Abstract
Magnetotelluric soundings in the range (100-0.01Hz. ) have been carried out along a
140 km. profile across S.Scotland and N.England. Following discussion of the steps
taken to reduce cultural noise in the response functions, the results for 13 stations
along the traverse are presented. Since the study of dimensionality parameters
supports 1-D modelling, rotationally invariant resistivity-depth data have been
subjected to two 1-D inversion schemes and the different parameters of the model
derived have been examined using the Most squares approach. Two dimensional
numerical modelling has also been carried out considering both the present data and
those of earlier induction studies along the profile.
The resulting geoelectrical model confirms the existence of a crustal conducting layer
(10-50 Ohm.m.) as found in earlier induction studies. Results of this study show
additionally that- a) the conductor rises to a depth of nearly 3.5 km. about 20km.
south of the SUF i.e., near the axis of the Eskdalemuir magnetic variation anomaly b)
the conductor is at a depth of about 8-10 km. in region of the Weardale granite in
N. England, c) there is a marked resistivity-depth variation below S.Scotland and d) the
surface resistivity along the profile decreases from NW to SE and corresponds well to
the Ordovician, the Silurian and the Carboniferous sediments. The geophysical
interpretation of the model has been discussed in association with the results of
earlier induction studies, seismic and gravity studies and the various tectonic models
of the Iapetus suture zone. Based on the present study and comparison with electric
structures of other tectonic regions, a hypothetical tectonic model has been proposed
for this study region. It comprises a resistive triangular wedge surrounded by a
conducting layer which is incorporated as a signature for the presence of a subducting
plate in an island arc environment.
In addition to these field investigations, several computational analyses have also been
undertaken. The main studies were - a) procedures to reduce noise from the
recorded data using digital filtering techniques and b) determination of the
distortion of telluric field measurements due to a conducting hill using a conformal
mapping method. Digital filters have been discussed with special reference to
persistent noise signals, eg. power lines, electrical fences etc. Delay line filtering,
notch filtering and the maximum entropy method have been applied to both synthetic
and field data and the results are discussed. The effect of a two-dimensional
conducting hill on the telluric fields measured on a horizontal surface has been
studied using the Schwartz-Christoffel conformal transformation technique. The
results indicated that the distortion depends strongly on the inclination, height of the
hill and depth to the basement. The distortion of apparent resistivity values computed
from MT field measurements has also been discussed.
In addition, some consideration has been given to - a) the processing of the data in
the time domain using adaptive filters, b) the application of joint inversion of DC-AMT
data to resolve shallow structures in N.England and c) the distortion effects of near
surface inhomogeneities in N.England.
These various studies are presented in 7 chapters with 3 Appendices and the results
are discussed in chapter-8, where the conclusions and suggestions for further work
are also presented.
