Magnetotelluric and Geomagnetic Depth Sounding Methods Compared

Lilley, F. E. M.; Tammemagi, Hans

doi:10.1038/physci240184a0

Cited by 19 publications

(12 citation statements)

References 5 publications

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“…For a considerable part of the Russian or Easteuropean Platform she proposed a depth of 200-350 km for the UCL. Similar values have been found in the Siberian Platform by POSPEEV and MIKHALEVSKY (1976), in the W part of the North American Platform by PoRATH and GoUGH (1971), in the SE part of the Canadian Shield by DOWLING (1970) and on the Australian Craton by LILLEY and TAMMEMAGI (1972).…”

Section: Revision Of the Available Data Setsupporting

confidence: 69%

Connection between the Electric Conductivity Increase due to Phase Transition and Heat Flow

Ádám

1980

J. geomagn. geoelec

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The deepest electric conductivity increase determined by electromagnetic induction methods can be attributed to the conductivity increase observed in the laboratory during rock phase transition. It can be called the Ultimate Conducting Increase (or UCL), since no further increase of conductivity has yet been detected at greater depths. A positive dP/dT gradient is characteristic of rock phase transition, where P is the pressure in BARS, T the temperature in C.The aim of the research reported here was to look for an indication of a positive dP/dT gradient in the data set available, i.e. whether greater depths of the UCL (or greater pressures) correspond to greater temperatures at the depth of the phase transition. The surface heat flow (q in HFU), the characteristic part of which is coming from the upper mantle, can be used as an indicator of the temperature.Since in the upper mantle of the territories with lowest heat flow (q<1 HFU) there is no partial melting, it can be assumed that the depth of 250-300km of the conductivity increase determined on platforms and crystalline shields corresponds to the depth of the rock phase transition. Using these data, which are very critical for our interpretation, an empirical formula was calculated by least squares fit between the depth of the conductivity increase HULL (km) and the surface heat flow q (HFU): HULL=16.3+292.5 q. This formula indicates a positive dP/dT gradient. On the basis of the data set, the average HUCL is 420km and the average heat flow 1.38 HFU. From this formula, taking dP/dT=30 BARS/C, it can be concluded that the temperature at a depth of 300-400km below the platform areas is about 1,000C less than the average.

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Section: Revision Of the Available Data Setsupporting

confidence: 69%

Connection between the Electric Conductivity Increase due to Phase Transition and Heat Flow

Ádám

1980

J. geomagn. geoelec

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“…4.6. CA's iy AUSTRALTA From two-dimensional array and MT studies in southern Australia, a conductivity anomaly was found below the Adelade Geosyncline region where seismic activity is high (Lilley and Tammemagi, 1972). An anomalous conductor appears to be situated in the crust and its conductivity ranges from 1 to 10 S/m, although a proposed model does not account for the observed data very well.…”

Section: Ca's In Africamentioning

confidence: 97%

Electrical conductivity anomalies in the earth

Honkura

1978

Geophysical Surveys

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Abstract. Anomalies of short-period geomagnetic variations have been found in various regions over the world. It is known that such anomalies arise from electromagnetic induction within an electrical conductivity anomaly or from local perturbation of induced electric currents by a conductivity anomaly. In order to investigate a regional electric state in the Earth, conductivity anomaly (CA) studies based on anomalous behaviors of geomagnetic variations have been extensively undertaken, as well as studies based on magnetotellurics in which induced currents are directly used.Some of the geomagnetic variation anomalies, however, turned out to be caused by surface conductors, such as sea water and sediments. Anomalies of this sort have been intensively studied and classified into coast, island, peninsula, and strait effects in the case of sea effects. Three-dimensional conduction or channelling of induced electric currents is sometimes observed in the cases of sediments and some crustal conductivity anomalies. However, anomalies of such surface origins often provide some information of the underground conductivity structure.Electrical conductivity anomalies can be classified into two types: anomalies originating in the crust and in the upper mantle. Many of crustal anomalies are well correlated with metamorphic belts, fracture zones, and hydrated layers, and magnetic and gravity anomalies are also often found over the conductivity anomalies. Most of mantle anomalies have been interpreted mainly in terms of high temperature and partial melting, since conductivity anomalies coincide well with anomalies in heat flow and seismic wave velocities.

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“…Previous Geomagnetic Depth Sounding (GDS) studies, with Telluric data at some sites, using magnetic variometer data have broadly mapped the axis of a crustal conductor, the Flinders Conductivity Anomaly (FCA), which crosses the present line at its southern extremity from the south-west and then trends to the north (Lilley and Tammemagi, 1972;Gough et al, 1974;Chamalaun, 1986). Seven resistivity soundings were also made in the area south of Lake Frome by Constable (1985) which showed that the surface sediments have very low resistivity.…”

Section: Introductionmentioning

confidence: 99%