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9 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1973

Theory for magnetotelluric observations on the surface of a layered anisotropic half space

The conductivities for the magnetotelluric model described are taken to be constant vertically in each of a large number of horizontal layers. The conductivities and the associated magnetic, electric, and current vector fields are considered to be laterally homogeneous within a vertical cylinder centered about the observation site. The electromagnetic fields are, therefore, only functions of the vertical coordinate. A relatively simple algorithm is presented for calculation of the magnetotelluric complex impedance tensor.

Loewenthal, D.↗
NASA NTRS1977

Three-dimensional modelling in magnetotelluric and magnetic variational sounding

The Galerkin finite-element method is used to obtain approximate solutions for the three-dimensional induction problem. A rectangular conductive prism is considered as an example, and solutions are obtained for linear and circularly polarized incident plane-wave fields. Magnetotelluric tensor impedances and magnetic transfer functions are computed. Polar diagrams of the tensor impedances and magnetic transfer functions along with their amplitude contour maps are presented. The dimensionality parameter, skew, is contoured at the surface of the earth. It is shown that the relative amplitudes and shapes of the additional and principal impedance polar diagrams can be used to determine the dimensionality of geoelectrical structures. Stations with skew values greater than 0.2 are significantly influenced by the three-dimensionality of the geoelectric structure. The amplitudes of the magnetic transfer function and the orientations of its polar diagrams exhibit large anomalies in the vicinity of the intersection of the lateral contacts.

Reddy, I. K.↗
NASA NTRS2017

Methodology for Time-Domain Estimation of Storm Time Geoelectric Fields Using the 3-D Magnetotelluric Response Tensors

Geoelectric fields at the Earth's surface caused by magnetic storms constitute a hazard to the operation of electric power grids and related infrastructure. The ability to estimate these geoelectric fields in close to real time and provide local predictions would better equip the industry to mitigate negative impacts on their operations. Here we report progress toward this goal: development of robust algorithms that convolve a magnetic storm time series with a frequency domain impedance for a realistic three-dimensional (3-D) Earth, to estimate the local, storm time geoelectric field. Both frequency domain and time domain approaches are presented and validated against storm time geoelectric field data measured in Japan. The methods are then compared in the context of a real-time application.

Kelbert, Anna↗
NASA NTRS1977

Electrical structure in a region of the Transverse Ranges, southern California

Magnetotelluric sounding at a site in the Transverse Ranges province in southern California indicates a low-resistivity region in the lower crust and possibly also the upper mantle. A two-dimensional model fit to the data indicates that the resistivity of this region is between 1 and 10 ohm-meters. The depth to the top surface of this zone is between 15 and 20 km. The lateral extent of this feature, which strikes N65 deg W, appears to be confined to the Transverse Ranges province. The petrological characteristics of this region cannot be deduced unambiguously from the magnetotelluric sounding alone.

Reddy, I. K.↗
NASA NTRS1979

The problem of solar induced currents

The production of surface electric fields by geomagnetic variations is discussed with reference to magnetotelluric theory; and evidence is presented to show that the severity of solar-induced currents (SIC) at Cornerbrook, Newfoundland is due to channeling of currents induced in the sea through a region adjacent to the power line. An 'isolated loop' approximation is proposed for calculating the quasi-dc currents produced in the power system by the surface electric fields. Identification of the processes causing the geomagnetic perturbations responsible for SIC would be aided by investigation of the local time dependence of SIC occurrence. Such knowledge could potentially be used to provide several hours warning of SIC.

Boteler, D. H.↗
NASA NTRS1983

Electromagnetic induction studies

Recent developments in electromagnetic induction studies of the lithosphere and the asthenosphere are reviewed. Attention is given to geoelectrical studies of active tectonic areas in terms of the major zones of crustal extension, the basin and range province along western regions of North America, and the Rio Grande rift. Studies have also been performed of tectonic activity around Iceland, the Salton Trough and Cerro Prieto, and the subduction zones of the Cascade Mountains volcanic belt, where magnetotelluric and geomagnetic variation studies have been done. Geomagnetic variations experiments have been reported in the Central Appalachians, and submarine electromagnetic studies along the Juan de Fuca ridge. Controlled source electromagnetic and dc resistivity investigations have been carried out in Nevada, Hawaii, and in the Adirondacks Mountains. Laboratory examinations on the conductivity of representative materials over a broad range of temperature, pressure, and chemistry are described.

Hermance, J. F.↗
NASA NTRS2016

Geoelectric Hazard Maps for the Continental United States

In support of a multiagency project for assessing induction hazards, we present maps of extreme-value geoelectric amplitudes over about half of the continental United States. These maps are constructed using a parameterization of induction: estimates of Earth surface impedance, obtained at discrete geographic sites from magnetotelluric survey data, are convolved with latitude-dependent statistical maps of extreme-value geomagnetic activity, obtained from decades of magnetic observatory data. Geoelectric amplitudes are estimated for geomagnetic waveforms having 240 s sinusoidal period and amplitudes over 10 min that exceed a once-per-century threshold. As a result of the combination of geographic differences in geomagnetic activity and Earth surface impedance, once-per-century geoelectric amplitudes span more than 2 orders of magnitude and are an intricate function of location. For north-south induction, once-per-century geoelectric amplitudes across large parts of the United States have a median value of 0.26 Vkm; for east-west geomagnetic variation the median value is 0.23 Vkm. At some locations,once-per-century geoelectric amplitudes exceed 3 Vkm.

Love, Jeffrey J.↗