3D Magnetotelluric Inversion Research Articles

Three‐dimensional electrical structure of the crust and upper mantle in Ordos Block and adjacent area: Evidence of regional lithospheric modification

AbstractLong‐period magnetotelluric (MT) data from project SINOPROBE were acquired and modeled, using three‐dimensional (3D) MT inversion, to study the electrical structure of Ordos Block, a component of the North China Craton. For the first time, a high‐resolution 3D resistivity model of the lithosphere is defined for the region. Contrary to what would be expected for a stable cratonic block, a prominent lithospheric conductive complex is revealed extending from the upper mantle to the mid‐to‐lower crust beneath the northern part of Ordos. Correlating well with results of seismic studies, the evidence from our independent magnetotelluric data supports regional modification of the lithosphere under the north Ordos and lithosphere thinning beneath Hetao Graben. The abnormally conductive structure may result from upwelling of mantle material in mid‐to‐late Mesozoic beneath the northern margin of the Ordos block.

Three-dimensional magnetotelluric inversion using a heterogeneous smoothness-constrained least-squares method

This paper presents a fast algorithm for electromagnetic data inversion to three-dimensional (3D) resistivity models. The algorithm is distinctive for the level of accuracy it attains while bypassing the sensitivity matrix update. A common sensitivity matrix for homogeneous half-space is used in all iterations. Instead of updating the sensitivity matrix, the smoothness filter coefficients at each model element are updated, based on the spatial variations in resistivity in the model derived from the latest iteration. This substitution is expected not only to reduce the computation time required for large-scale inversions, such as those for 3D surveys, but also to allow the resolution of sharp boundaries in resistivity structures. Our algorithm was applied to 3D magnetotelluric inversion in order to confirm its effectiveness. Using synthetic examples under several conditions, we demonstrated that the method can reduce the number of forward calculations required to reduce data misfits to noise level, and that the method is robust for constructing target models even with sharp boundaries without generating fatally false resistivity structures or boundaries under noisy conditions.

Geophysical Inversion Enhancement Based on Blind Deconvolution

AbstractGeophysical systematic errors inevitably occur during acquisition and processing of data because of noise and discretization as well as incorrect geophysical forward modeling. In this paper, a new technology is proposed to enhance the geophysical inversion model without introducing any additional information. The relationship between the inversion model resolution matrix and the enhancing model is described. We suggest that more information could be extracted from inversion results by using the blind deconvolution method. Then, a mix‐norm TV regularization enhancement algorithm is proposed. Finally, we use the 1D and 2D magnetotelluric inversion models to prove the correctness and the effectiveness of the proposed methods. The results show that the proposed algorithm can increase geophysical model precision by 88.57%.

Three-dimensional inversion of large-scale EarthScope magnetotelluric data based on the integral equation method: Geoelectrical imaging of the Yellowstone conductive mantle plume

[1] Interpretation of the EarthScope MT (magnetotelluric) data requires the development of a large-scale inversion method which can address two common problems of 3D MT inversion: computational time and memory requirements. We have developed an efficient method of 3D MT inversion based on an IE (integral equation) formulation of the MT forward modeling problem and a receiver footprint approach, implemented as a massively parallel algorithm. This method is applied to the MT data collected in the western United States as a part of the EarthScope project. As a result, we present one of the first 3D geoelectrical images of the upper mantle beneath Yellowstone revealed by this large-scale 3D inversion of the EarthScope MT data. These images show a highly conductive body associated with the tomographically imaged mantle plume-like layer of hot material rising from the upper mantle toward the Yellowstone volcano. The conductive body identified in these images is west-dipping in a similar way to a P-wave low-velocity body.

Application of the 3D magnetotelluric inversion code in a geologically complex area

ABSTRACTThe WSINV3DMT code makes the implementation of 3D inversion of magnetotelluric data feasible using a single PC. Audio‐magnetotelluric data were collected along two profiles in a Cu‐Ni mining area in Xinjiang, China, where the apparent resistivity and phase curves, the phase tensors and the magnetic induction vectors indicate a complex 3D conductivity structure. 3D inversions were carried out to reveal the electrical structure of the area. The final 3D model is selected from the inversion results using different initial Lagrange values and steps. The relatively low root‐mean‐square (rms) misfit and model norm indicate a reliable electrical model. The final model includes four types of low resistivity areas, the first ones coincide with the known location of an orebody and further forward modelling indicates that they are not in full connectivity to form a low resistivity zone. The second ones are not controlled by magnetotelluric sites and embody little information of the observed data, they are considered as tedious structures. The third one is near to the regional Kangguer fault and should be treated carefully considering the effect of the fault. The last ones are isolated and existing at a limited level as the first ones, they should be paid more attention to.

Electrical Structure of the Middle Segment of the Tancheng‐Lujiang Fault Zone and Its Geological Implications

AbstractTancheng‐Lujiang fault zone is an important strike‐slip fault system in eastern China continent, and has significant status to study the forming, evolution and current structure of China continent. The magnetotelluric (MT) sounding profile from Alxa Zuoqi to Rizhao city of Shandong province crossed the middle part of Tancheng‐Lujiang fault zone at Juxian of Shandong province, where its strike angle is about NE20 degree as shown by strike analysis. The resistivity model of 2D MT inversion shows that the width of Tancheng‐Lujiang fault zone is about 30 km at where the profile crossed it, the main part of Tancheng‐Lujiang fault zone consists of two deep and steep faults, the western one dips to west steeply with a dip angle about 70 degrees and its penetration depth is about 60 km, the eastern one dips to east with a dip angle 60~80 degrees and its penetration depth is between 80 and 100 km. Both of these two faults cut through the crust, but limited in the lithosphere. The crust is resistive from eastern edge of Tancheng‐Lujiang fault zone to Rizhao city, which is very different to the west of the fault zone, this fact indicates that Tancheng‐Lujiang fault zone is the boundary between North China block and Jiao‐Liao‐Korea block.

Automatic Construction of a Site‐Centered Grid (SCG) for 2D Magnetotelluric Inversion

AbstractIn magnetotelluric (MT) inversion, when the used grid is not uniform or the positions of observation sites in the grid are not appropriate, site shift will occur which can affect the accuracy of inversion. Besides, in order to ensure accuracy and stability of forward modeling, spacing of the inversion grid cannot vary too much, requiring smoothness of some degree. To solve these issues, this paper suggests several methods of automatic constructing a site‐centered grid to eliminate site shift and using regularization constraints to control the smoothness of the grid. To realize this object this paper also proposes a set of test indexes for grid quality. Comparison of case calculations indicates that different site sets may require varied methods to construct optimal grids. Among them the “optimized dynamic allocation under flattest model constraint”, and “optimized dynamic allocation under smoothest model constraint” are most adaptive, by which the constructed grids have fairly smooth grid spacing and smaller site shifts. The comparison also shows that using a grid of high quality (smooth, little site shift) can enhance the reliability of inversion as well as efficiency of inversion calculation. The methods proposed in this paper provide a simple and effective approach to produce a proper grid for 2D MT inversion.

Parallel rapid relaxation inversion of 3D magnetotelluric data

We implement a parallel algorithm with the advantage of MPI (Message Passing Interface) to speed up the rapid relaxation inversion for 3D magnetotelluric data. We test the parallel rapid relaxation algorithm with synthetic and real data. The execution efficiency of the algorithm for several different situations is also compared. The results indicate that the parallel rapid relaxation algorithm for 3D magnetotelluric inversion is effective. This parallel algorithm implemented on a common PC promotes the practical application of 3D magnetotelluric inversion and can be suitable for the other geophysical 3D modeling and inversion.

Identifying the lithospheric structure of a Precambrian orogen using magnetotellurics: The Capricorn Orogen, Western Australia

A 300-km long magnetotelluric (MT) survey was carried out across the Capricorn Orogen in Western Australia. The Capricorn Orogen includes reworked crust of the Archaean Yilgarn and Pilbara Cratons, and the allochthonous latest Archaean to Palaeoproterozoic Glenburgh Terrane in the southern Gascoyne Complex and the Palaeoproterozoic northern Gascoyne Complex. The survey aimed to increase our understanding of the juxtaposition of these crustal elements by obtaining information about their electrical structure with depth. Phase tensor analysis of the data showed that 2D MT inversion could only be carried out for stations in the centre of the profile, crossing the Gascoyne Complex. The resulting model showed that there is no electrical distinction between the Glenburgh Terrane and the northern Gascoyne Complex, suggesting that the Glenburgh Terrane may form basement to the whole of the Gascoyne Complex. 3D forward modelling was carried out for the entire survey line, incorporating the basement blocks as well as the surrounding ocean and sedimentary basins. Station data are most closely reproduced by a model in which the margin between the Glenburgh Terrane and the Yilgarn Craton (the Errabiddy Shear Zone) dips south, with crust of the Glenburgh Terrane wedged beneath the northern Yilgarn Craton. Forward modelling results also suggest that the main boundary between the Gascoyne Complex and the Pilbara Craton to the north is a steeply dipping structure beneath the Edmund and Collier Basins that may correlate with the Talga Fault.

Hydrogeologic assessment of the Amchitka Island nuclear test site (Alaska) with magnetotellurics

Amchitka Island, in Alaska, was used for underground nuclear testing from 1965 to 1971. Since the test program concluded, there have been concerns about the possible release of radionuclides into the marine environment of the Aleutian Islands. The hydrogeology of islands such as Amchitka is characterized by a layer of freshwater overlying a saltwater layer, with the salinity increasing across a transition zone (TZ). Hydrogeologic modeling can provide an estimate of the timing and amount of radionuclide release from the explosions beneath Amchitka Island. This modeling is inconclusive because of a lack of information regarding subsurface structure. To address this problem, magnetotelluric (MT) data were collected on Amchitka Island in 2004. Broadband MT data were recorded on profiles passing through three explosion sites to give information about subsurface porosity and salinity. A 2D MT inversion produced models of sub-surface electrical resistivity and showed a pattern of increasing, decreasing, and increasing resistivity with depth at each test site. The depth at which resistivity begins to decrease defines the top of the TZ. The deeper increase in resistivity approximates the base of the TZ. The depths of the top and bottom of the TZ were determined as follows: Cannikin 900–2500 m; Long Shot 600–1700 m; Milrow 900–1700 m. Uncertainties were estimated for these depths. Effective porosities were also estimated and ranged from 10%–20% at the surface to 1%–3% at 3-km depth. These porosities are higher than those assumed in several hydrogeologic models, and give longer transit times from the explosion to the marine environment. Subject to the limits of the analysis, it appears that each of the cavities resulting from underground nuclear explosions is located in the TZ from fresh to saltwater. This implies shorter transit times to the marine environment than if the detonations had been located in the saltwater layer.

Three-dimensional magnetotelluric inversion: data-space method

A three-dimensional magnetotelluric (MT) minimum structure inversion algorithm has been developed based on a data-space variant of the Occam approach. Computational costs associated with construction and inversion of model-space matrices make a model-space Occam approach to 3D MT inversion impractical. These difficulties are overcome with a data-space approach, where matrix dimensions depend on the size of the data set, rather than the number of model parameters. With the transformation to data space it becomes feasible to invert modest 3D MT data sets on a PC. To reduce computational time, a relaxed convergence criterion is used for the iterative forward modeling code used to compute the sensitivity matrix. This allows reduction in computational time by more than 70%, without affecting the inversion results. Numerical experiments with synthetic data show that reasonable fits can be obtained within a small number of iterations, with a few additional iterations required to eliminate unnecessary structure and find the model with minimum norm.

Minimum support nonlinear parametrization in the solution of a 3D magnetotelluric inverse problem

In this paper we describe a new approach to sharp boundary geophysical inversion. We demonstrate that regularized inversion with a minimum support stabilizer can be implemented by using a specially designed nonlinear parametrization of the model parameters. This parametrization plays the same role as transformation into the space of the weighted model parameters, introduced in the original papers on focusing inversion. It allows us to transform the nonquadratic minimum support stabilizer into the traditional quadratic minimum norm stabilizer, which simplifies the solution of the inverse problem. This transformation automatically ensures that the solution belongs to the class of models with a minimum support. The method is illustrated with synthetic examples of 3D magnetotelluric inversion for an earth conductivity structure. To simplify the calculations, in the initial stage of the iterative inversion we use the quasi-analytical approximation developed by Zhdanov and Hursán (2000 Inverse Problems 16 1297–322). However, to increase the accuracy of inversion, we apply rigorous forward modelling based on the integral equation method at the final stage of the inversion. To obtain a stable solution of a 3D inverse problem, we use the Tikhonov regularization method with a new nonlinear parametrization. This technique leads to the generation of a sharp image of anomalous conductivity distribution. The inversion is based on the regularized conjugate gradient method.

3D inversion of a scalar radio magnetotelluric field data set

A radio magnetotelluric (MT) field data set, acquired in scalar mode, over a buried waste site has been successfully analyzed using a 3D MT inversion scheme using nonlinear conjugate gradients. The results of this analysis demonstrate the utility of the scheme where more than 4800 data points collected on multiple measurement profiles have been inverted simultaneously. The resulting image clearly detects the buried waste; when receiver profiles cross pit boundaries, the image maps the lateral extent of the pit. However, the base of the pit is poorly resolved, and depends upon the starting model used to launch the inversion. Hence, critical information on whether contamination is leaching into a resistive gravel bed lining the base of the pit, as well as the deeper geological horizons consisting of brown coal, clay, and tertiary sands, is inconclusive. Nevertheless, by incorporating within the inversion process a priori information of the background media that is host to the waste, sharper images of the base of the pit are obtained, which are in good agreement with borehole data. The 3D analysis applied in this paper overcomes previous limitations in the radio magnetotelluric (RMT) method using 2D data analysis and inversion. With 3D analysis, it is unnecessary to make assumptions regarding geological strike, and near‐surface statics can be accommodated in both source polarizations. Our findings also indicate that 2D MT interpretation can overestimate the pit's depth extent. This may lead to the erroneous conclusion that the geological horizons beneath the pit have been contaminated.

Deep structure of the northeastern margin of the Parnaiba Basin, Brazil, from magnetotelluric imaging

ABSTRACTThe magnetotelluric (MT) method has been applied to the determination of the deep resistivity structure of the northeastern margin of the Parnaiba Basin. Transient electromagnetic (TEM) and MT data were collected in early 1999 along a 95 km long N–S line, extending from the coast across the projected subcrop position of a discontinuous fault found to the west of the study area that is believed to be a possible basin‐bounding fault. The MT data were processed to yield the TE‐ and TM‐mode responses and then corrected for static shift using central‐loop and single‐loop TEM data, respectively. Regularized 2D MT inversion was subsequently undertaken using a structured initial model with the near‐surface constrained by TEM inversion results. As a consistency check, we performed another set of 2D inversions using different smooth initial models. The various optimal 2D inversion models show clearly the presence of a major basement trough, over 2 km deep, located about 70 km from the coast. We interpret it as possibly marking the main basin margin and suggest that it may have implications for groundwater resource development in the area.