TEM Soundings Research Articles

Cryohydrogeological settings appraisal by shallow near-field TEM sounding: a case study of the western part of the Yakut artesian basin

Cryohydrogeological settings appraisal by shallow near-field TEM sounding: a case study of the western part of the Yakut artesian basin

Petrophysical substantiation of geological section elastic-velocity property recovery potential based on TEM data

The geological section of Eastern and Western Siberia (Russia) is a very complicated object for seismic exploration. The research presented in the article is aimed at studying a petrophysical relation between electrical resistivity and P-wave velocity, as a basis for predicting the velocity model of the upper part of the section based on TEM sounding. Having performed a numerical modeling of petrophysical function, the authors calculated the dependency curves of electrical resistivity on the P-wave velocity. The obtained results of mathematical modeling and field experiments have proved the effectiveness of the proposed methodology as it increases the accuracy of geological model construction and enhances prediction reliability. Based on the dependences obtained, conclusions were drawn about the geological conditions favorable for a steady transition of section geoelectric characteristics to the velocity ones. The proposed technology is shown to provide a reliable reconstruction of the velocity model of the upper part of the section. The use of the developed methodology allows to improve the quality of seismic data processing and increase the accuracy of geological section boundary mapping at minimum cost based on the nature of the problem under investigation.

Multiscale geoelectrical characteristics of seawater intrusion along the eastern coast of the Gulf of Aqaba, Jordan

Jordan's eastern coast of the Aqaba gulf is an example of seawater intrusion in extremely arid conditions and escalating urban development. The presented research is the first to use a large dataset of ground–based transient electromagnetic, which consists of 160 TEM soundings of various loop dimensions (100 m × 100 m, 50 m × 50 m, and 25 m × 25 m) that were conducted in a quasi–grid layout (35 km2 surveyed area), and also with dense TEM sites combined along three ERT profiles. To obtain valid and reliable resistivity interpretation, geological and hydrogeological analyses of existing boreholes were used in the interpretation of multiscale dimensions of geophysical data, such as 1D–quasi 3D–inversion of TEM data and 2D inversion of ERT data.1D TEM resistivity models (at depths ranging from 40 to 360 m) show potential seawater intrusion of several kilometers from the gulf of Aqaba into a terrestrial unconfined sandy aquifer of resistivity <3 Ω·m. A resistivity >3 Ω·m and <70 Ω·m, on the other hand, best describes brackish seawater–bearing sediments with a wide range of total dissolved solids (1400–7000 mg/l). Resistivity models suggest upwelling along faults where the deep terrestrial semi–confined Kurnub sandstone aquifer is uplifted and salinized from direct seawater intrusion and/or sub–Cretaceous aquifers. The combined interpretation of resistivity models along profiles (ERT and TEM data) near coast areas demonstrated that the clastic sequence is a heterogeneous medium at the survey scale, with saline water–bearing (0.6–1 Ω·m) sediments having a very sharp interface with brackish water–bearing (10–30 Ω·m) sediments. Several geoelectrical signatures appear in the intrusion zone as an irregular pattern like alluvial sediments thickening and abrupt lateral resistivity changes. These patterns have been associated to lithological or structural control factors, which allowed the explanation of seawater intrusion characteristics.

Identification of Groundwater Bearing Zones Using Geoelectrical and Electromagnetic Techniques at Tourah Area, South of Cairo - Egypt

The complex geology of Tourah area, south Cairo is a serious challenge for groundwater exploration, where the risk of unsuccessful groundwater drilling excavation for industrial purposes is well dressed, raising the need for better geophysical subsurface detection and characterization approaches in terms of water bearing and aquitard zones. Datasets from the study area were acquired using integrated geophysical techniques comprising of one Dimensional Vertical Electrical Soundings (1D VES) and one Dimensional Transient Electromagnetic (1D TEM) as well as two-Dimensional Electrical Imaging (2D ERI) which was restricted to the available spreading space as well as subsurface infrastructure noise. The results of 1D VES and 1D TEM soundings detected three to five geoelectric layers used to generate geoelectrical cross sections and maps of resistivity, thickness, and depth to water bearing zone. Moreover, the 2D ERI inversion profiles were able to image the first three layers and the depth of the water bearing zone [A] with significantly greater resolution due to their higher lateral and vertical resolutions compared to the traditional 1D VES and TEM interpolated cross section. The integration of the three geophysical methods displayed a smooth distribution of both marly limestone and fractured shaly limestone and claystone bands water bearing zones [A and B]. Resistivity values of the water bearing zone [A] ranges from 0.1 to 24 Ω m, with average thickness of 4 to 12 m, and depth to water of 1.5 to 13.6 m. In addition, [zone B] is the main water bearing zone of fractured shaly limestone and claystone bands (2 to 14 Ω m) which is detected at deep depth of 40 and 75 m. Two normal fault systems inferred from 1D interpretation are considered responsible for local recharging of the main Eocene aquifer over the study area. In conclusion, the outlined water bearing thickness as well as limited sources of groundwater recharge indicate that these water bearing zones may not provide a sustainable supply for the industrial development purposes in the future.

Three-dimensional time-lapse inversion of transient electromagnetic data, with application at an Icelandic geothermal site

SUMMARY 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.

Dealing with automation and optimization problems when processing data measured by array with fixed transmitter in interpretation system &amp;lt;TEM-IP&amp;gt;

The paper discusses upgraded version of the computer system for joint inversion transient electromagnetic data in conductive and polarizable geological medium for squared and multi-components TEM soundings. New version is dealing with batch inversion challenges and direct problems optimizations.

Investigation of the structural control of a deltaic valley with geophysical methods. The case study of Pineios river delta (Thessaly, Greece)

The current study aims to clarify the structural regime of the deltaic valley of Pineios river (Thessaly, Greece). The structural control of a deltaic area is usually a crucial parameter for its Palaio-geographical evolution and the latter needs to be clear for the contemporary conservation of such a sensitive ecotope environmentally. The investigation of the concealed subsurface tectonic structures was accomplished through the combined interpretation of gravity measurements, VES and TEM soundings. The standard gravity data reduction has been carried out and the residual anomaly was isolated with the contribution of the Fourier filters. The Euler deconvolution has been applied, providing the corresponding depth solutions between 159.8 and 1716.6 m. In the context of the qualitative interpretation, we produced several structural maps (THDR, VDR, Tilt and Theta) in order to enhance the edges of density sources that may reflect fault zones. Severe indications for the delineation of fault zones of the area were provided by these maps. Moreover, 3D density models of the area have been constructed illustrating the subsurface density distribution, up to depths of 3370 m. A main zone of lower densities in the central part of the delta has been revealed, surrounded by three zones of higher densities. Afterwards, the densities of the majority of the existing geological formations were determined with laboratory measurements from geological specimens. Therefore, three geophysical-geological profiles have been constructed, based not only on the gravity modelling but also on the geological interpretation of the geoelectrical layered models from the VES and TEM measurements. In conclusion, concealed and unknown tectonic structures of the Pineios deltaic valley have been identified beneath the surface.

Characterization of Valley-Fill Alluvial Aquifers in Plutonic and Volcanic Semi-arid Andes Using Electromagnetic Methods: The Case of the Limarí Catchment (Chile)

Many areas of the world are facing sustained periods of water resource stress during which the enhanced exploitation of groundwater is key to maintaining irrigation and drinking water supplies. A good knowledge about groundwater resources is therefore essential to develop sustainable water management strategies. In this study, we aimed to characterize a mountainous watershed in the semi-arid Chilean Andes. The area of interest is distinguished by a high topographic gradient and narrow valleys filled with sedimentary deposits of various origins and surrounded by plutonic and volcano-sedimentary rocks. To characterize the hydrostratigraphy of this complex sedimentary system and to estimate the volume of groundwater stored, we implemented a multidisciplinary approach integrating geophysical data from transient electromagnetic sounding (TEM), hydrogeological, geological, geomorphological and groundwater quality information. The results indicate the presence of two aquifer layers in the majority of the investigated area: a superficial unconfined aquifer and a deeper confined (or semi confined) aquifer. We found that the width and depth of the sedimentary deposits increase with decreasing topography, while the proportion of fine material increases, in coherence with the sedimentation processes. Finally, we quantified the groundwater contribution of the different areas of the catchment and identified the main aquifer potential area in the pediplanes of the coastal mountain range (storing approximately 67% of the water available for extraction). The main contributions to the total uncertainties on the groundwater storage (ranging between 30 and 80% of the estimated volumes) are due to the propagation of the uncertainty on the thickness and porosity/specific yield of the modeled hydrostratigraphic layers. Due to the large spacing between TEM soundings and the limited number of stratigraphic bore logs in part of the studied area, the obtained characterization should be integrated with additional data for precise borehole sittings. Nevertheless, the implementation of TEM allowed to cover an extensive area and to reach large depth of exploration, so that it was possible to extract general information about the hydrostratigraphy of the different areas of the catchment.

Forearc density structure of the overriding plate in the northern area of the giant 1960 Valdivia earthquake

Abstract. The objective of this work is to analyse the density structure of the continental forearc in the northern segment of the 1960 Mw 9.6 Valdivia earthquake. Regional 2D and local 3D density models have been obtained from available gravity data in the area, complemented by new gravimetric stations. Models are constrained by independent geophysical and geological information and new TEM and MT soundings. The results show a segmentation of the continental wedge along and perpendicular to the margin, highlighting a high-density anomaly, below the onshore forearc basin, that limits the late Paleozoic–early Mesozoic metamorphic basement in the region where Chaitenia terrane has been proposed. A progressive landward shift of this anomaly correlates with the high slip patch of the giant 1960 Mw 9.6 Valdivia earthquake. Based on these results, we propose that the horizontal extension of the less rigid basement units conforming the marine wedge and Coastal Cordillera domain could modify the process of stress loading during the interseismic periods, and also that changes in position and extension of the late Paleozoic–early Mesozoic accretionary complex could be linked with the frictional properties of the interplate boundary. This analysis provides new evidence of the role of the overriding plate structure in the seismotectonic process in subduction zones.

Integrated TEM and 2DERI techniques to delineate groundwater bearing zones in fractured carbonate rocks at the upstream portion of wadi Halazeen, Northwestern Coast, Egypt

The increasing reliance on groundwater as supplementary water resource for the decreasing rainfall in the Northwestern Coastal Zone of Egypt (NWC) makes groundwater exploration and aquifers characterization in that area of particular importance. A representative locality for the conditions under which groundwater occurs in NWC is the upstream portion of Wadi Halazeen, which lies some 37 km to the west of Mersa Matruh city. In the present work, integrated Transient electromagnetic (1DTEM) and two-dimensional electrical resistivity imaging (2DERI) has been conducted to explore and characterize the groundwater of Miocene aquifer in the upstream portion of Wadi Halazeen. A total of 25 TEM soundings along with 5 normal and reciprocal profiles of 2DERI were measured. The lithological data of the borehole has been used to parameterize the inversion of the geophysical data. In order to maximize the reliability of the inversion results, the acquired data is subjected to some processing such as smoothing, editing and removing depending on TEM sounding significance plus computing reciprocal error of 2DERI measurements. The inversion results of both geophysical methods indicated that the geoelectrical succession consists of five layers along with the surface weathered layer (Dolomitic limestone, Sandy limestone, limestone, marly limestone and clay). The results indicated also that facies change is a common feature characterizes most of the layers. This feature controls groundwater occurrence within the fifth layer where the lithological composition of this layer changes from fractured limestone to marly limestone to marl and clay. Groundwater exists in a channel-like zone of fractured limestone and marly limestone which extends from south to north in the study area. Generally, the groundwater salinity of the study area and neighboring groundwater wells tapping the fractured Miocene aquifer are range between 2900 and 4268 ppm, while the groundwater of DRC5 well which lays in the watershed area has a relatively lower salinity value (1482 ppm) where it receives large recharge related to annual precipitations and surface runoff water than area of outside basin boundary divides. Integration of the results of 1DTEM and 2DERI delivered plausible information about the groundwater occurrence in fractured carbonate rocks.

Three dimensional inversion of magnetotelluric (MT) resistivity data from Reykjanes high temperature field in SW Iceland

The resistivity structure of Reykjanes is presented, derived from a three dimensional (3D) model developed in 2016. The model is the final results of a 3D inversion of MT data, corrected for static shift by joint inversion with TEM soundings at the same location as the MT soundings. In the model presented here (REYSAND), a total of 136 sounding pairs are used for the inversion. The model covers Reykjanes area and Sandvík potential geothermal area. The resistivity structure of a high enthalpy system reflects the thermal alteration of the rock. The Reykjanes models reveal a conventional resistivity structure for a high enthalpy geothermal system, i.e. a low resistivity cap underlain by a high resistivity core. The low resistivity cap reaches surface in the hot spring area, Gunnuhver, and dips down in all directions forming an elongated area of 2.5 km × 3 km in strike direction down to 1 km depth bounded by the low resistivity. The resistivity of the low resistivity cap is less 3 Ωm and is underlain by a high resistivity core with resistivity of 10–30 Ωm down to 3 km depth. At greater depth, high resistivity anomalies are prominent with a zone of lower resistivity, in strike direction, under the main field. This zone coincides with an aseismic zone at depth and may indicate a fracture zone with higher temperature and/or permeability within the geothermal system. Comparison to data from the IDDP-2 drilling as well as seismic monitoring confirms that suggestion.

Krýsuvík high temperature geothermal area in SW Iceland: Geological setting and 3D inversion of magnetotelluric (MT) resistivity data

The Krýsuvík area in SW Iceland is characterized by a 40 km long and around 6–8 km wide fissure swarm and two NE-SW trending tindars complexes. Associated with the fissure swarms are fumarole fields located primarily on parallel postglacial eruptive fissure segments. There are indications that Krýsuvík hosts a buried caldera and an intrusion complex in its roots. The geothermal surface manifestations cover an area of around 50–60 km2 and has potential for future exploitation. Faulting accompanies the ridges and postglacial volcanic fissures, with normal faults and open fissures with a maximum throw of 10 m.A 3D inversion was performed of the static shift corrected off-diagonal impedance tensor elements of 102 MagnetoTelluric (MT) resistivity soundings from the Greater Krýsuvík geothermal area. In the inversion, 21 periods were used, evenly distributed from 0.01 to 100 s on a logarithmic scale. The robustness of the inversion was tested by using three different initial models: A model compiled from a joint 1D inversion of TEM and MT soundings, with a homogeneous Earth of resistivity 100 Ω m and a homogeneous Earth of resistivity 10 Ω m. A flat topographic surface was assumed in the 3D inversion. The resulting models were later elevation corrected. The results of the 1D inversion and the 3D inversion are strikingly similar, where the 1D inversion reproduces the main resistivity structures while the 3D inversion shows considerably more details. An analysis of the electrical strike directions based on the vertical magnetic field data is in fairly good agreement with the final resistivity model.The subsurface resistivity structure in Krýsuvík has the same main features as other high temperature areas in Iceland and in general where the host rocks are basaltic. Above a shallow conductive cap, a high resistivity zone is seen, reflecting unaltered rock. The conductive cap reflects smectite hydrothermal mineral alteration. Below the low-resistivity cap, a resistive core is found, reflecting chlorite-epidote alteration. Good correlation is observed in the Krýsuvík field between the subsurface resistivity structure and the hydrothermal alteration revealed by boreholes cuttings. Parts of the field have cooled down and therefore the resistivity structure indicates alteration mineralogy but not necessarily the present rock temperature. The geothermal up-flow zones are therefore most likely where the hydrothermal alteration and the resistive core reach the highest elevation.The 3D inversion shows an indication of a relatively deep-seated conductive body below the central part of the high temperature area. This body coincides horizontally with the centre of the 2010–2011 inflation source at 4–5 km depth. It has been suggested that the 2007–2016 inflation/deflation periods are linked to gas flux, as no signs of S-wave attenuation have been found. It is therefore unlikely that the deep-seated conductive body resolved by our resistivity models comprises partial melt. The body is probably connected to the heat source of the geothermal field and could be due to emission of gas or dehydration.

Analysis of Long-offset Transient Electromagnetic (LOTEM) Data in Time, Frequency, and Pseudo-seismic Domain

Long-offset transient electromagnetic (LOTEM) has received great attention in mineral, hydrocarbon and hydrogeological investigations for the last several years. Conventionally, TEM soundings have been presented as apparent resistivity curves as function of time. With development of sophisticated computational algorithms, it became possible to extract more realistic geoelectric information by applying inversion programs to 1-D and 3-D problems. Here, we analyze LOTEM data by carrying out analysis in time, frequency and pseudo-seismic domain supported by borehole information. At first, H, K, A &amp; Q type geoelectric models are processed using a proven inversion program (1-D Occam inversion). Second, time-to-frequency transformation is conducted from TEM ρa( f) curves for the same models based on all-time apparent resistivity curves. Third, 1-D Bostick's algorithm was applied to the transformed resistivity. Finally, EM diffusion field is transformed into propagating wave field and constructed pseudo-seismic section. The transformed seismic-like wave indicates that some reflection and refraction phenomena appear when the EM wave field interacts with a geoelectric interface at different depth intervals due to contrast in resistivity. In all three cases, synthetic tests showed that conductive anomaly in resistive host environment can be retrieved more clearly than resistive target. A case study illustrates the successful application of proposed approach in recovering a water-filled mined-out area in a coal field located in Ye county, Henan province, China. The results support the introduction of pseudo-seismic imaging technology in long-offset version of TEM which can also be an useful aid if integrated with seismic reflection technique to explore possibilities for high resolution EM imaging in future. [Figure: see text]

Geophysical and Hydrogeological Evaluation of Pliocene Aquifer in East Esna, Egypt

The current study of East Esna area was selected due to its prosperous conditions. In this area, the reclamation of agricultural land is increasing and the population is growing, which necessitate an equivalent development of groundwater. The main aim of the study was to estimate geometrical and qualitative characteristics of the study aquifer. This will help to have a systematic view of the hydrogeological setting in the area of investigation, categorize and evaluate the influential factors of existence, quality, and protection of the groundwater. The geometrical characteristics of the local aquifer were revealed by using 45 VES and TEM soundings. The study area has two main aquifers. Both hosted in sandstone of Issawia formation. The brackish groundwater lies above the fresh groundwater, which is shielded by Esna shale at the bottom. The source of feeding to these aquifers is direct leakage of runoff and rain on the east side with sporadic leaks from the waters of the River Nile on the west side. The analyzed groundwater samples are geochemically homogenous, indicating that their genesis is rain water. They also belong to Na–Ca–SO4–Cl type. The groundwater in the study area is assessed for drinking, household, livestock, and agricultural purposes. The current study recommends some advises for groundwater development in the study area.

Delimiting a saline water zone in Quaternary fluvial\u2013alluvial deposits using transient electromagnetic: a case study in Punata, Bolivia

The semiarid Punata alluvial fan is located in the central part of Bolivia. The main activity of this region is the extensive agriculture, and groundwater is the main water supply. Local villagers who use groundwater reported that in some places groundwater has a salty taste. In order to investigate the origin of this problem, several TEM soundings were performed in the study area, and they were complemented with ERT surveys. The results show top layers with resistivity values ranging from 30 to 200 Ωm and a bottom layer with resistivity values ranging from 1 to 20 Ωm, which might be interpreted as the main aquifer and a layer with high clay content, respectively. Between the top and bottom layer, a transition zone with saline water has been identified, with resistivity values ranging from 0.1 to 1 Ωm. The origin of this closed-basin brine might be a product of the evaporation of paleolakes during the lower Pliocene, where saline clays were deposited. This study demonstrated the effectiveness of TEM sounding for mapping very low resistivity zones such as saline water.

The Effects of Negative Values of Apparent Resistivity in TEM Surveys

The calculation of apparent resistivity in electrical methods is relatively simple and consolidated in the literature. Furthermore, in the case of DC methods, the apparent resistivity values are for the most part, intuitive and enable an initial interpretation of the results. On the other hand, in TEM method the apparent resistivity values are not very intuitive and interpretation based only on the apparent resistivity values cannot be done reasonably. In this way, this paper presents a discussion about the reasons and effects of the negatives values of apparent resistivity in TEM soundings. The main objective of this paper is to clarify the meanings of these negatives values and the possible effects in TEM inversion.

TEM surveys for search of taliks in areas of strong fast-decaying IP effects

Lenses of water-saturated unfrozen rocks (taliks) in permafrost are important sources of freshwater in high-latitude regions. Taliks stand out against the host frozen rocks in much lower resistivity and thus are detectable by resistivity surveys. TEM soundings are especially efficient in this application as they can go without galvanic grounding, have small offsets, and are sensitive to buried conductors. Early-time TEM data in the Taz area of the Yamal-Nenets district bear strong effects of fast-decaying inductively induced polarization (IIP), which rules out the use of nonpolarizable earth assumption for their interpretation. The TEM responses are inverted by means of the TEM-IP software using the model of a polarizable earth with Cole-Cole complex frequency-dependent conductivity. The resulting earth model mainly includes three layers, with a 100 to 250 m thick highly resistive polarizable upper layer. The polarization parameters of the layer (chargeability, time constant and exponent) are typical of frozen sedimentary rocks, while the presence of a talik reduces notably the effective resistivity and chargeability. This feature can be used as a guide to taliks, as it was confirmed by TEM surveys and subsequent drilling.© 2014, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

Geoelectrical characterization using joint inversion of VES/TEM data: A case study in Paraná Sedimentary Basin, São Paulo State, Brazil

This article presents the results of re-interpretation of VES and TEM soundings data set acquired in Bebedouro region in São Paulo State — Brazil. A total of 46 VES/TEM soundings were acquired in a region of 90 km 2 located in the Paraná Sedimentary Basin . This article has two main goals, compare the joint inversion and the single inversion of VES and TEM in two large areas and obtain a new and more precisely geoelectrical model of the Bebedouro region, focusing the exploration in the shallow and deep sedimentary aquifers of the study area. The previous interpretation was based in geoelectrical models obtained by single inversion of the VES and TEM soundings. In this work we present the results with single inversion of VES and TEM sounding inverted by the Curupira Program ( Bortolozo and Porsani, 2012 ) and a new interpretation based in the joint inversion of the both methodologies. The results from joint inversion permit to obtain a more reliable geoelectrical model and serve as an accurate guide to the exploration of groundwater. • Use of joint inversion of VES and TEM soundings for hydrogeological purposes • Compare the joint inversion and single inversion of VES and TEM in two large areas • Obtain a new and more precise geoelectrical model for water exploration

Three-dimensional interpretation of TEM soundings

We describe the approach to the interpretation of electromagnetic (EM) sounding data which iteratively adjusts the three-dimensional (3D) model of the environment by local one-dimensional (1D) transformations and inversions and reconstructs the geometrical skeleton of the model. The final 3D inversion is carried out with the minimal number of the sought parameters. At each step of the interpretation, the model of the medium is corrected according to the geological information. The practical examples of the suggested method are presented.

On the estimation of the maximum depth of investigation of transient electromagnetic soundings: the case of the Vizcaino transect, Mexico

We test an approach proposed in the literature for estimating the maximum depth of investigation (MDI) of in-loop transient electromagnetic soundings (TEM) with data from the Vizcaino transect, a profile of 38 TEM soundings crossing the Baja California peninsula. The validity of this approach is confirmed by comparing the MDI with the deepest interface of 16 stratified models. In these soundings, all located over the Vizcaino basin, the measured data are not affected by induced polarization. The models indicate the presence of a dipping conductor interpreted as a zone of seawater intrusion with a large lateral extension of over 70 km. The remaining 22 soundings, located over igneous and metamorphic rocks, show reversals in the voltage polarity, indicating the presence of induced polarization effects. The layered Cole-Cole models for these soundings suggest a significant decrease in the MDI. This is confirmed by analyzing the depth behavior of the subsurface current densities. We further analyze the noise level of a data set comprising close to 2000 late-time voltages of about 400 TEM sites acquired in northwestern Mexico. No difference was found between the stationary noise levels of winter and summer, presumably because near thunderstorms are practically absent in this part of Mexico.