EMRP2.16

Controlled-Source Electromagnetic (CSEM) methods have the potential to be powerful geophysical tools for imaging and monitoring the distribution of electrically resistive fluids, such as freshwater aquifers, CO₂ injected into the subsurface or hydrocarbons during oil and gas production.  However, the presence of metallic infrastructure (steel well casings, pipelines etc.) presents an enormous challenge, because the highly conductive metal masks the electromagnetic response of subsurface geology and distorts any associated time-lapse changes. Therefore, numerical techniques to predict and mitigate the contamination caused by pipelines and casings on CSEM surveys are critical for real world imaging and 4D applications near any such metal objects.

In a collaborative project between the Colorado School of Mines and Shell, we have developed CSEM modeling and inversion tools that can handle realistic scenarios with multiple vertical as well as deviated casings and complex pipeline networks, as will be encountered in mature oil field environments. First, we implemented a forward modeling code based on the Method of Moments technique, which effectively turns the casings into extra sources, such that we do not need to discretize them into excessive numbers of very small model cells. We used this modeling tool to demonstrate quantitatively how steel casings impact synthetic and real time-lapse EM data. The forward modeling code was then combined with a newly developed Gauss-Newton inversion engine, which by itself has been demonstrated to provide images of superior resolution, depth penetration and data fit with less dependency on initial conditions compared to previous quasi-Newton inversion engines.

In this contribution, we first demonstrate on synthetic data that the combination of these two algorithms provides high-quality electrical resistivity images in the immediate vicinity of well casings. Then, we show encouraging results of applying the new tools to field trial data acquired over known casings under semi-controlled conditions. The images obtained are nearly free of casing imprint and subsurface geology could be recovered. These results suggest that this technology may enable us to explain severely distorted field data that were previously uninterpretable.

How to cite: Streich, R., Orujov, G., and Swidinsky, A.: Advances in electromagnetic imaging in the presence of well casings: algorithms and field experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2196, https://doi.org/10.5194/egusphere-egu22-2196, 2022.

The Ubaye Region is a seismically active region in the Western Alps (France), regularly struck by seismic swarms characterized by a high number of small to moderate earthquakes, such as in 2003–2004 or 2012–2015. While some earthquakes could be associated with known faults, the character of the observations (high seismicity – low deformation rate) requires complex driving processes beyond local or regional tectonics. Most conceptual models involve fluids present down to depths of several km, and/or long-range transport.

Magnetotellurics (MT) is known to be an efficient imaging method sensitive to crustal fluids. During 2020/21, a data set of 30 MT sites was acquired, covering a signal period ranging between 10^-4 to 10⁴ s, with generally all 5 components measured. Data quality was generally satisfactory up to 3 s and sometimes up to 100 s. Major problems were related to topography (including logistics), and to the presence of electromagnetic noise, only to be mitigated by advanced processing methods (FFMT). For the 3-D inversion required by the data (phase tensors, WAL, topography), we have chosen a joint inversion of induction vectors, phase tensors and off-diagonal impedances (previously corrected for static shift with help of phase tensor inversion). This allowed us to obtain the best 3-D model using the ModEM inversion code, explaining all three data types reasonably well.

The main findings from this investigation include (a) a prominent conductor (down to 20 Ωm) located along the axis of the swarm zone, though generally above it; (b) a regional dominance of the Penninic Front in the East and the overridden Mesozoic (Dauphinoise) sediments in the West, both not fully covered by the current survey; (c) strike directions that agree well with most of the mapped faults and focal mechanisms of the strongest seismic events.

Uncertainties associated with the insufficient data coverage in some of the most interesting zones were studied by analysing the sensitivities provided by the inversion and direct forward modelling of significant model features. In general, this led to the conclusion that our sensitivity does reach the border of the seismic swarm activity, but does not cover its depth extent. Due to the gap in data in the central area of interest, the geometry and connectivity of the main conductor remains a hypothesis. Thus, a truly quantitative characterization of prominent identified structures is not currently possible and should be better assessed with additional measurement sites. The different conceptual models proposed for the origin of the seismic swarm activity will be discussed in the light of the MT imaging, and the associated uncertainties.

How to cite: Byrdina, S., Got, J.-L., Metral, L., Hering, P., Baques, M., De Barros, L., Garambois, S., Gueguen, P., and Rath, V.: Magnetotelluric investigations in the Ubaye seismic swarm region, Western Alps: a connection between electrical conductivity, fluids, and earthquakes?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3498, https://doi.org/10.5194/egusphere-egu22-3498, 2022.

Geothermal energy extracted from hydrothermal systems can play a key role in mitigating the effects of climate change, while meeting the world’s increasing energy demand. Moreover, not only does it represent a highly economic and adaptive source of renewable energy, but it is genetically related with the formation of ore deposits which are necessary to fuel our energy transition.

The presented study is part of a larger multidisciplinary and international project that aims at investigating the geothermal potential of the Baia Mare region, in northern Romania and for which geological, geochemical, hydrogeological and geophysical data have been. In this framework, the magnetotelluric (MT) method is used to study the hydrothermal system and more specifically to locate heat sources, highlight the presence of fluids and identify the system’s structure.

The largest cluster of measurements with anomalously high heat flow values in Romania (100-160 mW/m²) is situated in the greater Baia Mare region, revealing this area as a prime interest for geothermal exploration. The study area is located within the Neogene Inner Carpathians volcanic arc. Crustal hydraulic properties and associated hydrothermal systems are possibly controlled by the regional Dragos-Voda strike-slip fault zone, which could also provide a pathway for late Miocene magmatic intrusions and lava flows. The associated magmatic plumbing system crosscuts the Neogene sedimentary deposits of the Pannonian Basin. The region is known for surface hot springs, salt and metal mining, which all suggest the presence of a hydrothermal system that could be exploited for geothermal energy.

Broadband magnetotelluric transfer functions have been obtained at 24 sites in the Baia Mare region ranging from 300 Hz up to around 1000s using two MTU instruments from Luleå University of Technology. In addition, we collected continuous telluric broadband recordings at the Surlari National Geomagnetic Observatory to be used as remote reference data together with data from other INTERMAGNET observatories. We applied non-stationary, robust remote processing, which allowed to improve poorly constrained estimates of the impedance tensor in the MT dead band (1 to 10s). Phase tensor and impedance maps provide data overview and a first glimpse of subsurface structures. The preliminary 3D inversion results are presented.

How to cite: Neukirch, M., Minakov, A., Smirnov, M., Gaina, C., Panea, I., Isac, A., Collignon, M., Zlibut, A., Coverca, C., Paralescu, B. S., and Henriuc Morosan, A.-M.: Geothermal Exploration of the Baia Mare Region (Romania) with Magnetotellurics – Responses, Analysis and 1D Models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1034, https://doi.org/10.5194/egusphere-egu22-1034, 2022.

Long period magnetotelluric (LMT) stations were deployed on an array format covering much of the Parnaíba basin and western edge of the Borborema Province in NE Brazil. A grid with 56 LMT stations (from 10 s to over 10⁴ s) with 70 km spacing were acquired during periods of acquisition varying from 2-3 weeks up to 6 months through two field campaigns between November 2018 and July 2019. This study is the first of this kind undertaken in Brazil, much in line with the American EarthScope, the Chinese SiinoProbe and AUSLAMP - Australia array initiatives suggesting the way forward for a comprehensive understanding of large 3D electrical structures of the continental crust and the lithospheric mantle in the entire country.  The results already published show that the resolution of the models obtained is comparable to the inversions of seismic tomography, the sensitivity of the MT method being superior in sensing the melting fraction, temperature and water content in the mantle. As an example, the 70 km spacing between EarthScope stations proved adequate to delineate the main structural features of the middle crust to the upper mantle of the United States. The Parnaíba Basin is a cratonic basin that has been formed by sedimentary mega sequences deposited along the Phanerozoic, after the formation of the Gondwana Supercontinent, with sedimentation expanding over the Borborema Province. The basin's evolution was conditioned by subsidence processes along unstable crustal areas at the end of the Brazilian cycle and demarcated by grabens, fault zones and magmatism. The shallow sediments of the basin (around 2.5 to 3.4 km maximum) however widely spread 60,000 km² cover prominent crustal features including the location of the trans-Brazilian lineament, limits of the Borborema Province, Amazon craton and other important shear and suture zones that may be important to better understand the evolutionary geodynamics of the supercontinents. The time series of electric and magnetic fields recorded in the field have been passed through extensive quality control analysis and then were robustly processed in the frequency domain generating good quality impedance tensor and tipper transfer functions. Currently we are concentrating our efforts on testing several parameters (e.g. mesh design, starting resistivity, damping and covariance analysis) involved in the 3D inversion nonlinear conjugate gradient ModEM code. Following the best practical procedures suggested by previous MT studies to avoid bad impact on the inversion results, misfits have been achieved nRMS ~ 2.2 in joint inversion of impedance tensor and tipper and nRMS < 2 for separate inversion of these transfer functions. Notwithstanding, preliminary resistivity models present good agreement with previous geophysical studies in the area and portray remarkable large-scale middle crust and deep conductive structures inside the covered area. Since there are several geodynamic processes associated with this area and electrical signatures may still exist the MT data set can be an important key to understand the evolution of the pan-African cycle and unravel new findings related to the formation of the Parnaiba basin.

How to cite: Fontes, S. L., Benevides, A., Panetto, L., Maurya, V. P., La Terra, E. F., and Padilha, A.: Deep images of electrical conductivity in Parnaiba basin - NE Brazil, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6586, https://doi.org/10.5194/egusphere-egu22-6586, 2022.

The marine controlled-source electromagnetic (CSEM) method has been used for offshore natural resource exploration for a few decades, and it has the potential to be used for offshore CO₂ storage monitoring. Airwaves, previously treated as distortions, often dominate marine CSEM data when the offshore seawater is shallower than a couple of kilometers. Therefore, different methods have been proposed to distinguish and then correct the airwaves in marine CSEM data. In this study, we analyzed the airwave features by different model parameter perturbations with two-dimensional (2D) modeling. After a thorough study of differentiated EM fields and Poynting vectors for each single model component, we summarize the airwave propagation features and sensitivities regarding different modeling parameters. Particularly, the scenario with and without an oil reservoir or CO₂ storage is carefully studied. It turns out that the airwave can provide useful information at certain transmitter and receiver offsets. Nevertheless, we propose to model the airwave as what it is in the marine CSEM data rather than correct it before feeding the offshore CSEM data to inversion. This idea is demonstrated with a field example from offshore of Svalbard, Norway.

How to cite: Wang, S., Constable, S., Orange, A., and Johansen, S. E.: New insights of airwave in controlled-source electromagnetic offshore data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2268, https://doi.org/10.5194/egusphere-egu22-2268, 2022.