Electromagnetic sounding experiments in the Schwarzwald central gneiss massif

Geomagnetic and telluric pulsations were observed at nine stations, partially at the same time, and at two sites with additional recordings of variations. They occupied a 20 x 30 km2 area of high-grade metamorphism. There is a nearly perfect spatial uniformity of the magnetic variation field except for a small local anomaly attributable to the Rheingraben. The telluric field is highly polarized in a N47 ± 7W direction but with local differences in amplitude. In contrast, telluric phases are spatially uniform and, as a function of period, distinctly different for the N47W orientation of the telluric field (= "B-polarisation") and the N43E orientation (= "E-polarisation"). Telluric and magnetic observations are not explainable by one-dimensional (1-D) models for the Schwarzwald alone. Therefore a 2-D model is derived, comprising Schwarzwald and Rheingraben, which can account for the graben Z anomaly and the phase curves in both polarisations. An unscaled 1-D model is obtained from the telluric phases in E-polarisation and then a 2-D model for Schwarzwald and Rheingraben in E- and B-polarisation. This latter model allows the scaling of the 1-D Schwarzwald model and shows a thin conductive layer under the gneiss massif at a depth of 12 km with a conductance of 650 S.           ARK: https://n2t.net/ark:/88439/y056011 Permalink: https://geophysicsjournal.com/article/121 &nbsp

Two-dimensional basement modeling of central loop transient electromagnetic data from the central Azraq basin area, Jordan

Thick sedimentary sequences are deposited in the central area of the Azraq basin in Jordan consisting mostly of hyper-saline clay and various evaporates. These sediment successions form the 10 km x 10 km large Azraq mudflat and are promising archives for a palaeoclimatical reconstruction. Besides palaeoclimatical research, the Azraq area is of tremendous importance to Jordan due to groundwater and mineral resources. The heavy exploitation of groundwater has lead to a drastic decline of the water table and drying out of the former Azraq Oasis. Two 7 and 5 km long transects were investigated from the periphery of the mud flat across its center using a total of 150 central loop transient electromagnetic (TEM) soundings. The scope of the survey was to detect the thickness of sedimentary deposits along both transects and to provide a basis for future drilling activities. We derive a two-dimensional model which can explain the TEM data for all soundings along each profile simultaneously. Previously uncertain depths of geological boundaries were determined along both transects. Particularly the thickness of the deposited mudflat sediments was identified and ranges from 40 m towards the periphery down to approximately 130 m at the deepest location. Besides that, the depth and lateral extent of a buried basalt layer was identified. In the basin center the groundwater is hyper-saline. The lateral extent of the saline water body was determined precisely along both transects. In order to investigate the detectability of the basement below the high conductive mudflat sediments an elaborate two-dimensional modeling study was performed. Both, the resistivity and depth of the basement were varied systematically. The basement resistivity cannot be determined precisely in most zones and may range roughly between 1 and 100 Omega m without deteriorating the misfit. In contrast to that, the depth down to the basement is detected accurately in most zones and along both transects. Varying the depth of the basement or removing it completely results in a poor data fitting and, therefore, proves its significance. From the modeling study we derived bounds for the resistivity and depth of the base layer as a measure of their uncertainty. (C) 2016 Elsevier B.V. All rights reserved

Meshfree magnetotelluric modelling

We present a new approach for 2-D magnetotelluric forward numerical modelling in contrast to traditional numerical methods like finite elements or finite differences. The method used for solving the partial differential equations is based on a mesh-free technique which does not need an underlaying mesh or grid. We use the Meshless Local Petrov-Galerkin (MLPG) method in combination with radial basis functions to simulate the response of a given conductivity model to a plane-wave source. We compare the mesh-free solution with known simulation programs and simple analytical solutions. Furthermore, we discuss the new magnetotelluric modelling method in terms of implementation and stability. First, we study the convergence and discretization errors of the new method with a simple half-space conductivity model. Then we compare our mesh-free simulation results with simple 2-D conductivity models with the results of a well-known finite element program. In the end, we provide a smooth conductivity model calculated with the mesh-free approach. The modelling results, even with randomly distributed nodes, are in a good agreement with those obtained by the finite element method

A Newly Developed 2D DC and Time-domain IP Inversion Algorithm -Application on Data Acquired on an Ore Deposit in Turkey

We present the application of a newly developed 2D direct current (DC) resistivity and time-domain induced polarization (TDIP) inversion algorithm on synthetic and on field data. The algorithm applies local smoothness constraints and error weighting. The inversion is conducted in two steps. First, an effective resistivity model is obtained from inversion of the measured apparent resistivity data. Subsequently, a non-linear IP inversion is applied resulting in a chargeability model of the subsurface. The implemented forward calculation uses the Finite Element (FE) method. Unstructured triangular meshes are used for all forward and inverse calculations allowing the incorporation of complex surface topography in the inversion mesh. A modeling study with synthetic data shows that the algorithm is capable of resolving chargeability anomalies independent from the corresponding resistivity anomaly. This is important for disseminated deposits, since the resistivity signature of finely distributed material might be insignificant whereas the IP effect of ores, especially sulfides, is strong. The inversion of DC/TDIP field data acquired on a (mainly disseminated) copper ore deposit in Turkey reveals a strong IP effect, as expected, which is interpreted to be caused by sulfidic copper ore content

Two-dimensional meshless modelling and TE-mode inversion of magnetotelluric data

We present a new 2-D forward modelling and inversion scheme to interpret magnetotelluric/radio-magnetotelluric data by utilizing a novel meshless forward operator. We use this discretization technique within an inverse scheme to recover conductivity structures from given magnetotelluric data. To approximate solutions of the partial differential equations that describe the magnetotelluric experiment, we discretize the subsurface only in terms of nodes. These node sets, which are simple to generate, are used to derive the differential operators' approximations in a generalized meshless framework. First, we study and compare forward modelling calculations to an analytical and known solution from the literature. Several example calculations are given, which validate the proposed meshless forward operator. We then formulate our inverse scheme for TE-mode data, which uses only subsets of the nodal subsurface parametrization to generate conductivity structures from this given data. The inverse scheme consists of a Gauss-Newton algorithm combined with the generalized meshless framework. To validate the algorithm, we present inversion results from synthetic and field data. We compare our results to conductivity models calculated by established, well-known inversion schemes and literature results. We report that our algorithm can accurately model magnetotelluric responses and recover meaningful conductivity models, explaining given magnetotelluric data

Mapping of buried faults using the 2D modelling of far-field controlled source radiomagnetotelluric data

Controlled source radiomagnetotellurics (CSRMT) is a relatively new geophysical method for near-surface applications. A rectangular signal with base frequencies between 0.1 and 150kHz is injected through a grounded electric dipole which is used as a transmitter. Electric and magnetic field components are observed at these frequencies and at their subharmonics, usually in the far-field zone so that apparent resistivities and impedance phases can be obtained in a broad frequency range between 1 and 1000kHz. Inline or broadside configuration can be used for measurements. Similar to the controlled source audiomagnetotelluric method, tensor measurements are also possible when locating two transmitters perpendicular to each other. A scalar CSRMT survey was carried out on the buried faults in the Vuoksa region, 110km north of St. Petersburg to test the applicability of this method to the mapping of near-surface faults. A 700m electric dipole with base frequencies of 0.5, 11.3, 30 and 105kHz was used as a transmitter. Smooth apparent resistivity and phase values as a function of frequency from 1kHz to 1MHz were observed in the far-field zone for the inline configuration at 57 stations using a station distance of 20m. Electric fields observed in the direction of the transmitter were perpendicular to the assumed strike direction of the buried faults so that they could be associated with the TM mode. The observed apparent resistivity and phase TM mode data were interpreted using the 2D inversion algorithm, and a good data fitting could be obtained. The resistivity structure beneath the survey area (down to a depth of 80m) could be derived and the buried faults could be mapped successfully. In addition to the CSRMT observations, a conventional radiomagnetotelluric (RMT) survey was also carried out on the same profile. An excellent correlation of the observed RMT and CSRMT transfer functions and 2D conductivity models was achieved

Investigation of the Azraq sedimentary basin, Jordan using integrated geoelectrical and electromagnetic techniques

The Eastern Mediterranean has been used as a passageway for human migration from Africa to the Middle East, the Balkans and Europe. The Azraq basin in the eastern desert of Jordan has been a major spot for human settlements since the middle Pleistocene. The former lake in the basin centre has developed to a hyper‐saline alluvial mudflat, the Qa’ Al Azraq. In the mudflat thick sequences of alluvial sediments are deposited. Such sediment successions are promising archives used for reconstructing a paleoclimate. In order to identify geological structures and to derive suitable borehole locations in the area for a paleoclimatical reconstruction, the Transient Electromagnetic (TEM) and the Electrical Resistivity Tomography (ERT) methods were utilized. Two transects were investigated from the edge of the basin to the basin centre, crossing three geological formations. The data sets of both methods are interpreted by 1D and 2D inversion algorithms and appraised by inversion statistics. Previously uncertain geological boundaries are determined from geoelectrical models along both transects. Furthermore, a transition zone from fresh to saline groundwater is clearly detected

Loop source transient electromagnetics in an urban noise environment: A case study in Santiago de Chile

We have developed a transient electromagnetic (TEM) study in the noisy urban environment of the megacity Santiago de Chile. Our investigation characterizes the electrical conductivity structure of the Santiago Basin down to 300 m depth, providing key information about the sedimentary infill, hydrogeologic aspects, and geomorphological units. In total, 52 TEM soundings were recorded over roughly 900 km(2). We identified different transient characteristics and noise patterns, spatially correlated to the investigation area. This step allowed classification of the soundings into three groups: highly distorted data, transients severely affected by cultural electromagnetic noise sources, and good-quality data with low noise levels. Conventional 1D inversion techniques were then used to derive resistivity depth models. The models were interpreted using sedimentary units, alluvial fan, static groundwater level information, and also were verified using borehole data. Based on 1D models, different minimum sedimentary thicknesses are ob-served, ranging between 50 and 300 m in depth. The average derived sedimentary thickness is approximately 210 m. In addition, seven soundings indicated the basement. These are mostly located toward the periphery of the Santiago Basin. We found a high conductive zone toward the north and a low-conductive area toward the south of the basin. The southern area is characterized by the absence of conductive layers at a depth between 100 and 150 m. This is likely related to the presence of a thick gravel layer and the absence of shallow clay layers acting as aquitards. The resistivity distribution and the sedimentary thickness obtained for this noisy and urban environment are key inputs for groundwater research as the water supply becomes more limited, specifically in central Chile. The overall consistency of the derived subsurface models high-lights the suitability of the TEM method for investigating urban noise environments