Comparison of 3D, 2D, and 1D Magnetotelluric Inversion Results on the Example of Data from Fore-Sudetic Monocline

This study’s main objective is to better define and understand results for the most commonly used inversion algorithms in magnetotelluric data interpretation as part of geological exploration of the region of the Dolsk fault and the Odra fault. The data obtained from the eastern part of Fore-Sudetic Monocline measurements were used to describe the boundaries of lithospheric blocks (terranes) and recognize their origin. The magnetotelluric (MT) soundings were carried out to achieve this goal. There were conducted 51 soundings on five quasiparallel profiles. That allows constructing a quasiregular mesh in the area of the Fore-Sudetic Monocline. This arrangement of the measuring grid allowed reducing the influence of the largest sources of disturbances on MT data. 1D and 2D models were created by using the inverse algorithms. The models were prepared for each profile separately. Further, parallel (ModEM) 3D inversion codes were applied. The area where the investigation was done involves the region of the Dolsk fault and the Odra fault. These zones are essential geologic borders of a regional nature, and they pull apart the crust blocks with different origins. It was vitally needed to correctly identify the crust and upper mantle structure around a part of the Fore-Sudetic Monocline. The paper shows how these key features of the geological structures are revealed using 1D, 2D, and 3D algorithms.

Application of Infrared Remote Sensing and Magnetotelluric Technology in Geothermal Resource Exploration: A Case Study of the Wuerhe Area, Xinjiang

As a new green energy source, geothermal resource’s exploration, development, and utilization are an important direction in geophysical exploration at present. In this study, the actual land surface temperature was inferred based on the thermal infrared band of Landsat8 remote-sensing images, and the information about the surface anomalies and their spatial distribution was obtained through a multifactor analysis. In addition, three magnetotelluric sounding profiles were deployed in the study area, and the geo-electric sections in the study area were obtained through inversion of the measured data. Then, based on the inverse geo-electric information and the land surface temperature anomaly information, we analyzed and verified the geothermal resource genesis of the thermal anomaly area and inferred the favorable geothermal resource area in the study area. The results show that these two methods can be used to compare and analyze the possible distribution of the geothermal resources in the study area in two dimensions: the spatial distribution on the surface and the vertical distribution in the subsurface. Moreover, the results of the geothermal anomalies inferred from the thermal infrared remote sensing and the geo-electric results inferred from the magnetotelluric data are in good agreement. This study demonstrates that the integrated application of thermal infrared remote sensing and magnetotelluric technology is a promising tool for geothermal exploration.

Necessity of Terrain Correction in Magnetotelluric Data Recorded from Garhwal Himalayan Region, India

The magnetotelluric (MT) method is one of the useful geophysical techniques to investigate deep crustal structures. However, in hilly terrains, e.g., the Garhwal Himalayan region, due to the highly undulating topography, MT responses are distorted. Such responses, if not corrected, may lead to the incorrect interpretation of geoelectric structures. In the present paper, we implemented terrain corrections in MT data recorded from the Garhwal Himalayan Corridor (GHC). We used AP3DMT, a 3D MT data modeling and inversion code written in the MATLAB environment. Terrain corrections in the MT impedance responses for 39 sites along the Roorkee–Gangotri profile in the period range of 0.01 s to 1000 s were first estimated using a synthetic model by recording the topography and locations of MT sites. Based on this study, we established the general character of the terrain and established where terrain corrections were necessary. The distortion introduced by topography was computed for each site using homogenous and heterogeneous models with actual topographic variations. Period-dependent, galvanic and inductive distortions were observed at different sites. We further applied terrain corrections to the real data recorded from the GHC. The corrected data were inverted, and the inverted model was compared with the corresponding inverted model obtained with uncorrected data. The modification in electrical resistivity features in the model obtained from the terrain-corrected response suggests the necessity of terrain correction in MT data recorded from the Himalayan region.

Crust and Upper Mantle Inhomogeneities Beneath Western North Island, New Zealand: Evidence from Seismological and Electromagnetic Data.

<p>Three geophysical techniques have been used to investigate the location and the nature of a large-scale change in crust and uppermost mantle properties below the western North Island of New Zealand. Receiver function analysis reveals a step like change in crustal thickness from ~ 25 km below the northwestern North Island to ≥ 32 km in the southwestern North Island. P-wave attenuation is elevated north of this change in crustal thickness (1000/Qp ≈ 1.9 for α = 0) and is compatible with a wet mantle at near solidus temperatures (T ≈0.97 melting temperature). Attenuation decreases by at least a factor of 2 for the southwestern North Island to values closer to those expected for normal continental lithosphere (1000/Qp ≤ 1 for α = 0). A region of extremely high attenuation (1000/Qp ≈ 5 for α = 0) is observed below the Central Volcanic Region. This value of attenuation is compatible with a wet mantle at temperatures just above melting (T ≈ 1.02 melting temperature). Finally 2D modelling of magnetotelluric data reveals a region of low electrical resistivity (100 Ωm) in the mantle below the region of thinned crust. Like the P-wave attenuation, this region of low resistivity can be explained by a water-saturated mantle at near solidus temperatures (T=0.88-0.97 melting temperature). The changes in crustal thickness, attenuation and electrical resistivity are all coincident with the southern limit of volcanism (~ 39.3°S) at a boundary that runs approximately east-west, perpendicular to the present plate boundary. The only surface expressions of this boundary are the termination of volcanism and the dome-like uplift of the North Island, which has previously been explained by the presence of a buoyant low-density mantle beneath the northwestern North Island. Elevated temperatures and water content inferred from this study are in agreement with this explanation. The sudden transition displayed in all three data sets, but particularly the crustal thickness step seen in the receiver function, calls for a special explanation. Thermal processes are too diffuse to explain the step and instead a mechanical process is called for. One possibility is that the step was created by convective removal of thickened lithosphere.</p>