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1
Zhang, P., R. G. Roberts i L. B. Pedersen. "Magnetotelluric strike rules". GEOPHYSICS 52, nr 3 (marzec 1987): 267–78. http://dx.doi.org/10.1190/1.1442301.
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The distortion of the magnetotelluric impedance tensor by complex “near‐surface” structure leads to leakage between the elements of the tensor. The magnetotelluric impedance tensor for our principal model, which has both a local and a regional strike, can be written in the long‐period limit as a sum of the regional, undistorted impedance and a perturbed impedance. The latter can be written as a product of a local distortion (which can be regarded as thin‐sheet distortion in the long‐period range) tensor and the regional impedance. Local and regional strikes are found by rotating the impedance tensor into directions in which diagonal elements are proportional and column elements are proportional, respectively. The regional impedance tensor is calculated assuming that the strikes are uniquely defined. An example from a crystalline area with well conducting fracture zones illustrates the model concepts. A weighted least‐squares procedure is used for the estimation of distortion parameters.
2
Yin, Changchun. "Inherent nonuniqueness in magnetotelluric inversion for 1D anisotropic models". GEOPHYSICS 68, nr 1 (styczeń 2003): 138–46. http://dx.doi.org/10.1190/1.1543201.
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This paper investigates the inherent nonuniqueness in the inversion of magnetotelluric (MT) measurements over an earth with a layered anisotropic resistivity distribution. This kind of nonuniqueness arises from the fact that even accurate and complete data contain insufficient information to uniquely resolve the earth parameters. For a very simple anisotropic case, where all layers in the earth have at least one horizontal principal axis in common coinciding with the x‐ or y‐axis of the Cartesian coordinate system, the resolvable parameters or parameter combinations from MT surface observations are identified by analyzing the structure of the conductivity tensor of the earth and the differential equations the electromagnetic (EM) fields satify. It is found that for a layered anisotropic earth model, several parameters can be uniquely resolved from the MT inversion while others are nonresolvable. However, particular combinations of the nonresolvable parameters can be uniquely resolved. These theoretical findings on the parameter resolvability of MT anisotropic inversion are further verified by a number of numerical experiments using synthetic data and a nonlinear parameter estimation technique. For this purpose, I parameterize the resistivity tensor assigned to each layer of the earth with three principal resistivities and three Euler angles, and then use the three Euler angles, the logarithms of three principal resistivities, and the logarithmic thickness as inverse parameters. This guarantees that the layer thickness is positive and the resistivity tensor is positive‐definite, while at the same time the inverse parameters may span the whole real axis. Numerical inversion results for different anisotropic models also reveal the fact that the resolvable parameters and parameter combinations are invariant with respect to the starting model. This means that for different starting models, the inversion results for some earth parameters might be individually quite different; however, particular combinations of these parameters remain unchanged.
3
Yee, E., P. R. Kosteniuk i K. V. Paulson. "The reconstruction of the magnetotelluric impedance tensor: An adaptive parametric time‐domain approach". GEOPHYSICS 53, nr 8 (sierpień 1988): 1080–87. http://dx.doi.org/10.1190/1.1442544.
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Magnetotelluric (MT) data processing for the estimation of the impedance tensor is usually carried out in the frequency domain using nonparametric methods of spectral analysis. In this paper, the impedance tensor is reconstructed using an adaptive parametric time‐domain approach, whereby the observed MT data are used directly in the estimation process without the need for a frequency‐domain transformation. In such a reconstruction, the impedance tensor is represented by a rational‐form or matrix‐fraction model. The parameters for this model are determined using a recursive instrumental variables (RIV) adaptation algorithm, allowing on‐line real‐time application. This adaptation algorithm is capable of providing consistent estimates for the impedance tensor from only the observed (i.e., noisy) MT field data and auxiliary information in the form of measurements of the contemporaneous components of the magnetic field at some remote site. Hence, the RIV algorithm provides a time‐domain implementation of the remote‐reference MT method, which has been applied with good success for unbiased impedance tensor determination in the frequency domain for moderate‐to‐high‐level noise conditions.
4
Boisse, I., X. Dumusque, N. C. Santos, M. Oshagh, X. Bonfils, M. Montalto, G. Boué i C. Lovis. "Stellar Variabilities: Challenges for the Detection and Characterization of Exoplanets". Proceedings of the International Astronomical Union 8, S293 (sierpień 2012): 388–92. http://dx.doi.org/10.1017/s1743921313013227.
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AbstractThe photometric and RV techniques, although extremely efficient to detect and characterize planets, are, however, indirect techniques (as well as astrometry). Phenomena such as stellar pulsation, inhomogeneous convection, spots or magnetic cycles can prevent us from finding planets or they might degrade the parameters estimation. We will consider the challenges related to the knowledge of stellar activity for the next decade: detect telluric planets in the habitable zone of their stars (from G to M dwarfs), understand the activity in the low-mass end of M dwarf (on which will focus future near-infrared high-resolution spectrograph like SPIRou or CARMENES), limitation to the process of summing several transit observations (in order to characterize the atmospheric components) due to the variability of stellar activity (from the ground or with Spitzer or JWST), as well as the methods proposed and used to overcome this issue.
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Aboud, Essam, Aref Lashin, Faisal Zaidi, Abdulaziz Al-Bassam, Nassir Al Arifi, Mohamed Abu Anbar i Emad Al-Homadhi. "Audio Magnetotelluric and Gravity Investigation of the High-Heat-Generating Granites of Midyan Terrane, Northwest Saudi Arabia". Applied Sciences 13, nr 6 (8.03.2023): 3429. http://dx.doi.org/10.3390/app13063429.
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The Midyan Terrane (northwest Saudi Arabia) is characterized by the presence of a massive belt of radioactive granitic rocks and thick sedimentary cover near the coastal areas. The area is greatly influenced by the tectonic activities of the Red Sea and Gulf of Aqaba, implying its high potentiality of geothermal energy. In the present work, geophysical surveys, including audio magnetotelluric and gravity methods, were integrated to investigate the subsurface structural pattern of the study area, which identified regional deep and shallow fault systems and detected the subsurface geometry/extension of the granitic rocks as well as detecting the thickness of the sedimentary basins near the coastal area. A total number of 80 audio magnetotelluric and 246 gravity stations were recorded, analyzed, and interpreted. Two high-potential geothermal targets were indicated: high-heat-generating granites and thick anomalous sedimentary basins near the coastal areas. High-heat-generating granites are significant in terms of enhanced geothermal systems (EGSs) whereas sedimentary basins play a crucial role in the formation of conventional geothermal systems. Both areas require more exploration plans to evaluate the energy potential of geothermal reservoirs. The results also contribute to the identification of the subsurface orientation and geometry of radioactive granites, providing the necessary parameters to enhance a volumetric estimation for geothermal reserves.
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Maryadi, M., P. Bramanthyo, A. Zarkasyi i H. Mizunaga. "Estimation of near-surface temperature in Suwawa Geothermal Prospect, Gorontalo, Sulawesi, Indonesia, based on magnetotelluric and artificial neural network". IOP Conference Series: Earth and Environmental Science 851, nr 1 (1.10.2021): 012018. http://dx.doi.org/10.1088/1755-1315/851/1/012018.
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Abstract A geophysical survey using broadband magnetotelluric (MT) technology was carried out in Suwawa Geothermal Prospect Area, Gorontalo Province, Sulawesi Island, Indonesia. The target of that research is to evaluate the geothermal potential hidden below the surface, based on underground resistivity distribution. However, the information about resistivity alone is not enough to get a proper understanding of the geothermal system in this area. Another important subsurface feature that could be useful for the evaluation is temperature. In this study, an attempt to predict the subsurface temperature using resistivity and limited information from a shallow borehole thermogram was carried out. Employing the dependency between resistivity and temperature an indirect temperature estimator was built, thanks to the applicability of artificial neural network (ANN) to learn the pattern connecting both parameters. Comparing some neural network training data shows that the predictive powers of the calibrated neural network highly influenced by the geological difference between the location of borehole and MT station. The best trained ANN was then used to predict the temperature below the other MT stations. The result shows that a proper ANN architecture is important to improve the deeper temperature estimation. The best ANN estimator was obtained from the BT01 and AMT39 data pair, which has the highest correlation as well. This preliminary study gives useful insight into how resistivity could be an alternative tool to delineate the near-surface temperature profile, in order to get a more comprehensive image of the subsurface geothermal system.
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Ważny, Jakub, Michał Stefaniuk i Adam Cygal. "Estimation of electrical resistivity using artificial neural networks: a case study from Lublin Basin, SE Poland". Acta Geophysica 69, nr 2 (1.03.2021): 631–42. http://dx.doi.org/10.1007/s11600-021-00554-0.
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AbstractArtificial neural networks method (ANNs) is a common estimation tool used for geophysical applications. Considering borehole data, when the need arises to supplement a missing well log interval or whole logging—ANNs provide a reliable solution. Supervised training of the network on a reliable set of borehole data values with further application of this network on unknown wells allows creation of synthetic values of missing geophysical parameters, e.g., resistivity. The main assumptions for boreholes are: representation of similar geological conditions and the use of similar techniques of well data collection. In the analyzed case, a set of Multilayer Perceptrons were trained on five separate chronostratigraphic intervals of borehole, considered as training data. The task was to predict missing deep laterolog (LLD) logging in a borehole representing the same sequence of layers within the Lublin Basin area. Correlation between well logs data exceeded 0.8. Subsequently, magnetotelluric parametric soundings were modeled and inverted on both boreholes. Analysis showed that congenial Occam 1D models had better fitting of TM mode of MT data in each case. Ipso facto, synthetic LLD log could be considered as a basis for geophysical and geological interpretation. ANNs provided solution for supplementing datasets based on this analytical approach.
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McNeice, Gary W., i Alan G. Jones. "Multisite, multifrequency tensor decomposition of magnetotelluric data". GEOPHYSICS 66, nr 1 (styczeń 2001): 158–73. http://dx.doi.org/10.1190/1.1444891.
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Accurate interpretation of magnetotelluric data requires an understanding of the directionality and dimensionality inherent in the data, and valid implementation of an appropriate method for removing the effects of shallow, small‐scale galvanic scatterers on the data to yield responses representative of regional‐scale structures. The galvanic distortion analysis approach advocated by Groom and Bailey has become the most adopted method, rightly so given that the approach decomposes the magnetotelluric impedance tensor into determinable and indeterminable parts, and tests statistically the validity of the galvanic distortion assumption. As proposed by Groom and Bailey, one must determine the appropriate frequency‐independent telluric distortion parameters and geoelectric strike by fitting the seven‐parameter model on a frequency‐by‐frequency and site‐by‐site basis independently. Although this approach has the attraction that one gains a more intimate understanding of the data set, it is rather time‐consuming and requires repetitive application. We propose an extension to Groom‐Bailey decomposition in which a global minimum is sought to determine the most appropriate strike direction and telluric distortion parameters for a range of frequencies and a set of sites. Also, we show how an analytically‐derived approximate Hessian of the objective function can reduce the required computing time. We illustrate application of the analysis to two synthetic data sets and to real data. Finally, we show how the analysis can be extended to cover the case of frequency‐dependent distortion caused by the magnetic effects of the galvanic charges.
9
Kushnir, Anton, Tatiana Burakhovych, Volodymyr Ilyenko i Bogdan Shyrkov. "GEODYNAMICS". GEODYNAMICS 2(31)2021, nr 2(31) (29.12.2021): 92–101. http://dx.doi.org/10.23939/jgd2021.02.092.
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In order to study the deep structure of the southwestern Ukrainian Carpathians, where the Carpathian conductivity anomaly is located, in 2015 and 2020, modern synchronous magnetotelluric studies were carried out on the profiles of Mukachevo-Skole, Seredne-Borynya and Karpatsky at twenty-three points and the spatiotemporal distribution and the electric field on the Earth's surface, which can be used to assess the conductivity and geoelectrical structure of the region, was obtained. Processing of experimental data was performed using the software PRC_MTMV, which provides a common noise-canceling impedance estimation for synchronous magnetotellurical recordings. Curves of apparent electrical resistivity (amplitude values and phases of impedance) from 10 to 10000 s were obtained reliably. A joint analysis of the apparent resistivity and impedance phases and the formal interpretation of the deep magnetotellurical sounding curves using the Niblett transformation indicate the presence of the spatially inhomogeneous conductor both in the earth's crust and in the upper part of the upper mantle. The chain of local conductive sections in the earth's crust coincides with the axial part of the Carpathian conductivity anomaly. High conductivity of the upper mantle was recorded in the Ukrainian Carpathians from the Transcarpathian Depression to the Skiba cover. It is shown that it is not a homogeneous layer, there is a general deepening of the upper edge to the northeast from 40-60 km (Transcarpathian depression) to 90-100 km (Krosno cover). Sharp deepening along the Porkulets and Dukla covers is revealed. Information about the existence of a deep conductor and its parameters should be the basis for quantitative interpretation and construction of the 3D deep geoelectrical model.
10
LaTorraca, G. A., T. R. Madden i J. Korringa. "An analysis of the magnetotelluric impedance for three‐dimensional conductivity structures". GEOPHYSICS 51, nr 9 (wrzesień 1986): 1819–29. http://dx.doi.org/10.1190/1.1442228.
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The eigenstate analysis of Lanczos, also known as singular value decomposition (SVD), is used to define eight parameters which uniquely describe the magnetotelluric impedance Z. These parameters are independent of a priori assumptions about Z and can be interpreted in terms of three‐dimensional conductivity structures. Through SVD, the impedance is represented by two characteristic states. These states consist of two pairs (E and H) of complex vectors and two corresponding, real, singular values which together describe the extremal properties of Z. The singular values are the maximum and minimum |E|/|H| ratios possible at the observation site and therefore yield the true maximum and minimum apparent resistivities. We use a variation of SVD analysis by incorporating phases in the singular values, which are then called characteristic values. These phases reflect the delay (caused by the earth’s conductivity) of the electric fields relative to their associated magnetic fields. In this analysis of Z, the characteristic values contain four parameters, two singular values and two phases. The characteristic vectors contain the remaining four parameters, two principal axis directions and two ellipticities. The principal axis directions for the E and H vectors need not be at right angles as in biorthogonal analysis. The deviation of these axes from orthogonality is called the “skew angle” S. From a model by Park, we have found S to be closely related to distortions in the telluric current system caused by current gathering due to a good conductor. From the same model, we have found the ellipticity parameters to be the largest in regions of high current distortion and at the shorter periods. Consequently, we speculate that the ellipticity parameters are associated with local induction.
11
Doumouya, V., J. Vassal, Y. Cohen, O. Fambitakoye i M. Menvielle. "Equatorial electrojet at African longitudes: first results from magnetic measurements". Annales Geophysicae 16, nr 6 (30.06.1998): 658–66. http://dx.doi.org/10.1007/s00585-998-0658-9.
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Abstract. In the framework of the French participation in the International Equatorial Electrojet Year (IEEY), ten magnetotelluric stations were installed between November 1992 and November 1994 along a 1200-km-long meridian profile, between Lamto (latitude 6.2°N, Côte d'Ivoire) to the south and Tombouctou (latitude 16.7°N, Mali) to the north. These stations measured digitally the three components of the magnetic field and the two components of the telluric electric field, and operated over a period of 20 months. The magnetic data is used to study the features of the equatorial electrojet (EEJ) in West African longitude. The measurement of the telluric electric field variations will be presented elsewhere. Hourly mean values are used to study the morphological structure of the regular diurnal variation SR of the three components (H, D, and Z) of the earth magnetic field and to characterize the EEJ during magnetically quiet days. The occurrences of the counter-electrojet (CEJ) are set forth, emphasizing its seasonal variability. Assumed to be a current ribbon, the EEJ main parameters (the position of its center, its width, and the amplitude of its current density at the center) are determined. A preliminary analysis of the time variations of these parameters is presented over period of 20 months (from January 1993 to August 1994). Results are compared with those obtained during previous experiments of the same kind.
12
Prystai, A. M., i V. O. Pronenko. "Improving of electrical channels for magnetotelluric sounding instrumentation". Geoscientific Instrumentation, Methods and Data Systems Discussions 5, nr 1 (2.04.2015): 63–81. http://dx.doi.org/10.5194/gid-5-63-2015.
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Abstract. The study of deep structure of the Earth's crust is of great interest for both applied (e.g. mineral exploration) and scientific research. For this the electromagnetic (EM) studies which enable to construct the distribution of electrical conductivity in the Earth's crust are of great use. The most common method of EM exploration is magnetotelluric sounding (MT). This passive method of research uses a wide range of natural geomagnetic variations as a powerful source of electromagnetic induction in the Earth, producing there telluric currents variations. It includes the measurements of variations of natural electric and magnetic fields in orthogonal directions at the surface of the Earth. By this, the measurements of electric field are much more complicated metrological process, and namely they are limiting the precision of MT prospecting. This is especially complicated at deep sounding when measurements of long periods are of interest. The increase of the accuracy of the electric field measurement can significantly improve the quality of MT data. Because of this the development of new version of instrument for the measurements of electric field at MT – both electric field sensors and the electrometer – with higher relative to the known instruments parameters level were initiated. The paper deals with the peculiarities of this development and the results of experimental tests of the new sensors and electrometer included as a unit in the long-period magnetotelluric station LEMI-420 are given.
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Cristofari, P. I., J.-F. Donati, T. Masseron, P. Fouqué, C. Moutou, X. Delfosse, E. Artigau i in. "Estimating fundamental parameters of nearby M dwarfs from SPIRou spectra". Monthly Notices of the Royal Astronomical Society 511, nr 2 (17.12.2021): 1893–912. http://dx.doi.org/10.1093/mnras/stab3679.
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ABSTRACT We present the results of a study aiming at retrieving the fundamental parameters of M dwarfs from spectra secured with SPIRou, the near-infrared high-resolution spectropolarimeter installed at the Canada–France–Hawaii Telescope (CFHT), in the framework of the SPIRou Legacy Survey (SLS). Our study relies on comparing observed spectra with two grids of synthetic spectra, respectively, computed from PHOENIX and MARCS model atmospheres, with the ultimate goal of optimizing the precision at which fundamental parameters can be determined. In this first step, we applied our technique to 12 inactive M dwarfs with effective temperatures (Teff) ranging from 3000 to 4000 K. We implemented a benchmark to carry out a comparison of the two models used in this study. We report that the choice of model has a significant impact on the results and may lead to discrepancies in the derived parameters of 30 K in Teff and 0.05 dex to 0.10 dex in surface gravity (log g) and metallicity ($\rm {[M/H]}$), as well as systematic shifts of up to 50 K in Teff and 0.4 dex log g and $\rm {[M/H]}$. The analysis is performed on high signal-to-noise ratio template SPIRou spectra, averaged over multiple observations corrected from telluric absorption features and sky lines, using both a synthetic telluric transmission model and principal component analysis. With both models, we retrieve Teff, log g, and $\rm {[M/H]}$ estimates in good agreement with reference literature studies, with internal error bars of about 30 K, 0.05 dex, and 0.1 dex, respectively.
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A. Meloni, P. Palangio, M. Marchetti, E. Teisseyre, J. Marianiuk i T. Ernst. "Central Italy magnetotelluric investigation. Structures and relations to seismic events: analysis of initial data". Annals of Geophysics 39, nr 1 (18.01.1996). http://dx.doi.org/10.4401/ag-3959.
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A scientific collaboration between the Warsaw Academy of Science, (Poland) and the National Institute of Geophysics (Italy), gave rise to the installation of few stations for the long term measurement of magnetotelluric fields in central Italy. The selection of investigation sites was determined by the individual seismic interest of each location. The project began in the summer of 1991, with the installation of 2 magnetotelluric stations in the province of Isernia, (Collemeluccio and Montedimezzo). In 1992, 2 more stations became operative, one in the province of Rieti, (Fassinoro), the other in the province of L'Aquila, (S. Vittoria). For the purpose of this project, the magnetic observatory in L'Aquila was also equipped with electric lines, for the measurement of the telluric field. The aim of the analysis here presented, is to show that is possible to follow the temporal evolution of magnetotelluric characteristic parameters. At Collemeluccio this evolution was compared with the seismic released energy for events recorded within the study area.
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T. Ernst, R. Teisseyre, A. Meloni, P. Palangio i M. Marchetti. "Magnetic variation analysis for the June 1993 seismic events in Central Italy". Annals of Geophysics 40, nr 2 (18.03.1997). http://dx.doi.org/10.4401/ag-3930.
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A scientific collaboration between the Istituto Nazionale di Geofisica (Italy) and the Warsaw Academy of Science (Poland) gave rise to the installation of a few stations for the long-term measurement of magnetotelluric fields in Central Italy. The investigation sites were determined following the individual seismic interest of each location. For this project, the magnetic observatory in L'Aquila was also equipped with electric lines, for simultaneous measurements of the telluric field. After a few years of experience some of the installed stations had to be removed for their high noise level that made this study almost impossible. A first time interval was considered from January 1992 to February 1993 and showed the existence of significant changes in magnetotelluric parameters related to earthquake occurrence time, an extension of that analysis was made to include the event of June 1993 using the magnetic field time variation.
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Manthilake, Geeth, Ye Peng, Kenneth T. Koga i Mainak Mookherjee. "Tracking slab surface temperatures with electrical conductivity of glaucophane". Scientific Reports 11, nr 1 (9.09.2021). http://dx.doi.org/10.1038/s41598-021-97317-0.
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AbstractSlab surface temperature is one of the key parameters that incur first-order changes in subduction dynamics. However, the current thermal models are based on empirical thermal parameters and do not accurately capture the complex pressure–temperature paths of the subducting slab, prompting significant uncertainties on slab temperature estimations. In this study, we investigate whether the dehydration-melting of glaucophane can be used to benchmark the temperature in the slab. We observe that dehydration and melting of glaucophane occur at relatively low temperatures compared to the principal hydrous phases in the slab and produce highly conductive Na-rich melt. The electrical properties of glaucophane and its dehydration products are notably different from the hydrous minerals and silicate melts. Hence, we conclude that the thermodynamic instability of glaucophane in the slab provides a unique petrological criterion for tracking temperature in the present-day subduction systems through magnetotelluric profiles.
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Bravo-Osuna, Ana G., Enrique Gómez-Treviño, Olaf J. Cortés-Arroyo, Nestor F. Delgadillo-Jauregui i Rocío F. Arellano-Castro. "Reframing the magnetotelluric phase tensor for monitoring applications: improved accuracy and precision in strike determinations". Earth, Planets and Space 73, nr 1 (2.02.2021). http://dx.doi.org/10.1186/s40623-021-01354-y.
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AbstractThe magnetotelluric method is increasingly being used to monitor electrical resistivity changes in the subsurface. One of the preferred parameters derived from the surface impedance is the strike direction, which is very sensitive to changes in the direction of the subsurface electrical current flow. The preferred method for estimating the strike changes is that provided by the phase tensor because it is immune to galvanic distortions. However, it is also a fact that the associated analytic formula is unstable for noisy data, something that limits its applicability for monitoring purposes, because in general this involves comparison of two or more very similar datasets. One of the issues is that the noise complicates the distribution of estimates between the four quadrants. This can be handled by sending all values to the same quadrant by adding or subtracting the appropriate amount. This is justified by showing that the analytic formula is also a least squares solution. This is equivalent to define penalty functions for the matrix of eigenvalues and then select the minima numerically. Contrary to the analytic formula, this numerical approach can be generalized to compute strikes using windows of any number of periods, thus providing tradeoffs between variance and resolution. The performance of the proposed approach is illustrated by its application to synthetic data and to real data from a monitoring array in the Cerro Prieto geothermal field, México.