Application of Magnetotelluric (MT) Resistivity to Imaging of Regional Three-Dimensional Geologic Structures and Groundwater Systems

Widespread definition of a groundwater system in three dimensions is necessary for the management and maintenance of groundwater resources. A magnetotelluric (MT) survey can be an effective geophysical prospecting method for imaging regional geological structures by measuring both shallow and deep resistivity. To demonstrate the capability of an MT survey to characterize a groundwater system, the Kumamoto area of central Kyushu in southwestern Japan was selected as a case study site because of its rich groundwater resources. Three-dimensional (3D) MT resistivity structure to a depth of 5?km was modeled by 1D inversion analysis of raw MT data and 3D interpolation of the resultant resistivity column data by the optimization principle method. Consequently, both deep and shallow aquifers were detected. A high-resistivity zone appears at depths between 500 and 2,000?m between the Futagawa?CHinagu faults and the Usuki?CYatsushiro tectonic line, which supports the existence of an aquiclude under the aquifer. The most important characteristic inferred from the 3D resistivity model is that the deep groundwater system below a depth of 1,000?m has two main flow paths. One path is likely to be through porous rocks because the low resistivity zone is regarded as tuff with sand and gravel, and the other flow path is interpreted to be through fractured zones along the Hinagu faults. Hence, the path and direction of the groundwater flows are probably controlled by geologic structures and the configuration of the active faults. These findings support the effectiveness of the MT method for investigating groundwater systems.     

高密度电阻率法在二次滑坡预报中的应用

松散土体、破碎岩体、较陡的地形以及大量的降雨容易引起山体的滑坡,怎样判断和预测滑坡的可能性则成为了人们越来越关心和重视的问题.高密度电阻率法是近些年引入的物探方法,广泛用于地质灾害调查及工程勘察中,本文分析了高密度电阻率法的原理和野外施工方法,通过应用实例说明高密度电阻率法在二次滑坡预报中的作用.     

Magnetotelluric interpretation of crustal and mantle structure in the Grenville Province

Summary Accurate determinations of depths and conductivities of electrical structures in shield regions are often difficult because of the inhomogeneity of the uppermost crust. A magnetotelluric (MT) station (BAT) in the Grenville Province of the Precambrian Shield in eastern Canada has been in operation since 1975 for time-dependency studies of electrical resistivity changes related to earthquakes. The MT response of the station displays low skew with small to moderate anisotropy. One-dimensional inversion of the apparent resistivity and phase reveals two well-defined conductors in the crust, one at 10 km and the second at the base of the crust. The latter has a resistivity less than 50 Ω m. These results are substantiated by three additional MT stations located up to 40 km distant.
Data from other new MT stations and from stations previously published in the literature are compared with two-dimensional computer model results and with the three-dimensional analogue scale model results of Dosso et al. While additional data for periods less than 100 s would be desirable the results from a number of the MT stations are not inconsistent with a widespread occurrence of a conducting zone at the base of the crust in the Grenville. The inversion analysis also indicates the existence of a conductor at some depth greater than 100 km with a resistivity less than 30 Ω m. This may coincide with a seismic low-velocity zone observed in the mantle under the Canadian Shield.     

Magnetotelluric studies in the Market Weighton area of eastern England

Summary Magnetotelluric measurements at periods from 30 to 1000s were made at eight locations in the Market Weighton (MW) area, along an east—west profile across gravity and magnetic anomalies. Dimensional parameters were developed for assessing the structural dimensionality of the electrical conductivity of the Earth from the data. One-dimensional inversion modelling techniques were employed to interpret the data at each site, and four-layer models were obtained to explain the main structure of the crust in the area studied. If it is assumed that all strata are unmagnetized then the results show that there is a highly resistive layer in the crust, the thickness of the highly resistive layer ranges from 12 km in the east to 44 km in the west with a large change in the middle near the MW site. A structural boundary lying north—south near MW was also indicated by the principal directions of rotated apparent resistivities and transfer functions. Both electrical conductivity and magnetic permeability contrast in the ground were considered in an attempt to interpret the observed variations in apparent resistivity at different periods.     

Soil Characterization Using Joint Interpretation of Remote Sensing,Resistivity and Induced Polarization Data along the Coast of the Nile Delta

Natural Resources Research - Soil characterization in coastal areas is essential for strategic coastal engineering implementation and for understanding the paleo-environmental conditions. However,...     

Magnetotelluric sounding study in the region of Zhongshan Station，East Antarctica

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Magnetotelluric image of the crust and upper mantle in the backarc of the northwestern Argentinean Andes

Magnetotelluric data from the backarc of the Central Andes in NW Argentinawere re-examined by employing impedance tensor decomposition and 2-D inversion and modelling techniques. The data in the period range of 50–15 000 s were collected on a profile of 220 km length reaching from the Eastern Cordillera across the Santa Barbara System to the Andean foreland of the Argentinean Chaco.
After a dimensionality analysis, data from most sites were treated as regional 2-D. The exception was the eastern section of the profile, where the magnetotelluric transfer functions for periods ≤ 1000 s reflect a 3-D earth. Application of two tensor decomposition schemes yielded a regional strike direction of N–S, which is the azimuth of the Central Andean mountain chains. Several 2-D models were obtained by pseudo- and full 2-D Occam inversion schemes. Special emphasis was placed on the inversion of phase data to reduce the influence of static shifts in the apparent resistivity data. The smooth inversion models all show a good conductor at depth. A final model was then calculated using a finite element forward algorithm.
The most prominent feature of the resulting model is a conductor which rises from depths of 180 km below the Chaco region to 80 km beneath the Santa Barbara System and the Eastern Cordillera. Its interpretation as a rise of the electrical asthenosphere is supported by seismic attenuation studies. Magnetotelluric results, surface heat-flow distribution in the area, and the electrical properties of crustal and mantle rocks suggest that the upper mantle is predominantly ductile beneath the Eastern Cordillera and the western Santa Barbara System. This generally agrees with anelastic seismic attenuation models of the area and is useful in discriminating between models of Q quality factor distribution.     

Changes in the Geographical Distributions of Global Human Settlements

In recent decades, the continuous growth in the population has significantly changed the area of human settlements across the globe. The change of human settlements has brought great challenges to human development, environmental change, resource allocation, and disaster prediction and prevention. In the current paper, we integrate data products provided by the European Commission, Joint Research Centre with multi-source remote sensing data to analyze the changing trends of global human settlements under varying geographical distributions from 1990 to 2014. The results demonstrate that on the global scale, human settlements are generally distributed in Europe, East Asia, Southeast Asia, South Asia, the eastern United States, the Gulf Coast and the coast of Oceania, with most of them distributed in urban agglomerations and coastal areas. Global human settlements have continued to grow over the past 25 years, mainly in East Asia, Western Europe and the United States. The area of human settlements in eastern Europe has been slightly reduced. The distribution of human settlements is affected by climate, water and terrain conditions. Humans were more likely to have settled in temperate regions with wetter climates, and most of the human settlements are located within 500 km of the coastline and 30 km of land-based water sources. Our results can provide insights into further investigations of the spatio-temporal dynamics of human settlements and its connections to ecological and environmental issues in a changing world.     

Traces: Philosophy,Interpretation, and Method in Postqualitative Human Geography

This article attempts a double reflection: a methodological interrogation of myself and an autointerrogation of my methodology. Following Ernst Bloch, I structure this reflection around the idea of traces, which are brief, narrative, aphoristic speculations on a particular theme. In this article, I (re)produce my own narrative traces, engaging with and representing several moments of strangeness in my methodological praxis as they are recorded in field notes from prior fieldwork with urban secession movements in black and white communities of Atlanta. Building from Bloch’s hermeneutic, I treat these moments as traces to be pursued, rather than simple social artifacts of the relational, intersubjective activity of research. Finally, I demonstrate how a geographer might develop that which crystallizes in the interpretation of the trace (i.e., through the intentional reconsideration of the uncanny and recurrent moments of everyday experience) toward the methodological worlding of philosophy as a vibrant, reflexive, human praxis. Key Words: Bloch, interpretation, method, postqualitative analysis, praxis.