MT Data Research Articles

2D Magnetotelluric Resistivity Structure Modeling Using Finite Element Method Based on Vector Triangular Grid and Its Application to Lembang Fault MT Data

Abstract Magnetotellurics is one of the geophysical exploration techniques that relies on the natural fluctuations of electromagnetic waves to delineate their influence on the Earth. The primary focus of this method is to reveal the structure of Earth’s subsurface with the value of resistivity. The application of numerical approaches in magnetotelluric modeling has proven to be an efficient method in various theoretical studies in the field of geophysics, particularly in the context of modeling two-dimensional structures. This research explains a 2D resistivity structure modeling using a vector finite element method. This approach utilizes the edges of elements as vector bases. The presented results include response values such as apparent resistivity and impedance phase at the surface. The study employs the standard model from COMMEMI as a reference to validate the modeling program. Furthermore, the results from this modeling program are compared with the outcomes of the modeling program developed by Weaver et al. The good results were obtained with error values for each model for layered and homogeneous Earth < 3%. Additionally, for the reference model COMMEMI, errors of 3.4393% and 1.4050% were obtained for TE and TM modes, respectively. Furthermore, apparent resistivity and impedance phase results closely approximated the reference values for the topography model. Subsequently, in the application to field data, specifically the Lembang Fault, errors were obtained for the TE and TM modes within the range of 1.16 – 9.16% for each MT data acquisition site.

MFF‐DenseNet: Densely Connected Convolutional Network With Multi‐Scale Feature Fusion for Magnetotelluric Noise Suppression

AbstractMagnetotelluric (MT) is a geophysical technique for detecting subsurface electrical structures. However, MT data collected in areas with frequent human activity often encounter various types of electromagnetic (EM) noise, which can mask or distort the signals we aim to analyze. Over the past decades, data processing methods based on deep learning has become the focus of multiple disciplines. Training neural networks to identify and handle noise has been proven effective in reducing the impact of noise. Therefore, ensuring the neural network accurately learns the noise and signal characteristics during the training is crucial. Against this background, we propose a multi‐scale feature fusion technique based on the densely connected network and apply it to processing MT data. First, we construct a data set resembling the noise in field data and use it to train the network. Leveraging dense connections, we extract feature maps of EM noise from noisy data and utilize Spatial Pyramid Pooling to integrate feature maps of various scales, enabling the network to capture features of the noise precisely. At the same time, we reduce the computation of feature fusion by introducing the Channel‐wise Squeezed Layer to compress the channels of the feature maps. Ultimately, we apply the trained model to the field noisy data. The results of synthetic and field data demonstrate that our method suppresses low‐amplitude and continuous high‐amplitude noise while preserving low‐frequency valuable signal. Apparent resistivity‐phase curves and polarization direction shows a noticeable improvement in the mid and low‐frequency bands with our method.

Inversion of Gravity Data Constrained by a Magnetotelluric Resistivity Model: Application to the Asal Rift, Djibouti

AbstractBefore exploiting a geothermal resource in a volcanic setting such as the Asal rift, it is necessary to acquire a better knowledge of the subsoil, with the objective of locating the geothermal reservoir and evaluating the resource characteristic (permeability, temperature, etc.). For this type of resource, geophysical exploration methods are essential (such as gravimetry, magnetotellurics, etc.). However, a particular data type does not necessarily have the resolution and sensitivity. Furthermore, individual inversions of these geophysical data face the ambiguity of the non‐uniqueness of the inverse solution. In this paper, we present a new linear approach of gravity data using the constraint of a MT resistivity model. We coupled the resistivity and density using inversion cross‐gradients and the linear correlations. The approach was tested and validated on synthetic data and applied to gravity and MT data in the Asal Rift. Multiple inversions with different levels of coupling provided a series of density models. We applied the principal component analysis (PCA) technique to assess these models. We were able to define two dominant processes acting differently on the density and resistivity distribution at depth, namely the geothermal activity of the rift and the structural control of active tectonics.

Lithospheric electrical structure and geodynamic model of the red river fault zone and its adjacent areas in southwest China: Constraints from 3-D magnetotelluric imaging

The Red River Fault (RRF) zone is situated on the southeastern boundary of the Tibetan Plateau. To further explore the geodynamic model of the region and the collision mechanism of the Eurasian continent, the electrical structure model of the lithosphere in the RRF was obtained through 3-D inversion of MT data. The preferred model indicated that fault zones were primarily distinguished by low resistivity anomalies, with some exhibiting high resistivity characteristics. It was hypothesized that the low resistivity anomalies may be attributed to the presence of saline fluids or metal sulfide within the fault zones, on the other hand, the high resistivity characteristics suggested inactivation due to fault cooling. Rapid changes in the electrical structure occurred on both sides of the fault zones, causing the upper and middle crust to be divided into distinct secondary tectonic units. Based on the effective viscosity calculation, we analyzed the crust-mantle rheology of the RRF area. We assumed that the RRF area was composed of three geodynamic models: the “crustal flow” model was distributed from the northern part of the Lancang River Fault (LCRF) zones to the southern part of the RRF zones. The “ductile continuum rheology” model was distributed from the north of the RRF to the western part of the Xiaojiang Fault (XJF) zones. The “mantle upwelling” model was distributed in the southeast part of the RRF.

Deep basin conductor characterization using machine learning-assisted magnetotelluric Bayesian inversion in the SW Barents Sea

SUMMARY In this paper, we use a new workflow to substantiate the characterization of a prominent, deep sediment conductor in the hyperextended Bjørnøya Basin (SW Barents Sea) previously identified in smooth resistivity models from 3-D deterministic inversion of magnetotelluric data. In low-dimensionality environments like layered sedimentary basin, 1-D Bayesian inversion can be advantageous for a thorough exploration of the solution space, but the violation of the 1-D assumption has to be efficiently handled. The primary geological objectives of this work is therefore preceded by a secondary task: the application of a new machine learning approach for handling the 1-D violation assumption for 21 MT field stations in the Barents Sea. We find that a decision tree can adequately learn the relationship between MT dimensionality parameters and the 1-D–3-D residual response for a training set of synthetic models, mimicking typical resistivity structures of the SW Barents Sea. The machine learning model is then used to predict the dimensionality compensation error for MT signal periods ranging of 1–3000 s for 21 receivers located over the Bjørnøya Basin and Veslemøy High. After running 1-D Bayesian inversion, we generated a posterior resistivity distribution for an ensemble of 6000 1-D models fitting the compensated MT data for each 21 field stations. The proportion of 1-D models showing ρ < 1 Ω·m is consistently beyond 80 per cent and systemically reaches a maximum of 100 per cent in the Early Aptian–Albian interval in the Bjørnøya Basin. In hyperextended basins of the SW Barents Sea, the dimensionality compensation workflow has permitted to refine the characterization of the deep basin conductor by leveraging the increased vertical resolution and optimal used of MT data. In comparison, the smooth 3-D deterministic models only poorly constrained depth and lateral extent of the basin anomaly. The highest probability of finding ρ < 1 Ω·m is robustly assigned to the syn-tectonic Early Aptian–Albian marine shales, now buried at 6–8 km depth. Based on a theoretical two phase fluid-rock model, we show that the pore fluid of these marine shales must have a higher salinity than seawater to explain the anomaly ρ < 1 Ω·m. Therefore, the primary pore fluid underwent mixing with a secondary brine during rifting. Using analogue rift systems in palaeomargins, we argue that two possible secondary brine reservoir may contribute to deep saline fluid circulation in the hyperextended basin: (1) Permian salt-derived fluid and, (2) mantle-reacted fluid from serpentinization.

The Pannon LitH2Oscope magnetotelluric array in the Pannonian Basin

The Pannonian Basin is one of the best natural laboratories in the world to study the lithospheric response to continental extension and subsequent tectonic inversion. Here we address the topic of lithospheric structure by a combined geochemical and magnetotelluric analysis, which has been carried out in the framework of the Pannon LitH2Oscope project. The main objective was to detect the resistivity distribution over the entire lithosphere by magnetotelluric measurements, considering the lithological resistivity properties and relate the results to the structure and evolution of the Pannonian Basin. The Pannon LitH2Oscope MT array was used to estimate the depth of the Lithosphere-Asthenosphere Boundary (LAB), considering the legacy MT data and compared to previous estimates for the region. Using the MT and geomagnetic response functions, major structural zones of the Pannonian basin, such as the Mid-Hungarian Shear Zone or fault systems like the Makó Trough and the Békés Basin, were also imaged. In addition, we used the apparent resistivity soundings to compare 1D resistivity models computed from geochemistry and obtained from field MT measurements. This comparison provided new constrains for the composition, fluid and melt content variations at the local lithosphere-asthenosphere boundary. The Pannon LitH2Oscope MT dataset and the results presented in this paper provide input for more complex 3D inversions and further investigations of the lithospheric structure in the Carpathian-Pannonian region.

3-D Joint Inversion of MT and CSEM Data Based on Adaptive Finite-Element Forward Modeling

3-D Joint Inversion of MT and CSEM Data Based on Adaptive Finite-Element Forward Modeling

Three-Dimensional Joint Inversion of MT and Gravity Data Based on Unstructured Tetrahedron Discretization

Three-Dimensional Joint Inversion of MT and Gravity Data Based on Unstructured Tetrahedron Discretization

Integrative Analysis of the Geothermal Structure in Kepahiang: Insights from Magnetotelluric, Gravity, and Remote Sensing Techniques

This research was conducted to determine the structure and depth of the reservoir using remote sensing as an initial survey to assess the geological alignment direction, employing ALOS PALSAR radar imagery. Subsequently, further surveys were conducted using the magnetotelluric and gravity methods to analyze the structure and depth of the geothermal reservoir. The magnetotelluric data were processed using Phoenix software, where the data was transformed from the time domain to the frequency domain using Fourier transformation, and processed to obtain apparent resistivity and phase. The MT data was integrated with gravity data, and the gravity data underwent standard correction procedures to obtain the Complete Bouguer Anomaly (CBA) map. Two-dimensional (2D) inversion using the NLCG algorithm and 2D forward modeling of the gravity data were performed. The dominant alignment pattern obtained was northwest-southeast, with an orientation of 320° NW or 140° SE. Based on the results of geological alignment, a profile is produced that is perpendicular to the straightness. The results from the 2D inversion and gravity forward modeling indicated that the geothermal reservoir is likely located beneath the caprock at an estimated depth of approximately 1800 m, with resistivity values ranging from 32 to 256 Ohm-m and a density value of 2.6 gr/cc.

Crustal resistivity structures beneath the western flank across the Central Main Ethiopian Rift: Its implications for partial melt and pathways

The Wonji Fault Belt (WFB) and the Silti Debre Zeit Fault Zone (SDFZ) are two linear belts in the Central Main Ethiopian Rift (CMER) where recent magmatic and tectonic activities have been concentrated. To examine deep resistivity patterns, magma storage, and its path beneath the Earth's crust across the CMER, the researchers used a two-dimensional (2D) inversion model of magnetotelluric data. The time series magnetotelluric data ranging from 0.003125 to 1450.0 s were collected during >24 h of installations at 21 sites, with an average station separation of 2.5 km along a profile perpendicular to the rift from the western margin of the rift (near Butajira) to AdamiTulu (near the rift axis). This work describes the use of phase tensor analysis to determine the dimensionality and geoelectrical strike direction, as well as the estimation of geoelectric strike parameters and the 2D inversion model of MT data. The results of dimensionality analysis of the majority of our MT data show a small beta value (|β|<30) at low periods (nearly below 10 s), which indicates 1D or 2D structures, while at long periods (almost above 10 s), the data show 3D structures with a large value of |β| >30. However, from the results of the dimensionality analysis of MT data, the 2D inversion model is suitable to image the resistivity of subsurface structures. The dominant geo-electrical strike is estimated to be N150E using a Z-invariant and phase tensor azimuth. The 2D inversion model was performed after the MT data were rotated to the proper geoelectric strike direction of N150E. The desired 2D inversion of the resistivity model was obtained for the MT profile using joint inversion of TE (transverse electric) and TM (transverse magnetic) mode data with an RMS misfit of 1.11. The results of the 2D inversion model reveal partial melt and upper crustal fracture zone (fault). They are attributed to a large mass of batholithic or magmatic intrusion found at a depth of 12–22 km that is characterised by a resistivity of <5 Ωm. The partial melt extends beneath stations B07 to B11 along SDFZ with a horizontal width of approximately 10 km. It could be related to the source of heat for the Ashute and Aluto geothermal fields.

Geoelectrical model of the Earth’s crust along the Shu-Sarysu geotraverse according to magnetotelluric soundings

Purpose. To study and assess the possibility of effective application of electrical prospecting by magnetotelluric sounding (MT-sounding) and to determine the range of geological problems to be solved in the geoelectric conditions of the Shu-Sarysu geotraverse. Methodology. The issues of methodology of modern technologies of field observations, peculiarities of processing and interpretation of MT-sounding data are considered, which showed that modern instrumental and methodical electrical survey technologies of MT-sounding allow obtaining results of field measurements of increased reliability, due to their accuracy, productivity, mobility, noise immunity, level of automation. To identify the main features of the geoelectric structure of the Earth’s crust and the upper mantle, the modern technique of interpreting MT data allows for 1, 2D inversion of the effective curves of apparent resistivity and impedance phase. Findings. Based on MT-sounding data, a geoelectric section of the Shu-Sarysu geotraverse was constructed, characterizing the position and morphology of five geoelectric boundaries in the crustal section in the 0–40 km depth interval. The identified five geoelectric horizons show good coincidence with the conventional seismic horizons and confidently correlate with the geological references established by GIS data. Originality. On the basis of MT-sounding studies, the geological interpretation of the structure of suprasalt and subsalt structural-formation complexes is presented; the geological efficiency of MT-sounding electrical prospecting in revealing the complex structures in the conditions of the Shu-Sarysu geotraverse is shown. On the basis of model constructions of the geoelectric section, new data on the deep geological structure of the Shu-Sarysu geotraverse were obtained: new data on the structure of Mesozoic terrigenous-sedimentary rock complexes and the Paleozoic part of the section with the allocation of the supposed faults; on the basis of MT-sounding results, recommendations for further detailed geological exploration work were given. Practical value. A comprehensive interpretation is recommended of the obtained MT-sounding data together with the results of geological and geophysical research on the regional profile in order to establish search criteria for the discovery of mineral deposits and the prospects of gas content in the area.

Geoelectric Model of the Northwestern Caucasus: Three-Dimensional Inversion

Interpretation of magnetotelluric observations made within the Northwestern Caucasus, by means of one-dimensional, two-dimensional inversion and three-dimensional mathematical modeling programs, has helped build testing and starting geoelectric models that are necessary to test and adapt the program of three-dimensional inversion of the impedance tensor components. The performed three-dimensional inversion of the experimental magnetotelluric data has significantly changed the parameters of conducting blocks within the folded structures of the region, identified at the previous stages of MT data interpretation. In the resulting three-dimensional geoelectric model, the position of low-resistance blocks correlates with: suture zones, deep faults, mud volcanoes, and domains characterized by an increased absorption of transverse and longitudinal seismic waves. The electrical resistivity of the most low-resistance anomalies is explained by the degree of their saturation with the water fraction of the fluid.

Three-dimensional magnetotelluric signatures and rheology of subducting continental crust: Insights from Sikkim Himalaya, India

3D inversion of broad band MT data present variation of electrical signatures across the subducting Indian crust in Sikkim Himalaya. The vertical and horizontal geoelectric cross-sections are dominated by north-east dipping conductive zones. Two high conductivity zones (4–8 Ω m) at a depth of 5–18 km in Lesser Himalayan Domain (LHD) are explained by conductive mineral assemblage associated with abundant low saline and entrapped fluids. Another conductive feature (6–16 Ω m) in Main Himalayan Thrust Zone close to Main Himalayan Thrust ramp could have arisen from entrapment of CO2-H2O fluids and fluids released by metamorphic reactions. The high conductive anomaly (4–10 Ω m) at a depth of 5–16 km in Greater Himalayan Sequence (GHS) is caused by the presence of partial melts/aqueous fluids derived by present day fluid-absent melting of leucogranite source rocks. A combination of leucogranite intrusion, shear heating, and radiogenic heat production (4–17 μW/m3) are the heat sources for inferred partial melting. Though, the constrained melt fractions of 1.4–3.8% in GHS are lower than the estimation in south Tibet that might be due to the less intrusion of leucogranites. The obtained moderate viscosities of (104.19-105.49 Pa.s) from empirical relation with low melt and fluid fractions of 5–6 wt% in high conductive zone suggest viscous/ductile deformation and weakening mid-crust beneath northern Sikkim Himalaya. However, the estimated values of melt fractions and viscosities at mid-crustal depth of GHS are insufficient to develop a melt channel to flow southward between Main Central Thrust-1(MCT-1) and South Tibet Detachment (STD) envisaged by channel flow model.

A Method for Reducing Ocean Wave-Induced Magnetic Noises in Shallow-Water MT Data Using a Complex Adaptive Filter

A Method for Reducing Ocean Wave-Induced Magnetic Noises in Shallow-Water MT Data Using a Complex Adaptive Filter

Effect Of Soil Shear Strength on Shoreline Changes at Batu Pahat Coastal Area

This study focuses on the effect of soil shear strengthdue to the shoreline changes in Batu Pahat Coastal Area. The shorelines of Pantai Punggur were chosen as the subject location for this investigation on soil shear strength and shoreline changes of eroded areas in Batu Pahat, Johor. Pantai Punggur is situated on the west coast of Johor, with a latitude of 1.62° to 1.87° N and a longitude of 102.78° to 103.19° E. Aerial photographs and field observation data were used to determine the geomorphological components of the Pantai Punggur shoreline using an unmanned aerial vehicle (UAV). The study conducted is to produce shoreline changes mapping diagram and analysis soil shear strengths across different sampling locations were measured and correlate with shoreline changes at zones A, B, C, D and E. The aerial image that has been captured by the drone was analyzed using Pix4D and Global Mapper software. Based on the data of shoreline changes zone A portrays the highest changes followed by zone B and C. As for zone D and E its shows quite a small change. Within one month interval, Pantai Punggur coastline experiences changes in about 1.57 meters. From the undrained shear strength test at 0.5 m depth at HT and MT data for December 2021, zone A with Cu reading are more than 4.5 kPa followed by zones B, C, D and lowest at zone E. Next, at depth 0.5 m and 1.0 m data for December 2021 at HT level, zone A with Cu reading is more than 4.5 kPa at the 0.5 m and 1.0 m depth and other zones are below than 3.0 kPa. The depth of soil also influences the data of this study. The correlation of changes in soil shear strength with shoreline changes is reported to have a linear correlation with R2 value is 0.9261. It can conclude that the effect of soil shear strength on shoreline changes at Batu Pahat coastal area cause by the changes in soil shear strength (Cu) gives a large effect of shoreline changes.

Deep electrical structure of the northern UAE: An investigation along the Dibba zone and the foreland basin using magnetotellurics

Recent geophysical studies are focused on imaging the present-day architecture of the subsurface structures in the northern United Arab Emirates. Previous geophysical data have not fully investigated the deep structures in the middle and lower crust. The foreland basin flanking the UAE-Oman mountains to its west has been investigated by oil companies, mainly using seismic techniques. These studies have been vital in providing a constraint on the subsurface structure of the northern Emirates. This paper reports the results of using broad-band magnetotelluric (BBMT) data acquired for the first time in the UAE to map deep electrical crustal structures. The study aims at determining the geometry of the Semail Ophiolite, the underlying thrust sheets, and the basement structure beneath the allochthonous units and the foreland basin. The MT data were collected from 15 stations along an E-W profile across the Semail Ophiolite and the Dibba zone in the east and the foreland basin in the west. The MT data achieve greater crustal penetration, revealing electrical structures within the upper, mid, and lower crust. 2D and 3D inversion of the data map the high resistivity Ophiolite structure and the associated conductive Hwasina-Haybi thrust sheets in the mountain ranges and in the Dibba zone. In the foreland basin, the Quaternary and Tertiary sediments in the Pabdeh basin are characterized by low resistivity. Furthermore, the MT models detect a high resistivity zone related to the basement structure of the northern UAE in the middle and lower crust. The Ophiolite near the Dibba zone has >7 km thickness as shown in the 2D MT model, and the Hawasina-Haybi sheets have a maximum thickness of ⁓10 km beneath the Ophiolite. The allochthonous units indicate an oceanward dipping as similarly reported in the previous geophysical and geologic studies.

Resistivity analysis of geothermal system at cubadak village west sumatera using magnetoteluric method

Cubadak area was located in depression zone of great Sumatra fault zone which affected by normal faults. Geothermal in this area was characterized by the appearance of hot springs type chloride-bicarbonate on Cubadak, Sawah Mudik and Talu. The manifestation temperature on this area is about 37,1° - 74,8° C. This study purpose to recontruct model of subsurface geothermal system in Cubadak area based on MT data. Model 1-D was processed by apply Bostick inversion and NLCG (Nonlinear Conjugate Gradient) for model 2-D. The result showed that lowest resistivity was on Southwest to Northeast research area. Highest resistivity spread at West to Northeast. Geothermal system was composed by caprock (2-10 ohm), reservoar (20-300 ohm) and heat source (>300 ohm).

Seismic Survey in Lesser Himalayan Thrust Belt, Western Nepal

Two hundred km of 2D seismic survey was carried out at the Lesser Himalayan Thrust Belts in Dailekh district, western Nepal. The main motivation is to elucidate the geologic relationship between the known oil and gas seeps, subsurface structure, and stratigraphy in the area. This is a challenging task which is from its extreme structural and geological complexity such as thrust faulting, tight folding, steep dip layers, and strong lateral variations in seismic velocity. Seismic data were acquired with SERCEL 428XL system and processed by GEOEAST computer software. In order to increase signal-to-noise ratio (SNR), suppress interference, and search for optimum acquisition parameters, a series of comparative tests on the different charge depth and size, group interval, CDP fold, geophone array, and single high-sensitivity geophone were conducted. We also tested 2S3L (two lines shooting and three lines receiving) wide line profiling. The results indicate that single hole with charge depth of 12 m, 4-16 kg charge size (less charge size for the densely populated areas), single high-sensitivity geophone, and 1S2L wide line profiling with 132 folds are the optimum acquisition parameters. On the basis of comparative process experiment, data processing workflow consisting of data preparation, prestack denoising, amplitude compensation, deconvolution, tomography static correction, velocity analysis, residual static correction, CRS stack, poststack migration, prestack time migration (PSTM), and prestack depth migration (PSDM) was selected. Maybe affected by problem of conflicting dip in complex media, CRS stack section does not show satisfactory geological characteristics. PSTM profile has moderate signal-to-noise (S/N) ratio; the shallow, medium, and deep continuous reflections can be observed in section. More details of the geological structures can be observed in PSDM section, especially in medium and shallow layers (less than 3000 ms or 4000 m), but PSDM method is more expensive and highly time consuming than that of CRS stack and PSTM. So, the PSTM section can be reasonably used for geological interpretation. By reference to field mapping, thrust characteristics, and MT data, the final interpretation to the PSTM section identified the interfaces of 6 geological units (Paleoproterozoic Nabhisthan Fm., Paleoproterozoic Dubidanda Fm., Neogene to Late Cretaceous Surkhet group, Late Carboneferous to Early Cretaeous Gondwana group, Mesoproterozoic Upper Lakharpata group, and Lower Lakharpata group) and delineated Main Boundary Thrust (MBT), Ramgarh Thrust (RMT), Padukasthan Thrust (PT), and Dailekh Thrust (DT). The bottom of Surkhet group which is our top target zone is about 4250 meters deep.

Electrical resistivity structure of Danau Ranau geothermal prospect area based on integrated 3-D Inversion of impedance tensor and tipper vector

Subsurface electrical resistivity distribution needs to be revealed to explore the geothermal potential in The Danau Ranau prospect area, as the components in geothermal systems could be easily characterized by their resistivity contrasts. The magnetotelluric method is a geophysical method commonly used to perform this task. This paper performs a three-dimensional inversion based on the complete component of magnetotelluric transfer functions, including impedance tensor and tipper vector. Prior dimensionality analysis using phase tensor revealed that the MT data contains a high dimensionality distortion and shows significant horizontal geoelectrical variations. The analysis suggests using the MT full components in the inversion process to effectively recover all the geoelectrical features in such a geologically complex area. The three-dimensional inversion was then carried out after taking control of the model mesh dimension and the regularization parameter. The 3-D resistivity model showed an overburden layer with varying resistivity in the shallow depth. In the subsequent layer, a conductive anomaly representing impermeable cap rock is distributed from the center to the northern and western parts of the study area, bordered by faults and manifestations. The reservoir zone is located under the center of Mount Seminung at an approximate depth of 1500 m. The heat source is expected to be the residual eruption magma of the volcano, which sits far under the reservoir zone and is unreachable by the MT signal. All the findings agree with the existing geological and geochemical survey data and can explain the hydrothermal activities in the Danau Ranau geothermal prospect area.

Investigation of the main reservoir of the Tulehu geothermal system (Indonesia) using 3-D inversion of MT data

Tulehu geothermal prospect area is located in Maluku Province, Indonesia. The existence of the geothermal system in this area is recognized by the appearance of hot springs on the western side of Mt Eriwakang and the northern side of Mt. Huwe. Most of these hot springs occur along inferred geological structures. The geological conditions of this area are dominated by Quaternary volcanic rocks, limestone, alluvium, and metamorphic rocks. Several authors have conducted several studies to propose various geothermal conceptual models, from structurally controlled to volcanic-hosted geothermal systems. However, the geoscientific data supporting the hypotheses are not comprehensive yet. There are still gaps in the data coverage as well as in the interpretation of geological, geochemical, and geophysical data, so a comprehensive study should be done before deciding on further stages. Therefore, this paper proposes a new conceptual model of the Tulehu geothermal area by reducing the existing data gap. Interpretation of remote sensing data, geological field observations, age dating analysis, reanalysis of geochemical data, and additional MT data covering Mt. Eriwakang are conducted to reconstruct the conceptual model. Graben-like structures are identified based on the geological data where the product of Mt. Eriwakang is in the middle. Based on the age dating data, the product of Mt. Eriwakang erupted in the Quaternary time (413±6 kA), so it is still quite potential as a heat source. This indication is also supported by the trend of Cl, HCO3, and SiO2, which describe the possible flow from Mt. Eriwakang to the surrounding hot springs. Based on the Na-K-Mg and Silica geothermometers, the reservoir temperature is estimated at 210-240 °C. A three-dimensional (3-D) MT inversion was applied to all the existing and new MT data to obtain a 3-D subsurface resistivity model. The oceanic bathymetry data was also incorporated to mitigate the possible coastal effect from the surrounding seawater to the MT data. The new 3-D MT inversion results show a good agreement with the geological and geochemical indications. The updome-shaped pattern of the subsurface resistivity distribution beneath Mt. Eriwakang supports the geological and geochemical data analysis results concerning the indications of the prospects area around Mt. Eriwakang. These interpretations can strengthen the recommendation to focus on the area around Mt. Eriwakang for further development.