A one-dimensional model of solid-earth electrical resistivity beneath Florida
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Abstract:
First posted November 19, 2015 For additional information, contact: Director, Geologic Hazards Science Center U.S. Geological Survey Box 25046, MS–966 Denver, CO 80225-0046http://geohazards.cr.usgs.gov/ An estimated one-dimensional layered model of electrical resistivity beneath Florida was developed from published geological and geophysical information. The resistivity of each layer is represented by plausible upper and lower bounds as well as a geometric mean resistivity. Corresponding impedance transfer functions, Schmucker-Weidelt transfer functions, apparent resistivity, and phase responses are calculated for inducing geomagnetic frequencies ranging from 10−5 to 100 hertz. The resulting one-dimensional model and response functions can be used to make general estimates of time-varying electric fields associated with geomagnetic storms such as might represent induction hazards for electric-power grid operation. The plausible upper- and lower-bound resistivity structures show the uncertainty, giving a wide range of plausible time-varying electric fields.Keywords:
Electrical Resistivity Tomography
Geopotential
Geomagnetic secular variation
Secular Variation
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What is the appropriate coordinate system for magnetometer data when analyzing ionospheric currents?
In this paper we investigate which coordinate representation is most appropriate when analyzing ground magnetometer data in terms of ionospheric currents, in particular the westward electrojet. The $\textit{AL}$ and the recently introduced $\textit{SML}$ index are frequently used as monitors of the westward electrojet. Both indices are based on ground magnetometers at auroral latitudes. From these magnetometers, the largest perturbation in the southward direction is selected as the $\textit{ AL/SML }$ index at 1 min cadence. The southward component is defined as antiparallel to the orientation of the horizontal part of the Earths' main field, $\textbf{B}_{0, \textit{H}}$. The implicit assumption when using these indices as a monitor of the westward electrojet is that the electrojet flows perpendicular to $\textbf{B}_{0, \textit{H}}$. However, $\textbf{B}_{0, \textit{H}}$ is, in general, not perpendicular to the westward direction in coordinate systems that take nondipole terms of the Earth's magnetic field into account, such as apex and the Altitude Adjusted Corrected Geomagnetic coordinate systems. In this paper we derive a new $\textit{SML}$ index, based on apex coordinates. We find that the new index has less variation with longitude and universal time (UT), compared to the traditionally defined $\textit{SML}$. We argue that when analyzing ionospheric currents using magnetometers, it is appropriate to convert the components to a corrected geomagnetic system. This is most important when considering longitudinal or UT variations, or when data from a limited region are used.
Electrojet
Longitude
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Geothermal exploration involves geology, geochemistry and geophysics. In geophysical exploration, resistivity surveying plays a most important role in delineating the reservoir. The parameters that control the geothermal system show a strong response to electrical resistivity. The resistivity methods that are mostly used in geothermal exploration in Iceland are TEM (Transient electromagnetics) and MT (Magnetotellurics). The resulting resistivity cross sections and resistivity depth slices, show a shallow lying low resistivity layer and deep lying low resistivity towards the end of the cross sections
Magnetotellurics
Geothermal exploration
Electromagnetics
Electrical Resistivity Tomography
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An active landslide investigation using 2D electrical resistivity tomography has been under taken in Garhwal Himalaya. Six electrical resistivity tomography profiles spanning the landslide were<br>conducted. The resistivity tomograms reveal the presence of slip zones at a depth range of 10 to 20 m from ground level. The inferred lithological depth sections clearly outline the importance of electrical resistivity tomography in landslide studies.
Electrical Resistivity Tomography
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Electrical resistivity is known as a good indicator for geo-fluid distribution especially in the crust and upper mantle. In this paper, we introduced physical concept of electrical resistivity in solid, liquid, and their mixing law, which can explain resistivity of crust and upper mantle. We also introduced magnetotelluric method, a common exploration method to image resistivity distribution in the earth, and modeling (inversion) method for resistivity distribution. Because resolution of inverted resistivity model from the magnetotelluric data depend on depth, resistivity, density of observation station and smoothness constraint, the model should be carefully interpreted. The magnetotelluric method has been applied for various tectonic settings. Many studies discovered low resistivity zones probably indicating fluid-rich area in or beneath the earthquake faults. In the volcanic zones, partial melt and hydrothermal areas were inferred based on three-dimensional modeling. Intensive MT surveys and newly developed interpretation techniques such as correction method of bathymetry effect and 3-D inversion method enable us to image resistivity of subduction slab and oceanic plate.
Magnetotellurics
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Rectangular harmonic analyses are applied to the vector data acquired by MAGSAT to make altitude correction and to obtain anomaly maps over the area of the Japanese islands at a specific altitude. For the analyses local Cartesian coordinates are adopted with the origin at 35°N, 140°E, taking the x-axis to the north, the y-axis to the east, and the z-axis vertically downward. Not only the position of measuring points but also each component of the magnetic field are transformed from geocentric coordinates to these local Cartesian coordinates. From the component data, a magnetic scalar potential is derived by expanding in double Fourier series with x and y coordinates, of which each term decays in the negative z-direction.The magnetic potential is first computed for the area of 4000×4000km, terminating the series at n=m=3. Then, in order to see finer structures of the magnetic anomaly, the potential is derived for the area of 2000×2000km with the maximum order of n=m=3. Synthesized maps drawn for the altitude of 300km show three features of magnetic anomalies over Japan and its surrounding area. 1) An intense positive anomaly of the vertical component which runs along the Kuril trench from the Kuril Islands to southeastern Hokkaido. 2) A pair of positive and negative vertical anomalies that cover the area from northern Korea to East China, suggesting the existence of a very localized magnetization in the southwestern tip of the Korean Peninsula. 3) The Sea of Japan anomalies consisting of a negative anomaly of the vertical component that covers the northeastern part, and a positive anomaly covering the southwestern part.
Anomaly (physics)
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Electrical Resistivity Tomography
Embedment
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We determine the occurrence times of geomagnetic impulses (jerks) around the year 1991 in the three geomagnetic secular variation components for the Earth's surface by a simple optimization algorithm. The geomagnetic field models we use are the low-degree parts of the models CM4 and C3FM. We find that the temporal jerk pattern can be detected in fields (n ≤ 4), from which the spherical harmonic degrees n = 2 or n = 3 (tangential) and n = 4 (radial) are representative.
Jerk
Geomagnetic secular variation
Secular Variation
Core–mantle boundary
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