The Topographic Effect in Electromagnetic Fields

Ku, Chayong; Hsieh, M.; Lim, Sung-Jin

doi:10.1139/e73-065

Cited by 33 publications

(15 citation statements)

References 2 publications

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“…With a resistivity of the order of 100 Q . m, the model gave apparent resistivities (Ku et al 1973) that were close to the E-parallel minimum observed between stations 8 and 12 (Fig. 8).…”

Section: Audio-frequency Magnetotelluric Survey Across the Wopmay Fausupporting

confidence: 68%

Electromagnetic sounding and crustal electrical conductivity in the region of the Wopmay Orogen, Northwest Territories, Canada

Camfield¹,

Gupta²,

Jones³

et al. 1989

Can. J. Earth Sci.

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Temporal variations of the three components of the geomagnetic field were recorded at eight sites along a 240 km profile across the Early Proterozoic Wopmay Orogen. After an empirical separation of these data into normal and anomalous parts, horizontal-to-vertical-field transfer functions in the period range 40 -1200 s display evidence for a minor anomaly spatially located near the allochthonous shelf margin at the eastern edge of the Hepburn Batholith. The observations can be partially simulated by a two-dimensional 20 Q . m body (30 km wide, 2 km thick) embedded in the surface of a very resistive layered Earth model derived from inversion of magnetotelluric sounding data at a central station. The body correlates spatially with metamorphosed graphitic pelites of the Odjick Formation (Epworth Group), a unit of deep-water facies interpreted as continental slope-rise deposits. Laboratory measurements on samples of the pelite yielded resistivity values of the order of lo4 Q . m, so the enhanced conductivity of the body is more likely caused by water filling cracks associated with the pelites' welldeveloped cleavage and schistosity, rather than by the graphite. A scalar audiomagnetotelluric survey across the Wopmay fault zone, a prominent structure that bisects the orogen, gave results very much distorted by three-dimensional effects. The electric-polarization apparent resistivities of these data indicate a shallow conductor 2 km east of the fault scarp, 1-2 km wide. Models of the feature suggest that its vertical extent is at least 1-2 krn.Des variations temporelles des trois composantes du champ gComagnCtique ont Ct C enregistrkes 2 huit points sur un profil long de 240 km, en travers de l'orogkne Wopmay du Protkrozoique prkcoce. Aprts avoir sCparC empiriquement la partie normale de la partie anomale de ces donnkes, on a calculC des fonctions de transfert des champs horizontaux au champ vertical pour des pkriodes de 40 2 1200 s. Ils montrent, pour de faibles pkriodes, une anomalie mineure prts de la marge de la plateforme allochtone % extrCmitC est du batholite Hepburn. On a partiellement modClisk les observations par un corps bi-dimensionnel (30 km de large par 2 km d'Cpais) de rksistivitk 20 Q . m, encastrk dans un modkle de la Terre constituk de couches trts rCsistives, modele qu'on a tirC d'un sondage magnCtotellurique effectuC 2 une station centrale. Il y a une corrClation spatiale entre le corps conducteur et les pklites graphitiques mktamorphisCes de la Formation Odjick (du Groupe Epworth), une unite ii faciks d'eau profonde interprktke cornrne Ctant des dCp6ts de talus ou de glacis continental. Des mesures en laboratoire de la rCsistivitC des Cchantillons des pClites ont donne des valeurs trks Clevkes, de l'ordre de lo4 Q . m. Pour cette raison, on ne peut pas associer la haute conductivitk du corps ii la prCsence de graphite. I1 est plus probable que la cause de la conductivitC ClevCe est l'eau qui remplit les pentes du clivage et de la schistositk bien dtveloppCs des pklites. Un lev6 audio-magnCtotelluriq...

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“…With a resistivity of the order of 100 Q . m, the model gave apparent resistivities (Ku et al 1973) that were close to the E-parallel minimum observed between stations 8 and 12 (Fig. 8).…”

Section: Audio-frequency Magnetotelluric Survey Across the Wopmay Fausupporting

confidence: 68%

Electromagnetic sounding and crustal electrical conductivity in the region of the Wopmay Orogen, Northwest Territories, Canada

Camfield¹,

Gupta²,

Jones³

et al. 1989

Can. J. Earth Sci.

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“…Local and shallow resistivity distribution (RIKITAKE, 1972;BERDICHEVSKY and DMITRIEV, 1976;JIRACEK et al, 1983) or topography (e.g., KU et al, 1973) affects the MT sounding curves. The apparent resistivity curves are often shifted upward or downward.…”

Section: On the Static Effectmentioning

confidence: 99%

Two-dimensional resistivity modeling based on regional magnetotelluric survey in the northern Tohoku district, northeastern Japan.

Ogawa

1987

J. geomagn. geoelec

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In 1984, a regional Magnetotelluric (MT) survey was carried out in the northern Tohoku district in the frequency range from 17.4 kHz to 1/256 Hz. MT stations wereThe shallow lateral inhomogeneity was evaluated from ELF-MT (8 to 20 Hz) and VLF-MT (17.4 kHz) survey at 35 sites, and the deep structure was investigated using ULF-MT (1/4 to 1/256 Hz) survey at 7 sites.Two-dimensional modelings with north-south strike are carried out. Analyses are done with special reference to the anisotropic behavior of phase of impedance. From the modeling with north-south strike, the following features are found. (1) For TM mode, where electric field is directed eastward, frequency characteristics and spatial dependence of impedance are primarily due to effect of oceans and shallow resistivity distribution. For 1/16 Hz, coastal effect alone cannot account for the observed impedance and conductor (300 ohm-m) is required beneath the eastern part of the profile whose top is located at 10 km depth in the crust. (2) On the other hand, for TE mode, where electric field is directed northward, the spatial and frequency dependence of impedance requires the existence of a conductor (10 ohm-m) in lower crust beneath the volcanic region whose depth ranges from 20 km to 30 km. This conductor is in harmony with low seismicity, low velocity, and shallow Curie isotherm depth.

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“…Greater progress in correcting for topography appears for de resistivity in both two and three dimensions (Fox et al, 1980;Holcombe and Jiracek, 1984;Oppliger, 1984). Implementing transmission surfaces, Ku et al (1973) and Ngoc (1980) present MT responses of several two-dimensional (2-D) geometries for both transverse electric (TE) and transverse magnetic (TM) modes. Parry and Ward (1971) give transverse electric (TE) results over topography using integral equations.…”

Section: Introductionmentioning

confidence: 99%

Two‐dimensional topographic responses in magnetotellurics modeled using finite elements

1986

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We simulate the magnetotelluric response to twodimensional earth topography using finite elements. Linear interpolation of the secondary field parallel to strike over triangular elements allows accurate modeling of inclined resistivity boundaries, including topographic surfaces. To avoid discontinuities in field derivatives or resistivity, care must be taken that the nodal values of the field parallal to strike used to obtain the auxiliary secondary fields are kept within uniform earth media. The nodal locations may be shifted, but the derivatives still are evaluated at the field points of interest. Correct values may be returned at gentle breaks in slope as well as along straight surfaces. The finite-element program is verified by comparison with the analytic transverse magnetic response of a hemicylindrical depression and with Rayleigh scattering and transmission surface results for transverse electric and transverse magnetic polarization. Agreement with the other methods generally is excellent, with the exception of some results of the transmission surface technique (especially the transverse magnetic mode). A result presented shows that placing the H-field sensors horizontally reduces topographic anomalies compared to locating sensors parallel to the slope. Moreover, if earth resistivity increases with elevation, the apparent resistivity is relatively nonanomalous near the base of the topography.

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The Topographic Effect in Electromagnetic Fields

Cited by 33 publications

References 2 publications

Electromagnetic sounding and crustal electrical conductivity in the region of the Wopmay Orogen, Northwest Territories, Canada

Electromagnetic sounding and crustal electrical conductivity in the region of the Wopmay Orogen, Northwest Territories, Canada

Two-dimensional resistivity modeling based on regional magnetotelluric survey in the northern Tohoku district, northeastern Japan.

Two‐dimensional topographic responses in magnetotellurics modeled using finite elements

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