Electrical conductivity anomalies in the earth

Abstract

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.

The long-period electromagnetic response of the Earth

“…The similarity of the effect at the two stations confirms that the effect is due t o a real physical process. It is possible that the upper mantle structure in the Japan area (see Honkura 1978) contributes to the anomalous responses at these two stations. However, simple models (e.g.…”

Pressure-driven conductivity of lizardite-implication to the high conductive layers in craton lithosphere

“…Serpentine enriched in H 2 O (~13 wt.%) universally distributes in the crust and upper mantle, and is the primary contributor to the global water cycle in the subduction zone and the hydrothermal alteration of oceanic lithosphere (Schmidt & Poli, 1998;Rüpke et al, 2004). Magnetotelluric survey data reveal pervasive existence of lower resistive zones that are related to the earthquakes in the crust and upper mantle in the continental region (Wei et al, 2001;Xu, 2003;Gürer & Bayrak, 2007;Naganjaneyulu et al, 2013;Yang et al, 2020), subduction zone (Cordell et al, 2019), and throughout the world (Honkura, 1978;Ichiki et al, 2009;Özaydıın & Selway, 2020). The lower resistive domains by far are commonly attributed to fluids enrichment.…”

The effect of the depth of the non-conducting layer on the induced magnetic field at the Peruvian dip equator.

“…In zones where the existence of a lateral inhomogeneity in the earth's conductivity, that is to say a "conductivity anomaly", is not suspected, a very simple conductivity model is usually assumed; a non-conducting layer up to a depthp, and a perfectly conducting half space beyond it (see for example ONWUMECHILLI 1967;SAMPATH and SASTRY, 1979). * Member of CONICET (Consejo Nacional de Investigaciones Cientificas y Tecnicas) , Two main conductivity anomalies exist in the South American equatorial zone : the well known Andean anomaly (SCHMUCKER, 1969;ALDRICH et al, 1975) and the one produced by the ocean coast (ALDRICH et al, 1975;HONKURA, 1978).…”

“…* Member of CONICET (Consejo Nacional de Investigaciones Cientificas y Tecnicas) , Two main conductivity anomalies exist in the South American equatorial zone : the well known Andean anomaly (SCHMUCKER, 1969;ALDRICH et al, 1975) and the one produced by the ocean coast (ALDRICH et al, 1975;HONKURA, 1978). Nevertheless, the simple model mentioned above for the conductivity distribution is applied in the present paper to reproduce the horizontal component of the internal part of the geomagnetic daily variations at the ends of the profiles measured by FoRBUSH and CASAVERDE (1961) since this component is not appreciably affected by the coast effect (SCHMUCKER, 1969;Cox and FILLOUX, 1974) and the Andean anomaly seems to be localized near the dip equator (SCHMUCKER, 1969;ALDRICH et al, 1975;HONKURA, 1978).…”