Transient Electromagnetic Measurements Research Articles

Sensitivity analysis of inverted model parameters from transient electromagnetic measurements affected by induced polarization effects

We investigate the application of the distance-based global sensitivity analysis (DGSA) to evaluate the sensitivity of electrical model parameters obtained from transient electromagnetic (TEM) data including induced polarization (IP) effects. We propose novel open-source forward modeling and inversion routines for single-loop TEM data including IP effects with the maximum phase angle model to model the frequency dependence of the complex resistivity. In a first step, we evaluate the accuracy of our forward modeling and inversion routines using numerical studies, where the actual variations of layer thicknesses and resistivities, as well as the frequency dependence of the complex resistivity is known. In a second step, we extend our investigation to field data and apply our approach to three distinct case studies in layered media: 1) a confined aquifer corresponding to conductive non-polarizable media, 2) a graphite deposit corresponding to highly conductive and polarizable anomalies in a resistive host rock and 3) an ice glacier corresponding to highly resistive polarizable media. Our DGSA results reveal that standard depth of investigation (DOI) approaches may overestimate the true sensitivity of the model obtained from the inversion. TEM data collected in conductive layered media without IP effects show reduced sensitivity above the predicted DOI. The case studies in polarizable media demonstrate that the maximum phase angle is more influential on the TEM model response than the relaxation time and dispersion coefficient. Our DGSA results for polarizable media reveal that TEM field data collected at the graphite deposit and at the ice glacier are sensitive to the geometry of the polarizable layer.

Rapid Saline Permafrost Thaw Below a Shallow Thermokarst Lake in Arctic Alaska

AbstractPermafrost warming and degradation is well documented across the Arctic. However, observation‐ and model‐based studies typically consider thaw to occur at 0°C, neglecting the widespread occurrence of saline permafrost in coastal plain regions. In this study, we document rapid saline permafrost thaw below a shallow arctic lake. Over the 15‐year period, the lakebed subsided by 0.6 m as ice‐rich, saline permafrost thawed. Repeat transient electromagnetic measurements show that near‐surface bulk sediment electrical conductivity increased by 198% between 2016 and 2022. Analysis of wintertime Synthetic Aperture Radar satellite imagery indicates a transition from a bedfast to a floating ice lake with brackish water due to saline permafrost thaw. The regime shift likely contributed to the 65% increase in thermokarst lake lateral expansion rates. Our results indicate that thawing saline permafrost may be contributing to an increase in landscape change rates in the Arctic faster than anticipated.

A feasibility study on time-lapse transient electromagnetics for monitoring groundwater dynamics

Time-lapse geophysical studies have been used as noninvasive tools to study subsurface processes. However, compared to direct current resistivity methods, the transient electromagnetic (TEM) method has not been explored in much detail for time-lapse investigations. Here, we introduce the use of a monitoring TEM (mTEM) system and methodology for long-term groundwater monitoring purposes. The system can run measurements automatically and the modeling approach uses a time-constrained inversion (TCI) scheme based on an existing laterally constrained inversion scheme. Instead of lateral constraints that traditionally ensure lateral coherency between measurements, temporal constraints are used to control the temporal coherency between measurements. Synthetic analysis demonstrates that very tight time constraints on resistivities and a time constraint on the depth to the water table fixed at a fraction of the expected annual water table variation is needed to track water table changes at the decimeter level. Furthermore, the synthetic analysis indicates that the mTEM method can track annual variations in the water table with amplitudes as low as 0.6 m with a mean absolute error of 0.2 at depths between 16 m and 30 m. A field example consisting of 233 1D TEM measurements acquired over 10 months using the mTEM instrument supports the synthetic results. The estimated water table depth from the TCI scheme reproduces the variation in the water table accurately, as confirmed by measurements from a water level data logger in a nearby well. The differences between the results from the TEM monitoring system and the diver measurements are in the range of ±0.1 m with an average absolute difference of 0.05 m. This study demonstrates the potential to monitor changes in the groundwater level noninvasively.

Mapping Hydrogeological Structures Using Transient Electromagnetic Method: A Case Study of the Choushui River Alluvial Fan in Yunlin, Taiwan

We used transient electromagnetic (TEM) to map the hydrogeological structures in the Choushui River Alluvial Fan in Yunlin County of central Taiwan. A total of 63 TEM measurements were collected using the FASTSNAP system with 50 × 50 m in-loop configurations in the middle and distal fan. The 1D model, based on prior information, was constructed from the inverted soundings. Results showed a thin, resistive shallow layer and a 40 m low-resistive (6–42 ohm-m) zone beneath it. High-resistive zones (50–170 ohm-m) were found from 50–120 m depth, and low-resistive zones were revealed below 120 m in some areas. Results were consistent with resistivity and lithology logs from nearby wells. The inverted TEM models provide reliable subsurface information when prior informations of vertical electrical sounding and TEM were considered. We interpolated resistivity at 10 m, 50 m, 100 m, and 160 m depth from the 1D model results to produce a slice map of the area, which indicated variations, trends, and depths of the sediment deposits. The TEM method successfully identified the hydrogeological structures, showing that the upper 40 m of sediment acts as a confining layer for the aquifer structure from 50 m to 120 m depth.

The Potential of the Horizontal Component TEM Data in the Detection of Polarizable Mineral: Synthetic Cases

Induced polarization (IP) effects in transient electromagnetic (TEM) measurement play a significant role in the detection of minerals, such as sulfide ore, clay ore, etc. However, due to the complex impacts caused by the polarization parameters, the structure of the earth, and the measurement system configuration, the IP effects cannot be easily measured and interpreted. We paid more attention to the vertical component of the ungrounded-TEM system in previous work; however, we will now attempt to utilize the horizontal component to enhance the precision of the measurement and interpretation. We quantitatively discussed the behavior of the horizontal and vertical magnetic fields of the towed-TEM (tTEM) system. We simultaneously introduced the vertical and horizontal components into the Jacobian matrix of the 1D laterally constrained inversion to build the joint inversion. Compared to the vertical component, the horizontal component always shows earlier and bigger sign reversals and contains more IP information. Inversion with the single horizontal or single vertical component showed poor accuracy in the deep and shallow layers, respectively. The joint inversion with both components could correct the errors. Including the horizontal component in the detection and interpretation facilitates an improved resolution for polarizable minerals.

Inversion of TEM measurement data via a quantum particle swarm optimization algorithm with the elite opposition-based learning strategy

The fine interpretation and inversion of transient electromagnetic method measurement data have the problems of nonlinearity, multi-solution, and ill condition. However, the conventional particle swarm optimization (PSO) nonlinear inversion methods suffer from prematurity, slow convergence, and low calculation accuracy. To solve these problems, a quantum PSO (QPSO) algorithm based on the elite opposition-based learning (EOL) strategy is proposed. Firstly, three performances tests of the EOL-QPSO algorithm are carried out with Peaks, Schaffer and Rastrigin functions. The results show that the EOL-QPSO algorithm has excellent solution accuracy, efficient calculation speed and balanced exploitation and exploration capability. Secondly, the conventional PSO algorithm and the EOL-QPSO algorithm are used to compare the inversion of the theoretical model and the synthetic data with noise, and combined with Bayesian method, the posterior model probability statistics of the synthetic data are carried out. The research shows that the EOL-QPSO inversion algorithm is improved in terms of calculation accuracy, calculation efficiency, anti-noise performance and exploitation and exploration capability, and it can accurately obtain the posterior estimates of the real model. Finally, the inversion of field-measured data demonstrates that the EOL-PSO inversion method accurately reflects the position of the water-accumulated goaf.

Combined geophysical methods to investigate seawater intrusion in the Souss-Massa coastal area, Morocco

Groundwater are strategic water resources for irrigation and drinking purposes in the Souss-Massa semi-arid region (Morocco). These resources have been threatened during the last decades by different factors such as drought, over-pumping and marine intrusion. The latter is responsible for the degradation of the groundwater quality along the coastal area, which is reducing its usability. Therefore, the localisation of the freshwater/saline water interface is essential for better sustainable management of groundwater resources in the area. The objective of the present study is to assess the temporal changes, the extent, and the depth of the seawater intrusion in the Souss-Massa coastal area. To fulfil this objective, two monitoring sites were equipped with water conductivity measurement devices (Subsurface Monitoring Device – SMD) to observe the temporal changes of the salinity at a local scale, as well as to support the interpretation of a transient electromagnetic (TEM) survey. This survey consisted of 83 TEM soundings, along thirteen profiles located perpendicularly to the ocean and the two rivers delineating the study area. The data from the SMD stations highlight the temporal changes of electrical conductivity (EC) with depth, clearly illustrating the freshwater/saltwater interface location in the central part of the study area (−35.55 m NGM at the Tifnit location, ca. 1 km from the coast). In addition, the results of the TEM measurements show that the marine intrusion is already advanced towards the land, over a maximum distance of 2500 m. Finally, some new monitoring sites have been proposed to improve the current monitoring network for seawater intrusion and assess whether mitigation solutions lead to a slower progression of the seawater intrusion. Decision-makers can use these results as a basis for the development and implementation of effective policies to ensure a sustainable and integrated water resources management in this region.

Magnetic on-time transient electromagnetic method (MoTEM) for imaging magnetic susceptibility

SUMMARY We present the magnetic on-time transient electromagnetic (MoTEM) method for obtaining information about subsurface magnetic susceptibility from transient electromagnetic (TEM) measurements. The method uses the primary field produced by a TEM system as the inducing magnetic field and extracts from the measured late on-time response the magnetic responses produced by the subsurface magnetic susceptibility. The method assumes that the transmitter waveform is long enough to allow for the dissipation of eddy currents until the electric conductivity effect becomes negligible towards the end of the on-time transmission and that the measured magnetic field or its time derivative is dominantly the response to magnetic susceptibility. Using numerical simulations, we study the dissipation of the field produced by the conductivity anomaly for different times of the transmitter waveform and demonstrate that the signal becomes dominated by the susceptibility anomaly (MoTEM signal) at late on-time for both moving and fixed-loop sources. We present observational evidence for the presence of MoTEM responses by showing that they can be identified in field data measured from a fixed-loop ground TEM survey over the ultramafic units in the Raglan nickel belt. Our study lays the foundation for the MoTEM method, demonstrates that it can be realized with existing TEM acquisition systems, and provides the guidance for new system development. The MoTEM response could enable the mapping and imaging of subsurface susceptibility from the airborne EM data and, therefore, has the potential to double the information content in such data by imaging both conductivity and magnetic susceptibility.

Determination of formation dip and strike from transient LWD electromagnetic measurements

An original method has been developed for estimating formation dip and strike from transient induction LWD data, based on focusing in the time domain. The focusing consists in decomposing the measured signals into a time series and diagonalizing the matrix of focused magnetic field components. We have implemented the method and comprehensively tested it in horizontally-layered media used for LWD data inversion to solve geosteering problems and evaluate the formation resistivity. Estimates of the angles contribute to reliable geosteering when choosing a direction of drilling, as well as when inverting data for a complex earth model. A significant reduction in the resource intensity of inversion and model equivalence is achieved by reducing the number of determined parameters.

Innovative Surface-Borehole Transient Electromagnetic Method for Sensing the Coal Seam Roof Grouting Effect

With the purpose of monitoring coal seam roof grouting, an innovative surface–borehole transient electromagnetic (TEM) method to compare the Earth resistivity distribution of the coal seam roof before grouting and after grouting is proposed. The TEM secondary field time–decay curve of the proposed surface–borehole TEM method is compared with that of the traditional surface TEM method. It is found that the time–decay curve of the former is very different from that of the latter. For the first time, forward modeling algorithm has been proposed to evaluate the TEM response characteristics of Earth, with an inversion algorithm developed to invert the measured data into resistivity distribution. A field work has been implemented with the customized equipment to evaluate the surface–borehole TEM method. In the field work, the proposed surface–borehole TEM measurement is performed twice, i.e., before and after the coal seam roof grouting. The analysis results demonstrated that the resistivity distribution had a distinct change near the depth of grouting with low resistivity distribution replaced by high resistivity distribution, which validates the effectiveness of the proposed surface–borehole TEM method.

Research on Airborne Electromagnetic Whole-area Apparent Resistivity Imaging Algorithm in the Detection of Goaf in Rail Transit

Depth apparent resistivity imaging is an important process of data processing and analysis in the aviation transient electromagnetic method. It can provide reference value of conductor depth, vertical extension, and other information, and can accurately provide the measurement of each aviation transient electromagnetic measurement system. The structural section of the apparent conductivity of the one-dimensional layered medium on the line. As an advanced geophysical exploration technology, the aerial transient electromagnetic method has been applied significantly in the exploration of polymetallic minerals abroad in recent years. In this paper, based on the theory of ground-to-air transient electromagnetic method with multiple radiation sources, a corresponding multi-component global apparent resistivity definition method is established. The advantages of using the magnetic field strength to define the global apparent resistivity of the multi-radiation field source ground-air system are analysed. For each component of the magnetic field strength, respective global apparent resistivity algorithms are proposed to realize the multi-component, full-time, and full-space visual resistivity. The resistivity is calculated, and the influence of the offset on the global apparent resistivity is analysed. By adjusting the relative position of the source and the current direction and other parameters, the multi-radiation source transient electromagnetic ground-air system can not only strengthen the signal strength of different components, weaken random interference, but also better distinguish the location of underground anomalies

An efficient preprocessing method to suppress power harmonic noise for urban towed transient electromagnetic measurement data

Towed transient electromagnetic method (TEM) is an efficient geophysical method for urban underground space measurement. However, compared with the field environment, the power harmonic noise as urban electromagnetic noise is the key reason for the low signal-to-noise ratio (SNR) of the towed TEM data. We propose an efficient preprocessing method based on a multi-step processing strategy, which can fast suppress power harmonic noise and improve SNR. Firstly, a pure power harmonic noise sample is obtained by the least-square fitting. Secondly, the noise component of the measurement data that is most relevant to this sample is quickly searched by an improved genetic algorithm. Finally, the power harmonic noise component is subtracted from the measurement data, and the SNR is significantly improved. The simulation and field experiments show that the new method can fast and effectively suppress power harmonic noise without signal distortion, and it is especially suitable for towed data processing.

Geophysical Observations of Taliks Below Drained Lake Basins on the Arctic Coastal Plain of Alaska

AbstractLakes and drained lake basins (DLBs) together cover up to ∼80% of the western Arctic Coastal Plain of Alaska. The formation and drainage of lakes in this continuous permafrost region drive spatial and temporal landscape dynamics. Postdrainage processes including vegetation succession and permafrost aggradation have implications for hydrology, carbon cycling, and landscape evolution. Here, we used surface nuclear magnetic resonance (NMR) and transient electromagnetic (TEM) measurements in conjunction with thermal modeling to investigate permafrost aggradation beneath eight DLBs on the western Arctic Coastal Plain of Alaska. We also surveyed two primary surface sites that served as nonlake affected control sites. Approximate timing of lake drainage was estimated based on historical aerial imagery. We interpreted the presence of taliks based on either unfrozen water estimated with surface NMR and/or TEM resistivities in DLBs compared to measurements on primary surface sites and borehole resistivity logs. Our results show evidence of taliks below several DLBs that drained before and after 1949 (oldest imagery). We observed depths to the top of taliks between 9 and 45 m. Thermal modeling and geophysical observations agree about the presence and extent of taliks at sites that drained after 1949. Lake drainage events will likely become more frequent in the future due to climate change and our modeling results suggest that warmer and wetter conditions will limit permafrost aggradation in DLBs. Our observations provide useful information to predict future evolution of permafrost in DLBs and its implications for the water and carbon cycles in the Arctic.

Integrated use of well and geophysical data for constructing 3D geological models in shallow aquifers: a case study at the Tymbakion basin, Crete, Greece

3D geological models increase our understanding of the subsurface structures and the spatial distribution of sediment’s properties. The models can be applied in different aspects of geology and assist in reaching to more robust conclusions. In this study, a methodology for constructing a 3D geological model of the Tymbakion basin, Crete, Greece, is presented. The model is based on a dense array of newly acquired Transient Electromagnetics measurements (TEM), legacy Vertical Electrical Soundings (VES), and well data that cover an area of approximately 35 km2 and 500 m in depth. The model is representative of the Plio-Quaternary section, which hosts the Tymbakion shallow aquifer. The study area is one of the main agricultural sites of Greece with touristic activity. The model reveals the existence of 12 faults, which are not observed on the surface, and provides the lithological and resistivity distribution on the subsurface. The importance of the model relies in the fact that it gives a better insight into the hydrogeological setting of the shallow aquifer and the problems related to the intensive water exploitation during the last decades.

Application of Semi-Airborne Transient Electromagnetic Method in Ancient River Area

Semi-airborne transient electromagnetic method (SATEM) is a new type of transient electromagnetic measurement method. Compared with the ground transient electromagnetic system, the system has the characteristics of convenience, high efficiency, large detection range, repeatable observation, high lateral resolution, etc. At the same time, this ground-space model shows that this method is suitable for detecting missions in mountainous and undulating terrain, and has obvious advantages in dividing underground space structure. In this paper, the theory of semi-airborne transient electromagnetic method is systematically studied and forward modeling is used to simulate the diffusion law of the end value under different resistivity and the characteristics of the response curve under different influencing factors of G model. The identification capability of SATEM for multi-layer model is verified by theoretical inversion, which lays a foundation for the practical application of SATEM in an ancient river channel. Finally, the application of SATEM to an example of an ancient river channel verifies the practicability and efficiency of the method, and the detection results are in good agreement with the local geological data.

Application of TEM Based on HTS SQUID Magnetometer in Deep Geological Structure Exploration in the Baiyun Gold Deposit, NE China

Exploration of deep mineralization, particularly where the mineralization of interest is covered by a conductive overburden, is still a challenge for the conventional transient electromagnetic (TEM) method, which measures TEM response using induction coils as the sensor. However, sensors such as fluxgate and superconductive quantum interfere device (SQUID) magnetometers can measure the B-field directly, which can provide more reliable deep information for mineralization exploration. In this paper, we report on the research and development of our newly developed high-temperature superconductor (HTS) SQUID magnetometer, which is cooled by liquid nitrogen at 77 K, and its application in TEM measurement for deep exploration in a gold deposit in China. This improved SQUID magnetometer version has a good performance with noise (60 fT/ $$\sqrt {{\rm{Hz}}} $$ ), slew rate (0.8 mT/S), dynamic range (100 dB), sensitivity (6.25 mV/nT), and bandwidth (DC-20 kHz). To find deep and peripheral ore in the Baiyun gold deposit located in Liaoning Province, NE China, both the SQUID magnetometer and induction coil were used for TEM data acquisition. Results show that TEM can detect the distribution of local strata and the faults contained within them. Results also indicate that the SQUID magnetometer has superior response performance for response over geological targets with slower decay time when compared to the induction coil signals. The SQUID magnetometer is more sensitive at observing the induced-polarization effect which is closely related to the ore-controlling faults.

Choosing between Gaussian and MPS simulation: the role of data information content\u2014a case study using uncertain interpretation data points

Integration of geophysical data with information such as from boreholes and expert domain knowledge is often performed as cognitive or explicit geological modeling when generating deterministic geological models of the subsurface. However, such cognitive geological models lack the ability to express the uncertainty of layer boundaries. To remedy the shortcomings of this strategy we propose a novel stochastic methodology combining the efforts of probabilistic data integration and cognitive modeling. We treat geological interpretation points from the cognitive model as uncertain “soft” data. These data are then combined with analogous geology in a probabilistic model. We test two ways of combining and sampling from such a probabilistic model. Firstly, a high-entropy setup based on Gaussian distributions simulation. Secondly, lower entropy (and conceivable more realistic) geological solutions are obtained from multiple-point geostatistics (MPS). We apply both ways of solving the problem at a study site near Horsens, Denmark, where airborne transient electromagnetic measurements, seismic data, and borehole information are available and interpreted in cognitive modeling. We explain the complete framework for integrating the uncertain interpretations in geostatistical simulation. Results show that both the Gaussian simulation and multiple-point geostatistical approach allows satisfactory simulations of uncertain geological interpretations and are consistent with prior geological knowledge. Our results suggest that the number of uncertain data points and their information content play a pivotal role in selecting the most appropriate simulation method for the given framework. MPS simulations allow connectivity in scenarios with few data points due to the low entropy of the model. When the number of soft data increases, Gaussian simulation is less prone to produce simulation artifacts, faster to apply, and could be considered preferential to using MPS simulation.

Stratigraphy of the Gorstian and Ludfordian (upper Silurian) Hemse Group reefs on Gotland, Sweden

ABSTRACT The Hemse Group is one of the least understood stratigraphic units of the Silurian sequence of Gotland, Sweden. New results from airborne transient electromagnetic (ATEM) measurements in combination with previously published data from field studies and geophysical investigations shed new light on carbonate platform development during the early- to mid-Ludlow Hemse Group. ATEM reveals a transgressive phase that began near the Wenlock-Ludlow boundary, which resulted in deposition of marls and corresponds roughly to the Hemse limestone units a-c and the Hemse Marl NW. In this phase little or no reef development occurs. The end of the transgressive phase coincides with the weak Linde Event. The following highstand favoured extensive reef growth forming a reef barrier system of both fringing reefs and more rampiform settings with stromatoporoid biostromes and occasional biohermal buildups. The Kuppen-Snabben Unconformity Complex marks an erosional (karstic) sequence boundary and rocky shoreline and the transition from a rampiform setting with reef biostromes towards a more rimmed setting with patch reefs.

Double Trapezoidal Wave Transmitting System with Controllable Turn-Off Edge

For time domain transient electromagnetic measurement, the negative sign often appears in the polarization region, which contains the induced polarization information. It is considered that the polarization effect is caused by the capacitance charge of the earth. Extending the turn-off time of the emission current means increasing the charging time, and reducing the charging voltage, which makes the polarization effect easier to observe. Therefore, a double trapezoidal wave transmitting system with a controllable turn-off edge is designed in this paper. In the process of current transmitting, the turn-off time can be controlled by changing the clamping voltage depending on the passive clamping technology. By cutting into the absorption resistance, the current oscillation can be eliminated under the condition of ensuring linearity. To verify the effectiveness of the system, we designed a polarized wire loop based on the filament model simulating the polarized earth. Comparing the response of the wire loop, the emission current with short and long turn-off times contributes to inducing the induction and polarization fields respectively. The double trapezoidal wave transmitting system with a controllable turn-off edge is suitable for measuring the induced polarization effect.

Multi-Layered High Permeability Conduits Connecting Onshore and Offshore Coastal Aquifers

Groundwater resources in coastal regions are facing enormous pressure caused by population growth and climate change. Few studies have investigated whether offshore freshened groundwater systems are connected with terrestrial aquifers recharged by meteoric water, or paleo-groundwater systems that are no longer associated with terrestrial aquifers. Distinguishing between the two has important implications for potential extraction to alleviate water stress for many coastal communities, yet very little is known about these connections, mainly because it is difficult to acquire continuous subsurface information across the coastal transition zone. This study presents a first attempt to bridge this gap by combining three complementary near-surface electromagnetic methods to image groundwater pathways within braided alluvial gravels along the Canterbury coast, South Island, New Zealand. We show that collocated electromagnetic induction, ground penetrating radar, and transient electromagnetic measurements, which are sensitive to electrical contrasts between fresh (low conductivity) and saline (high conductivity) groundwater, adequately characterize hydrogeologic variations beneath a mixed sand gravel beach in close proximity to the Ashburton River mouth. The combined measurements - providing information at three different depths of investigation and resolution - show several conductive zones that are correlated with spatial variations in subsurface hydrogeology. We interpret the conductive zones as high permeability conduits corresponding to lenses of well-sorted gravels and secondary channel fill deposits within the braided river deposit architecture. The geophysical surveys provide the basis for a discharge model that fits our observations, namely that there is evidence of a multilayered system focusing groundwater flow through stacked high permeability gravel layers analogous to a subterranean river network. Coincident geophysical surveys in a region further offshore indicate the presence of a large, newly discovered freshened groundwater system, suggesting that the offshore system in the Canterbury Bight is connected with the terrestrial aquifer system.