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Title: An electromagnetic induction study of south Cornwall, England
Author: Jones, Philip Charles
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1992
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Twenty one magnetotelluric and 16 magnetovariation soundings were taken on or near the Carnmenellis granite over periods in the range of 0.0078 seconds to 3000 seconds. The measured impedance tensors were analysed in detail using decomposition methods. All the data are distorted by galvanic electric charges which build up on the granite country rock contact. The amount of distortion varies between sites with distance from the edge of the batholith. The vast majority of the data are at least two dimensional and the effects of three dimensional induction are increasingly sensed by periods greater than 1s. Short period soundings indicate that the anisotropy in the magnetotelluric field is caused by electric current being channelled along fluid filled cracks. One dimensional modelling of the E-pol response indicates that the bottom of the granite is not flat, but slopes downwards to the south. This finding is evidence to support the theory that the Cornubrian granites originated SSE of their present position and were rafted NNW as a thin sheet. Two dimensional modelling suggests that at least a portion of the gradient of this slope is caused by the neglection of 3D induction in models used in the study. The pattern of regional azimuths between 0.1 and 10 seconds is caused by a combination of lateral, near surface, conductivity contrast, such as the surrounding seas, and conductivity contrasts at depth due to the slope of the bottom of the granite. Mainly due to the effects of conductivity contrasts perpendicular to the regional azimuth, it was found impossible to find a model which fitted the E-pol data at both on and off granite sites. The 2D model indicates that there is a steep rise in the resistivity depth profile of the granite from 800 ohm-m at the surface to 20000 ohm-m at 4 km. The closure of fluid filled joints due to the increase in lithological load with depth is interpreted to be the cause of this increase in resistivity.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available