Regularization strategy for the layered inversion of airborne TEM data: application to VTEM data acquired over the basin of Franceville (Gabon)

Airborne transient electromagnetic (TEM) is a cost-effective method to image the distribution of electrical conductivity in the ground. We consider layered earth inversion to interpret large data sets of hundreds of kilometre. Different strategies can be used to solve this inverse problem. This consists in managing the a priori information to avoid the mathematical instability and provide the most plausible model of conductivity in depth. In order to obtain fast and realistic inversion program, we tested three kinds of regularization: two are based on standard Tikhonov procedure which consist in minimizing not only the data misfit function but a balanced optimization function with additional terms constraining the lateral and the vertical smoothness of the conductivity; another kind of regularization is based on reducing the condition number of the kernel by changing the layout of layers before minimizing the data misfit function. Finally, in order to get a more realistic distribution of conductivity, notably by removing negative conductivity values, we suggest an additional recursive filter based upon the inversion of the logarithm of the conductivity. All these methods are tested on synthetic and real data sets. Synthetic data have been calculated by 2.5D modelling; they are used to demonstrate that these methods provide equivalent quality in terms of data misfit and accuracy of the resulting image; the limit essentially comes on special targets with sharp 2D geometries. The real data case is from Helicopter-borne TEM data acquired in the basin of Franceville (Gabon) where borehole conductivity loggings are used to show the good accuracy of the inverted models in most areas, and some biased depths in areas where strong lateral changes may occur

Determining Geological Properties by a Hybrid Seismic-Magnetotelluric Approach

This paper concerns the controlled source audio magnetotelluric technique (CSAMT) for imaging subsurface structure. Given the short time available, we limited our investigation to a simple 1D earth model where regional seismic and well logs suggest discrete layers, each with constant seismic velocity and constant electrical conductivity. In addition, the well logs provide rough estimates of velocity and conductivity for use as a starting point in the seismic and MT inversions

Mapping localised freshwater anomalies in the brackish paleo-lake sediments of the Machile–Zambezi Basin with transient electromagnetic sounding, geoelectrical imaging and induced polarisation

A recent airborne TEM survey in the Machile-Zambezi Basin of south western Zambia revealed high electrical resistivity anomalies (around 100 Omega m) in a low electrical resistivity (below 13 Omega m) background. The near surface (0-40 m depth range) electrical resistivity distribution of these anomalies appeared to be coincident with superficial features related to surface water such as alluvial fans and flood plains. This paper describes the application of transient electromagnetic soundings (TEM) and continuous vertical electrical sounding (CVES) using geoelectrics and time domain induced polarisation to evaluate a freshwater lens across aflood plain on the northern bank of the Zambezi River at Kasaya in south western Zambia. Coincident TEM and CVES measurements were conducted across the Simalaha Plain from the edge of the Zambezi River up to 6.6 km inland. The resulting TEM, direct current and induced polarisation data sets were inverted using a new mutually and laterally constrained joint inversion scheme. The resulting inverse model sections depict a freshwater lens sitting on top of a regional saline aquifer. The fresh water lens is about 60 m thick at the boundary with the Zambezi River and gradually thins out and deteriorates in water quality further inland. It is postulated that the freshwater lens originated as a result of interaction between the Zambezi River and the salty aquifer in a setting in which evapotranspiration is the net climatic stress. Similar high electrical resistivity bodies were also associated with other surface water features located in the airborne surveyed area

Airborne EM in northern Italy for sustainable and resilient management of groundwater resources

Climate change and anthropogenic pollution pose a significant challenge to the management of groundwater resources in northern Italy, where in 2022 alone rainfall decreased by 40% compared to the average of the previous 30 years and 67% of the utilized agricultural area is vulnerable to nitrate pollution. To face these challenges, starting in the spring of 2021, A2A Ciclo Idrico initiated a large-scale Airborne electromagnetics (AEM) campaign for mapping the groundwater resources in the Brescia province, Italy, consisting of more than 2000 linear kilometers of AEM data flown over an area of approximately 200 km2. This mapping campaign is the largest AEM campaign carried out in Italy for groundwater management, both in terms of extension and data density. Despite of the large anthropization of Brescia province, with the related problems of inductive couplings that disturb AEM data in the vicinity of buildings and infrastructures, excellent resistivity models have been retrieved in the flown areas, in a range of depths between 300 meters to 400 meters, thanks to careful data processing. This has allowed to map the groundwater resources with unprecedented coverage and resolution, both discovering new aquifers and highlighting their vulnerability. In particular, pathways for nitrate contamination in existing wells have been imaged, and the location for a new well for public water supply has been identified within an extended aquifer overlain by a clay layer that protects against pollution percolating from shallower aquifers. Moreover, 3D geological and hydrogeological models informed by AEM have been constructed in the valley of the Chiese river, and incorporated in a groundwater flow model of the area. These results triggered an even larger AEM campaign currently ongoing over all the plain and a mountainous sector of Brescia province, which we believe are the beginning of a paradigm shift in groundwater management in Italy towards resilient and sustainable exploitation of geo resources

Laterally constrained inversion of ground roll from seismic reflection records

Seismic reflection data contain surface waves that can be processed and interpreted to supply shear-wave velocity models along seismic reflection lines. The coverage of seismic reflection data allows the use of automated multifold processing to extract high-quality dispersion curves and experimental uncertainties in amoving spatial window. The dispersion curves are then inverted using a deterministic, laterally constrained inversion to obtain a pseudo-2D model of the shear-wave velocity. A Monte Carlo global search inversion algorithm optimizes the parameterization. When the strategy is used with synthetic and field data, consistent final models ith smooth lateral variations are successfully retrieved. This method constitutes an improvement over the individual inversion of single dispersion curve

High-resolution subsurface imaging and neural network recognition: Non-intrusive buried substance location. Final report, January 26, 1997

A high-frequency, high-resolution electromagnetic (EIVI) imaging system has been developed for environmental geophysics surveys. Some key features of this system include: (1) rapid surveying to allow dense spatial sampling over a large area, (2) high-accuracy measurements which are used to produce a high-resolution image of the subsurface, (3) measurements which have excellent signal-to-noise ratio over a wide bandwidth (31 kHz to 32 MHZ), (4) elimination of electric-field interference at high frequencies, (5) large-scale physical modeling to produce accurate theoretical responses over targets of interest in environmental geophysics surveys, (6) rapid neural network interpretation at the field site, and (7) visualization of complex structures during the survey. Four major experiments were conducted with the system: (1) Data were collected for several targets in our physical modeling facility. (2) We tested the system over targets buried in soil. (3) We conducted an extensive survey at the Idaho National Engineering Laboratory (INEL) Cold Test Pit (CTP). The location of the buried waste, category of waste, and thickness of the clay cap were successfully mapped. (4) We ran surveys over the acid pit at INEL. This was an operational survey over a hot site. The interpreted low-resistivity region correlated closely with the known extent of the acid pit

Eletrodo de aterramento HVDC do Rio Madeira - Bipolo 1 : modelagem geoelétrica da crosta terrestre para projeto do eletrodo

Orientador: Sueli Yoshinaga PereiraTese (doutorado) - Universidade Estadual de Campinas, Instituto de GeociênciasResumo: Um sistema de transmissão HVDC é composto por duas Subestações Conversoras, interligadas pela linha HVDC, cada uma com um eletrodo de aterramento separado do seu pátio CC, geralmente localizado de 15 km a 150 km de distância e conectado por meio da linha do eletrodo. Os eletrodos HVDC proporcionam redução de custos e agregam confiabilidade ao sistema de transmissão de energia. Os eletrodos geralmente dissipam na terra a corrente de desequilíbrio do bipolo, entre 20 A a 40 A. No caso de perda de um polo da linha HVDC, a energia pode ser transmitida pelo polo remanescente com retorno pela terra, utilizando os eletrodos de aterramento para a injeção de correntes que chegar a quase 4 kA, o que pode resultar em interferências em uma área ampla, dependendo da estrutura geológica. A seleção dos locais de construção dos eletrodos deve ser realizada dentro de um raio de algumas dezenas de quilômetros ao redor das subestações, nas duas extremidades da linha HVDC. O melhor local em cada extremidade é aquele que apresenta a estrutura geoelétrica com resistividades mais baixas, desde a superfície do solo até pelo menos o meio da crosta. Esta tese apresenta o desenvolvimento do modelo geoelétrico 1D para o eletrodo sul do sistema HVDC do Rio Madeira, bipolo 1, localizado em Araraquara, na Bacia Sedimentar do Paraná, sul do Brasil. O eletrodo é constituído por um anel aproximadamente retangular de poços (cerca de 820 m x 560 m), cada um revestido por tubos de aço com profundidades variáveis, entre 20 m e 40 m de profundidade. O modelo geoelétrico deve ser representativo da média do solo raso, até a profundidade dos poços, combinada com um modelo profundo. A modelagem do solo raso foi desenvolvida a partir de uma campanha de sondagens Schlumberger e da perfilagem por indução de poços de monitoramento perfurados no local. O modelo profundo foi construído a partir de uma campanha magnetotelúrica (MT). Os modelos geoelétricos são aprimorados ao longo do projeto, à medida que mais dados geofísicos e geotécnicos são levantados. O modelo de projeto tem um ajuste final após o comissionamento do eletrodo, pois o desempenho elétrico medido permite um ajuste complementar do desvio estático da curva de resistividades aparentes MT. Uma medição independente do potencial tubo-solo foi feita no gasoduto Bolívia-Brasil, a 26 km do eletrodo, sendo o valor medido comparado com o potencial calculado a partir da simulação do eletrodo com o modelo geoelétrico final, com ambos os valores apresentando boa compatibilidadeAbstract: A HVDC transmission system comprises two Converter Substations, interconnected by the HVDC line, each one requiring a separate grounding electrode for its DC switchyard, which usually is located from 15 km to 150 km away and connected by means of the electrode line. HVDC electrodes allow for cost reduction and add reliability to the energy transmission system. The electrodes usually dissipate into the ground the unbalance current of the bipole, about 20 A to 40 A. In case of the loss of one pole of the HVDC line, the energy can be transmitted by the remaining pole with ground return, using grounding electrodes for the injection into the ground currents that may reach almost 4 kA, which may produce interferences within a wide area, depending on the tectonic setting. The electrodes Site Selection shall be carried up within a radius of some tens of kilometers around the substations at the two ends of the HVDC line. The best site at each end is the one with the geoelectric structure that presents lower resistivities, from soil surface down to at least mid-crust. This thesis presents the development of the 1D geoelectric model for the South electrode of Rio Madeira HVDC system, bipole 1, located at Araraquara, in the Paraná Sedimentary Basin, South of Brazil. The electrode is constituted by an approximately rectangular ring of wells (about 820 m x 560 m), each one lined with steel pipes with varying depths, between 20 m to 40 m deep. The geoelectric model shall represent the average of the shallow ground, down to the depth of the wells, combined with a deep model, down to the mid-crust. The modeling of the shallow ground was developed from a Schlumberger survey and from the induction profiling of monitoring wells drilled in the site. The deep model was built from a magnetotelluric (MT) survey. The models are improved along the project, as more geophysical and geotechnical data are surveyed. The design model has a final adjustment after the electrode commissioning, because the measured electrical performance allows for a complementary adjustment of the MT static deviation. An independent measurement of pipe-to-ground potential was done at the Bolivia-Brazil pipeline, 26 km away from the electrode, which was compared with the potential calculated from the electrode simulation using the final geoelectric model, with both values presenting good compatibilityDoutoradoGeologia e Recursos NaturaisDoutor em Geociência

Multi-dimensional Resistivity Models of the Shallow Coal Seams at the Opencast Mine 'Garzweiler I' (Northwest of Cologne) inferred from Radiomagnetotelluric, Transient Electromagnetic and Laboratory Data

The entire Cenozoic unconsolidated fill of the Lower Rhine Embayment in Germany hosts the largest single lignite, or brown coal, deposit in Europe which covers an area of some 2,500 km2 to the northwest of Cologne. Rhineland brown coal is mined in large-scale opencast mining and accounts for around one-quarter of the public electricity supply in Germany. The present study was devoted to carrying out radiomagnetotelluric (RMT) and transient electromagnetic (TEM) investigations over the shallow coal seams at the opencast mine 'Garzweiler I.' The main objectives of the survey were to highlight the applicability and efficiency of RMT and TEM methods in an area like brown coal exploration, and to image the vertical electrical resistivity structure of these coal seams. Therefore, the vertical and lateral resolution capabilities of such methods were as necessary as the ability to cover large areas. Consequently, a total of 86 azimuthal RMT and 33 in-loop TEM soundings were carried out along six separate profiles over two opencast benches at the 'Garzweiler I' mine. The local stratigraphy at the survey areas comprises a layer-cake sequence, from top to bottom, of Garzweiler, Frimmersdorf and Morken coal seams embedded in a sand background, consisting of Surface, Neurath, Frimmersdorf and Morken Sands. A considerable amount of clay and silt intervenes the whole succession. The data were interpreted extensively and consistently in terms of one-dimensional (1D) RMT and TEM resistivity models, without using any complex multi-dimensional interpretation. However, the presence of thin, surficial clay masses (or lenses) broke down such interpretation scheme. In this case, to greatly improve the resistivity resolution for these surficial masses and the underlying coal seams, two-dimensional (2D) RMT and three-dimensional (3D) TEM interpretations have been carried out. They could be used effectively to study the local EM distortion on the measured data, where these surficial masses were found, as well as to cross-check the nearby-topography effect. Because the RMT data are usually skin-depth limited, they only provided a resolution depth between 25 and 30 m for the shallow resistivity structures. Whereas, the TEM data still have sufficiently early- to late-time information, and therefore resulted in a better resolution depth of about 100 m for the shallow to sufficiently-deep resistivity structures. The final 1D/2D RMT and 1D/3D TEM resistivity models displayed a satisfied correlation with both thicknesses derived from the stratigraphic-control boreholes and resistivities measured from direct-current (DC) and spectral induced polarization (SIP) laboratory techniques on 16 rock samples. As demonstrated, the integrated use of azimuthal RMT and in-loop TEM soundings was highly successful and effective at mapping the major stratigraphic units at the survey areas, i.e. the shallowest conductive Garzweiler and Frimmersdorf Coals within their fairly resistive sand background. They could not distinguish between Neurath Sand and the underlying sand/silt or between Frimmersdorf Coal and the underlying organic clay. The deepest Morken Coal was beyond the depth-of-investigation of the present measurements. Finally, the resistivity models revealed that both coal seams gently dip in the southwesterly direction. This should be in fairly good agreement with the regional structural makeup of the Rhineland brown coal. However, they showed that Garzweiler Coal is gradually thinned northeastwards, while Frimmersdorf Coal still has almost a regular thickness

Generation of synthetic wide-band electromagnetic time series

The estimation of the earth transfer functions in MT prospecting method poses the greatest difficulty. As in the seismic prospecting method this task requires the development of advanced processing techniques. In order to assess the performance of each technique, controlled synthetic data and different noise types, which simulate the observed signals, are required. This paper presents a procedure to generate a wide-band noise-free electromagnetic field to be used both for magnetotelluric and audio-magnetotelluric studies. Furthermore, an effort was made to extend the simulation procedures to generally stratified and simple inhomogeneous earth structures. The discrete-time magnetic field values are generated through the inverse Fourier transform of a continuous amplitude spectrum and a sampling procedure. The electric field time series are obtained by the convolution of the magnetic field time series, calculated in the interested frequency band, with a non-causal impedance impulse response. Polarized fields, which are important when inhomogeneous media are considered, are also generated