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Identification of preferential flow‐paths, such as fractures, is required for various issues in geosciences. When chemicals are injected into the subsurface, monitoring the resulting structural and chemical changes remains a challenge. The ability of geophysical tomography to tackle this problem is not fully explored due to the lack of numerical methods suitable for modeling narrow structures. We explore how discrete representation of preferential flow‐paths provides innovative ways to invert electrical resistivity data collected during reagent injection at a contaminated site. The data set is inverted with a scheme where a new fracture is added at every iteration. This allows identifying newly created narrow conductive structures from the field data collected before and after injection. Fracture location remains overall consistent despite using different starting points for the fracture search. A prior constraint on fracture length improves convergence. These results show the potential of discrete inversion for identifying narrow structures from electrical resistivity monitoring. Plain Language Summary: In some geoscience activities, such as water extraction, geological storage, and contaminant fate, chemical solutions are injected into wells, which potentially leads to modify the system properties. Identifying preferential flow‐paths is therefore necessary even though monitoring structural and chemical changes in the subsurface remains a challenge. Electrical geophysical imaging techniques may be adapted to provide information for this purpose. However, their potential is not fully explored due to the limitations of numerical models to represent small‐scale structures. In this work, we explore the representation of preferential flow‐paths by discrete elements for the inversion of electrical resistivity data collected during a reagent injection at a contaminated site. The inversion scheme based on discrete forward simulations successively identifies small‐scale structures and focuses on evaluating their localization and length. The results show the need for further efforts to use discrete electrical resistivity inversion in the presence of narrow structures. Key Points: Standard modeling strategies are not well suited to identify narrow geological structures from cross‐borehole electrical data inversionInversion scheme based on discrete‐dual‐porosity forward simulations enable to image narrow preferential flow‐paths from electrical dataPrior assumptions are needed to invert the position and length of narrow preferential flow‐paths [ABSTRACT FROM AUTHOR]

The Valgarður database is a compilation of data describing the physical and geochemical properties of Icelandic rocks. The dataset comprises 1166 samples obtained from fossil and active geothermal systems as well as from relatively fresh volcanic rocks erupted in subaerial or subaqueous environments. The database includes petrophysical properties (connected and total porosity, grain density, permeability, electrical resistivity, acoustic velocities, rock strength, and thermal conductivity) as well as mineralogical and geochemical data obtained by point counting, X-ray fluorescence (XRF), quantitative X-ray diffraction (XRD), and cation exchange capacity (CEC) analyses. The database may be accessed at 10.5281/zenodo.6980231 (Scott et al., 2022a). We present the database and use it to characterize the relationship between lithology, alteration, and petrophysical properties. The motivation behind this database is to (i) aid in the interpretation of geophysical data, including uncertainty estimations; (ii) facilitate the parameterization of numerical reservoir models; and (iii) improve the understanding of the relationship between rock type, hydrothermal alteration, and petrophysical properties. [ABSTRACT FROM AUTHOR]

Transient electromagnetic (TEM) is an efficient non-invasive method to map electrical conductivity distribution in the subsurface. This paper presents an inversion scheme for 3-D TEM time-lapse (TL) data using a generalized minimum support (MS) norm and its application to monitoring conductivity changes over time. In particular, two challenges for TL TEM applications are addressed: (i) the survey repetition with slightly different acquisition position, that is, because systems are not installed and (ii) non-optimal data coverage above the TL anomalies, for instance, due to the presence of infrastructure that limits the acquisition layout because of coupling. To address these issues, we developed a new TEM TL inversion scheme with the following features: (1) a multimesh approach for model definition and forward computations, which allows for seamless integration of data sets with different acquisition layouts; (2) 3-D sensitivity calculation during the inversion, which allows retrieving conductivity changes in-between TEM soundings and (3) simultaneous inversion of two data sets at once, imposing TL constraints defined in terms of a generalized MS norm, which ensures compact TL changes. We assess the relevance of our implementations through a synthetic example and a field example. In the synthetic example, we study the capability of the inversion scheme to retrieve compact time-lapse changes despite slight changes in the acquisition layout and the effect of data coverage on the retrieval of TL changes. Results from the synthetic tests are used for interpreting field data, which consists of two TEM data sets collected in 2019 and 2020 at the Nesjavellir high-temperature geothermal site (Iceland) within a monitoring project of H2S sequestration. Furthermore, the field example illustrates the effect of the trade-off between data misfit and TL constraints in the inversion objective function, using the tuning settings of the generalized MS norm. Based on the results from both the synthetic and field cases, we show that our implementation of 3-D TL inversion has a robust performance for TEM monitoring. [ABSTRACT FROM AUTHOR]

The Valgarður database is a compilation of data describing the physical and geochemical properties of Icelandic rocks. The dataset comprises 1072 samples obtained from fossil and active geothermal systems, as well as relatively fresh volcanic rocks erupted in sub-aerial or sub-aqueous environments. The database includes petrophysical properties (effective and total porosity, grain density, permeability, electrical resistivity, acoustic velocities), as well as mineralogical and geochemical data obtained by point-counting, X-ray Fluorescence (XRF), quantitative X-ray Diffraction (XRD), and Cation Exchange Capacity (CEC) analyses. The motivation behind this database is threefold: (i) aid in the interpretation of geophysical data including uncertainty estimations, (ii) facilitate the parameterization of numerical reservoir models, and (iii) improve our understanding of the relationship between rock type, hydrothermal alteration and petrophysical properties [ABSTRACT FROM AUTHOR]

Processing of geophysical data is a time consuming task involving many different steps. One approach for accelerating and automating processing of geophysical data is to look towards machine learning (ML). ML encompasses a wide range of tools, which can be used to automate complicated and/or tedious tasks. We present strategies for automating the processing of time-domain induced polarization (IP) data using ML. An IP data set from Grindsted in Denmark is used to investigate the applicability of neural networks for processing such data. The Grindsted data set consists of eight profiles, with approximately 2000 data curves per profile, on average. Each curve needs to be processed, which, using the manual approach, can take 1–2 hr per profile. Around 20 per cent of the curves were manually processed and used to train and validate an artificial neural network. Once trained, the network could process all curves, in 6–15 s for each profile. The accuracy of the neural network, when considering the manual processing as a reference, is 90.8 per cent. At first, the network could not detect outlier curves, that is where entire chargeability curves were significantly different from their spatial neighbours. Therefore, an outlier curve detection algorithm was developed and implemented to work in tandem with the network. The automatic processing approach developed here, involving the neural network and the outlier curve detection, leads to similar inversion results as the manual processing, with the two significant advantages of reduced processing times and enhanced processing consistency. [ABSTRACT FROM AUTHOR]

We investigate the complex conductivity behaviour of natural volcanic rocks containing variable amounts of smectite in multi‐salinity experiments. We compare the results with relationships established for sandstones. Considering only samples with little volume of metallic particles, we observe similar and small phase‐angles at low frequency for all samples at all salinities (less than 25 mrad at 1 Hz). Yet, a wide range of cation exchange capacity, porosity and formation factor is covered by the sample set: 0.5–50 meq/100 g, 4–40% and 18–780, respectively. Our results show that, in the absence of metallic particles, the ratio between imaginary conductivity and surface conductivity is significantly lower for altered volcanic rocks than for sandstones and decreases with the smectite content. These observations indicate that an increased smectite content causes more conduction and less polarization, which could be explained by the onset of a continuous conduction pathway throughout connected interfoliar spaces of smectite. Due to this pathway, cations from the pore fluid may penetrate the solid lattice, for example through connected smectite aggregates clogging the fracture network, thus preventing polarization. We also observe that the relationship between imaginary conductivity and surface conductivity, at one salinity or over the whole salinity range, is not more significant than the relationship between the imaginary conductivity and the total real conductivity. Therefore, we suggest that the imaginary conductivity cannot be used to discriminate the contributions from smectite and pore water to the total conductivity of altered volcanic rocks. [ABSTRACT FROM AUTHOR]

In volcanic environments, the presence of smectite may indicate recent hydrothermal circulations. Smectite is also responsible for enhanced rock electrical conductivity, as well as mechanical weakening. Therefore, quantifying smectite is important in geothermal exploration. Smectite identification requires X-ray diffraction (XRD) but quantification based on XRD is time-consuming and not always accurate. In the present study, we investigate the use of an optimized unbuffered Cation Exchange Capacity (CEC) determination, by back-titration of the Copper-triethylenetetramine(II) "Cu-trien" molecule, to quantify the smectite content of altered volcanic rock samples. We establish that a satisfying trade-off between the instrument uncertainty and an independant systematic error is theoretically reached for a fraction of reactants consumed of about 30% at the end of the exchange reaction. We suggest a modification to classical protocols to fall in that range. Finally, we show that optimized CEC determination by Cu-trien are a direct measure of the smectite weight fraction in altered volcanic samples, with an average CEC of pure smectite of 90 ± 5 meq/100 g. [ABSTRACT FROM AUTHOR]