Summary The application of d.c. resistivity, magnetotelluric—both natural and artificial source—and geomagnetic variation techniques has provided lithospheric electrical resistivity models which generally display a relatively low resistivity region at lower crustal/upper mantle depths. In this paper, model parameters are summarized for the lower crust in tectonically inactive regions and discussed in more detail with respect to specific studies undertaken in Europe, N. America and Africa. Special attention is drawn to the resolution of the model parameters and to the fact that the depth to the low resistivity layers and the ratio of their thickness to their resistivity can normally be well determined. The petrological interpretation is non-unique. Although electrical resistivity is strongly temperature dependent, the temperature within the lower crust in inactive regions is too low to give an in situ decrease in resistivity of the major rock phases. A more acceptable interpretation involves compositional variations—both lateral and vertical—within the crust associated with an increased amount of free fluids (e.g. water) or of free carbon (e.g. graphite). The correlation of the low resistivity structures in the lower crust with sedimentary layers involved in large-scale overthrusting or related tectonic processes is an attractive idea while the generation of low resistivity shear zones during metamorphism could also be an effective explanation of the observations. Some of the elongated anomalies of low resistivity which are discussed may correspond to Proterozoic plate boundaries and subduction zones.

A Monte-Carlo inversion procedure is developed and applied to magnetotelluric data from six locations, two of which are in the Midland Valley of Scotland, three in the Southern Uplands, and one in northern England. The method is described in full in respect of one of the six locations to illustrate both the importance of satisfying the phase as well as the amplitude data and the effect of model acceptance level. The electrical resistivity profiles resulting from application of the method indicate that; (a) there is a conducting zone under the Midland Valley at a depth no greater than 12 km, (b) the crust under the Southerri Uplands is mainly resistive, (c) there is a conductor at a depth greater than 24 km in this region, and (d) under northern England there is probably a very highly conducting region very close to the surface. A brief discussion of the possible geophysical and tectonic significance of these models follows. Monte-Carlo inversionAs stated by Jackson (1973), the complete solution to an inverse problem consists of two parts: (1) finding a solution, and (2) representing, in a meaningful way, the degree of non-Present address: lnstitut fur

Magnetotelluric measurements have been made at 13 locations in southern Scotland along lines perpendicular and parallel to the strike of a major electrical conductivity anomaly detected by previous geomagnetic deep sounding studies.Following a brief account of the fieldwork and data analysis procedure, the justification for one-dimensional interpretation at certain sites is presented. Six of the 13 sites fall into this category. Their magnetotelluric responses were found to fall into three groups which appear to be representative of (a) the Midland Valley, (b) the Southern Uplands and (c) northern England. Limited forward modelling of the well-estimated data was undertaken and is discussed in this paper. It indicates marked lateral variations in conductivity structure across the region.

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General theoretical expressions have been obtained for time variations of the magnetic and electric fields in the vicinity of the magnetic dip equator. From these, magneto-telluric ratios have been determined for a number of earth models, including the general case of n stratified layers, and these are found to be functions of the dimension of the inducing field as well as of the earth conductivity structure and the period of the variation. Model apparent resistivity curves have been computed for the same earth models as used by other research workers and the differences between them are discussed. Some of the problems which have to be considered in the practical application of the magneto-telluric expressions are noted and attention is drawn to the rapid change in the value of the apparent resistivity with electrojet width for variations of period greater than several hours.