Chaojian Chen

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Chen

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Chaojian

ORCID

0000-0001-7869-0724

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Publications1 - 10 of 13

Enormously large tippers observed in southwest China: can realistic 3-D EM modeling reproduce them?
Xu, Shan; Chen, Chaojian; Kruglyakov, Mikhail; et al. (2023)
Earth, Planets and Space
Vertical magnetic transfer functions (tippers) estimated at a global/continental net of geomagnetic observatories/sites can be used to image the electrical conductivity structure of the Earth’s crust and upper mantle (down to around 200 km). We estimated tippers at 54 geomagnetic observatories across China, aiming eventually to invert them in terms of subsurface three-dimensional (3-D) conductivity distribution. Strikingly, we obtained enormously large tippers at three inland observatories in southwest China. Large tippers are often observed at coastal/island observatories due to high conductivity contrasts between resistive bedrock and conductive seawater. However, tippers at those inland observatories appeared to be a few times larger than coastal/island tippers. As far as we know, such large tippers (reaching value 3) were never reported in any region worldwide. We perform electromagnetic simulations in 3-D conductivity models mimicking the geological setting and demonstrate that enormously large tippers are feasible and can be attributed to a current channeling effect.
Improved shift-invariant sparse coding for noise attenuation of magnetotelluric data
Li, Guang; Liu, Xiaoqiong; Tang, Jingtian; et al. (2020)
Earth, Planets and Space
Magnetotelluric (MT) method is widely used for revealing deep electrical structure. However, natural MT signals are susceptible to cultural noises. In particular, the existing data-processing methods usually fail to work when MT data are contaminated by persistent or coherent noises. To improve the quality of MT data collected with strong ambient noises, we propose a novel time-series editing method based on the improved shift-invariant sparse coding (ISISC), a data-driven machine learning algorithm. First, a redundant dictionary is learned autonomously from the raw MT data. Second, cultural noises are reconstructed using the learned dictionary and the orthogonal matching pursuit (OMP) algorithm. Finally, the de-noised MT data are obtained by subtracting the reconstructed cultural noises from the raw MT data. The synthetic data, field experimental data and measured data are tested to verify the effectiveness of the newly proposed method. The results show that our new scheme can effectively remove strong cultural noises and has better adaptability and efficiency than the predefined dictionary-based methods. The method can be used as an alternative when a remote reference station is not available.
Imaging oceanic electrical conductivity structures by jointly inverting multi-source electromagnetic transfer functions. An approach, implementation and results
Chen, Chaojian (2022)
Multi-channel geomagnetic signal processing based on deep residual network and MVMD
Li, Guang; Zheng, HaoHao; Cai, HongZhu; et al. (2023)
Chinese journal of geophysics
The geomagnetic data are of great value in earthquake prediction, space weather monitoring, mineral resources exploration, and deep structure exploration of the earth. However, the geomagnetic data are increasingly polluted by cultural noise, which greatly complicates the high-precision imaging of the earth's interior. Therefore, we extend the deep residual network (ResNet) and multivariate variational mode decomposition (MVMD) to the processing of geomagnetic signals and propose a novel multi-channel geomagnetic signal processing method. Firstly, a large number of manually labeled data sets are trained by ResNet to obtain a signal-to-noise recognition model. Then the trained model is used to identify the noisy fragments from the raw observation signal. Hereafter, MVMD is adopted to perform multi-channel signal-to-noise separation on noisy segments, and the denoised segments are obtained. Finally, the noisy segments in the original observation signal are replaced by the denoised segments to obtain a complete high-quality signal. To verify the effectiveness of the method, we designed simulation experiments. The results show that the proposed method can improve the signal-to-noise ratio of the observed signal by about 15 dB, which has obvious advantages over VMD, complementary ensemble empirical mode decomposition (CEEMD), mathematical morphological filtering (MMF), and Wavelet, and is suitable for the batching processing of multi-channel signals. We apply the proposed method to the geomagnetic data observed in the Philippine Sea and the Western Pacific Ocean. The results show that the recognition accuracy of the proposed method is about 98%, and can greatly improve the signal quality. The normalized cross-correlation between the processed signal and the high-quality signal of the adjacent station at the same time has increased from 94.75% before denoising to 97.34%, indicating that the result is reliable. Our method is expected to improve the accuracy and reliability of geomagnetic data imaging.
Constraining the Crustal and Mantle Conductivity Structures Beneath Islands by a Joint Inversion of Multi‐Source Magnetic Transfer Functions
Chen, Chaojian; Kuvshinov, Alexey; Kruglyakov, Mikhail; et al. (2023)
Journal of Geophysical Research: Solid Earth
In this study, we present a tool to simultaneously invert multi-source magnetic transfer functions (TFs), including tippers, solar global-to-local TFs originating from the signals due to ionospheric source, and global Q-responses originating from the signals due to magnetospheric source. We jointly invert the aforementioned TFs to constrain the local conductivity structures beneath three islands in the Atlantic (Tristan da Cunha), Indian (Cocos), and Pacific (Oahu) Oceans. The recovered conductivity profiles appeared to be consistent with the presence of upper mantle plumes beneath the Tristan da Cunha and Oahu Islands. Our results indicate resistive lithosphere of different thicknesses beneath considered three islands. Besides, new conductivity profiles suggest warmer-than-average mantle temperatures and the presence of a small fraction of melt beneath Tristan da Cunha Island. At the same time, the conductivities beneath Cocos Island are in good agreement with estimates expected for ambient mantle conditions.
The Western Segment of the Precambrian Suture Between the Yangtze and Cathaysia Blocks: Constraints From Magnetotelluric Data in Southwest China
Xu, Shan; Hu, Xiangyun; Chen, Chaojian; et al. (2023)
Geophysical Research Letters
The western segment of the suture zone between the Yangtze and Cathaysia blocks, which is the most important tectonic boundary related to the formation and rifting of south China, is enigmatic and not fully understood due to the sporadic exposure of Precambrian strata and ophiolites. Three-dimensional electrical resistivity models derived from inversion of magnetotelluric data identified a lithospheric-scale conductive zone extending northeastwards beneath the Youjiang basin, which was interpreted as the western segment of the suture zone. The high conductivity and coincident high magnetic anomalies closely match the location of Carlin-type gold deposits, which can be explained by fluids and gold-bearing sulfide minerals in a fossil suture zone. Inconsistent with the southeast-dip resolved at the eastern segment of the suture zone (the Jiangshan-Shaoxing fault) in the earlier study, the slightly north-dipping geometry at the western suture zone implies the reactivation by northward subduction and closure of the Paleo-Tethys Ocean.
Advanced three-dimensional electromagnetic modeling using a nested integral equation approach
Chen, Chaojian; Kruglyakov, Mikhail; Kuvshinov, Alexey (2021)
Geophysical Journal International
Most of the existing 3-D electromagnetic (EM) modelling solvers based on the integral equation (IE) method exploit fast Fourier transform (FFT) to accelerate the matrix–vector multiplications. This in turn requires a laterally uniform discretization of the modelling domain. However, there is often a need for multiscale modelling and inversion, for instance, to properly account for the effects of non-uniform distant structures and, at the same time, to accurately model the effects from local anomalies. In such scenarios, the usage of laterally uniform grids leads to excessive computational loads, in terms of both memory and time. To alleviate this problem, we developed an efficient 3-D EM modelling tool based on a multinested IE approach. Within this approach, the IE modelling is first performed at a large domain and on a (laterally uniform) coarse grid, and then the results are refined in the region of interest by performing modelling at a smaller domain and on a (laterally uniform) denser grid. At the latter stage, the modelling results obtained at the previous stage are exploited. The lateral uniformity of the grids at each stage allows us to keep using the FFT for the acceleration of matrix–vector multiplications. An important novelty of the paper is the development of a ‘rim domain’ concept that further improves the performance of the multinested IE approach. We verify the developed tool on both idealized and realistic 3-D conductivity models, and demonstrate its efficiency and accuracy.
Revisiting Piezoelectric Sensor Calibration Methods Using Elastodynamic Body Waves
Wu, Rui; Selvadurai, Paul Antony; Chen, Chaojian; et al. (2021)
Journal of Nondestructive Evaluation
The application of absolutely calibrated piezoelectric (PZT) sensors is increasingly used to help interpret the information carried by radiated elastic waves of laboratory/in situs acoustic emissions (AEs) in nondestructive evaluation. In this paper, we present the methodology based on the finite element method (FEM) to characterize PZT sensors. The FEM-based modelling tool is used to numerically compute the true Green’s function between a ball impact source and an array of PZT sensors to map active source to theoretical ground motion. Physical-based boundary conditions are adopted to better constrain the problem of body wave propagation, reflection and transmission in/on the elastic medium. The modelling methodology is first validated against the reference approach (generalized ray theory) and is then extended down to 1 kHz where body wave reflection and transmission along different types of boundaries are explored. We find the Green’s functions calculated using physical-based boundaries have distinct differences between commonly employed idealized boundary conditions, especially around the anti-resonant and resonant frequencies. Unlike traditional methods that use singular ball drops, we find that each ball drop is only partially reliable over specific frequency bands. We demonstrate, by adding spectral constraints, that the individual instrumental responses are accurately cropped and linked together over 1 kHz to 1 MHz after which they overlap with little amplitude shift. This study finds that ball impacts with a broad range of diameters as well as the corresponding valid frequency bandwidth, are necessary to characterize broadband PZT sensors from 1 kHz to 1 MHz.
Exact gravity field for polyhedrons with polynominal density contrasts of arbitrary orders
Ren, Zhengyong; Chen, Chaojian; Zhong, Yiyuan; et al. (2018)
arXiv
New analytical expression of the magnetic gradient tensor for homogeneous polyhedrons
Ren, Zhengyong; Chen, Huang; Chen, Chaojian; et al. (2019)
Geophysics

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Chaojian Chen

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