The telluric-magnrtotelluric method uses magnetotrlluric measurements at the base site, but only telluric mrnsurements at remote sites. It thus combines the economy, simplicity, and speed of the traditional telluric method with the quantitative advantages of the traditional magnetotelluric method. The dominant features of the combined method are the following: First, the time required to set up a telluric site is less by a factor of at least, 5 than the time for a complete magnetotelluric site. Second, one does not need to record magnetic field data at the base site simultaneously wit' 11 the electric field recorded at each remote site. One needs only enough magnetic data to adequately determine the base tensor. A tclluric transfer tensor coupling electric field measurements at t#he base site and each remote site can be used to transfer the base impedance tensor to an impedance tensor at each rem&e site. By being much more selective of the magnetic data used in the analysis, one can significantly improve the signal-to-noise rat#io. Third, the data are analyzed to determine each element of the complex imlwdance tensor so that, important, phase information as well as cLm#plitude information is availablr for interpretations which are more sophisticated than those currently attempted in convent ional telluric surveys. Finally, in making the ultimate interpretation in terms of the impedance tensor rather than the telluric tensor usrd in convent,ional telluric surveys, one essentially refers t#he interpretation of remote electric field observations to the ma.gnefic field at the base site rather than to the electric field. Both rsperience and model studies demonstrate t#hat the magnetic field is much more homogeneous than the electric field in the vicinity of lateral hricrogeneities; thus the selection of a proper base site is not as critical in the combined method as it is in the conventional telluric method.

Regional telluric-magnetotelluric measurements performed as a cooperative research project between Brown University and the National Energy Authority of Iceland are interpreted in terms of physical processes in the crust and upper mantle associated with magma genesis and crustal spreading. First, these results confirm, over a larger region than was previously studied, the presence of a relatively conducting crust (25-30 ohm-m) at shallow depth (d G 4.5 km) reflecting electrolytic conduction in hydrothermal pore fluids on a regional scale. Secondly, and perhaps of greatest interest to current work on processes of accretion along plate boundaries, is the presence of an anomalous conducting layer (T s 4 km, p s 10 ohm-m) at the base of the crust (8-15 km) beneath the neovolcanic zone. This layer we feel represents an accumulation of molten magma. There is some suggestion that it is at shallower depth beneath the most active portion of the neovolcanic zone, and that its depth increases with age of the crust. The presence of this feature suggests that the crust is being underplated by the accretion of molten material to its base which, as it solidifies, contributes a significant component of crustal thickening. We feel that the molten phase residing in this basal zone represents an available source of magma for high level intrusions. Finally, our results confirm that, beneath the zone of magma accumulation, the resistivity changes little with depth in the mantle. Although there may be an indication that the upper few tens of kilometres of the mantle is somewhat more conducting, reflecting the presence of a minor amount of melt, this is not a firmly resolved feature of our analysis. Nevertheless, the mantle has almost a constant resistivity to a depth exceeding 100 km. Assuming a homogenous composition for the mantle beneath Jceland, the vertical gradient of temperature is of the order of only a few "C km-', or less. 424

An analytic solution of the electric field near a two‐dimensional offset in the earth’s surface is presented as a means of estimating the effects of nonplanar topography on telluric measurements. Results indicate that, for an offset height d, the field at the earth’s surface is within ten percent of its undisturbed value for distances, greater than 2.5 d from the offset on the lower side and for distances greater than 4 d on the upper side. The disturbance due to the offset extends even less far out in situations where the telluric currents are channeled close to the surface.

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