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The stability field of hydrous phases carried by subducting slabs regulates water transport along the subduction pathway. The ultimate vertical distribution of this water at different depths in Earth’s mantle is governed by the thermal state of the slab. A warm slab is considered to lose water mainly in the upper mantle. However, whether a warm slab can carry water into the lower mantle (LM) is uncertain because of the scarcity of geophysical observations. We report an electrical conductivity model of the mantle transition zone (MTZ) and uppermost LM beneath North America to confirm the water-transporting ability of a warm slab. A high-conductivity anomaly was identified beneath the western United States. The LM portion of this anomaly is interpreted as a hydrous region containing rehydrated stishovite, whereas the lower MTZ portion of the anomaly is ascribed to the presence of water (~0.8 wt%) released by the hydrous stishovite. We speculate that warm slabs, such as the Farallon slab, can cause the breakdown of large amounts of dense hydrous magnesium silicates, releasing water mainly into the upper mantle. This water rehydrates stishovite when percolating through the slab, enabling the slab to continue to transport water into the LM. The identified high-conductivity anomaly, together with previously recognized electrical features in the upper mantle, demonstrates the existence of a trans-MTZ water transport system associated with the warm subduction of the Farallon slab.

Details on data processing and 3-D GDS inversion, robust tests and rock physics analysis of conductivity anomalies, and supplemental table and figures.

The magma plumbing systems of volcanoes in subduction and divergent tectonic settings are relatively well known, whereas those of intraplate volcanoes remain elusive; robust geophysical information on the magma pathways and storage zones is lacking. We inverted magnetotelluric data to image the magma plumbing system of an intraplate monogenetic volcanic field located above the stagnant Pacific slab in northeast China. We identified a complex, vertically aligned, low-resistivity anomaly system extending from the asthenosphere to the surface consisting of reservoirs with finger- to lens-like geometries. We show that magma forms as CO2-rich melts in a 150-km-deep asthenospheric plume crossing the whole lithosphere as hydrated melt, inducing underplating at 50 km depth, evolving in crustal reservoirs, and erupting along dikes. Intraplate volcanoes are characterized by low degrees of melting and low magma supply rates. Their plumbing systems have a geometry not so different from that of volcanoes in subduction settings.

Songyuan is the most earthquake prone area in northeast China. Since 2006, earthquakes have occurred in the area in the form of swarms, with a maximum magnitude of M L5.8. There is much controversy about the cause of the Songyuan earthquakes. We attempted to determine the cause using a three-dimensional electrical conductivity structure inverted from a regional network of magnetotelluric data in the Songyuan area. The L-BFGS inversion method was applied, with a full-impedance tensor data set used as the inversion input. Combined with an evaluation of the earthquake locations, the resistivity model revealed a northeast-oriented hidden fault running through the Songyuan earthquake area (SEA), which was speculated to be the preexisting Fuyu-Zhaodong Fault (FZF). Our resistivity model also found an apparent lithospheric low-resistivity anomaly beneath the earthquake area, which breached the high-resistivity lithospheric mantle and stalled at the base of the crust. A petrophysical analysis showed that this lower crustal low-resistivity anomaly was most likely attributed to hydrated partial melting, which could release water into the lower crust during later magma emplacements. While weakening the strength of the FZF, these ascending fluids also increased the pore pressure in the fault, further reducing the shear strength of the fault. Shear stress action (a fault strike component of the east-west regional compress), together with possible near-surface disturbances, may drive the fault to slip and trigger the earthquakes in Songyuan. It is possible that the continuous replenishment of fluids from the deeper mantle forces the Songyuan earthquakes into the form of swarms. We infer that the Songyuan earthquakes could be attributed to a combination of preexisting faults, regional stress, and deep fluids associated with plate subduction, and near surface disturbances might induce the earthquakes in advance. The Songyuan earthquakes are inherently induced earthquakes, fed by deep fluids.

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.

The three-dimensional (3-D) electrical structure of the upper-mantle was used to examine the deep origins of and relationship among the Cenozoic volcanoes located in Northeast China (NEC). High-quality, long-period magnetotelluric (LMT) full-impedance tensor data were collected in NEC and subjected to 3-D Gauss-Newton inversion in order to construct a resistivity model. The resulting model reveals the presence of multiple localized low-resistivity anomalies (LRAs) within the high resistivity lithosphere beneath NEC. These LRAs partially coincide with Cenozoic volcanoes on the surface. Three LRAs that form a larger, annular LRA were observed in the deep upper mantle beneath the Songliao Basin, whereas vein-like LRAs were found in the asthenosphere that connect the lithosphere and deep upper mantle. Petrophysical analyses suggest that the LRAs may have been caused by fluid-induced melting. Based on our electrical model, we propose that, following dehydration of the subducted Western Pacific slab into the mantle transition zone (MTZ) beneath NEC, the released water migrated upward and caused partial melting at the top of the MTZ beneath the Songliao Basin. Under the effect of buoyancy, the melted mantle formed a thermal upwelling that caused melting of asthenosphere before diapiring at the base of the dry lithosphere. The magma then penetrated structural boundaries (such as thinner, weaker, or activated suture zones) and finally reached the Earth’s surface. This melting and upwelling of hot mantle materials may have resulted in large-scale volcanism in the region throughout the Cenozoic, including the eruption of Changbai Mountain and Halaha Volcanoes. Our results suggest that the Cenozoic NEC volcanoes may all share a similar mode of genesis, and probably originated from the annular LRA in the deep upper mantle.

We describe a three‐dimensional electrical exploration system, and its use in carrying out a three‐dimensional survey on a polymetallic deposit located in China's Gansu Province. A time‐domain‐induced polarization method, along with a controlled‐source audiomagnetotelluric method, was used to survey the same station points for comparison. Then, a three‐dimensional inversion based on the incomplete Gauss–Newton method (for time‐domain‐induced polarization) and a nonlinear conjugate gradient (for controlled‐source audiomagnetotelluric) method were developed and applied in order to model the data sets. Four subsurface targets were mapped by the three‐dimensional electrical exploration method. A strong correlation was found between the zones of the revealed ore‐bearing anomalies and the geological setting. Using the results of the three‐dimensional electrical exploration, we revealed the spatial distribution of the ore‐bearing stratum, as well as the relationship between the ore bodies and the stratum. The results potentially provided evidence for future interpretations of deep geological structures, along with evaluations of mineral resources.

Details on the 3-D magnetotelluric inversion, sensitivity test of resistivity anomalies and melt fraction estimation, and Figures S1–S11.

Previous inversions of surface magnetometric resistivity (MMR) could obtain only the ratio, not the absolute value of earth electrical conductivity. In this paper, instead of solving Poisson's equation, the magnetic fields generated by low frequency alternating current(AC) line are looked as MMR data and calculated from electric-type Helmholtz's equation in frequency domain, so that the absolute conductivity could be taken into account. Forward modeling scheme is adapted from staggered-grid finite-difference method. Inversion algorithm is non-linear conjugate gradient (NLCG). The magnetic field perpendicular to line source is selected as observed data. To improve depth resolution, we introduce a model reconstruction strategy based on reference model. In the strategy, the upper portion of the resultant model from previous inversion is replaced by a homogeneous layer to construct a new model to trigger next inversion. The thickness of the homogeneous layer is called reconstruction depth, it is equal to the depth where the models difference between last two iterations after the run of the i th inversion is almost none-zero. The reconstruction could be applied until the model difference is almost zero or the difference of model differences between two adjacent inversions is less than a predefined threshold. Inversion results from synthetic and real data have proved that the proposed multi-reconstruction inversion strategy is able to obtain absolute resistivity, as well as enhance the deep resolution of surface MMR data inversion .

The computationally expensive cost and complexity are practical challenges in solving the three-dimensional (3-D) inverse problem in the time domain, and many real-world surveys are still inverted in one dimension (1-D). We present a new way to invert transient electromagnetic (TEM) data from a rectangular fixed loop to image subsurface electrical conductivity, which overcomes this limitation and permits efficient implementation of the inversion process on a personal computer. A TEM signal was transformed to the real and imaginary components of a frequency-domain (FD) electromagnetic (EM) response. The transformation was performed by a regularization inversion method. The recovered FD EM response was used as the data for the 3-D inversion. The forward problem and sensitivity were solved by using a staggered-grid finite-difference technique in the FD, and the background Green's function was carried out using the virtual interface method. The non-linear conjugate gradient method (NLCG) was employed to solve the 3-D inverse problem. A synthetic example revealed the basic functionality of the approach, and the FD inversion of a TEM field dataset acquired in the northeastern Qinshui Basin, Shanxi Province, China demonstrated a real-world application. Our inversion approach is general and suitable for different-type sources, such as a vertical magnetic dipole or horizontal electric dipole .

In this paper, we introduce a technique to obtain smoother C-responses in geomagnetic depth sounding. Instead of frequency by frequency, C-responses in entire frequency range are estimated simultaneously. In order to do so, after obtaining the spectrum of Hy and Hz, the equations relating C-response with Hy and Hz are combined for a batch frequency to form a linear equation set, from which, incorporating the requirement of smooth constraint on C-responses, we could define a regularized least-squared inversion problem. And to minimize influence of noise, data squared coherencies between Z and H signals are used to weight prediction error in order to suppress unreliable data. Synthetic data tests indicate that using the optimal regularization parameter determined by V-curve method other than L-curve method, regularization method could produce rather smooth C-responses from noise interfered signals. C-responses in Northeast China are thus obtained and then 1-D inversion is applied, from the obtained electrical and combining the Global and regional tomographic results, it is speculated that the stagnant plate in MTZ leads the high conductivity anomaly in Northeast China.

We present a means of processing data from frequency-domain controlled-source electromagnetic method observations via a Nyquist diagram. For a controlled-source electromagnetic field in the frequency domain, the x component of the electric field and the y and z components of the magnetic fields were chosen for discussion. Typical 1-D models and a 3-D model, which have a single anomalous body embedded in a half space, have been used to calculate the complex electromagnetic fields. The corresponding Nyquist diagrams have been plotted and analyzed. The diagrams can reveal the magnitude-phase frequency characteristics of an electromagnetic field and can be used to guide data processing by considering the constraints of the intrinsic connection between amplitude and phase. The results further show that: (1) the Nyquist diagrams of both electric and magnetic responses, varying from low to high frequencies, are all characterized by continuous, single-direction, clockwise spiral curves; (2) for the magnetic field, the opening direction of the Nyquist curve is upward for the y and downward for the z components, regardless of the change in resistivity; (3) for the electric fields, the resistivity changes also lead the spiral structure to take on complicated shapes, such as a snail-like shape or a W, and the starting points of the Nyquist curves depend on the variations in the resistivity with depth. According to the aforementioned rules, we can obtain reliable and reasonably complex field data during the data processing of frequency-domain controlled-source electromagnetic method, which lays a foundation for 3-D frequency-domain data inversion.

Wide applications of time-domain electromagnetic (TEM) data require 3D inversion. A possible strategy is to use the developed 3D inversion algorithms in frequency-domain (FD) electromagnetic (EM) methods. Thus, the key of the strategy is how to transform the time-domain ([Formula: see text]-[Formula: see text]) EM signal into the FD. An inversion algorithm has been developed to transform the [Formula: see text]-[Formula: see text] signal into a corresponding FD response. In this method, a step-off current is presumed. Under this assumption, the Fourier transform relating the EM FD response to the [Formula: see text]-[Formula: see text] signal becomes a sine or cosine transformation. Using the polynomial approximation method, the transformation turns into a linear equation. From a set of [Formula: see text]-[Formula: see text] signals, FD responses could be obtained by solving these linear equations in the least-squares sense. To reduce the nonuniqueness of the solution, and enhance the solution stability, an additional smoothness constraint on the FD response is imposed, thus converting the minimization problem into a regularization inversion problem. The algorithm is applied to synthetic and field vertical magnetic data in the in-loop TEM surveying mode. The numerical results show that in the entire audio-frequency range, the relative errors between the inversed and theoretical FD responses of the real and imaginary parts are almost all less than 1%, with the largest discrepancy of 5% occurring at high frequencies. There are two significances behind our work: First, the possibility of accurately transforming [Formula: see text]-[Formula: see text] response into FD response in audio-frequency range is coming into true, thereby (from the mathematical perspective) implementing the equivalence between the responses of the EM method in the time domain and the FD. Second, the algorithm provides a new approach to interpret TEM data in 3D mode by using developed 3D FEM inversion techniques.

We present a method to unify the calculation of Green’s functions for an electromagnetic (EM) transmitting source embedded in a homogeneous stratified medium. A virtual interface parallel to layer interfaces is introduced through the source location. The potentials for Green’s function are derived by decomposing the partial wave solutions to Helmholtz’s equations into upward and downward within boundaries. The amplitudes of the potentials in each stratum are obtained recursively from the initial amplitudes at the source level. The initial amplitudes are derived by coupling with the transmitting sources and following the discontinuity of the tangential electric and magnetic fields at the source interface. Only the initial terms are related to the transmitting sources and thus need to be modified for different transmitters, whereas the kernel connected with the stratified media stays unchanged. Hence, the present method can be easily applied to EM transmitting sources with little modification. The application of the proposed method to the marine controlled-source electromagnetic method (MCSEM) demonstrates its simplicity and flexibility.

Impact of the shape of grounded-line sources on the frequency domain response observed in the controlled source electromagnetic (CSEM) method has not been addressed thoroughly. In this paper, we use the electromagnetic (EM) fields from a half space as examples to discuss the impact of shape. The EM fields from current dipoles along a curved source are summarized. To illustrate the impact, the ratios of the fields from a curved line source to that of a straight line source with the same current are computed. Numerical results show that the impact of source shape on EM fields occurs mainly from a few hertz to higher frequencies. In our examples, shape deformation causes about 7% variation in the real part, and up to a relative error of ~22.5% in the imaginary part at middle frequencies. However, for the derived impedance, the impact can be ignored. Spatially, in the coaxial zone, the impact of source curvature can also be ignored; but in the equatorial zone near the source, the impact needs to be considered. In our examples, the maximal distance between a curved source and the corresponding straight line source is about 1/4 of the spacing between two current poles, more curved than usually seen in field work. Therefore, our results indicate that we do not need to care about source shape when working in a far zone or with source curvature smaller than that of the sources here. There are two possible ways to lessen the shape impact. One is to lay out grounded line source in a Z shape, but a more complete way is by using impedance derived from orthogonal electrical and magnetic fields to present observed data.

Controlled-source electromagnetic method can now survey along several profiles in a working area to span a 3D data set. However, these data are commonly interpreted by 2D models along each profile, and combined together to create a persudo-3D resistivity volume. Meanwhile, to obtain reasonable results, data collection must meet special requirements and undergo corrections, e.g., far-field and static correction (Zonge, 1989). Besides, other factors, like source effect and inhomogeneous between source and receiver, can distort the 2D inversion model seriously. In this case, 3D inversions are helpful.