Journal articles on the topic 'Geomagnetic depth sounding'
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1
Armadillo, E., E. Bozzo, V. Cerv, A. De Santis, D. Di Mauro, M. Gambetta, A. Meloni, J. Pek, and F. Speranza. "Geomagnetic depth sounding in the Northern Apennines (Italy)." Earth, Planets and Space 53, no. 5 (May 2001): 385–96. http://dx.doi.org/10.1186/bf03352395.
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Li, Shiwen, and Yunhe Liu. "Wavelet-Based Three-Dimensional Inversion for Geomagnetic Depth Sounding." Magnetochemistry 8, no. 12 (December 12, 2022): 187. http://dx.doi.org/10.3390/magnetochemistry8120187.
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The complexity of Earth’s structure poses a challenge to the multiscale detection capability of geophysics. In this paper, we present a new wavelet-based three-dimensional inversion method for geomagnetic depth sounding. This method is based on wavelet functions to transfer model parameters in the space domain into the wavelet domain. The model is represented by wavelet coefficients containing both large- and fine-scale information, enabling wavelet-based inversion to describe multiscale anomalies. L1-norm measurement is applied to measure the model roughness to accomplish the sparsity constraint in the wavelet domain. Meanwhile, a staggered-grid finite difference method in a spherical coordinate system is used to calculate the forward responses, and the limited-memory quasi-Newton method is applied to seek the solution of the inversion objective function. Inversion tests of synthetic data for multiscale models show that wavelet-based inversion is stable and has multiresolution. Although higher-order wavelets can lead to finer results, our tests present that a db6 wavelet is suitable for geomagnetic depth sounding inversion. The db6 inversion results of responses at 129 geomagnetic observatories around the world reveal a higher-resolution image of the mantle.
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Neska, Anne, Jan Tadeusz Reda, Mariusz Leszek Neska, and Yuri Petrovich Sumaruk. "On the relevance of source effects in geomagnetic pulsations for induction soundings." Annales Geophysicae 36, no. 2 (March 7, 2018): 337–47. http://dx.doi.org/10.5194/angeo-36-337-2018.
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Abstract. This study is an attempt to close a gap between recent research on geomagnetic pulsations and their usage as source signals in electromagnetic induction soundings (i.e., magnetotellurics, geomagnetic depth sounding, and magnetovariational sounding). The plane-wave assumption as a precondition for the proper performance of these methods is partly violated by the local nature of field line resonances which cause a considerable portion of pulsations at mid latitudes. It is demonstrated that and explained why in spite of this, the application of remote reference stations in quasi-global distances for the suppression of local correlated-noise effects in induction arrows is possible in the geomagnetic pulsation range. The important role of upstream waves and of the magnetic equatorial region for such applications is emphasized. Furthermore, the principal difference between application of reference stations for local transfer functions (which result in sounding curves and induction arrows) and for inter-station transfer functions is considered. The preconditions for the latter are much stricter than for the former. Hence a failure to estimate an inter-station transfer function to be interpreted in terms of electromagnetic induction, e.g., because of field line resonances, does not necessarily prohibit use of the station pair for a remote reference estimation of the impedance tensor.
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Li, Shiwen, Yunhe Liu, and Jianping Li. "A Mantle Plume Beneath South China Revealed by Electrical Conductivity Obtained from Three-Dimensional Inversion of Geomagnetic Data." Sensors 23, no. 3 (January 21, 2023): 1249. http://dx.doi.org/10.3390/s23031249.
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A three-dimensional electrical conductivity model of the mantle beneath South China is presented using the geomagnetic depth sounding method in this paper. The data misfit term in the inversion function is measured by the L1-norm to suppress the instability caused by large noises contained in the observed data. To properly correct the ocean effect in responses at coastal observatories, a high-resolution (1° × 1°) heterogeneous and fixed shell is included in inversion. The most striking feature of the obtained model is a continuous high-conductivity anomaly that is centered on ~(112° E, 27° N) in the mantle. The average conductivity of the anomaly appears to be two to four times higher than that of the global average models at the most sensitive depths (410–900 km) of geomagnetic depth sounding. Further analysis combining laboratory-measured conductivity models with the observed conductivity model shows that the anomaly implies excess temperature in the mantle. This suggests the existence of a mantle plume, corresponding to the Hainan plume, that originates in the lower mantle, passes through the mantle transition zone, and enters the upper mantle. Our electrical conductivity model provides convincing evidence for the mantle plume beneath South China.
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Hanchinal, A. N., P. B. V. Subba Rao, Nandini Nagarajan, D. R. K. Rao, and B. P. Singh. "Geomagnetic depth sounding over the Singhbhum and the surrounding regions of eastern India." Journal of Earth System Science 103, no. 3 (September 1994): 383–400. http://dx.doi.org/10.1007/bf02841528.
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Zhang, Yanhui, and Yue Yang. "Three-dimensional inversion resolution in detecting stagnant slabs using a dense geomagnetic depth sounding method." Physics of the Earth and Planetary Interiors 333 (December 2022): 106955. http://dx.doi.org/10.1016/j.pepi.2022.106955.
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Zhang, Yanhui, Yue Yang, Xueqiu Wang, Shiwen Li, and Aihua Weng. "C-responses estimation of geomagnetic depth sounding using regularization method and its application in Northeast China." Journal of Applied Geophysics 195 (December 2021): 104475. http://dx.doi.org/10.1016/j.jappgeo.2021.104475.
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Li, Shiwen, and Yunhe Liu. "Cold and Wet Mantle Transition Zone beneath the Mediterranean Sea Revealed by the Electrical Image." Applied Sciences 13, no. 2 (January 4, 2023): 689. http://dx.doi.org/10.3390/app13020689.
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A three-dimensional electrical conductivity model of the mantle beneath North Africa and Southern Europe is obtained by geomagnetic depth sounding. C-responses are estimated from geomagnetic data observed at observatories in and around the region and converted to the electrical structure of the mantle transition zone. The limited-memory quasi-Newton method is chosen to minimize the nonlinear objective function of inversion, while the forward modeling relies on a staggered-grid finite difference method in the spherical coordinate system. The data misfit term of the inversion function is measured using the L1-norm in order to suppress the response instability caused by the significant noise contained in the observed data. In order to adequately correct the ocean effect in observatories near the coast, a fixed shell comprised of ocean and land is incorporated in inversion. A banded zone with reduced conductivity is present in the three-dimensional model, primarily seen in the lower mantle transition zone and lower mantle beneath the Mediterranean Sea. Combining laboratory-measured conductivity models, we propose that subducted slabs causing reduced temperature and a water reservoir in the mantle transition zone should be responsible for the observed electrical model.
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Banerjee, B., P. B. V. Subba Rao, Gautam Gupta, E. J. Joseph, and B. P. Singh. "Results from a magnetic survey and geomagnetic depth sounding in the post-eruption phase of the Barren Island volcano." Earth, Planets and Space 50, no. 4 (April 1998): 327–38. http://dx.doi.org/10.1186/bf03352119.
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Guo, Junhao, Xinbao Lian, and Xueqiu Wang. "Electrical Conductivity Evidence for the Existence of a Mantle Plume Beneath Tarim Basin." Applied Sciences 11, no. 3 (January 20, 2021): 893. http://dx.doi.org/10.3390/app11030893.
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This paper proposes using a simulated annealing (SA) calculation to perform one-dimensional inversion of Geomagnetic Depth Sounding (GDS) to obtain the conductivity information of the lower mantle beneath the Tarim area, to calculate the temperature of the lower mantle according to the relevant formula of the petrophysical experiment, and to provide evidence of the existence of the Tarim mantle plume. The data used for inversion originate from the China Geomagnetic Network Center. This article uses theoretical data to prove that the simulated annealing algorithm can invert the true conductivity model when the data do not contain noise. However, when the data contain noise, it is more accurate to use the statistical expected value of the high-quality conductivity model during the simulated annealing inversion process as the optimal conductivity model rather than the classic simulated annealing algorithm. The simulated annealing inversion results of only four stations in Tarim area show that the conductivity of the top of the lower mantle and the upper part of the mantle transition zone in Tarim area is higher than the global average, and it is speculated that the temperature is 150k–450k higher than the global average. This is important evidence for the existence of the mantle plume beneath the Tarim Basin.
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Liperovskaya, E. V., V. V. Bogdanov, P. F. Biagi, C. V. Meister, V. A. Liperovsky, and M. V. Rodkin. "Day-time variations of foF2 connected to strong earthquakes." Natural Hazards and Earth System Sciences 9, no. 1 (January 19, 2009): 53–59. http://dx.doi.org/10.5194/nhess-9-53-2009.
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Abstract. The statistical analysis of the characteristic frequency foF2 of the Earth's ionosphere averaged over mid-day hours – from 11:00 till 17:00 h LT – is carried out. Disturbances of foF2 connected to earthquakes are considered on the background of seasonal, geomagnetic, 11-years and 27-days solar variations. A special normalized parameter F is introduced, which represents the almost seasonal-independent part of foF2. Days with high solar (Wolf number >100) and geomagnetic (∑Kp>30) activity are excluded from the analysis. Events with magnitude M>5, distance from the sounding station R<500 km and depth h<70 km are taken into account. The superimposed epoches' method is used to determine the temporal dependence of F. It is found that F increases about 3–6 days before the earthquakes and then decreases one day – two days before the shock. The decreased values of F continue to exist two-three days after events with M>5.5. The obtained phenomenon depends on the magnitude of the earthquake. For events with M>5.5, the reliability of the effect is larger than 95%. For data of more than 80 earthquakes in the vicinity of Petropavlovsk-Kamchatsky and more than 200 earthquakes in the vicinity of Tokyo analogous results are obtained.
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HANDA, Shun, Yoshikazu TANAKA, and Akira SUZUKI. "The Electrical High Conductivity Layer beneath the Northern Okinawa Trough, Inferred from Geomagnetic Depth Sounding in Northern and Central Kyushu, Japan." Journal of geomagnetism and geoelectricity 44, no. 7 (1992): 505–20. http://dx.doi.org/10.5636/jgg.44.505.
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Legchenko, Anatoly, Jean-Michel Vouillamoz, Fabrice Messan Amene Lawson, Christian Alle, Marc Descloitres, and Marie Boucher. "Interpretation of magnetic resonance measurements in the varying earth’s magnetic field." GEOPHYSICS 81, no. 4 (July 2016): WB23—WB31. http://dx.doi.org/10.1190/geo2015-0474.1.
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At the scale of a magnetic resonance sounding (MRS) field setup, the earth’s magnetic field in the subsurface may vary laterally with depth and over time. These variations can be caused by different natural factors and generally cannot be compensated for by accurate tuning of the measuring device. The varying geomagnetic field (GMF) causes nonresonance conditions of excitation that affect the amplitude and phase of the MRS signal. Usually, variations of the GMF do not exceed a few hertz and their effect on the amplitude is relatively small, permitting us to assume near-resonance conditions for inversion. However, in some cases, the results may be erroneous if a varying GMF is not taken into account. Motivated by possible improvements in MRS inversion, we have developed a procedure for measuring and interpreting MRS data that considers a varying GMF. Our results showed that it is relatively easy to take a time-varying GMF into account. As a demonstration, we have developed the inversion of MRS data measured in Benin (West Africa). A depth-varying GMF is a more complex problem, and to consider this, we have developed an algorithm of nonlinear inversion. We have tested this approach on synthetic data, which resulted in an improved inversion. Field validation of this procedure awaits the discovery of a suitable test site with known variations of the earth’s magnetic field in the subsurface.
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Al-Halbouni, D. "The European Alps as an interrupter of the Earth's conductivity structures." Solid Earth Discussions 5, no. 2 (July 17, 2013): 1031–79. http://dx.doi.org/10.5194/sed-5-1031-2013.
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Abstract. Joint interpretation of magnetotelluric and geomagnetic depth sounding results in the period range of 10–105 s in the Western European Alps offer new insights into the conductivity structure of the Earth's crust and mantle. This first large scale electromagnetic study in the Alps covers a cross-section from Germany to northern Italy and shows the importance of the alpine mountain chain as an interrupter of continuous conductors. Poor data quality due to the highly crystalline underground is overcome by Remote Reference and Robust Processing techniques and the combination of both electromagnetic methods. 3-D forward modeling reveals on the one hand interrupted dipping crustal conductors with maximum conductances of 4960 S and on the other hand a lithosphere thickening up to 208 km beneath the central Western Alps. Graphite networks arising from Palaeozoic sedimentary deposits are considered to be accountable for the occurrence of high conductivity and the distribution pattern of crustal conductors. The influence of huge sedimentary Molasse basins on the electromagnetic data is suggested to be minor compared with the influence of crustal conductors. Dipping direction (S–SE) and maximum angle (10.1°) of the northern crustal conductor reveal the main thrusting conditions beneath the Helvetic Alps whereas the existence of a crustal conductor in the Briançonnais supports theses about its belonging to the Iberian Peninsula. In conclusion the proposed model arisen from combined 3-D modeling of noise corrected electromagnetic data is able to explain the geophysical influence of various structural features in and around the Western European Alps and serves as a background for further upcoming studies.
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Tjulin, A., M. André, A. I. Eriksson, and M. Maksimovic. "Observations of lower hybrid cavities in the inner magnetosphere by the Cluster and Viking satellites." Annales Geophysicae 22, no. 8 (September 7, 2004): 2961–72. http://dx.doi.org/10.5194/angeo-22-2961-2004.
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Abstract. Observations by the Viking and Cluster satellites at altitudes up to 35000km show that Lower Hybrid Cavities (LHCs) are common in the inner magnetosphere. LHCs are density depletions filled with waves in the lower hybrid frequency range. The LHCs have, until recently, only been found at altitudes up to 2000km. Statistics of the locations and general shape of the LHCs is performed to obtain an overview of some of their properties. In total, we have observed 166 LHCs on Viking during 27h of data, and 535 LHCs on Cluster during 87h of data. These LHCs are found at invariant latitudes from the auroral region to the plasmapause. A comparison with lower altitude observations shows that the LHC occurrence frequency does not scale with the flux tube radius, so that the LHCs are moderately rarer at high altitudes. This indicates that the individual LHCs do not reach from the ionosphere to 35000km altitude, which gives an upper bound for their length. The width of the LHCs perpendicular to the geomagnetic field at high altitudes is a few times the ion gyroradius, consistent with observations at low altitudes. The estimated depth of the density depletions vary with altitude, being larger at altitudes of 20000-35000km (Cluster, 10-20%), smaller around 1500-13000km (Viking and previous Freja results, a few percent) and again larger around 1000km (previous sounding rocket observations, 10-20%). The LHCs in the inner magnetosphere are situated in regions with background electrostatic hiss in the lower hybrid frequency range, consistent with investigations at low altitudes. Individual LHCs observed at high altitudes are stable at least on time scales of 0.2s (about the ion gyro period), which is consistent with previous results at lower altitudes, and observations by the four Cluster satellites show that the occurrence of LHCs in a region in space is a stable phenomenon, at least on time scales of an hour.
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Rokityansky, I. I., and A. V. Tereshyn. "Donbas conductivity anomaly in the Karpinsky Swell." Geofizicheskiy Zhurnal 44, no. 6 (February 22, 2023): 101–11. http://dx.doi.org/10.24028/gj.v44i6.273642.
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Donbas Foldbelt (DF) and Karpinsky Swell (KS) are contiguous parts of a system of elongated sedimentary basins forming lineament from Poland through Pripyat Trough in Belarus, the Dnieper-Donets Basin (DDB) and DF in Ukraine, KS in Russia, across the Caspian Sea, through Mangyshlak in Turanian plate in Asia. In DF, the Mesozoic-Cenozoic sediments were raised, and subsequent erosion exposed the Carboniferous coal-bearing strata. In contrast to DF, the Paleozoic rocks in KS are covered by 1—3 km thick sediments of Mesozoic-Cenozoic age; productive structures of the earlier age cannot be confidently studied in KS by geological methods. So, geophysical methods are the promising approach for the KS deep structure studies. This work is devoted to electromagnetic (EM) studies of the electrical conductivity of rocks by the methods of magnetic variation profiling (MVP) and magnetotelluric sounding (MTS). Previously [Rokityansky, Tereshyn, 2022], we described the results of the EM research on DF in detail. MVP reveals the intense Donbas electrical Conductivity Anomaly (DCA) running along the main anticline of the folded Donbas. DCA parameters: maximum possible depth of the anomalous currents center h=18±2 km. Frequency response maximum Т0≈3600 s yields the total longitudinal conductance G=(8±2)∙108 S∙m. 70 MTS at periods 0.1—3000 s yield two conductive stripes, with the upper edge varying from 0.3 to 5 km. The stripes are parallel to the DCA axis and are considered as part of DCA. A very large value of G suggests that the anomalous body extends to some considerable depth. The DCA axis spatially coincides with an intense (up to 90 mW/m2) deep heat flow anomaly. Thus, the nature of the DCA lower part can be a partial melting. Theoretical estimates show that intense anomalous fields of geomagnetic variations arise over highly elongated conductors. Therefore, there is reason to expect that the anomaly continues eastward. We found two MTS profiles crossing the Karpinsky swell, and under both profiles strong conductivity anomalies are clearly seen. We re-interpreted original data and presented parameters of all 3 anomalies in an identical style. The main conclusion: the anomaly parameters on the three profiles are approximately the same, and one can assume with a high probability the existence of a single anomaly of electrical conductivity in the Donbas and Karpinsky Swell with a common length of more than 500 km and longitudinal conductance G≈8×108 S×m.
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Rokityansky, I. I., and A. V. Tereshyn. "Donbas geoelectrical structure." Geofizicheskiy Zhurnal 44, no. 1 (April 3, 2022): 158–72. http://dx.doi.org/10.24028/gzh.v44i1.253717.
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The Donbas was formed as the result of Late Devonian rifting of the East European craton. During the Carboniferous, the subsidence of the basin and sedimentation were at their maximum, and a 15-kilometer stratum of Carboniferous deposits formed in the Donbas. The total thickness of the deposits reaches more than 20 km. The next important event was folding, which occurred in the Late Triassic — Early Jurassic and Late Cretaceous — Early Tertiary. The inversion lifted the upper part of the folded Donbas and subsequent erosion exposed the Carboniferous coal-bearing strata on the surface of the earth. Deep electrical conductivity was studied by the methods of magnetovariational profiling (MVP) and magnetotelluric sounding (MTS). The first large survey (13 long-period MTS sites and 32 MTS points) was carried out in 1986; in 1988 another 30 MTS were performed. In 2012-2013, a profile of 15 MVP-MTS sites was made with modern instruments that allow obtaining more accurate results. The data interpretation yields the following conclusions. The intense Donbass electrical conductivity anomaly (DAE) runs along the main anticline of the folded Donbass. In the northwest, it continues in the DDD, in the southeast — on the Karpinsky swell. DAE parameters obtained by the MVP method: Based on the frequency response of the anomalous field, the total longitudinal conductance G = (8±2)×108S×m was estimated. Profile graphs of the anomalous field of geomagnetic variations give an estimate of the maximum possible depth of the anomalous currents center hmax.c.curr, which depends on the variations period. For DAE at the maximum frequency response Т0≈3600 s, it turns out to be equal to hmax.c.curr=18±2 km. The upper edge of the anomaly is estimated from MTS data. Most of the available 70 MTS ρк curves begin at periods of 0.1—1 s from approximately the same level of 15 Ohm·m±half an order of magnitude. This means that in the Donbass, the rocks of the upper approximately half-kilometer layer have, as a rule, electrical resistivity in the range of 5—50 Ohm·m. Deeper, the resistivity can increase to hundreds and thousands of Ohm·m or decrease to units or fractions of Ohm·m. An analysis of the MTS by area showed that objects of low resistance (LRO) are located in two conductive bands, the upper edge of which varies from 0.3 to 5 km. The bands are parallel to the DAE axis and can be considered as some part of the anomaly. A very large value of G leads to the assumption that the anomalous body extends to a considerable depth. When compared with the data of other geophysical methods, it turned out that the DAE spatially coincides with an intense (up to 90 mW/m2) linear anomaly of the deep heat flow. This fact suggests that the nature of the DAE lower part can be a partial melting of fluid-enriched heated local rocks or intrusion of mantle magma. The obtained geoelectric results support the idea of the modern tectonic activation in Donbas.
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Padilha, Antonio L., and Ícaro Vitorello. "Magnetotelluric and geomagnetic depth soundings around the Torres Syncline Hinge, southeast Paraná Basin, Brazil." Geophysical Research Letters 27, no. 22 (November 15, 2000): 3655–58. http://dx.doi.org/10.1029/2000gl011507.
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Liperovskaya, E. V., P. F. Biagi, C. V. Meister, and M. V. Rodkin. "foF2 seismo-ionospheric effect analysis: actual data and numerical simulations." Natural Hazards and Earth System Sciences 8, no. 6 (December 11, 2008): 1387–93. http://dx.doi.org/10.5194/nhess-8-1387-2008.
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Abstract. On the background of seasonal and helio-geomagnetic disturbances, disturbances of the ionization density near noon connected to earthquakes are investigated. The study is performed for some tens of earthquakes with magnitudes M>5 and depths h<70 km, which occur at distances from the vertical sounding stations "Tokyo", "Akita", and "Yamagawa" not larger than R=exp(M)+150 km. The analysis is performed using data registered by the three sounding stations every hour during 30 years. Especially methods of the statistical analysis are applied to search for regularities in the lithospheric-ionospheric links before and after earthquakes. The growth of the mean ionization density N at the F-layer electron density maximum Fof2 five-three days before earthquakes, and the decrease of N approaching the eruption and during a few days after the earthquake is investigated in dependence on both the magnitude of the earthquake M and the ionization density N near the F-layer maximum. It is found that some days before earthquakes the decrease of the ionization density ΔN is proportional to the values of M and N. After the earthquakes, ΔN depends much weaker on the magnitude M, and it is not influenced by the ionization density N. The variations of the ionization density three days before earthquakes do not depend on N and M (within the same order of approximation).
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Zhang, Yanhui, Aihua Weng, Shiwen Li, Yue Yang, Yu Tang, and Yunhe Liu. "Electrical conductivity in the mantle transition zone beneath Eastern China derived from L1-Norm C-responses." Geophysical Journal International 221, no. 2 (February 3, 2020): 1110–24. http://dx.doi.org/10.1093/gji/ggaa059.
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SUMMARY Constraining the distribution of water in different regions of the mantle remains one of the significant challenges to comprehend the global deep water cycle. Geomagnetic depth soundings can provide such constraint through the electrical conductivity structure. Hence, this study aims to propose a regularization technique that can estimate previously unavailable C-response. In the method, the objective function comprised an L1-norm measured data prediction error and a spectral smoothness constraint term. We used the data error of C-response to weight the predicted error. The L-BFGS method was introduced to determine the minimum point of the objective function, and the regularization parameter decreased adaptively during inversion. Thus, the geomagnetic data processed yielded high-quality C-responses in 31 stations in Eastern China. In addition, we obtained 1-D electrical conductivity profiles in the mantle transition zone (MTZ) beneath Eastern China from C-responses using the L-BFGS method. Compared with the global 1-D model, the conductivity–depth profiles revealed that the MTZ beneath Eastern China is more conductive in the east but more resistive in the west. The conversion of these conductivities to water content based on the mineral physics suggested that the MTZ beneath Eastern China is characterized by a high water concentration, approximately 0.2 and 1 wt per cent in the upper and lower MTZ, respectively. Owing to the inclusion of more stations, the water-rich region could be constrained roughly to the east of the North–South Gravity Lineament (NSGL). Further considering seismic images in the same area, this water content distribution pattern suggested that the front of the stagnant Pacific Plate in the lower MTZ might have reached the NSGL. However, the dehydration reactions in the stagnant slab were more active in the eastern part. Perhaps, some of these fluids migrated into the upper MTZ and could be the source of the trapped water found in the xenoliths from the deep upper mantle beneath Eastern China.
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Vozar, Jan, Alan G. Jones, Joan Campanya, Chris Yeomans, Mark R. Muller, and Riccardo Pasquali. "A geothermal aquifer in the dilation zones on the southern margin of the Dublin Basin." Geophysical Journal International 220, no. 3 (November 26, 2019): 1717–34. http://dx.doi.org/10.1093/gji/ggz530.
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SUMMARY We present modelling of the geophysical data from the Newcastle area, west of Dublin, Ireland within the framework of the IRETHERM project. IRETHERM's overarching objective was to facilitate a more thorough strategic understanding of Ireland's geothermal energy potential through integrated modelling of new and existing geophysical, geochemical and geological data. The Newcastle area, one of the target localities, is situated at the southern margin of the Dublin Basin, close to the largest conurbation on the island of Ireland in the City of Dublin and surrounds. As part of IRETHERM, magnetotelluric (MT) soundings were carried out in the highly urbanized Dublin suburb in 2011 and 2012, and a description of MT data acquisition, processing methods, multidimensional geoelectrical models and porosity modelling with other geophysical data are presented. The MT time-series were heavily noise-contaminated and distorted due to electromagnetic noise from nearby industry and Dublin City tram/railway systems. Time-series processing was performed using several modern robust codes to obtain reasonably reliable and interpretable MT impedance and geomagnetic transfer function ‘tipper’ estimates at most of the survey locations. The most ‘quiet’ 3-hr subsets of data during the night time, when the DC ‘LUAS’ tram system was not operating, were used in multisite and multivariate processing. The final 2-D models underwent examination using a stability technique, and the final two 2-D profiles, with reliability estimations expressed through conductance and resistivity, were derived. In the final stage of this study, 3-D modelling of all MT data in the Newcastle area was also undertaken. Comparison of the MT models and their interpretation with existing seismic profiles in the area reveals that the Blackrock–Newcastle Fault (BNF) zone is visible in the models as a conductive feature down to depths of 4 km. The investigated area below Newcastle can be divided into two domains of different depths, formed as depth zones. The first zone, from the surface down to 1–2 km, is dominated by NE–SW oriented conductors connected with shallow faults or folds probably filled with less saline waters. The conductors are also crossing the surface trace of the BNF. The second depth domain can be identified from depths of 2–4 km, where structures are oriented along the BNF and the observed conductivity is lower. The deeper conductive layers are interpreted as geothermal-fluid-bearing rocks. Porosity and permeability estimations from the lithological borehole logs indicate the geothermal potential of the bedrock, to deliver warm water to the surface. The fluid permeability estimation, based on Archie's law for porous structures and synthetic studies of fractured zones, suggests a permeability in the range 100 mD–100 D in the study area, which is prospective for geothermal energy exploitation.
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Rashid, Mehboob Ur, Waqas Ahmed, Muhammad Waseem, Bakht Zamin, Mahmood Ahmad, and Mohanad Muayad Sabri Sabri. "Metallic-Mineral Prospecting Using Integrated Geophysical and Geochemical Techniques: A Case Study from the Bela Ophiolitic Complex, Baluchistan, Pakistan." Minerals 12, no. 7 (June 28, 2022): 825. http://dx.doi.org/10.3390/min12070825.
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An integrated geophysical and geochemical investigation was conducted to investigate the metallic minerals hosted in the mafic and ultramafic rocks in the Bela Ophiolitic Complex. Two thousand magnetic observations were made along with six vertical electrical soundings, with Induced Polarization (IP) targeting the anomalous magnetic zones. The magnetic raw field data were interpreted qualitatively and quantitatively, and two anomalous zones (A1 and A2) were identified on the magnetic maps. The residual magnetic values in the high-magnetic-anomalous zone (A2) ranged from 310 nT to 550 nT, while the magnetic signatures in the low-magnetic zone (A1) ranged from –190 nT to 50 nT. The high-anomalous zone (A2) was distinguished by a high IP value ranging from 3.5 mV/V to 15.1 mV/V and a low apparent and true resistivity signature of 50 ohm·m. Whereas, the low-anomalous zone (A1) was distinguished by very low IP values ranging from 0.78 mV/V to 4.1 mV/V and a very high apparent and true resistivity of 100 ohm·m. The Euler deconvolution was used to determine the depth of the promising zone, which for A1 and A2 was in the 100 m range. The statistical analysis was carried out using hierarchical classification to distinguish between background and anomalous data. The high-magnetic anomalous signature of probable mineralization was in the range of 46,181 nT–46,628 nT, with a total intensity range of 783 nT–1166 nT. The major and trace-element analysis of the 22 rock and stream sediments collected from the high-magnetic-anomalous zone confirmed the mineralization type. The geomagnetic and geophysical cross sections revealed that anomalous mineralization was concentrated with the anticlinal Bela Ophiolitic Complex. The generated results also aided in the identification of rock boundaries, depth, and hidden faults in the area. The findings revealed that the study area has excellent mineralization associated with the ultramafic-rock sequence.
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Guillemoteau, Julien, François-Xavier Simon, Guillaume Hulin, Bertrand Dousteyssier, Marion Dacko, and Jens Tronicke. "3-D imaging of subsurface magnetic permeability/susceptibility with portable frequency domain electromagnetic sensors for near surface exploration." Geophysical Journal International 219, no. 3 (August 26, 2019): 1773–85. http://dx.doi.org/10.1093/gji/ggz382.
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SUMMARY The in-phase response collected by portable loop–loop electromagnetic induction (EMI) sensors operating at low and moderate induction numbers (≤1) is typically used for sensing the magnetic permeability (or susceptibility) of the subsurface. This is due to the fact that the in-phase response contains a small induction fraction and a preponderant induced magnetization fraction. The magnetization fraction follows the magneto-static equations similarly to the magnetic method but with an active magnetic source. The use of an active source offers the possibility to collect data with several loop–loop configurations, which illuminate the subsurface with different sensitivity patterns. Such multiconfiguration soundings thereby allows the imaging of subsurface magnetic permeability/susceptibility variations through an inversion procedure. This method is not affected by the remnant magnetization and theoretically overcomes the classical depth ambiguity generally encountered with passive geomagnetic data. To invert multiconfiguration in-phase data sets, we propose a novel methodology based on a full-grid 3-D multichannel deconvolution (MCD) procedure. This method allows us to invert large data sets (e.g. consisting of more than a hundred thousand of data points) for a dense voxel-based 3-D model of magnetic susceptibility subject to smoothness constraints. In this study, we first present and discuss synthetic examples of our imaging procedure, which aim at simulating realistic conditions. Finally, we demonstrate the applicability of our method to field data collected across an archaeological site in Auvergne (France) to image the foundations of a Gallo-Roman villa built with basalt rock material. Our synthetic and field data examples demonstrate the potential of the proposed inversion procedure offering new and complementary ways to interpret data sets collected with modern EMI instruments.
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Kurtz, R. D., and J. C. Gupta. "Shallow and deep crustal conductivity studies in the Miramichi earthquake zone, New Brunswick." Canadian Journal of Earth Sciences 29, no. 7 (July 1, 1992): 1549–64. http://dx.doi.org/10.1139/e92-122.
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The occurrence in early 1982 of four earthquakes in north-central New Brunswick with magnitudes ranging from 5.0 to 5.7 mb prompted detailed electromagnetic surveys of the epicentral region. Scalar audiomagnetotelluric (AMT) measurements, spaced at 100 m intervals or less, along a 7.5 km east–west profile located two conductors but did not find anomalies that could be associated with the proposed fault plane defined by the earthquake hypocentres.The 150 AMT measurements, combined with broad-band tensor soundings at 11 sites in a confined region (6 km × 7.5 km), provided an opportunity to study the distorting effects of near-surface anomalies and also to determine the regional conductivity structure. The apparent resistivity and phase curves from all tensor stations, calculated in a common coordinate system, were remarkably similar (except for static shift of the apparent resistivity curves) to those derived from the rotationally invariant Berdichevsky determinant averages. These averages appear to be very effective for deriving a first-order estimate of the conductivity structure in areas for which near-surface anomalies are a problem.A geometric mean of the AMT measurements was used to estimate the correct level for the high-frequency asymptotes of the tensor apparent resistivity curves. A one-dimensional inversion of the tensor magnetotelluric (MT) data, with the apparent resistivity shifted to the AMT average, yields a four-layer electrical conductivity model for the crust, with depths from the surface of 2.4, 19, and 32 km and with resistivities of 10 000, >100 000, 10 000, and 300 Ω∙m. The crustal resistivities in the Miramichi region are considerably larger than those in other regions in eastern North America but are typical for the Precambrian Shield. As well, there is an indication of lower crustal and (or) upper mantle electrical anisotropy. The geomagnetic transfer function data suggest the presence of a north-northwest-trending structure 7–12 km east of the Miramichi survey area. At present there is little other geophysical or geological evidence for this conductive anomaly.
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D. Di Mauro, E. Armadillo, E. Bozzo, V. Cerv, A. De Santis, M. Gambetta, and A. Meloni. "GDS (Geomagnetic Depth Sounding) in Italy: applications and perspectives." Annals of Geophysics 41, no. 3 (August 18, 1998). http://dx.doi.org/10.4401/ag-4354.
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The analysis of geomagnetic field variations is a useful tool to detect electrical conductivity contrasts within the Earth. Lateral resolution of outlined patterns depends on the array dimensions and density of measurement sites over the investigated area. The inspection depth is constrained by the period of geomagnetic variations considered in data processing. Regions with significant geological features such as boundaries of continental plates, marginal areas of contact between tectonic units or other geodynamical processes, are of primary interest for the application of the MagnetoVariational (MV) method. In the last ten years, in the frame of the ElectroMagnetic (EM) sounding techniques in applied geophysics, this method has been applied in Italy by researchers of the Istituto Nazionale di Geofisica, Rome, the Dipartimento di Scienze della Terra, Universitá di Genova and the Czech Science Academy of Prague. The Ivrea body in the Northwestern Alps and their junction with the Apennine chain, the micro-plate of the Sardinian-Corsican system and, recently, the central part of the peninsula along Tyrrhenian-Adriatic lithospheric transects were investigated. Studies in time and frequency-domain used in the first investigations, have been followed by more refined analysis involving tests on the induced EM field dimension, computations of single site Transfer Functions (TFs) through Parkinson arrows' and Fourier maps in the Hypothetical Event technique (HE). It was possible to describe the electrical conductivity distribution in the inner part of the SW Alpine arc and to confirm the presence of lithospheric and asthenospheric anomalies obtained by other geophysical methods. For the Sardinia-Corsica system, 2D and 3D inversion models highlighted the existence of two major conducting bodies, one north of Corsica, and the other south of Sardinia. In Central Italy, the regional electrical conductivity distribution pointed out a deep conductive structure beneath the Apennines and a very resistive root for this part of the mountain chain.
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"Use of line current analogues in geomagnetic depth sounding." International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts 24, no. 2 (April 1987): 60–61. http://dx.doi.org/10.1016/0148-9062(87)92060-2.
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Wang, Ning, Changchun Yin, Lingqi Gao, Changkai Qiu, and Xiuyan Ren. "3D anisotropic modeling of geomagnetic depth sounding based on unstructured edge-based finite-element method." Geophysical Journal International, May 27, 2023. http://dx.doi.org/10.1093/gji/ggad224.
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Summary Geomagnetic depth sounding (GDS) is a geophysical electromagnetic (EM) method that studies the deep structure and composition of the earth by using long-period EM signals from geomagnetic observatories and satellites. In this paper, a three-dimensional (3D) anisotropic GDS modeling algorithm is developed. The curl-curl equation is discretized using the edge-based finite-element (FE) method on unstructured tetrahedral grids. In order to solve the computationally demanding problem of EM modeling on a global scale, the complex linear system is first separated into the equivalent real linear systems, and then the real system is iteratively solved by the flexible generalized minimum residual (FGMRES) method with a block diagonal preconditioner. This will greatly reduce the condition number of the linear system and thus speed up the solution process. We verify the accuracy of the proposed algorithm by comparing our results with the existing methods. After that, we design a subduction zone model to simulate the EM responses under isotropic and anisotropic environments, respectively. The numerical results show the high efficiency of the proposed algorithm and the response differences between isotropic and anisotropic models. This research can provide theoretical and technical support for the high-accuracy and efficient inversion of GDS data for the geo-dynamic study.
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Starzhinskii, S. S. "Magnetovariational Sounding in South Vietnam according to the Dalat Observatory Data." Russian Geology and Geophysics, April 25, 2023. http://dx.doi.org/10.2113/rgg20224481.
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Abstract ––This paper describes a three-dimensional inversion of magnetovariational tippers calculated for the Dalat observatory in South Vietnam on the basis of the geomagnetic variation records obtained from the INTERMAGNET network. The frequency dependences of the tippers are inverted using the ModEM software, which makes it possible to construct a geoelectric model of the Dalat zone in a 300 × 300 × 150-km spatial region (x, y, and z axes, respectively) with its center located at the observatory. The resulting model of the geoelectric section contains both surface and deep conductive blocks located at the top and bottom of the Earth’s crust and in the upper mantle. The central part of the upper local conducting block with a resistivity of 0.5–1.0 Ohm⋅m is located near the observatory at a depth of 12–14 km. A more massive conductive block is located to the east of the observatory in the coastal region. Its central part with a resistivity of 0.3 Ohm⋅m is located at a depth of 24–28 km and extends under the continent along the coast for about 80 km. The roots of this block are visible to depths of ~100 km in the South China Sea shelf region. It is assumed that the high electrical conductivity of these blocks is provided by melting in the presence of aqueous fluids and granite series rocks, which are widely distributed in the Dalat zone. The crystallization depths of the samples of these granites estimated from the geothermobarometry data are close to the depth intervals of conductive blocks in the model section, which was also observed in the interpretation of magnetotelluric soundings in Southern Tibet, where granitoid rocks are widespread.
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Chen, Hao, Hideki Mizunaga, and Toshiaki Tanaka. "Influence of geomagnetic storms on the quality of magnetotelluric impedance." Earth, Planets and Space 74, no. 1 (July 20, 2022). http://dx.doi.org/10.1186/s40623-022-01659-6.
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AbstractMagnetotelluric (MT) field data contain natural electromagnetic signals and artificial noise sources (instrumental, anthropogenic, etc.). Not all available time-series data contain usable information on the electrical conductivity distribution at depth with a low signal-to-noise ratio. If variations in the natural electromagnetic signal increase dramatically in a geomagnetic storm, the signal-to-noise ratio increases. A more reliable impedance may be obtained using storm data in a noisy environment. The field datasets observed at mid-latitudes were used to investigate the effect of geomagnetic storms on MT impedance quality. We combined the coherence between the electric and magnetic fields and the result of the MT sounding curve to evaluate the MT impedance quality across all periods and combined the phase difference among the electric and magnetic fields, the polarization direction, and the hat matrix to discuss the data quality for a specific period simultaneously. The case studies showed that the utilization of the data observed during the geomagnetic storm could overcome the local noise and bring a more reliable impedance. Graphical Abstract
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Yao, Hongbo, Zhengyong Ren, Jingtian Tang, Rongwen Guo, and Jiayong Yan. "Trans-dimensional Bayesian joint inversion of magnetotelluric and geomagnetic depth sounding responses to constrain mantle electrical discontinuities." Geophysical Journal International, January 28, 2023. http://dx.doi.org/10.1093/gji/ggad029.
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Summary Joint inversion of magnetotelluric (MT) and geomagnetic depth sounding (GDS) responses can constrain the crustal and mantle conductivity structures. Previous studies typically use either deterministic inversion algorithms that provide limited information on model uncertainties or using stochastic inversion algorithms with a predetermined number of layers that is generally not known a priori. Here, we present a new open-source Bayesian framework for the joint inversion of MT and GDS responses to probe one-dimensional (1D) layered Earth’s conductivity structures. Within this framework, model uncertainties can be accurately estimated by generating numerous models that fit the observed data. A trans-dimensional Markov chain Monte Carlo (MCMC) method is employed to self-parametrize the model parameters, where the number of layers is treated as an inversion parameter that is determined automatically by the data. This adaptability can overcome the under or over-parametrization problem and may be able to automatically detect the conductivity discontinuities in the Earth’s interior. To accelerate the computations, a large number of Markov chains with different initial states can be run simultaneously using the MPI parallel technique. Synthetic data sets are used to validate the feasibility of our method and illustrate how separate and joint inversions, as well as various priors affect the posterior model distributions. The trans-dimensional MCMC algorithm is then applied to jointly invert the MT and GDS responses estimated at the Tucson geomagnetic observatory, North America. Our results not only contain model uncertainty estimates but also indicate two distinct conductivity discontinuities at around 85 and 440 km, which are likely related to the lithosphere-asthenosphere boundary and the upper interface of the mantle transition zone, respectively.
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Rowland, Douglas, Michael Collier, John Keller, Robert Pfaff, Jeffrey Klenzing, Jason McLain, James Clemmons, and James Hecht. "Imaging Low-Energy Ion Outflow in the Auroral Zone." Frontiers in Astronomy and Space Sciences 9 (May 19, 2022). http://dx.doi.org/10.3389/fspas.2022.809367.
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The VISualizing Ion Outflow via Neutral atom imaging during a Substorm (VISIONS) sounding rocket mission investigated the factors leading to ion outflow following a geomagnetic substorm. In situ and remote sensing instrumentation provided complementary measurements that have been combined to yield an in-depth look at the phenomena associated with ion outflow. In particular, the inclusion of instrumentation that provided high spatial and temporal resolution “images” of low-energy neutral atom (ENA) emissions from the nightside auroral zone following a substorm has led to new insights. The observed ENAs were spatially structured, and strongly associated with regions of intense 630.0 nm auroral emissions. The ENAs in the auroral zone were predominantly up-going, consistent with thick-target scattering in the region where the ENA mean free path is close to or less than the atmospheric scale height.
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Heinson, Graham, Jingming Duan, Alison Kirkby, Kate Robertson, Stephan Thiel, Sasha Aivazpourporgou, and Wolfgang Soyer. "Lower crustal resistivity signature of an orogenic gold system." Scientific Reports 11, no. 1 (August 4, 2021). http://dx.doi.org/10.1038/s41598-021-94531-8.
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AbstractOrogenic gold deposits provide a significant source of the world’s gold and form along faults over a wide range of crustal depths spanning sub-greenschist to granulite grade faces, but the source depths of the gold remains poorly understood. In this paper we compiled thirty years of long-period magnetotelluric (MT) and geomagnetic depth sounding (GDS) data across western Victoria and south-eastern South Australia that have sensitivity to the electrical resistivity of the crust and mantle, which in turn depend on past thermal and fluid processes. This region contains one of the world’s foremost and largest Phanerozoic (440 Ma) orogenic gold provinces that has produced 2% of historic worldwide gold production. Three-dimensional inversion of the long-period MT and GDS data shows a remarkable correlation between orogenic gold deposits with > 1 t production and a < 20 Ω m low-resistivity region at crustal depths > 20 km. This low-resistivity region is consistent with seismically-imaged tectonically thickened marine sediments in the Lachlan Orogen that contain organic carbon (C), sulphides such as pyrite (FeS2) and colloidal gold (Au). Additional heat sources at 440 Ma due to slab break-off after subduction have been suggested to rapidly increase the temperature of the marine sediments at mid to lower crustal depth, releasing HS− ligands for Au, and CO2. We argue that the low electrical resistivity signature of the lower crust we see today is from a combination of flake graphite produced in situ from the amphibolite grade metamorphism of organic-carbon in the marine sediments, and precipitated graphite through retrograde hydration reactions of CO2 released during the rapid heating of the sediments. Thus, these geophysical data image a fossil source and pathway zone for one of the world’s richest orogenic gold provinces.
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Jennings, S., G. Heinson, D. Hasterok, and B. Kay. "Magnetotelluric support for edge-driven convection and shear-driven upwelling in the Newer Volcanics Province." Scientific Reports 13, no. 1 (April 4, 2023). http://dx.doi.org/10.1038/s41598-023-32403-z.
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AbstractIntraplate volcanic provinces present significant natural hazards to many populated regions globally but their origins are poorly understood. Though hypotheses involving mantle plumes are predominant, the Newer Volcanics Province of southeast Australia—a relatively young (< 4.5 Ma), EW trending collection of over 400 volcanic centres—is increasingly attributed to some combination of edge-driven convection (EDC) and shear-driven upwelling (SDU). In this paper, we provide magnetotelluric (MT) data in support of these geodynamic processes. Three-dimensional inversion of 49 new broadband MT sites, in combination with 143 previously collected broadband, long-period, and geomagnetic depth soundings, reveals an elongate zone of moderately low resistivity (∼ 10–300 Ω m) spanning the Mt Gambier subprovince at a depth of between 20 and 40 km. The newly defined Gambier Conductor is contiguous to, and orientationally aligned with, significant step in the seismically-defined lithosphere-asthenosphere boundary (LAB) presented by earlier studies. Moderately low resistivity is interpreted as fluid-catalysed alteration of iron-bearing crust resulting from percolating magmatic volatiles. We argue that localised low resistivity (< 10 Ω m) at ~ 25 km depth in the mid-lower crust is associated with 1.2–3.6% partial melt. Supporting evidence indicates possible crustal thickening from 5.8 Ma at a rate comparable to estimates of SDU-induced surface eruptions and previous NVP production rate estimates.