Green Geophysics is acquiring data, under a number of subawards through Oregon State University (OSU), for the Magnetotelluric (MT) Array . The MT Array is a national dataset of long-period MT data acquired on a 70-km grid across the USA under NSF Earthscope, NASA and USGS support. In support of this massive undertaking, Green Geophysics is the largest user of OSU's National Geoelectromagnetic Facility, the world's largest set of land MT instruments. The MT Array data are made publicly available through the IRIS Data Management System, where they are used to make fundamental discoveries about the structure and evolution of the North American continent. An important application of the MT Array is the production of GIC-hazard maps and to develop a real-time system to mitigate risk to critical infrastructure from space weather and electromagnetic pulses (EMP).
Geomagnetically Induced Current (GIC) are electrical currents induced in the Earth that result from the interaction of Solar magnetic storms (‘Space weather’) with the Earth’s magnetic field. The effects of Solar storms can be observed on the surface of the sun itself as ‘Sun Spots’; dark spots that are due to intense localized magnetic changes erupting on the surface. The largest space weather events result from coronal mass ejections (CME). As a result, the magnetic storm intensity is great enough to cast a large volume of ionized gas and an accompanying magnetic field (plasma) deep into space. If the surface of the sun produces a CME ‘aimed’ at the Earth, its effects can be strong enough to disrupt communication and electrical power transmission.
Significant economic effects can be caused by extreme space weather events. These include increased corrosion of steel pipelines, damage to high voltage power transformers and disruption of radio, as well as, telephone and telegraphic communications. Space weather has produced electromagnetic field fluctuations great enough that GIC in power transmission lines disrupted electrical power distribution and caused wide-spread blackouts.
The first linkage between solar flares and resulting magnetic storms occurred between September 1-2, 1859 detected by Carrington and Hodgson. The time delay between the initial flare and Earth interaction took 17 h and 40 min; the largest magnetic storm recorded in history which keyed the term, Carrington event. The hydro-quebec blackout in 1989 was the result of a powerful magnetic storm which caused loss of electrical power for several hours over much of the Provence. Further out in space, in July 2000, a CME caused a large enough charged particle burst to damage the camera and electrical sensors on the SOHO satellite.
Historic events related to GIC have captured the attention of scientists spanning disciplines from astrophysics, atmospheric sciences, and geophysics. Specifically, GIC have brought the attention to US Government agencies including: USGS, UCAR, NOAA, FEMA, NASA, IRIS. Preliminary research by these agencies has indicated that risk to ground based infrastructure is, to a degree, dependent on geology, were regions unladen by rocks having high electrical resistivity are most at risk.
Among the methods for exploring the subsurface electrical resistivity of the Earth, the Magnetotelluric (MT) technique is most efficient for obtaining information about the geoelectrical structure from the near surface to depths on the order of 100 Km. The magnetotelluric method is a passive geophysical technique that measures the Earth’s natural time variation of electric and magnetic fields. The MT survey consists of the measurement of the time variation of these electric and magnetic fields at multiple locations with instruments capable of recording these fields for periods as long a several days and resolving frequency variations ranging from several 10’s of Hertz to less than 1/1000th of a Hertz. The measurement instruments are quite sensitive in order to detect very small changes in the field’s strength and also have large dynamic range necessary to accommodate the large natural signal that results from extreme solar storm events.
At each MT measurement site (MT station) the electric field is measured using four chemical electrodes with one end attached to an insulated wire leading into the MT instrument reciever. The electrode makes electrical contact with the Earth buried in a shallow hole to protect the sensor from thermal variation resulting from daytime and nighttime cylces. The wire lengths are sufficient to extend to points 50m distant from the receiver in the north-south, and east-west directions forming a cross. Orientations of electriode configurations can be adjusted as long as the dipoles remain orthogonal and aligned with magnetic North. These electric dipole antennas are sensitive to electrical voltage changes in the Earth.
In addition to the dipole antennas, a fluxgate magnetometer is also buried in a shallow hole and positioned to measure magnetic field in three directional components: the North-South field component, the East-West component, and the Vertical component.
Collection of the electric and magnetic field components on the Earth’s surface are of practical importance for predicting and mitigating hazards related to GIC. Other benefits to obtaining knowledge of the geoelectrical structure over regional areas are related to exploration of geothermal energy resources and mineral resources. Of equal or greater importance, large scale MT surveying provides a basis for understanding the chemical makeup of the Earth’s crust and upper mantle by imaging its electrical resistivity structure. That information should prove useful in gaining a better understanding of continental evolution and the geologic history of North America.
Typically, the electric and magnetic field variation is recorded for approximately one month at each MT site. When field examination of the record indicate that the data of sufficient quantity and quality have been collected, the instruments are removed for installation at another site and the recorded data is sent in for laboratory processing. There, the time variation records of the fields are converted to frequency variation of the fields. This frequency domain data is then sorted for quality and used to calculate the MT transfer function (electromagnetic impedance) as it varies with frequency. Further processing (inversion) of the impedance results yields estimates of the electrically resistivity structure beneath the MT site -or sites-, depending on whether the inversion utilized a single site or groups of sites (remote reference). The inversion results provide an estimate of the ground resistivity structure for depths ranging from hundreds of meters to hundreds of kilometers below the surface.
MT installation in CA, 2019
MT extraction in CA, 2019
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