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Abstract Time-domain electromagnetic (TDEM) surveys commonly use diverse excitation waveforms, making accurate simulation of full-time three-dimensional (3D) responses necessary. This paper presents a fast 3D full-space TDEM forward solver that combines octree-based local refinement with a model order reduction (MOR) strategy. The governing equation is discretized using the mimetic finite-volume method in the magnetic flux density (b) formulation, and a source-decoupled shift-and-invert (SAI) Krylov-subspace MOR scheme is constructed to reduce the original high-dimensional system. Compared with tensor discretization, the octree mesh achieves a favorable trade-off between local resolution and memory consumption. Numerical results show that the proposed MOR solver accelerates simulations by factors of 10.5 and 4.2 relative to the conventional BDF and BDF-SAI schemes, respectively, while maintaining high accuracy. Comparisons between full-space and half-space responses reveal distinct electromagnetic field propagation characteristics in full-space environments. The results further indicate that anomalies outside the observation direction may affect measured responses, suggesting the need to mitigate interference from undesired directions in practical surveys.

An integrated hydro-geophysical and hydrochemical investigation was conducted to delineate aquifer geometry, assess groundwater potential, and evaluate water quality in the southern part of Wadi Qena, Eastern Desert, Egypt. Eighteen time-domain electromagnetic (TDEM) soundings, ground magnetic profiles, pumping-test data, and six groundwater chemical analyses were jointly interpreted. The integrated datasets reveal five geo-electrical layers and identify two main aquifer systems: a shallow Quaternary aquifer (50–300 m depth; 4.9–86 Ω m) and a deeper Nubian Sandstone aquifer (300–650 m depth; 4.7–17.6 Ω m). Magnetic modeling delineates a variable basement surface (350–850 m) that controls aquifer thickness and the spatial distribution of transmissive zones. Areas of deep basement lows coincide with high-transmissivity wells (655–1170 m 2 /day) and low resistivity, indicating thick, well-connected sandstone bodies. Hydrochemical data (TDS: 1447–1607 mg/L; Na–Cl facies) indicate increasing salinity toward the northwest, consistent with upward leakage along magnetic lineaments and the dissolution of salt-bearing formations. The integrated interpretation demonstrates that combining TDEM, magnetic, and geochemical approaches provides a robust framework for identifying productive aquifers, understanding salinity sources, and optimizing groundwater development in arid terrains.

In order to cope with the rise in human-caused demands, Saudi Arabia is exploring new groundwater sources. The groundwater potential of Wadi Ranyah was studied using a multi-dataset-integrated approach that included time-variable gravity data from the Gravity Recovery and Climate Experiment (GRACE), vertical electrical sounding (VES), and time-domain-electromagnetic (TDEM) data with other related datasets to examine the variations and occurrence of groundwater storage and to define the controlling factors affecting the groundwater potential in Wadi Ranyah in southwestern Saudi Arabia. Between April 2002 and December 2021, the estimated variation in groundwater resources was −3.85 ± 0.15 mm/yr. From 2002 to 2019, the area observed an average yearly precipitation rate of 100 mm. The sedimentary succession and the underlying fractured basement rocks are influenced by the structural patterns that run mainly in three different trends (NW, NE, and NS). The sedimentary cover varies from 0 to 27 m in thickness. The outputs of the electrical sounding revealed four primary geoelectric units in the study area: on top, a highly resistant geoelectrical unit with a resistivity of 235–1020 Ω.m, composed of unsorted, loose, recent sediments; this is followed by a layer of gravel and coarse-grained sands with a resistivity of 225–980 Ω.m; then, a water-bearing unit of saturated sediments and weathered, fractured, basement crystalline rocks with a resistivity of 40–105 Ω.m, its depth varying from 4 to ~9 m; and then the lowest fourth unit composed of massive basement rocks with higher resistivity values varying from 4780 to 7850 Ω.m. The seven built dams store surface-water runoff in the southwestern part of the wadi, close to the upstream section, in addition to the Ranyah dam, as the eighth one is located in the middle of the wadi. The subsurface NW- and NS-trending fault lines impede the groundwater from flowing downstream of the wadi, forming isolated water-bearing grabens. Minimal surface runoff might occur in the northern part of the wadi. The combined findings are beneficial because they provide a complete picture of the groundwater potential of Wadi Ranyah and the controlling structural patterns. Using this integrated technique, the groundwater potential in arid and semiarid regions can now be accurately assessed.

Time-Domain Electromagnetic (TDEM) soundings are widely recognized as an effective method for subsurface characterization at intermediate depths. This study applies TDEM surveying to the municipality of Alfaro (La Rioja, Spain), where ten stations with 200 × 200 m loops were acquired and processed using Occam 1D inversion. The resulting models were integrated into a 3D environment in geomatic software (Seequent, Oasis montaj 2025.1) to generate a continuous geoelectrical volume of the subsurface. Three major resistivity domains were identified: a shallow resistive unit (40–80 ohm·m), an intermediate unit (20–40 ohm·m), and a deep conductive domain (<20 ohm·m). The pseudo-3D model revealed a thickening of the intermediate–resistive domain toward the central and western sectors, interpreted as the most favorable zone for groundwater extraction. This workflow demonstrates that integrating 1D TDEM inversion results into a pseudo-3D geoelectrical framework improves the spatial interpretation of resistivity distributions and provides a practical decision-support framework for identifying optimal drilling sites.

[EN]The need to use groundwater resources for agricultural and livestock farms is a constantly growing issue, as these resources condition the socio-economic development of many areas with limited climatic regimes and surface water resources. Decisions on the location and characteristics of groundwater extraction works are based on hydrogeological studies, in which geophysical techniques are a very useful tool. In our study, the Time-Domain Electromagnetics technique (TDEM) was used, which significantly improves the precision and safety when planning to drill for water extraction in Mesozoic formations thanks to the significant resistivity contrast among its different hydrogeological units. The performance of a research campaign using TDEM allows for obtaining 2D geoelectrical profiles which define the hydrogeological structure of the study area and the materials of which it is composed, enabling the selection of a site where drilling can be carried out with the best prospects of success. This article shows the usefulness of this study methodology, applied to Mesozoic formations in a specific area located in the municipality of Liceras (Soria, Spain).

Nowadays in solving geological problems, the technologies of UAV-geophysics, primarily magnetic and gamma surveys, are being increasingly used. However, for the formation of the classical triad of airborne geophysics methods in the UAV version, there was not enough technology for UAV-electromagnetic sounding, which would allow studying the geological environment at depths of tens and hundreds of meters with high detail. This article describes apparently the first technology of UAV-electromagnetic sounding in the time domain (TDEM, TEM), implemented as an unmanned system based on a light multi-rotor UAV. A measuring system with an inductive sensor—an analogue of a 20 × 20 or 50 × 50 m receiving loop is towed by a UAV, and a galvanically grounded power transmitter is on the ground and connected to a pulse generator. The survey is carried out along a network of parallel lines at low altitude with a terrain draping at a speed of 7–8 m/s, the maximum distance of the UAV’s departure from the transmitter line can reach several kilometers, thus the created technology is optimal for performing detailed areal electromagnetic soundings in areas of several square kilometers. The results of the use of the unmanned system (UAS) in real conditions of the mountainous regions of Eastern Siberia are presented. Based on the obtained data, the sensitivity of the system was simulated and it was shown that the developed technology allows one to collect informative data and create geophysical sections and maps of electrical resistivity in various geological situations. According to the authors, the emergence of UAV-TEM systems in the near future will significantly affect the practice of geophysical work, as it was earlier with UAV-magnetic prospecting and gamma-ray survey.

Understanding the subsurface is of prime importance for many geological and hydrogeological applications. Geophysical methods offer an economical alternative for investigating the subsurface compared to costly borehole investigations. However, geophysical results are commonly obtained through deterministic inversion of data whose solution is non-unique. Alternatively, stochastic inversions investigate the full uncertainty range of the obtained models, yet are computationally more expensive. In this research, we investigate the robustness of the recently introduced Bayesian evidential learning in one dimension (BEL1D) for the stochastic inversion of time-domain electromagnetic data (TDEM). First, we analyse the impact of the accuracy of the numerical forward solver on the posterior distribution, and derive a compromise between accuracy and computational time. We also introduce a threshold-rejection method based on the data misfit after the first iteration, circumventing the need for further BEL1D iterations. Moreover, we analyse the impact of the prior-model space on the results. We apply the new BEL1D with a threshold approach on field data collected in the Luy River catchment (Vietnam) to delineate saltwater intrusions. Our results show that the proper selection of time and space discretization is essential for limiting the computational cost while maintaining the accuracy of the posterior estimation. The selection of the prior distribution has a direct impact on fitting the observed data and is crucial for a realistic uncertainty quantification. The application of BEL1D for stochastic TDEM inversion is an efficient approach, as it allows us to estimate the uncertainty at a limited cost.

The current study would use geophysical investigations and geo-informatics to assess the groundwater aquifer at Wadi Dawqah–Al Baha. The morphometric analysis of the drainage basins gives a complete picture of the best catchment areas for surface water, which consider the main feeder for the groundwater aquifer. For the area drainage basin, a digital elevation model, slope, aspect, and drainage pattern maps were developed. The drainage pattern of the study area appears to be dendritic, with five major tributaries converging at the study area. Various metrics for calculating drainage basin morphometry, such as stream number, flow order, flow length ratio, bifurcation ratio, basin length, basin area, relief ratio, streaming density, flow rate, shape factor, and circulation ratio have been studied using conventional mathematical formulas. Eighteen vertical electrical soundings (VES) were conducted using a Shlumberger array with a maximum AB/2 of 200 m. While the time domain electromagnetic (TDEM) survey was conducted at eight different locations along the Wadi using a coincident loop configuration in which signals were transmitted and received using the same loop. The effects of the interpreted VES and TDEM soundings are used to create a variety of cross-sections and maps of resistivity, thickness, and depth of the aquifer layer. These maps display a smooth distribution of aquifer resistivity values ranging from 30 to 190 Ω m. The average thickness of the water saturated Wadi deposits (aquifer) layer ranges from 16 to 50 m. The aquifer depth shows moderate values ranging from 14 to 26 m. On the other hand, the presence of four geoelectric layers is detected in the built resistivity cross-sections along some profiles. The resistivity values and average thickness of each layer were identified and their corresponding interpreted geological cross-sections were constructed.

ABSTRACT This study integrates Very Low Frequency Electromagnetic (VLF-EM) and Time-Domain Electromagnetic (TDEM) methods to characterise subsurface geological structures and aquifer conditions in the New Nubariya-2 City, Egypt. VLF-EM analysis, enhanced by Fraser filtering and Karous-Hjelt pseudo-sections, identified nineteen conductive linear anomalies (F1–F19) interpreted as fracture zones. TDEM 1D inversion validated several of these anomalies and delineated four distinct geoelectric layers: a heterogeneous surface layer composed of dry sands, silts, clay, or gravel (5–216 Ω·m), a low-resistivity clay layer (1.2–2.4 Ω·m) with thicknesses of 28–71 m, a water-saturated sand aquifer (13–33 Ω·m) with depths increasing from 34 m in the east to approximately 104 m in the northwest, and a basal clayey sand formation (3.7–9.6 Ω·m). The southeastern and northwestern regions were identified as optimal for well drilling due to greater aquifer thickness, higher resistivity, and the presence of deep water-bearing fracture zones. These findings enhance the understanding of groundwater resources in the West Nile Delta and provide valuable insights for urban planning and water resource management in the New Nubariya-2 City.

Santiago Island, the biggest and most populated island of the Cape Verde Republic, is characterised by limited surface waters and strong dependence on groundwater sources as the primary source of natural water supply for extensive agricultural activity and human use. However, as a consequence of the scarce precipitation and high evaporation as well as the intense overexploitation of the groundwater resources, the freshwater management is also in a delicate balance with saltwater at coastal areas. The time-domain electromagnetic (TDEM) method is used to locate the extent of saltwater intrusion in four important agricultural regions in Santiago Island; São Domingos, Santa Cruz, São Miguel, and Tarrafal. The application of this method in Santiago Island proves it to be a successful tool in imaging the fresh/saltwater interface location. Depths to the saline zones and extensions of saline water are mapped along eight TDEM profiles.