Two-dimensional inversion of magnetotelluric/radiomagnetotelluric data by using unstructured mesh

Geophysics ◽  
2017 ◽  
Vol 82 (4) ◽  
pp. E197-E210 ◽  
Author(s):  
Özcan Özyıldırım ◽  
Mehmet Emin Candansayar ◽  
İsmail Demirci ◽  
Bülent Tezkan
Keyword(s):  
Surface Topography ◽  
Unstructured Grid ◽  
Unstructured Mesh ◽  
Synthetic Data ◽  
Solution Technique ◽  
Inversion Algorithm ◽  
2D Inversion ◽  
Two Samples ◽  
Speed And Accuracy ◽  
Grid Based

We have compared structured and unstructured grid-based 2D inversion algorithms for magnetotelluric (MT) and radiomagnetotelluric (RMT) data in terms of speed and accuracy. We have developed a new 2D inversion algorithm for MT and RMT data by using a finite-element (FE) method that uses unstructured triangle grids. We compare the inversion results of our unstructured grid-based algorithm with those of the conventional algorithm, which uses either a structured FE or structured finite-difference (FD) numerical solution technique. The imaging of the surface topography and the underground resistivity structures by the new algorithm requires fewer elements than those that use FE and FD structured grids. We also find that when unstructured grids are used, the quality of the mesh is increased and the numerical errors are significantly reduced. Thus, the program runs faster and can simulate the complex surface topography in a more stable setting than the classic inversion algorithms. Furthermore, we implement a new smoothing matrix format for the unstructured triangle grids for the inversion procedure. We use two samples of synthetic data for the MT and RMT frequencies as well as a sample of field RMT data collected across a fault zone for comparison. In our synthetic data experiment, we find that the resistivity values and the boundaries obtained from the inversion of the unstructured mesh are closer to those of the true a priori synthetic model. Results of the synthetic and field data verify the computational advantages (speed and accuracy) of our inversion algorithm with respect to the conventional structured grid-based inversion algorithms.

Quasi-2D inversion of DCR and TDEM data for shallow investigations

Geophysics ◽  
2011 ◽  
Vol 76 (4) ◽  
pp. F239-F250 ◽  
Author(s):  
Fernando A. Monteiro Santos ◽  
Hesham M. El-Kaliouby
Keyword(s):  
Joint Inversion ◽  
Synthetic Data ◽  
Data Sets ◽  
Complex Environments ◽  
Inversion Algorithm ◽  
2D Inversion ◽  
New Approach ◽  
Earth Models ◽  
Time Domain Electromagnetic ◽  
Inversion Techniques

Joint or sequential inversion of direct current resistivity (DCR) and time-domain electromagnetic (TDEM) data commonly are performed for individual soundings assuming layered earth models. DCR and TDEM have different and complementary sensitivity to resistive and conductive structures, making them suitable methods for the application of joint inversion techniques. This potential joint inversion of DCR and TDEM methods has been used by several authors to reduce the ambiguities of the models calculated from each method separately. A new approach for joint inversion of these data sets, based on a laterally constrained algorithm, was found. The method was developed for the interpretation of soundings collected along a line over a 1D or 2D geology. The inversion algorithm was tested on two synthetic data sets, as well as on field data from Saudi Arabia. The results show that the algorithm is efficient and stable in producing quasi-2D models from DCR and TDEM data acquired in relatively complex environments.

Two-dimensional inversion of direct current resistivity data incorporating topography by using finite difference techniques with triangle cells: Investigation of Kera fault zone in western Crete

Geophysics ◽  
2012 ◽  
Vol 77 (1) ◽  
pp. E67-E75 ◽  
Author(s):  
Ismail Demirci ◽  
Erhan Erdoğan ◽  
M. Emin Candansayar
Keyword(s):  
Finite Element ◽  
Finite Difference ◽  
Surface Topography ◽  
Direct Current ◽  
Synthetic Data ◽  
Inversion Algorithm ◽  
Forward Solution ◽  
Data Set ◽  
Cpu Time ◽  
Resistivity Data

In this study, we suggest the use of a finite difference (FD) forward solution with triangular grid to incorporate topography into the inverse solution of direct current resistivity data. A new inversion algorithm was developed that takes topography into account with finite difference and finite element forward solution by using triangular grids. Using the developed algorithm, surface topography could also be incorporated by using triangular cells in a finite difference forward solution. Initially, the inversion algorithm was tested for two synthetic data sets. Inversion of synthetic data with the finite difference forward solution gives accurate results as well as inversion with finite element forward solution and requires less CPU time. The algorithm was also tested with a field data set acquired across the Kera fault located in western Crete, Greece. The fault location and basement depth of sedimentary units were resolved by the developed algorithm. These inversion results showed that if underground structure boundaries are not shaped according to surface topography, inversion using our finite difference forward solution with triangular cells is superior to inversion using our finite element forward solution in terms of CPU time and estimated models.

scholarly journals Three-Dimensional Inversion of Borehole-Surface Resistivity Method Based on the Unstructured Finite Element

2021 ◽  
Vol 2021 ◽  
pp. 1-13
Author(s):  
Zhi Wang ◽  
Sinan Fang
Keyword(s):  
Numerical Solution ◽  
Surface Topography ◽  
Current Source ◽  
Three Dimensional ◽  
Surface Resistivity ◽  
Normal Equation ◽  
Inversion Algorithm ◽  
Field Source ◽  
Resistivity Method ◽  
Borehole Surface

The electromagnetic wave signal from the electromagnetic field source generates induction signals after reaching the target geological body through the underground medium. The time and spatial distribution rules of the artificial or the natural electromagnetic fields are obtained for the exploration of mineral resources of the subsurface and determining the geological structure of the subsurface to solve the geological problems. The goal of electromagnetic data processing is to suppress the noise and improve the signal-to-noise ratio and the inversion of resistivity data. Inversion has always been the focus of research in the field of electromagnetic methods. In this paper, the three-dimensional borehole-surface resistivity method is explored based on the principle of geometric sounding, and the three-dimensional inversion algorithm of the borehole-surface resistivity method in arbitrary surface topography is proposed. The forward simulation and calculation start from the partial differential equation and the boundary conditions of the total potential of the three-dimensional point current source field are satisfied. Then the unstructured tetrahedral grids are used to discretely subdivide the calculation area that can well fit the complex structure of subsurface and undulating surface topography. The accuracy of the numerical solution is low due to the rapid attenuation of the electric field at the point current source and the nearby positions and sharply varying potential gradients. Therefore, the mesh density is defined at the local area, that is, the vicinity of the source electrode and the measuring electrode. The mesh refinement can effectively reduce the influence of the source point and its vicinity and improve the accuracy of the numerical solution. The stiffness matrix is stored with Compressed Row Storage (CSR) format, and the final large linear equations are solved using the Super Symmetric Over Relaxation Preconditioned Conjugate Gradient (SSOR-PCG) method. The quasi-Newton method with limited memory (L_BFGS) is used to optimize the objective function in the inversion calculation, and a double-loop recursive method is used to solve the normal equation obtained at each iteration in order to avoid computing and storing the sensitivity matrix explicitly and reduce the amount of calculation. The comprehensive application of the above methods makes the 3D inversion algorithm efficient, accurate, and stable. The three-dimensional inversion test is performed on the synthetic data of multiple theoretical geoelectric models with topography (a single anomaly model under valley and a single anomaly model under mountain) to verify the effectiveness of the proposed algorithm.

Unstructured Grid Based Monte Carlo Method for the Simulation of Light Propagation in Skin Tissues

2015 ◽  
Vol 42 (4) ◽  
pp. 0404001
Author(s):  
贾浩 Jia Hao ◽  
陈斌 Chen Bin ◽  
李东 Li Dong ◽  
张永 Zhang Yong
Keyword(s):  
Monte Carlo ◽  
Monte Carlo Method ◽  
Light Propagation ◽  
Unstructured Grid ◽  
Grid Based

scholarly journals An Unstructured-Grid Based Morphodynamic Model for Sandbar Simulation in the Modaomen Estuary, China

Water ◽  
2018 ◽  
Vol 10 (5) ◽  
pp. 611 ◽  
Author(s):  
Xiaozhang Hu ◽  
Fang Yang ◽  
Lixiang Song ◽  
Hangang Wang
Keyword(s):  
Unstructured Grid ◽  
Modaomen Estuary ◽  
Grid Based

3-D prestack full‐wavefield inversion

Geophysics ◽  
1995 ◽  
Vol 60 (6) ◽  
pp. 1805-1818 ◽  
Author(s):  
Tong Xu ◽  
George A. McMechan ◽  
Robert Sun
Keyword(s):  
Velocity Distribution ◽  
Synthetic Data ◽  
Compressional Wave ◽  
Multilayered Structure ◽  
Common Source ◽  
Inversion Algorithm ◽  
Reverse Time ◽  
Time Step ◽  
Unequally Spaced ◽  
Wavefield Inversion

A full‐wavefield inversion algorithm for direct imaging of a 3-D compressional wave velocity distribution is based on the full 3-D scalar wave equation and operates on common‐source data recorded by areal arrays. For each source, the method involves reverse‐time extrapolation of the residual wavefield. Application of the image condition by crosscorrelation with the source wavefield at each time step produces a 3-D image whose amplitude at each point is proportional to the required velocity update at that point. Convergence to local minima is mitigated against by gradually increasing the wavenumber bandwidth in the estimated 3-D velocity distribution as iterations proceed, starting from the smallest wavenumber. The algorithm is illustrated by successful application to synthetic data for a multilayered monocline, and for a multilayered structure with the geometry of the standard French model. The latter demonstrates good performance with noisy, unequally spaced data with significant elevation statics.

Simultaneous multifrequency inversion of full-waveform seismic data

Geophysics ◽  
2009 ◽  
Vol 74 (2) ◽  
pp. R1-R14 ◽  
Author(s):  
Wenyi Hu ◽  
Aria Abubakar ◽  
Tarek M. Habashy
Keyword(s):  
Seismic Data ◽  
Noise Suppression ◽  
Regularization Parameter ◽  
Synthetic Data ◽  
Frequency Data ◽  
Perfectly Matched Layers ◽  
Inversion Algorithm ◽  
Inversion Process ◽  
High Convergence Rate ◽  
Velocity Image

We present a simultaneous multifrequency inversion approach for seismic data interpretation. This algorithm inverts all frequency data components simultaneously. A data-weighting scheme balances the contributions from different frequency data components so the inversion process does not become dominated by high-frequency data components, which produce a velocity image with many artifacts. A Gauss-Newton minimization approach achieves a high convergence rate and an accurate reconstructed velocity image. By introducing a modified adjoint formulation, we can calculate the Jacobian matrix efficiently, allowing the material properties in the perfectly matched layers (PMLs) to be updated automatically during the inversion process. This feature ensures the correct behavior of the inversion and implies that the algorithm is appropriate for realistic applications where a priori information of the background medium is unavailable. Two different regularization schemes, an [Formula: see text]-norm and a weighted [Formula: see text]-norm function, are used in this algorithm for smooth profiles and profiles with sharp boundaries, respectively. The regularization parameter is determined automatically and adaptively by the so-called multiplicative regularization technique. To test the algorithm, we implement the inversion to reconstruct the Marmousi velocity model using synthetic data generated by the finite-difference time-domain code. These numerical simulation results indicate that this inversion algorithm is robust in terms of starting model and noise suppression. Under some circumstances, it is more robust than a traditional sequential inversion approach.

Accelerating unstructured grid-based seismic modeling on GPU

2014 ◽  
Author(s):  
Zhefeng Wei ◽  
ChengHong Zhu ◽  
Hongwei Gao ◽  
Jianfeng Zhang
Keyword(s):  
Unstructured Grid ◽  
Seismic Modeling ◽  
Grid Based
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