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Title: Magnetotelluric instrument development and application
Author: Nascimento, Darcy
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1997
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The magnetotelluric method (MT) has been used to study the conductivity structure of the earth. The method uses the time variations of the natural magnetic and the induced electric fields measured simultaneously at the earth's surface. The demand for new MT instrumentation has increased in the past few years. New applications require more complex sensor configurations. Characteristics like size, weight and real time data processing have become increasingly important to improve logistics and mobility, although it is becoming more and more difficult to measure electromagnetic fields without strong contamination from man-made sources. The ability to process MT results in the field is essential for a quick evaluation of the site conditions, enabling the time required to obtain a suitable amount of good quality data to be reduced. SPAM Mk-III (Short-Period Automatic Magnetotelluric) developed at The Edinburgh University introduces new concepts in terms of geophysical instrumentation. The equipment is based on a family of Transputer devices widely used in parallel processing, incorporating the recent technologies and techniques. Its modular design allows flexible network configurations. A complete description of the equipment, design concepts, features and the precautions needed in designing low-noise instrumentation are presented in this thesis. The distinctive SPAM Mk-III capabilities of multi-channel data acquisition were used in two field experiments. In the first experiment the effect of the electric dipole length on the impedance tensor was investigated. Results confirmed that the apparent resistivity and phase are quite consistent and that longer electric dipoles tended to be more susceptible to local interference. In the second experiment the rotation property of the impedance tensor was studied. The experiment reinforces the assumption that for a layered earth (1-D), the impedance tensor is independent of measuring co-ordinates. New Cu-CuSO4 electrodes were developed for the field work. These are to operate in the frequency range of the SPAM Mk-III (0.001 Hz to 2kHz). Several electrodes were tested and the results indicate that noise and stability does not depend on the internal resistance. Also the frequency response of these electrodes show they do not present any filter effects within the range of magnetotelluric application.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available