Subsurface hydrothermal alteration mapping in the Reykjanes Geothermal area using a combined geoelectrical approach

Geo-electric methods such as Electrical Resistivity Tomography (ERT) and Induced Polarization (IP) have become increasingly important in the characterization of volcanic and geothermal systems. The methods rely on the electrical properties of the subsurface. In volcanic settings, the main influences are temperature, gas content (i.e. saturation), mineralizations, and the presence of alteration clays. The ERupT project aims to assess the suitability of ERT to visualize the dynamics in hydrothermal systems. With the long-term aim of improving hazard assessment associated with phreatic/hydrothermal eruptions. In that context, a semi-permanent ERT setup was installed at the Reykjanes geothermal area in Iceland. In October 2022, the monitoring system was installed on-site, automatically measuring one profile per day, with currently 15 months of uninterrupted data, except for 5 days due to technical issues. The profile is 355 meters long and has a depth of investigation of 30 to 50 meters. The 2021 eruption at Fagradalsfjall has marked a new age of volcanism in the Reykjanes peninsula, 2023 was marked by two eruptions. The first eruption happened in the Fagradalsfjall system on July 10th. The second eruption started on 18 December, North of Grindavik, in the Svartsengi system. The eruption sites are located at respectively 30 and 10 km from the field site. Although the ERT system is located at a considerable distance from the eruption sites and the investigation depth is quite shallow, we observed signals possibly related to both eruptions and accompanying unrest, manifesting as a significant increase in resistance (figure1). The first peak in resistance happened between 19 and 26 June with an increase of more than 100%, shortly after that, the Fagradalsfjall system erupted. A second peak is observed at the end of August, here the increase happens slowly, as opposed to the sudden peak in June, with a steady increase starting on August 8th, reaching a peak on August 30th. Contradictory to the first example, no immediate eruption occurred after this second peak. In this context, an increase in resistance is likely caused by a drop in saturation due to high gas levels, which can be caused by magma degassing during the uprise. This raises the question of the time difference between the peaks and the subsequent eruptions, possible factors are the difference in morphological context (sill vs dyke intrusion) and preferential flowpaths relative to our monitoring site.  It should also be noted that this behavior is not observed in all data points, hence advanced processing is needed combined with interpretation using data from other methods. Tremor and soil temperature data are available along the ERT profile, together with one C02 sensor in the center of the profile. To our knowledge, this is the first time that ERT has been used for daily monitoring of a volcanic system. With joint interpretation to deduce the signal origin, we believe that ERT can be a valuable addition to volcanic monitoring networks. Figure 1: Resistance evolution in one measurement point, eruptions are indicated by the red lines.

Evaluation of flake graphite ore using self-potential (SP), electrical resistivity tomography (ERT) and induced polarization (IP) methods in east coast of Madagascar

Hard-rock investigation using a non-invasive geophysical approach

The Palaeo- to Meso-Proterozoic (1.0–2.4 Ga) North Singhbhum Mobile Belt (NSMB) is an assembly of multiphase folded, low- to medium-grade meta-sedimentary and meta-igneous rocks, lying within the north Singhbhum crustal province. The mobile belt is situated between the Archean Singhbhum craton in the south and the Meso/Neo-Proterozoic (0.9–1.7 Ga) Chotanagpur Gneissic Complex in the north. Gold mineralisation of moderate concentration is reported from different parts of NSMB along shear /fracture zones within the volcano-sedimentary and meta-sedimentary rocks sequence. The mineralisation is structurally controlled and is associated with higher order shear zones. Gold occurs in association with sulfides like pyrite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, etc. This work attempts to map the auriferous shear/fracture zone along the Babaikundi-Birgaon axis (BBA), which lies within the NSMB. In the present study, combined geophysical approach comprising self-potential (SP), electrical resistivity tomography (ERT) and induced polarisation (IP) methods were used for mapping of subsurface gold prospects. SP data were collected along 7 profiles at 10 m station interval with approximate profile separation of 100 m. The anomalies of possible ore bodies were found at depth of about 18–25 m using Euler deconvolution of SP data. The ERT and IP data were collected along four profiles at 10 m electrode spacing with approximate profile separation of 100 m. The inverted resistivity sections indicate very low resistivity value in the depth range of ∼20 m inferring highly conducting, while relatively high chargeability are also observed at the depth range of ∼25 m. The possible existence of Babaikundi-Birgaon lineation and the occurrences of sulfide minerals in quartz reef associated with gold mineralisation were mapped based on the horizontal locations and extensions of negative anomaly in SP data, low resistivity in 2-dimensional (2D)-inverted resistivity section and high chargeability in 2D-inverted IP section. Previous studies based on electron probe micro-analyzer (EPMA) and scanning electron microscope (SEM) analysis over the study area substantiate our combined geophysical interpretations for the presence of gold prospective zones along the BBA. Highlights Integrated geophysical approach comprising SP, ERT and IP methods Delineation of BBA Mapping of subsurface gold prospects/ mineralisation Validation of results using borehole data, EPMA and SEM–SED analysis of rock samples (a) Map showing profiles AA/, BB/, CC/ and DD/across the Babaikundi-Birgaon Axis (BBA). (b) Upward continued (60 m) self-potential (SP) anomaly map showing possible extension of a part of BBA. Schematic resistivity (c) and chargeability (d) distribution models showing possible extension of a part of the BBA. (e) mineralised sample with sulfide hosted quartz reef in BBA. The possible existence of BBA and the occurrences of gold-bearing sulfide minerals in quartz reef are mapped based on the horizontal locations and extensions of negative SP, high conducting / low resistivity in electrical resistivity tomography (ERT) and high chargeability in Induced Polarization (IP). Note that profiles C-C/ and D-D/ are not parallel to each other; their field orientation is shown in Figure 1(c) and Figure 2.

A series of catastrophic floods that have occurred over the last twenty years in Poland, brought an urgent need for taking preventive steps to monitor river embankments conditions. The main problem seems to be development of efficient, ie. fast and economical measurements for controlling the condition of the river embankments, because the execution of the full range of geotechnical measurements is lengthy and costly. In this situation, thecheap and quick geophysical survey has been proposed to undertake this purpose. In this article the results of geophysical surveys were described which were performed using geoelectric and electromagnetic methods along a section of the Vistula embankment, located near the Maniow area in the Malopolska province. According to the archival data, this region is situated at the high-risk flooding zone. Three geophysical methods were used to recognize geotechnical conditions of the levee: (i) Electrical Resistivity Tomography (ERT), (ii) Induced Polarization (IP) and (iii) Electromagnetic Conductivity Meters (ECM). Geoelectrical measurement results were presented in the form of resistivity and polarization cross-sections. Results of conductivity measurement were presented in the form of plots. These parameters effectively supplement geotechnical research providing information about the changes within the embankment and its substrate. It allows to predict potentially vulnerable area to water filtration during the flood risk.

Investigation of a hard rock site for the development of engineered structures mainly depends on the delineation of weathered and unweathered rock, and the fractures/faults. Traditionally, borehole tests are used in such investigations. However, such approaches are expensive and time-consuming, require more equipment, cannot be conducted in steep topographic areas, and provide low coverage of the area with point measurements only. Conversely, geophysical methods are non-invasive, economical, and provide large coverage of an area through both vertical and lateral imaging of the subsurface. The geophysical method, electrical resistivity tomography (ERT), can reduce a significant number of expensive drilling tests in geotechnical investigations. However, a geophysical method alone may provide ambiguity in the interpretation of the subsurface, such as electrical resistivity cannot differentiate between water and clay content. Such uncertainty can be improved by the integration of ERT with induced polarization (IP). Similarly, self-potential (SP) can be integrated with other geophysical methods to delineate the groundwater flow. In this contribution, we integrated three geophysical methods (ERT, IP and SP) to delineate the weathered and unweathered rock including the weathered/unweathered transition zone, to detect the fractures/faults, and to map the groundwater flow. Based on ERT, IP and SP results, we develop a geophysical conceptual site model which can be used by site engineers to interpret/implement the findings for build-out. Our approach fills the gaps between the well data and geological model and suggests the most suitable places for the development of engineered structures in the hard rock terrains.

The impacts of climate change, combined with population growth, necessitate practical and effective solutions for locating groundwater resources and ensuring drinking water quality. Our contribution explores recent advances in geoelectrical and electromagnetic imaging methods applied to investigate groundwater systems. Geoelectrical and electromagnetic imaging techniques are popular methods for characterising subsurface properties, such as electrical resistivity or dielectric permittivity. These electrical properties are strongly related to the hydrogeological characteristics of the subsurface. Therefore, geoelectrical and electromagnetic investigations can provide valuable insights into finding groundwater resources, assessing the water quality in terms of contaminations and conducting effective groundwater management.Our study examines state-of-the-art approaches in modelling and instrumentation of induced polarisation and electrical resistivity tomography, as well as time- and frequency-domain electromagnetics and ground-penetrating radar methods. We review recent impactful and innovative groundwater case studies where the above-mentioned methods were applied and further developed. Emphasising the combination of geoelectrical and electromagnetic methods, the studies provide insights into the variation of electrical subsurface properties at different scales, contributing to an improved understanding of the hydrological dynamics in the studied areas. Furthermore, we provide an outlook on the potential for applying geoelectrical and electromagnetic imaging techniques for large-scale groundwater investigations in the exascale computing area.

Introduction With only a few minor exceptions, electrical geophysical methods can be divided into two categories: (1) galvanic source methods, directly coupling or injecting electrical current into the ground via electrodes, and (2) inductive source methods, inducing eddy currents into the ground via time-varying magnetic fields using coils not in direct contact with the ground. Ground-penetrating radar (GPR) can be considered an extreme high-frequency, dielectric-properties-sensitive example of the latter. Self-potential (SP) effects, due to electrochemical mechanisms in the ground, and telluric current methods (where only ambient voltages, induced by natural EM sources such as the oscillating magnetosphere of the earth, are measured) can arguably be included in the former, but are both passive methods and are beyond the scope of this chapter.

The subsurface soil around Baruwa community was reportedly contaminated by hydrocarbon not because the area falls within oil-producing community but it suffers from inceasant leakage from petroleum pipeline that supply petroleum products to hydrocarbon terminar located within the area. Thus, to ascretain the extent of contamination of the subsurface soil around the area, an integrated geophysical methods involving vertical electrical sounding (VES) and electrical resistivity tomography (ERT) supported with induced polarization (IP) and spontaneous potential (SP) methods were carried out. The hydrocarbon contaminated layers were marked out beneath each VES point by high resistivity ranging between 943&Omegam and 4749&Omegam at a depth of 1 to 35.44m below the surface. Similarly, ERT result shows that the subsurface soil around the investigated area has been contaminated at a shallow depth of about 2m downward with resistivity value above 1000&Omegam. IP and SP data were Integrated to identify the sandy contaminated layers from clayey layer. IP value of 0 to 10mV/V and SP values of &saquo10mV were obtained over the sandy formation. The work shows that hydrocarbon leaking from the pipeline laid a few meters beneath the earth surface actually flow both upwardly (possibly due to seasonal variation in the water table which is usually close to the surface during rainy season) to the surface and downwardly at greater depth into the subsurface, through a porous medium - sandy layer. This may probably accounts for the reason while most of the handdug wells in the area are reportedly contaminated with hydrocarbon products.

Groundwater contamination monitoring with multichannel electrical and electromagnetic methods

Quantitative characterization of solute transport processes in the laboratory using electrical resistivity tomography

The proposed work is about the geophysical survey applied in three Italian waste dump (Savoia di Lucania and Pallareta in Basilicata region, Novellara in Emilia-Romagna Region). The Basilicata landfill are provided of HDPI liner while in the other one a compacted clay stratum is present at the bottom. In each site we carried out active and passive geophysical techniques: electrical resistivity tomographies (ERT), induced polarisation (IP), self potential (SP) and magnetic methods. The ERTs, inverted by using the Res2DInv software, provided information about the geometry of the waste basins where an HDPI liner is present. The IP results show high chargeability nuclei into the refuse indicated the metallic garbage cumuli. The SP signal is negative into the basins and slightly positive in the surrounding area. Moreover, the SP data show strong spikes at the borders of the basins. The SP data of Novellara waste dump (clay stratum coating) show positive values inside the basins. The magnetic method evaluates the dispersion of metallic garbage close to the waste dumps. Finally, the different acquired data allow to sketch the lateral and buried geometry of the waste basins and to infer their leachate-tight, at least where the HDPI liner is present.

filled caves or water-filled fissures underground. Recently, the audio frequency telluric method (AFTM) has been successfully employed to detect water in karst settings (Chen, 1988; CGS 2005; Gan, 2011), and to probe fissures of water in combination with the induced polarization (IP) method (Li, 2009). The typical advantages of this method are that they are both rapid and cost-effective; however, this is countered by disadvantages, including a shallow investigation depth and a lack of anomaly variations with depth. Electrical resistivity tomography (ERT) can be applied to define the water table (Zaidi, 2012), to delineate aquifers (Kumar, 2012), to search for karst geological structures (Leuccim, 2005) and find karst water (Metwaly, 2012; Vlahovic, 2011) using characteristics of the resistivity variation with depth. Compared with using a single geophysical method alone, the integrated approach usually provides more reliable information, and as a result has been widely applied for groundwater investigations. Alexopoulos (2011) employed both the very low frequency (VLF) electromagnetic method and ERT to map water pathways. Vargemezis (2011) even carried out VLF, Self-Potential (SP) and ERT surveys together to optimize locations for the construction of hydro wells.

BackgroundResidents of geothermal areas are exposed to geothermal emissions and water containing hydrogen sulphide and radon. We aim to study the association of the residence in high temperature geothermal area with the risk of cancer.MethodsThis is an observational cohort study where the population of a high-temperature geothermal area (35,707 person years) was compared with the population of a cold, non-geothermal area (571,509 person years). The cohort originates from the 1981 National Census. The follow up from 1981 to 2010 was based on record linkage by personal identifier with nation-wide death and cancer registries. Through the registries it was possible to ascertain emigration and vital status and to identify the cancer cases, 95% of which had histological verification. The hazard ratio (HR) and 95% confidence intervals (CI) were estimated in Cox-model, adjusted for age, gender, education and housing.ResultsAdjusted HR in the high-temperature geothermal area for all cancers was 1.22 (95% CI 1.05 to 1.42) as compared with the cold area. The HR for pancreatic cancer was 2.85 (95% CI 1.39 to 5.86), breast cancer 1.59 (95% CI 1.10 to 2.31), lymphoid and hematopoietic cancer 1.64 (95% CI 1.00 to 2.66), and non-Hodgkins lymphoma 3.25 (95% CI 1.73 to 6.07). The HR for basal cell carcinoma of the skin was 1.61 (95% CI 1.10 to 2.35). The HRs were increased for cancers of the nasal cavities, larynx, lung, prostate, thyroid gland and for soft tissue sarcoma; however the 95% CIs included unity.ConclusionsMore precise information on chemical and physical exposures are needed to draw firm conclusions from the findings. The significant excess risk of breast cancer, and basal cell carcinoma of the skin, and the suggested excess risk of other radiation-sensitive cancers, calls for measurement of the content of the gas emissions and the hot water, which have been of concern in previous studies in volcanic areas. There are indications of an exposure-response relationship, as the risk was higher in comparison with the cold than with the warm reference area. Social status has been taken into account and data on reproductive factors and smoking habits show that these do not seem to explain the increased risk of cancers, however unknown confounding can not be excluded.

ABSTRACTSinghbhum Shear Zone (SSZ) also referred as Copper Belt Thrust (CBT), located at the southern margin of North Singhbhum Fold Belt (NSFB) is well‐known for highly mineralized copper, uranium and other sulphide minerals deposits. In order to the identify favourable structures that could host the sulphide–uranium mineralization in Gurulpada area of SSZ, an integrated geophysical study was conducted using magnetic, self‐potential (SP), electrical resistivity tomography (ERT) and induced polarization (IP) surveys. The present study identifies pronounced magnetic anomalies in the central part due to presence of magnetite mineral along the shear planes of quartz‐chlorite‐schist (±sericite) and basic dykes, exhibiting an ENE–WSW orientation, which follows the geological strike and the trend of the shear zone within the area. Low magnetic intensity in the southern part of area suggests demagnetization caused by hydrothermal alteration, indicate mineralized zones. SP anomaly map has identified six zones exhibiting negative anomalies. Tilt derivative (TDR) and Euler deconvolution (ED) technique were applied on magnetic and SP data to depict geological structures that control mineralization and its depth. Magnetic and SP anomalies along the profile are plotted with a 2D inverted resistivity and chargeability section for comparative analysis. The inverted resistivity and chargeability model, illustrated as 2D cross sectional view and a 3D fence diagram, has delineated several anomalous zones at varying depths. The high magnetic anomaly, corroborated with negative SP values, is associated with low resistivity and high chargeability zones, indicating the disseminated sulphide ore bodies with quartz and magnetite mineral along the shear planes. Conversely, positive SP, high chargeability and high resistivity zones signify disseminated sulphide deposits that infilled quartz veins and intense silicification in the fractured zones. The 3D pseudo iso‐surface chargeability models indicate high chargeability values (M ≥ 15 mV/V) oriented in an ENE–WSW direction. The integration of geophysical (magnetic, SP, ERT and IP) anomalies and geological (bedrock and trench sampling) data, in conjunction with borehole analysis, confirms the presence of sulphide–uranium mineralization in the study area. The present study reaffirms the presence of ENE–WSW trending ductile‐brittle intense shear and hydrothermal alteration zones, which are key indicators of sulphide–uranium mineralization in the study area. The findings revealed that the mineralization accommodated within the quartz‐chlorite‐schist (±sericite) of the Chaibasa Formation of the Singhbhum Group, appearing as dissemination and fracture filling in association with quartz and magnetite in certain locations. Thus, these integrated geophysical studies are essential for understanding and delineating the complex structural and mineralogical framework of the SSZ. They provide a foundation for further exploration and exploitation of economically significant mineral deposits in this highly mineralized region.