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shows diagrams explaining the patterns of total force anomalies (∆F) caused by positive magnetic dipoles (orientated in the same direction as B o ) that are located in: (a) the northern hemisphere, (b) the southern hemisphere, (c) the magnetic south pole and (d) the magnetic
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The total force magnetic anomaly (termed in this chapter as ∆F) is defined by algebraic (scalar) subtraction of |Bo| (the strength “undisturbed” earth’s magnetic field) from |Bobs|, that is: ∆F = |Bobs|-|Bo|. As |Bo| cannot (are not) directly measured, the value |Bo| it is usually taken from computation of the IGRF (see Section 2). The ∆F computed...
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... There are several descriptions in the literature of low magnetic intensities observed over high enthalpy geothermal systems (Anderson et al., 1987;Hochstein et al., 1997;Soengkono, 2016). Direct measurements of magnetic susceptibility from core samples at the Wairakei field in New Zealand confirm the most intense demagnetization occurs at ~500 m deep (Allis, 1990), slightly above the producing reservoir. ...
Ormat has executed a comprehensive exploration program at Dixie Meadows, Nevada, leading to the discovery of a high permeability, 150°C+ geothermal resource. The reservoir is a low TDS (0.17 wt%) neutral-pH Na-Cl, low NCG fluid of meteoric origin, hosted within Triassic metasediments and Cretaceous intrusive rock. Exploration drilling during the early 1980's and by Ormat in 2012 identified outflow ranging in temperature from 70-140°C that is laterally channeled in the upper 50 m of the subsurface. This outflow is located near the trace of ENE-striking fault zones within permeable units in the alluvium and is also manifested as weak steam vents along the Dixie Valley range front fault scarp which ruptured in 1954 (M6.8). The thermal fluids discharge as hot springs in marshlands to the east. The initial exploration drilling during the 1980's and during the earliest exploration drilling by Ormat in 2011 failed to identify the source of upflow. A revised conceptual model was developed by Ormat based on detailed geologic mapping and synthesis with the available drilling, geophysical, and geochemical datasets. A deep core hole tested two competing conceptual models and proved the reservoir to be sourced from below the Dixie Valley Fault zone which bounds the eastern front of the Stillwater Mountains. Upflow for the resource is dominantly controlled by a series of ENE-striking faults that contain dilated fractures oriented ENE to ESE that occur at an extensional left step/bend of the Dixie Valley Fault. This paper serves as a case study to document the geoscientific data, conceptual model, and exploration strategy of an archetypal Basin and Range system in Nevada.
... For some cases such as the reconstruction of subsurface structures in geothermal regions, integration of gravity and magnetic methods is necessary, to know of subsurface geometry and plan the geothermal exploitation [13]. These two methods are complementary because the magnetic susceptibility of rocks is affected by temperature, susceptibility will decrease when rocks are exposed to high temperatures so that the existence of high subsurface temperatures can be analyzed indirectly from the distribution of susceptibility values [14]. ...
... where obs g is the observed gravity value, n g normal gravity value at the observation point, average density, h topographic height, dan TC terrain correction. magnitude of the Terrain correction (TC) using the cylinder ring approach can be written by equation (2) [14]. ...
Mt. Lamongan, East Java is located at near three major towns around, i.e. Probolinggo, Lumajang, and Jember. Its Mountain indicated saving the potential of geothermal energy. That seems from the existence of geothermal manifestations such as hot springs in Tiris Probolinggo. The hot spring temperature value at the surface is 67°C. Estimation of subsurface temperature distribution, model and the orientation of existing faults are very important to know, to the planning of geothermal exploitation. In this study, we used magnetic and gravity field anomaly data to approximate it. We use Mag3D and Grav3D software from the University of British Columbia (UBC) to make a 3D inversion of magnetic and gravity field data. The inversion is shown the pattern of subsurface susceptibility values and rock density clearly. The susceptibility values correlated with the presence of sub-surface heat source and the rock density is associated with the structural condition, i.e. model and orientation of fault in this area. In this study, we found the subsurface temperature distribution connected with the structural system, i.e. Tiris faults.
... By applying 3-D inversion of magnetic data, we can obtain an image of magnetic susceptibility distribution with a depth estimation which lead to the prediction of geothermal system and the reservoir depth. Several studies to determine the geothermal prospect zone using magnetic methods have been successfully carried out by Daud et al. [2] and Soengkono [3]. However, the previous geomagnetic research in Blawan-Ijen [2] could only determine location of a suspected reservoir area. ...
... Figure 3 shows the presence of low magnetic (blue color) which associated with hydrothermally demagnetized rock. The demagnetized rock usually occurred because of the interaction between host rock with a high temperature from the heat source [3]. So, it could be used to delineate the location of high temperature of geothermal prospect zone. ...
Blawan-Ijen geothermal prospect area is placed at Bondowoso, East Java. Geologically, the presence of geothermal system in this field is controlled by the huge ancient Kendeng caldera complex structure. The heat source of the geothermal system is predicted to be situated at the base of Kendeng caldera. Geomagnetic method was used for detecting the presence of the heat source. The high temperature zone as the main target of geothermal exploration mostly situated around the heat source location. The geomagnetic method was applied because of its ability to identify the magnetic variation of rocks such as the demagnetized zone which is caused by the high temperature condition in geothermal area especially in the center of geothermal reservoir. Geomagnetic survey was conducted in 2017 and 2018 with the total of 151 stations. The 3-D inversion of geomagnetic data was then carried out to image the subsurface magnetic variation which is representing the complex structure of geothermal system in this area. The subsurface magnetic variation structure revealed by 3-D inversion showed the distribution of demagnetization reservoir rock inside the Kendeng caldera which has low magnetic susceptibility. In the upper part of this layer that is a higher magnetic susceptibility layer which may associated with the clay alteration that is overlaying the geothermal reservoir in this area. Moreover, the very low magnetic susceptibility anomaly can be observed start from -1000 m elevation, which is interpreted as the heat source of the geothermal system in this area.
... There have IOP Conf. Series: Earth and Environmental Science 538 (2020) 012036 IOP Publishing doi:10.1088/1755-1315/538/1/012036 2 been several efforts to determine the geothermal prospect zone using magnetic and gravity methods [1][2][3]. Low magnetic anomaly caused by hydrothermal demagnetization in a geothermal prospect phenomenon is harnessed in the magnetic method by imaging this magnetic anomaly. Meanwhile, the 3-Dimensional inversion of gravity data is applied to image the density variation of the prospect area by detecting the presence of intrusive body which might be associated with a possible heat source of the geothermal system. ...
... The geothermal system in Blawan-Ijen Complex, East Java (UTM Easting 180,000-200,000 and Northing 9,100,000-9,120,000) is hosted by Quartenary andesitic volcanic rocks [1]. The old Ijen volcano is thought to form around Pleistocene and erupted in Plinian eruption [4]. ...
Integrated measurements of magnetic and gravity methods were carried out over Blawan-Ijen Geothermal Prospect area to discover the indication of prospect zone location more convincing. The acquisition was first conducted in 2017, followed by recent measurement in 2018 with a total of 151 stations for each method. In this research, Reduce to Pole for magnetic data was applied to identify the distribution of demagnetized rock which associated with reservoir zone. The prospect zone is usually indicated by low magnetic susceptibility value. Furthermore, to support the interpretation of prospect zone which usually located overlying the heat source, then 3D inversion of gravity data was conducted. Thus, by applying 3D inversion of gravity method, the distribution of density variation beneath the earth surface which represents the location of heat source can be obtained. The result of Reduce to Pole of magnetic data shows demagnetized zone with low magnetic susceptibility value located in the centre to the southeast at the study area. Moreover, 3D gravity data inversion delineates the existence of heat source indicated by high gravity anomaly which corresponds with demagnetized zone shown from magnetic data.
... The magnetic method itself is a geophysical method that measures the magnetic properties of rocks. This method is very good in identifying the structure and hydothermal demagnetization which is closely related to the gold mineralization process [4]. One analysis used is tilt derivative where this method can characterize magnetic anomalies against sudden changes in the value of magnetization, and illustrate the boundaries of the anomalous body (contrast anomaly). ...
Hydrothermal fluid is a fluid formed from the process of magmatism. Where this fluid will go to the surface through the geological structure and make contact with the surrounding rocks, the rock will be physically and chemically transformed. This process is called alteration and mineralization. The zone P region is a low epithermal sulfide deposition environment. This type of deposit is generally in the form of quartz veins that fill fractures and cavities, which indicate the existence of a structural control role. Magnetic data measurements have been carried out for 11 trajectories in the direction of measurement for each northeast-southwest trajectory. The purpose of this measurement is to identify the distribution of the geological structure that controls the mineralized zone in the P region. The two techniques used in this study are Lineament Density Analysis (LDA) to determine the distribution of structural density based on remote sensing data and tilt derivative analysis of magnetic data to identify the presence of faults and find out the orientation of the distribution of fractures. To support this research, forward 2D modeling was carried out. Based on the LDA results, the distribution of dominantly high structural density values was in the southeast of the study area, with a value of 0.0035–0.008 m/km2. While the results of tilt derivatives indicate faults have a maximum anomalous value and have a direction of continuity orientation from southeast to northwest.
... Demagnetization of the materials is a common feature in geothermal contexts (Rona, 1978 ;Nakamura et al., 2013 ;Bouligand et al., 2014 ;Soengkono, 2016). Accordingly, the offshore geology of the island where no surface data are available can be understood via magnetic analyses. ...
Cette thèse porte sur la méthode magnétique appliquée à l'exploration géophysique d'une zone à caractère géothermique. L'étude se concentre sur la zone volcanique de Basse-Terre, à l'Ouest de la Guadeloupe, dont les fractures, les fluides et les flux de chaleur engendrent un fort potentiel géothermique. Ainsi, le plan de la thèse suit le cours d'une étude magnétique complète, ce qui commence par la compréhension du contexte et des objets à considérer, autant d'un point de vue économique que géologique puis géophysique. S'ensuit un travail de collecte et de traitement des données magnétiques existantes, menant à la conception de nouvelles techniques d'acquisition ou de traitement pour répondre aux besoins spécifiques de la zone d’étude. Les techniques d’acquisition magnétique proposées, par drone (données onshore) ou par bateau (données offshore), permettent d’obtenir des données multi-altitudes haute résolution. Une fois les données obtenues et traitées, elles sont interprétées à différentes échelles en parallèle des connaissances géologiques existantes. Une méthodologie d'inversion spectrale est également proposée. Enfin, un rapprochement est opéré à partir de toutes les données géophysiques pouvant être obtenues : méthode magnétotellurique, électromagnétisme en domaine transitoire et gravimétrie.
... The RTP map can be seen in Fig 5. RTP anomaly is used to predict the position of geothermal prospect area which is indicated with low (negative) anomaly due to hydrothermal demagnetization [5]. From the result, the pattern of low magnetic anomalies (blue color) is appeared in the northern and continued to center and southern area. ...
The Blawan-Ijen volcanic complex is located in Bondowoso regencies, East Java province. The complex is expected to have geothermal system which is indicated by the occurrence of Blawan hotspring, acid lake on Ijen Crater and alterations. In 2017, measurements of gravity and magnetic methods have been conducted for the first time through the PITTA 2017 program. In 2018, further measurements are carried out to infill the previous data in order to strengthen the interpretation results. There are 151 stations obtained from each method until 2018. In this study, gravity method is used to detect the contrast density of an anomalous body while magnetic method is applied to discover the location of demagnetization zone. This paper presents the integration of both methods in geothermal exploration to determine the geothermal prospect area. The result of CBA and residual gravity indicated the existence of high gravity anomaly in the center to the southwest of the study area. Moreover, after processing RTP on magnetic data, there is the presence of low magnetic anomaly usually associated with demagnetization zone. Generally, the overall results supported one each other and pointed out the occurrence of the geothermal prospect possibly around the center of the study area.
Ijen Volcanic Complex (IVC) is located in the easternmost part of Java. This complex consist of three groups of volcanoes including the Old Ijen Caldera, Caldera Rim Volcanoes and Intra Caldera Volcanoes. The complex is expected to have geothermal system which is indicated by the occurrence of Blawan hotspring at the northern part of caldera rim and acid lake on Ijen Crater. The presence of surface manifestation is very rarely in this area. The Blawan hotspring represents typical outflow of geothermal system, while the existence of extremely acid lake on Ijen Crates indicates that the area should be avoided for geothermal development. Accordingly, the location of the geothermal system is still hidden and become a challenge to be explored. In order to identify the prospect zone of Blawan-Ijen geothermal area, a landsat data analysis and magnetic survey were conducted. From the landsat analysis, volcanoes and geological structures were delineated to understand the geological condition. Besides, a magnetic survey was also carried out in 2017 for 72 stations using aproximately 1 – 1.5 km grid spacing. The magnetic anomalies were processed using reduced to pole (RTP). The negative magnetic anomalies occur over the southeast part of the study area and also in the western part of the N-S structure. The negative anomaly in the southeast area is probably associated with the hydrothermal demagnetization. This is supported by landsat imagery that shows a volcanic feature right in that area.
