricity generation. In regions where such resources are available, geothermal energy often provides a large share of a country’s total power generation capacity. Sustainable geothermal energy utilization depends on the successful siting of geothermal wells, which in turn depends on prior geophysical subsurface imaging and reservoir characterization. Electromagnetic resistivity imaging methods have proven to be a key tool for characterizing magma-driven geothermal systems because resistivity is sensitive to the presence of melt and clays that form through hydrothermal alteration. Special emphasis is often given to the “clay cap,” which forms on top of hydrothermal reservoirs along the flow paths of convecting geothermal fluids. As an example, the Aluto-Langano volcanic geothermal field in Ethiopia was covered with 178 densely spaced magnetotelluric (MT) stations. The 3D electrical conductivity model derived from the MT data images the magma body that acts as a heat source of the geothermal system, controlling geothermal convection and formation of alteration zones (commonly referred to as clay cap) atop the geothermal reservoir. Detailed 3D imaging of the clay cap topography can provide direct insight into hydrothermal flow patterns and help identify potential “upflow” zones. At Aluto all productive geothermal wells were drilled into zones of clay cap thinning and updoming, which is indicative of underlying hydrothermal upflow zones. In contrast, nonproductive wells were drilled into zones of clay cap thickening and lowering, which is an indicator for underlying “outflow” zones and cooling. This observation is linked to fundamental characteristics of volcano-hosted systems and can likely be adapted to other geothermal fields where sufficiently detailed MT surveys are available. Therefore, high-resolution 3D electromagnetic imaging of hydrothermal alteration products (clay caps) can be used to infer the hydrothermal flow patterns in geothermal reservoirs and contribute to derisking geothermal drilling projects.
... This filter has been successfully applied in many areas (e.g. Saibi et al. 2006Saibi et al. , 2008Saibi et al. , 2012Saibi et al. , 2019Saadi et al. 2008;Azizi et al. 2015;Setyawan et al. 2015;Kafadar 2017;Kha et al. 2018;Eldosouky et al. 2020b) to enhance the presence of lineaments and geological boundaries. To improve the resolution of the edges, Fedi and Florio (2001) introduced an enhanced version of the total horizontal gradient, which is formed by taking the horizontal gradient of a sum of vertical derivatives of increasing order. ...
The enhancement techniques of potential field data are commonly used to detect the boundary locations of geological structures. There are many different techniques for estimating the source boundaries. Through synthetic examples and Bouguer data from the southern Red Sea, we have evaluated the performance of 15 enhancement techniques. The findings show that the tilt angle of horizontal gradient (TAHG) and fast sigmoid (FSED) techniques perform better than other techniques under almost all scenarios. Moreover, these two techniques can avoid producing false structures or connected structures as other techniques. The extracted lineaments from the TAHG and FSED were compared with surface faults of the study area. As a result, major differences are caused by rifting effect on the oceanic crust. The obtained results provide valuable information to better understand the structural features of the southern Red Sea and to introduce a more reliable structural interpretation.
... Geophysics has been widely applied to study shallow and deep faults around the world (Saibi et al., 2012;Zhang et al., 2015). Meanwhile, several methods can be used for shallow structures. ...
In the northern part of Sumatra Island, Indonesia, the Great Sumatran Fault, which can cause an earthquake, was divided into two segments: the Aceh and Seulimeum. An effort to reduce the risk is mapping the fault area, especially in the region that does not clearly show the sign on the surface, e.g., in the Lamtamot area, Indonesia. Electrical resistivity is widely used to study shallow structures, but the method requires more time when applied in a large area. This research explores the potential of an extremely low frequency (very low frequency-electromagnetic; VLF-EM) method to investigate the shallow fault of the Seulimeum segment. Here, the VLF-EM is compared with other geophysical methods such as resistivity and magnetic methods. For comprehensive results, the geomorphic observation that was conducted covered outcrops of the fault and trenching sites in the geophysical study for validating the model. A similar pattern of the VLF-EM and electrical resistivity data has been shown in a two-dimensional profile using data processing. The fault structure can be mapped at a distance of 20–24 m from the profile measurement, which is demonstrated by the low current density associated with the conductive zone from the VLF-EM, and low resistive anomaly in electrical resistivity. The fault can also be confirmed via magnetic intensity, which significantly increases at the same distance (20–25 m) of the VLF-EM and electrical resistivity. The geomorphic observation shows outcrops of fault activity, such as fault scarp, fractures, and faults, in the same direction as the Seulimeum segment, while scrap extends in the northwest direction up to ~20 m around the geophysical surveys. As revealed by the results, the VLF-EM method combined with other geophysical surveys and geomorphic observation can be used as a technique to image the fault that shows the shallow structure of the Seulimeum fault at 20–32 m along the profile.
... Aluto offers an ideal location to address some of these issues and understand the architecture of a rift-related alkaline, fault-controlled geothermal system. The volcanic complex has been a target for geothermal exploration and exploitation for several decades (Gebregzabher, 1986;Gianelli and Teklemariam, 1993;Gizaw, 1993;Hochstein et al., 2017;Teklemariam et al., 1996;Valori et al., 1992) as well as recent geological, geochemical, and geophysical investigations (Biggs et al., 2011;Braddock et al., 2017;Hutchison et al., 2015Hutchison et al., , 2016aIddon et al., 2019;Nowacki et al., 2018;Saibi et al., 2012;Samrock et al., 2015;Wilks et al., 2017) Although Aluto is well studied compared to many of Ethiopia's geothermal prospects, major uncertainties remain about how geothermal fluids are distributed in the subsurface and how they ascend along the mapped fault zones. Answering these questions has implications for understanding Aluto's geothermal system and for characterizing the fault network that might be exploited by future dike intrusions and volcanic events (Hutchison et al., 2015); more generally, it can provide important lessons on how to exploit the complex, highly-fractured alkaline geothermal systems that typify the East African Rift System. ...
Major challenges during geothermal exploration and exploitation include the structural-geological characterization of the geothermal system and the application of sustainable monitoring concepts to explain changes in a geothermal reservoir during production and/or reinjection of fluids. In the absence of sufficiently permeable reservoir rocks, faults and fracture networks are preferred drilling targets because they can facilitate the migration of hot and/or cold fluids. In volcanic-geothermal systems considerable amounts of gas emissions can be released at the earth surface, often related to these fluid-releasing structures. In this thesis, I developed and evaluated different methodological approaches and measurement concepts to determine the spatial and temporal variation of several soil gas parameters to understand the structural control on fluid flow. In order to validate their potential as innovative geothermal exploration and monitoring tools, these methodological approaches were applied to three different volcanic-geothermal systems. At each site an individual survey design was developed regarding the site-specific questions. The first study presents results of the combined measurement of CO2 flux, ground temperatures, and the analysis of isotope ratios (δ13CCO2, 3He/4He) across the main production area of the Los Humeros geothermal field, to identify locations with a connection to its supercritical (T > 374◦C and P > 221 bar) geothermal reservoir. The results of the systematic and large-scale (25 x 200 m) CO2 flux scouting survey proved to be a fast and flexible way to identify areas of anomalous degassing. Subsequent sampling with high resolution surveys revealed the actual extent and heterogenous pattern of anomalous degassing areas. They have been related to the internal fault hydraulic architecture and allowed to assess favourable structural settings for fluid flow such as fault intersections. Finally, areas of unknown structurally controlled permeability with a connection to the superhot geothermal reservoir have been determined, which represent promising targets for future geothermal exploration and development. In the second study, I introduce a novel monitoring approach by examining the variation of CO2 flux to monitor changes in the reservoir induced by fluid reinjection. For that reason, an automated, multi-chamber CO2 flux system was deployed across the damage zone of a major normal fault crossing the Los Humeros geothermal field. Based on the results of the CO2 flux scouting survey, a suitable site was selected that had a connection to the geothermal reservoir, as identified by hydrothermal CO2 degassing and hot ground temperatures (> 50 °C). The results revealed a response of gas emissions to changes in reinjection rates within 24 h, proving an active hydraulic communication between the geothermal reservoir and the earth surface. This is a promising monitoring strategy that provides nearly real-time and in-situ data about changes in the reservoir and allows to timely react to unwanted changes (e.g., pressure decline, seismicity). The third study presents results from the Aluto geothermal field in Ethiopia where an area-wide and multi-parameter analysis, consisting of measurements of CO2 flux, 222Rn, and 220Rn activity concentrations and ground temperatures was conducted to detect hidden permeable structures. 222Rn and 220Rn activity concentrations are evaluated as a complementary soil gas parameter to CO2 flux, to investigate their potential to understand tectono-volcanic degassing. The combined measurement of all parameters enabled to develop soil gas fingerprints, a novel visualization approach. Depending on the magnitude of gas emissions and their migration velocities the study area was divided in volcanic (heat), tectonic (structures), and volcano-tectonic dominated areas. Based on these concepts, volcano-tectonic dominated areas, where hot hydrothermal fluids migrate along permeable faults, present the most promising targets for future geothermal exploration and development in this geothermal field. Two of these areas have been identified in the south and south-east which have not yet been targeted for geothermal exploitation. Furthermore, two unknown areas of structural related permeability could be identified by 222Rn and 220Rn activity concentrations. Eventually, the fourth study presents a novel measurement approach to detect structural controlled CO2 degassing, in Ngapouri geothermal area, New Zealand. For the first time, the tunable diode laser (TDL) method was applied in a low-degassing geothermal area, to evaluate its potential as a geothermal exploration method. Although the sampling approach is based on profile measurements, which leads to low spatial resolution, the results showed a link between known/inferred faults and increased CO2 concentrations. Thus, the TDL method proved to be a successful in the determination of structural related permeability, also in areas where no obvious geothermal activity is present. Once an area of anomalous CO2 concentrations has been identified, it can be easily complemented by CO2 flux grid measurements to determine the extent and orientation of the degassing segment. With the results of this work, I was able to demonstrate the applicability of systematic and area-wide soil gas measurements for geothermal exploration and monitoring purposes. In particular, the combination of different soil gases using different measurement networks enables the identification and characterization of fluid-bearing structures and has not yet been used and/or tested as standard practice. The different studies present efficient and cost-effective workflows and demonstrate a hands-on approach to a successful and sustainable exploration and monitoring of geothermal resources. This minimizes the resource risk during geothermal project development. Finally, to advance the understanding of the complex structure and dynamics of geothermal systems, a combination of comprehensive and cutting-edge geological, geochemical, and geophysical exploration methods is essential.
