A resistivity-depth model of the central Azraq basin area, Jordan: 2D forward and inverse modeling of time domain electromagnetic data

The focus of this thesis is the geophysical exploration of the central part of the Azraq basin in the northeastern desert of Jordan. In addition to common 1D inversion techniques, further 2D forward modeling strategies and a rarely used 2D inverse modeling scheme are applied to transient electromagnetic data. The Azraq area is of potential interest for palaeoclimatical and archaeological research in the frame of the interdisciplinary Collaborative Research Centre 806, entitled "Our Way to Europe" (CRC 806). The project investigates the history of modern human, particularly population movements in the past 190,000 years before present. The center of the Azraq basin is covered by a 10 km x 10 km mudflat consisting thick sedimentary deposits. To provide the basis for probable future drilling projects within the CRC 806, a 7 km and a 5 km long transects were investigated in the mudflat area. An extensive survey was conducted consisting of 150 recorded central loop transient electromagnetic (TEM) sounding locations. The electrical resistivity tomography (ERT) was applied as a complementary method to validate the TEM results. Common 1D inversion techniques are applied to interpret the TEM field data and to investigate the uncertainty of the inverse models. The results are patched together to quasi 2D resistivity-depth sections. The derived quasi 2D sections are consistent and provide a detailed image of the subsurface electrical resistivity distribution down to approximately 100 m depth. The results identify a resistive buried basalt layer in the periphery of the mudflat and a resistivity increase inside the high conductive mudflat sediments, which obviously corresponds to the layer below. The subsurface models are in excellent agreement with lithological borehole data and the geological information. Moreover, a transition zone from moderate to very low resistivities is observed, which is of interest for the groundwater management in Azraq. To verify the derived 1D inverse models, a detailed 2D modeling study is performed. Although the subsurface resistivity structure varies significantly along both investigated transects, the study demonstrates that a 1D inversion is sufficient to interpret the TEM data. Due to reduced data quality at late transient times for a few sounding locations, the deep resistivity contrast inside the mudflat is not well resolved in those zones. Further systematical 2D forward modeling shows that the resistivity increase is in general required to fit the TEM field data. The 2D forward modeling approach is based on the prior selection of a model and, therefore, does not provide an independent validation of the subsurface resistivity distribution. For this reason, a rarely applied 2D TEM inversion scheme is used to interpret the field data. The obtained 2D inverse models reveal a remarkable agreement with the quasi resistivity-depth sections, which are derived from the 1D results. Moreover, the unsatisfactory resolved deep resistivity contrast below the mudflat is reconstructed by using a-priori information, which is integrated into the parameterization of the model. Accordingly, the 2D inversion provides a strong independent validation of the subsurface resistivity distribution

2-D Joint Inversion of Semi-Airborne CSEM and LOTEM Data in Eastern Thuringia, Germany

Various electromagnetic (EM) techniques have been developed for exploring natural resources. The novel frequency-domain semi-airborne controlled source electromagnetic (semi-AEM) method takes advantages of both ground and airborne techniques. It combines ground-based high-power electrical dipole sources with large scale and spatially densely covered magnetic fields measured via airborne receivers. The method can survey the subsurface down to approximately 1000 m and is particularly sensitive towards conductive bodies (e.g. mineralized bodies) in a more resistive host environment. However, the signal-to-noise ratio of semi-AEM is lower than that of ground-based methods such as long-offset transient electromagnetics (LOTEM), mainly due to the limited stacking time and motion induced noise. As a result, the semi-AEM often has reduced depth of investigation in comparison to LOTEM. One solution to overcome these flaws is to analyse and interpret semi-AEM data together with information from other EM methods using a joint inversion. Since our study shows that LOTEM and semi-AEM data have complementary subsurface resolution capabilities, we present a 2-D joint inversion algorithm to simultaneously interpret frequency-domain semi-AEM data and transient electric fields using extended dipole sources. The algorithm has been applied to the field data acquired in a former mining area in eastern Thuringia, Germany. The 2-D joint inversion combines the complementary information and provides a meaningful 2-D resistivity model. Nevertheless, obvious discrepancies appear between the individual and joint inversion results. Consequent synthetic modelling studies illustrate that the discrepancies occur because of i) differences in lateral and depth resolution between the semi-AEM and LOTEM data caused by different measuring configurations, ii) different measured EM components, and iii) differences in the error weighting of the individual datasets. Additionally, our synthetic study suggests that more flexible land-based configurations with sparse receiver locations are possible in combination with semi-AEM without a significant loss of target resolution, which is promising for accelerating data acquisition and for survey planning and logistics, particularly when measuring in inaccessible areas

3D forward modeling and analysis of the loop-source transient electromagnetic method based on the finite-volume method for an arbitrarily anisotropic medium

Electrical anisotropy of strata has been long recognized by field and laboratory observations. However, nearly all of interpretations of transient electromagnetic (TEM) data is based on the assumption of electric isotropy of media, which can cause misleading data interpretation in regions with strong electrical anisotropy. To clarify the influence of electrical anisotropy on loop-source TEM responses, we present a three dimensional (3D) robust finite-volume (FV) algorithm for simulating TEM responses in an arbitrarily anisotropic medium through solving Helmholtz equations in the time domain. The time domain Maxwell equations are discretized using the mimetic finite-volume method (MFV) on a conventional staggered grid in the space domain and discretized in the time domain using the backward Euler method. To reduce time steps required by computation, the modeling time is first divided into several intervals, each of which has a constant time step size. Then, the resulting system matrix in each interval is factored and solved by a direct solver, which allows the factored matrix to be used to calculate the TEM responses for subsequent time steps in the same time interval. The accuracy of our algorithm is validated against quasi-analytic solutions of a 1-D layered anisotropic model. Finally, by numerical experiments for 3D models with different types of electrical anisotropy, we analyze the influences of electrical anisotropy on TEM responses. The results demonstrate that TEM responses are mainly affected by the horizontal conductivity. The effect of dipping anisotropy on TEM responses is much greater than horizontal anisotropy. Besides, the horizontal principle axis direction of electrical anisotropy could be inferred from the TEM signal

Transdimensional Markov Chain Monte Carlo joint inversion of direct current resistivity and transient electromagnetic data

Joint inversion of multiple geophysical data sets with complementary information content can significantly reduce the non-uniqueness inherent to each individual data set and, therefore, can improve subsurface characterization. Gradient-based joint inversion methods depend on the choice of model regularization and usually produce one single optimal model, and rely on linearization to estimate model parameter uncertainty. However, a quantitative evaluation of the parameter uncertainty of the derived model parameters is crucial for reliable data interpretation. In this study, we present a transdimensional Markov Chain Monte Carlo (MCMC) method for the joint inversion of direct current resistivity and transient electromagnetic data, which provides a rigorous assessment of the uncertainty associated with the derived model. The transdimensional property of the algorithm allows the number of unknown model parameters to be determined adaptively by the data. This usually favours models with fewer parameters through the parsimony criterion of the Bayesian method by choosing suitable prior distributions. In this paper, we demonstrate that the transdimensional MCMC method combines complementary information contained in each data set and reduces the overall uncertainty using synthetic examples. Furthermore, we successfully applied the new joint inversion scheme to field data from Azraq, Jordan. The transdimensional MCMC inversion results are in good agreement with the results obtained by deterministic inversion techniques. From the MCMC inversion results we identified the thickness of a basalt formation and a conductive zone, which were uncertain and not interpreted in prior studies, adding to the geological interpretation

Analysis of measurement errors from electrical resistivity imaging investigation of First World War mining tunnels in La Boisselle, France

To derive reliable electrical resistivity subsurface models using error-weighted inversion schemes, a meaningful and correct error model is required. An over-estimated error leads to a lack of resolution and reduced target detectability. Furthermore, biased data are easily overlooked and can lead to artefacts and significant miss-interpretation. We carried out an electrical resistivity tomography survey to detect mining tunnels of World War I in La Boisselle, France. French, British and German troops extensively used mining warfare, such as tunnel constructions, to undermine opponents. While the location and orientation of some British tunnels are known from archaeological excavations, the exact location of the German tunnels is currently unknown. Due to systematic measurement errors resulting from a malfunction in the system, the acquired electrical resistivity tomography data in La Boisselle were significantly biased. Therefore, a detailed systematic error analysis was developed. Using a workflow of systematic error examination to identify biased data such as outliers or other bias, an unbiased dataset was retrieved. Subsequently, two-dimensional electrical resistivity tomography inversions using different error models provided a qualitative estimate of how the data errors influence the tunnel detectability within an inversion scheme. The field data from La Boisselle demonstrates the importance of correctly estimating measurement errors, especially in view of the detection of small-scale targets, such as tunnels

Characterization of the shallow structure of El Tatio geothermal field in the Central Andes, Chile using transient electromagnetics

This study presents the first high resolution geophysical survey conducted in The El Tatio geothermal field, northern of Chile, focused on the detection of shallow subsurface structures and identification of ascending fluid pathways. TEM data was collected along 5 profiles crossing the two main geothermal basins (Upper and Middle Basin) to obtain an electrical resistivity model up to 200 m depth. The models show important structures that allowed us to improve the conceptual model of the field connecting these geophysical observations with the geology and the geochemistry of the area. We found a shallow (<60 m) high conductivity layer in all profiles. This layer was interpreted as a shallow aquifer of thermal water, which is probably the water supplier of surface manifestations. In the Upper Basin a main permeable zone allows the ascent of fluids from deep aquifers to the shallower one, and a structure that probably act as impermeable geological barrier that forces the fluids to ascend has been detected. In the Middle Basin fluid ascent zones are less clear than in the Upper Basin but it is possible to observe areas of lower resistivity that could be associated with higher permeability. (c) 2021 Elsevier B.V. All rights reserved

Effects of electrical anisotropy on long-offset transient electromagnetic data

Electrical anisotropy of formations has been long recognized by field and laboratory evidence. However, most interpretations of long-offset transient electromagnetic (LOTEM) data arc based on the assumption of an electrical isotropic earth. Neglecting electrical anisotropy of formations may cause severe misleading interpretations in regions with strong electrical anisotropy. During a large scale LOTEM survey in a former mining area in Eastern Germany, data was acquired over black shale formations. These black shales are expected to produce a pronounced bulk anisotropy. Here, we investigate the effects of electrical anisotropy on LOTEM responses through numerical simulation using a finite-volume time-domain (FVTD) algorithm. On the basis of isotropic models obtained from LOTEM field data, various anisotropic models are developed and analysed. Numerical results demonstrate that the presence of electrical anisotropy has a significant influence on LOTEM responses. Based on the numerical modelling results, an isolated deep conductive anomaly presented in the 2-D isotropic LOTEM electric field data inversion result is identified as a possible artifact introduced by using an isotropic inversion scheme. Trial-and-error forward modelling of the LOTEM electric field data using an anisotropic conductivity model can explain the data and results in a reasonable quantitative data fit. The derived anisotropic 2-D model is consistent with the prior geological information

Capability of low-temperature SQUID for transient electromagnetics under anthropogenic noise conditions

Transient electromagnetics (TEM) is a well-established method for mineral, groundwater, and geothermal exploration. Superconducting quantum interference device (SQUID)-based magnetic-field receivers used for TEM have quantitative advantages and higher sensitivity compared with commonly used induction coils. Special applications are deep soundings with target depths > 1 km and settings with conductive overburden. However, SQUIDs have rarely been applied for TEM measurements in environments with significant anthropogenic noise. We compared a low-temperature SQUID with a commercially available induction coil in an area affected by anthropogenic noise. We acquired four fixed-loop data sets with totally 61 receiver stations close to Bad Frankenhausen, Germany. The high sensitivity of the SQUID enables low noise levels, which lead to longer high-quality transient data compared with the induction coil. The effect of anthropogenic and natural noise sources is more critical for the coil than for the SQUID data. In the vicinity of the transmitter loop, systematic distortion of the coil signals occurs at early times, most probably caused by sferic interferences. We have developed 1D inversion results of both receivers that matched well in general. However, the SQUID-based models were more consistent and showed greater depths of investigation. This led to a superior resolution of deeper layers and even enabled a potential detection of thin conducting targets at up to a 500 m depth. Moreover, we find that the SQUID data inversion revealed multidimensional effects within the conductive overburden. In this regard, we applied forward modeling to analyze systematic differences between inversion results of SQUID and coil data. We determine that low-temperature SQUIDs have the potential to significantly improve the reliability of subsurface models in suburban environments. Nevertheless, we recommend combined application of both types of receivers

Innovative boat-towed transient electromagnetics - Investigation of the Furnas volcanic lake hydrothermal system, Azores

Water-covered areas may lead to gaps in surface electromagnetic surveys, causing reduced resolution and, as a consequence, increased uncertainty in derived subsurface models. We have evaluated a boat-towed floating central loop time-domain electromagnetic technique that mitigates this problem. It facilitates obtaining data with a spatial sampling density, which is rarely possible with standard instrumentation on land, and it only requires moderate logistical effort. A unique field study on a shallow volcanic lake demonstrated that this method is feasible with only a minor loss of accuracy when compared to anchored and land soundings. We found that the noise sources arising from the moving instrument and the boat engine can be neglected. The field survey was performed on the Lagoa das Furnas (Sao Miguel, Azores Islands, Portugal), which is located within an active volcanic area and is characterized by fumarolic fields and CO2 degassing. Thus, the associated hydrothermal system is expected to extend below the lake. However, the character, geometry, and extent of this system are unknown because of the lack of boreholes and geophysical studies. In total, 600 soundings, combining towed profiles with anchored and land-based soundings, were acquired with an aim of imaging the hydrothermal system beneath the lake down to 200 m. The results from all three types of measurements compare well and thus led to consistent 1D inversion models. The inversion results delineate a highly conductive, smectite-rich cap layer dipping below the lake away from the main fumarole zone. Near this zone, the extent of the conductor agrees well with an area of intense dispersed CO2 degassing, which appears to be controlled by at least two electrically distinctive fault zones in which the conductor is found at greater depths

A novel semiairborne frequency-domain controlled-source electromagnetic system: Three-dimensional inversion of semiairborne data from the flight experiment over an ancient mining area near Schleiz, Germany

We have developed a novel semiairborne frequency-domain electromagnetic (EM) system and successfully tested it within the DESMEX project. The semiairborne approach relies on the fact that part of the system is positioned on the ground and the rest is airborne. This allows us to take advantage of ground and airborne techniques. In particular, a high-moment transmitter can be installed on the earth's surface, which enables us to inject and induce strong EM fields in the subsurface. Moreover, galvanic coupling is possible, which is an advantage if additional ground stations are deployed. The airborne receivers allow easier, significantly faster, and more uniform spatial coverage of the study area than the ground receivers. In our implementation, transmitters and electric field receivers are installed on the ground. Magnetic field sensors, such as commercially available fluxgate, total field magnetometers, and newly developed induction coils, are installed on a helicopter-towed bird. First, we describe the results of a semiairborne survey performed in a selected area with ancient mining located in the Saxothuringian zone near Schleiz, Germany. A 3D semiairborne inversion model represents several conductive anomalies, which agree well with the outcrop of alum shale formations at the surface. In addition, the shallow parts of the semiairborne model are compared with the result of an independent helicopter-borne survey, which consists of stepwise 1D models