TDEM Soundings Research Articles

TDEM Soundings as a Tool to Determine Seasonal Variations of Groundwater Salinity (Villafáfila Lakes, Spain)

Interaction between groundwaters with different salinities and lakes show seasonal variations driven by changes in precipitation and evapotranspiration. In the vicinity of Villafáfila lakes, local fresh and brackish regional groundwaters feeds the lakes, forming a brine in the lake sediments aquitard. Two TDEM surveys (summer 2019 and winter 2020) were carried out. Five TDEM soundings were acquired at the same location for each survey, forming a profile from the hills to the lake-shore. Simultaneously to the TDEM surveys, electric conductivity of lake water and groundwater was measured. The resistivity boundary between the local fresh (10–35 Ohm/m) and regional brackish groundwater (2–5 Ohm/m) is well marked at 600 m above sea level (masl) below the hills, and at 650 masl below the lowlands surrounding the lakes. During the summer, fresh-brackish groundwater interphase rises due to evaporative pumping occurring in the lowlands. This increases groundwater salinity close to the terrain surface favoring precipitation of halite efflorescences. Annual record of EC in a piezometer confirms the summer ascendant of the brine contained in the lake aquitard. TDEM sounding is fast and simple technique to monitor seasonal variations in fresh-brackish groundwater interphase and to detect possible salinization of consumption wells and environmental changes.

Applying of SP, DC-Resistivity, DC-TDIP and TDEM soundings in high saline coastal aquifer

Four surface geophysical techniques were utilized to study the geological and hydrogeological settings of highly saline a coastal aquifer system to the north-east of River Nile Delta, Egypt. These techniques include SP, DC-Resistivity, TDIP and TDEM methods. The first target was to determine the geological stratification as a differentiation among clay, clayey, sand and sandy layers of high saline water. These techniques reflect that there is a complicated lateral and vertical difference in sediments along study area. The surface layers with depth down to ~120 m have low to medium content of clay that change with depth. Then, the second target was the differentiation laterally and vertically for salinity with depth down to ~250m as an interesting hydrogeological setting. These techniques reported that the sediments consist of thin and thick clay and silts, clayey sand, and sandy clay strata. Investigation depth was up to ~210m due to high salinity and clay content effect. At shallow depths, soil texture (down to ~100m and sometimes down to ~160m) consists of clay and silt with sand intercalation. The TEM data indicate a zone of less saline water and low clay content starting from ~40 to ~100m. There may be an evidence for a significant high to medium clay content after these depths down to ~250m. All four methods were calibrated with each other. Accordingly, good matching between the inversion model of TEMSs and composite logs of new drilled well was found, especially in lithological layers identifications. Also, this calibration confirmed that the area was complicated regarding the geological and hydrogeological conditions and the TDIP and TEM are the best methods in studying the environmental, geological and hydrogeological settings as primary important engineering implications for studying coastal highly saline aquifers.

TDEM and VES soundings applied to a hydrogeological study in the central region of the Taubaté Basin, Brazil

TDEM and VES soundings applied to a hydrogeological study in the central region of the Taubaté Basin, Brazil

The Los Humeros (Mexico) geothermal field model deduced from new geophysical and geological data

The Los Humeros volcanic complex, a 21×15km diameter caldera edifice nesting volcanic domes and a complex faulting structure, is located at the eastern edge of the Trans-mexican volcanic belt (TMVB). It is a young edifice (<0.5Ma) that hosts one of the five main geothermal fields of Mexico with still an important energy production (∼65kW installed capacity). Being one of the more studied producing fields, the geothermal system of the caldera is largely unknown at depths greater than ∼2.4km, which is the approximate penetration range of the available geothermal wells. Here we present the results of a geophysical survey in Los Humeros caldera and surroundings with the aim to provide further insight on the physical characteristics of the geothermal system at depths greater than 2.4km. The survey comprised 70 broadband magnetotelluric (MT) soundings distributed within and in the periphery of the caldera edifice of which we present here three EW profiles. We also accomplished a mesh of 718 accurately leveled gravity stations. These data sets were complemented with 13 TDEM soundings for static shift control, as well as with the aero-magnetic digital chart of the area (#E14-3, SGM, 2004). The MT data analysis yielded an average electric azimuth of N23W for the central profile where the field production is concentrated, which is quite consistent with the mapped NW-SE geological structure. However, at individual frequency ranges the strike follows the local faulting structures, most of which are apparently controlled by the deeper crater structure. At the production zone, the conductivity model reveals the existence of an eastward dipping resistive body, which follows the isotherms registered at the wells. The inclined conductivity interface above it seems to play an important role in controlling the heat and fluid flow towards shallow depths. Petrographic studies of well samples provide evidence of mineralogical assemblages that suggest magnetite-metasomatose hydrothermal alteration. The production zone coincides with maximum gravity and magnetic gradients, at the western edge of the well-defined circular crater anomaly. At this point the central MT section (profile 1) shows the shallower depth (2–3km) to the relatively high resistivity (∼400–500ohm-m) and magnetized intrusive-like body. The interpreted geophysical and surface geological data backed by well data support a reservoir and plume model structure consisting of a resistive propylitic core that feds the geothermal field through fractures and deep- seated faults. Surface conductors associated with stratified mineralization produced from lixiviation of geothermal fluids are well differentiated from deeper conductors. The better preserved northern sector of the Humeros caldera yields an anomalous deep conductor at depths of 6–7km below sea level (profile 5) as well as at the central production sector (profile 1) whereas the anomalous conductivity zone at the southern profile (profile 6) could be as shallow as 5km below sea level. These anomalous conductivity zones are expected to be associated to the primary energy source of the Los Humeros geothermal system in the form of partial fusion or hypercritical trapped fluids within the upper crust, conditions that have been concluded to prevail along cordilleras of the American continent (e.g. Hyndman, 2017).

Identifying Ground-water Resources and Intrabasinal Faults in the Hueco Bolson, West Texas, using Airborne Electromagnetic Induction and Magnetic-field Data

We conducted a high-resolution airborne geophysical survey at the eastern margin of the basin-and-range province of the southwestern U.S. to examine the hydrostratigraphy of the Hueco Bolson and identify possible ground-water resources in this arid area. The survey, flown in 2001 over a 372-km2 area east of El Paso, Texas, acquired time-domain electromagnetic induction (TDEM) and magnetic field data. These data were used to map conductivity trends to depths of at least 200 m that are related to lateral and vertical changes in lithology, water content, water chemistry (EM data), basin geometry, and the location of intrabasinal faults (magnetic data). Pre-survey, ground-based TDEM soundings established the achievable exploration depths and demonstrated that the relatively deep groundwater (80 to 120 m) significantly influenced the transient signal and was within the exploration depth of the airborne system. Airborne EM and magnetic field data identified intrabasinal faults that influence basin-fill deposition. Conductivity-depth slices constructed from airborne TDEM data allowed lateral variations in water quality and lithology to be mapped that helped predict ground-water resource quality within these fault-bounded basins filled with alluvial, lacustrine, and eolian sediments.

Integrated geophysical survey for 3D modelling of a coastal aquifer polluted by seawater

ABSTRACTGeophysical surveys are carried out in the coastal area of Petrosino (south‐western Sicily) to study the time evolution of seawater contamination of the coastal aquifer, probably increased due to human impact. The overexploitation of the aquifer, due to an intensive agricultural use has affected significantly the natural hydro‐geochemical state of the basin. The study is based on a processing and integrated analysis of hydrogeological, geochemical and geophysical data. In particular in the last two years seasonal time‐lapse electrical resistivity tomographies (ERT), new TDEM soundings and Multi‐Analysis Surface Wave soundings (MASW) have been carried out. The interpretation of the total set of previously existing and new geophysical data made it possible to reconstruct a three‐dimensional model of the electrical resistivity of the aquifer, aimed at defining the extent and geometry of the seawater intrusion. Furthermore, the execution of a series of high‐resolution time‐lapse electrical tomographies and a correlation analysis between geophysical measures and geo‐chemical, geological and hydrogeological data allowed to discriminate the effects of the salt concentration in the groundwater and the porosity and saturation degree of the rock on the time variations of the measured electrical resistivity. Finally, the average porosity of the rocks forming the reservoir was determined.

TDEM survey in an area of seismicity induced by water wells in Paraná sedimentary basin, Northern São Paulo State, Brazil

This article presents TDEM results from an area with recent induced shallow seismicity. The purpose was to do a geoelectrical mapping of sedimentary and fractured basaltic aquifers for better understanding of the hydrogeologic setting. The study area is in the Paraná basin where flood basalts are overlain by sedimentary units near the city of Bebedouro, northern São Paulo State, Brazil. 86 TDEM soundings were acquired in an area of 90 km2 in the Andes and Botafogo study areas. The soundings were chosen next to wells for calibration, and also along profiles crossing the seismically active areas. 1-D interpretation results showed the general geoelectrical stratigraphy of this part of the Paraná basin. The upper geoelectrical layer is the shallow sedimentary aquifer (Adamantina formation) with less than 80 m thickness. The second geoelectrical layer contains the upper basalts of the Serra Geral formation at about 60–80 m depths. A saturated fractured basalt zone between 100 and 300 m depths was identifiable on various TDEM soundings. This depth range corresponds to the range of hypocentral depths for more than 3000 micro-earthquakes in this area. The lower basalt layer was estimated to lie between 400 and 650 m depth. The deepest geoelectrical layer detected by various TDEM soundings corresponds to the Botucatu sandstone (Guarani aquifer). Results suggest that the high-discharge wells are located in the fractured zone in the middle basalt of the Serra Geral formation. There is a good correlation between seismically active areas, high discharge wells (> 190 m3/h), and fracture zones in the middle basalt. The results reinforce the hypothesis that the shallow seismic activity in the Bebedouro region is being triggered by high rates of groundwater withdrawal.

Applying of the transient electromagnetic method to exploration of oil and gas fields in the area of Grabownica Starzeńska - Humniska - Brzozów

The paper presents the geologic and geoelectric characteristics of the Grabownica Starzenska – Humniska – Brzozow area (Polish Eastern Carpathians) based on electric logs and downhole geologic data. Synthetic TDEM sounding curves were calculated based on electric log data from four boreholes from the study area. The synthetic sounding curves were analysed from the point of view of possible interpretation of high-resistivity layers that could be prospective for oil and gas exploration. The 1D Occam interpretation of chosen TDEM soundings from profiles that ran across the Humniska Brzozow-7 borehole, was performed. The integrated data interpretation was made to estimate the capability of TDEM method to map hydrocarbon deposits at the study area.

Two Stages of Geological Evolution of East Asia Since Permian

This article presents TDEM results from an area with recent induced shallow seismicity. The purpose was to do a geoelectrical mapping of sedimentary and fractured basaltic aquifers for better understanding of the hydrogeologic setting. The study area is in the Paraná basin where flood basalts are overlain by sedimentary units near the city of Bebedouro, northern São Paulo State, Brazil. 86 TDEM soundings were acquired in an area of 90 km2 in the Andes and Botafogo study areas. The soundings were chosen next to wells for calibration, and also along profiles crossing the seismically active areas. 1-D interpretation results showed the general geoelectrical stratigraphy of this part of the Paraná basin. The upper geoelectrical layer is the shallow sedimentary aquifer (Adamantina formation) with less than 80 m thickness. The second geoelectrical layer contains the upper basalts of the Serra Geral formation at about 60–80 m depths. A saturated fractured basalt zone between 100 and 300 m depths was identifiable on various TDEM soundings. This depth range corresponds to the range of hypocentral depths for more than 3000 micro-earthquakes in this area. The lower basalt layer was estimated to lie between 400 and 650 m depth. The deepest geoelectrical layer detected by various TDEM soundings corresponds to the Botucatu sandstone (Guarani aquifer). Results suggest that the high-discharge wells are located in the fractured zone in the middle basalt of the Serra Geral formation. There is a good correlation between seismically active areas, high discharge wells (> 190 m3/h), and fracture zones in the middle basalt. The results reinforce the hypothesis that the shallow seismic activity in the Bebedouro region is being triggered by high rates of groundwater withdrawal.

Improvement in 1D TDEM Interpretation by Simultaneous Inversion of Data from Two Loop Configurations

This paper presents simultaneous inversion performed on data from central loop and offset receiver TDEM soundings. It is demonstrated that central loop and offset soundings complement each other and that the simultaneous 1D inversion of both data sets (1) yields rapid convergence and better resolution of the model parameters, (2) reduces the importance of distortion caused by induced polarization on interpretation, and (3) provides a good indication of the subsurface geometry in the measurement zone. Simultaneous inversion of central loop and different offset data sets are initially performed over a layered earth. The simultaneous inversion drastically improves interpretation obtained from the long offset data alone, but yields no noticeable improvement for the short offset sounding interpretation. Individual and simultaneous interpretation of central loop and offset soundings are then compared over a layered earth with a polarizable surface layer. The simultaneous interpretation reduces the importance of IP distortion affecting the central loop data and yields a more reliable model. Finally, central loop and offset data are calculated over a 3D conductive body embedded in layered host and individual and simultaneous inversions are performed. Lack of convergence and mediocre fit in simultaneous inversion indicate the presence of lateral conductivity variations. The simultaneous inversion is applied to two field examples from data collected in the Chalco Sub-Basin (Mexico). The first example consists in simultaneous inversion of two neighboring central loop soundings over an earth with no significant lateral conductivity changes. A slight enhancement in the resolution of model parameters is observed, but no substantial improvement in terms of misfit error or number of iterations is achieved compared to the single-site inversion. The second example shows simultaneous inversion of central loop and offset TDEM soundings when lateral conductivity changes occur. The combination of the two soundings never yields a final model that satisfies both data sets, showing the presence of a 2D or 3D heterogeneity in the vicinity of the two sounding sites.