GEOMAGNETIC INDUCTION STUDIES IN SOUTHERN SCOTLAND by Alan George Jones Thesis ...

411 Pages·2016·10.63 MB·English

GEOMAGNETIC INDUCTION STUDIES
IN SOUTHERN SCOTLAND
a
by
Alan George Jones
Thesis presented for the degree of
Doctor of Philosophy of the
University of Edinburgh in the Faculty of Science
1977
DECLARATION
I hereby declare that the work presented in this thesis
is my own unless otherwise stated in the text, and that
the thesis has been composed by myself.
Alan George Jones
(cid:9)(cid:9)(cid:9)
C.
ABSTRACT
The Southern Uplands of Scotland: is at present a region of great
interest to both geologists and geophysicists. The complex tectonic
history associated with the closing of the proto-Atlantic ocean has yet
to be determined. Previous geophysical studies have shown that the
Southern Uplands is atypical of normal continent. Geomagnetic investi-
gations have indicated a zone of anomalously high electrical conductivity
underlying the Southern Uplands region at a depth of 12-30 km.
In order to determine further the exact structure and spatial
variation of this conductivity zone., two geomagnetic studies have been
carried out in the region. A Geomagnetic Deep Sounding Array of 19
Gough-Reitzel variometers recorded the naturally varying Earth's magnetic
field during December 1913 and January 1914. This was followed by
• (cid:9) Magneto-Telluric observations on lines perpendicular and parallel to
the strike of the supposed anomaly.
• (cid:9) In this thesis, reviews of various regional MT studies, the geo-
physical significance of conductivity measurements and the known geology
and geophysics of the Southern Uplands region are given. The aims, and
relevant theory, of induction studies are also presented. The observa-.
• (cid:9) tional procedures for both the GDS and MT techniques, and the type and
form of the MT activity, are described. Techniques for analysing the
• (cid:9) MT data are developed in some detail. Methods are proposed, and
examples given, for (a) estimating, the gross structural strike direction,
• (cid:9) (b) averaging response function estimates, (c) estimating the confidence
• (cid:9)
intervals of the response functions, (d) estimating new forms of coherence
functions, which exhibit many desirable properties, and (e) a frequency-
time analysis for estimating the response functions for sub-intervals
of the data set.
a-
The full MT and GDS estimates from analysis of the single station
data - rotated major and rotated minor impedance estimates, azimuthal
angles, skew factors and real and imaginary induction vectors - are
presented and qualitatively discussed.
A one-dimensional interpretation, of two-dimensional MT data is
examined and shown to be valid for 'rotated major' impedance estimates
from locations sufficiently distant from gross lateral inhomogeneity.
Various methods for determining an 'optimum model' that best satisfies,
in some manner, the observed MT responses are reviewed. A Monte-Carlo
inversion procedure is developed and.applie& to the 'rotated major? data
from six of the thirteen locations. It was considered,,for various
reasons, unjustifiable to interpret an the data.
The GDS and MT results agree on the complexity of the conductivity
variations in the Midland Valley,- the Southern Uplands and Northern
England. The simple 'Eskdalernuiranomaly' proposed by Edwards et al.
----
(1971) cannot explain the observations.. A conductive layer is required
beneath the Midland Valley at a depth of no greater thaii 11 km. The
conductive zone underlying the Southern Uplands is at a depth greater
than 24 km. For the Northern England response, the top layer of highly
conducting sediments 'screen' the possible effects of a 'lower crustal/
upper mantle' conductive layer. The geological and geophysical impli-
cations of the acceptable MT models are discussed.
In this work, the quantitative information, offered by the MT
technique, is shown to be necessary fora full interpretation of the
conductivity distribution. Also, estimation of the phase response, as
well as the amplitude response, of the impedance tensor elements is
shown to resolve the surface structure of the acceptable models.
Various suggestions are made about further investigation of the
region.
ACKNOWLEDGEMENTS
I gratefully acknowledge the help and direction given me by my
supervisor, Dr. Rosemary Hutton. Dr. Hutton has not only encouraged me
throughout my research but also has ensured that I was not too side-
tracked by peripheral issues. I sin indebted to her for the years of
preparation and organisation she spent on the project before I came to
Edinburgh. I should like, to express my gratitude to the Geophysics
Department of Edinburgh University for providing the wherewithal for
my research. I wish to thank all members. of the Department for furnishing
a refreshing and stimulating atmosphere in which to work. In particular,
I would like to mention Drs. Jane Sik (co-supervisor), Dennis Rooney,
Dave Summers and Bruce Hobbs, all of whom gave me much food for thought.
Dennis Rooney is to be especially thanked for his guidance during the
initial stages of data processing,-and Bruce Hobbs for theoretical
assistance. The fieldwork and technical assistance -g4en me by Mr.
Alex Jackson and Mr. Hugh Muir did not go unnoticed. It was much
S (cid:9)
appreciated. The fieldwork for the GDS Array study was more amusing
because of the companionship of Mr. Surrinder Chita, whose help is
thankfully acknowledged. Also, I an grateful to my two colleagues,
Mr. Dave 14cKirdy and Ms. Sheila Kirkwood, for their patience and
assistance during the last six months.. Sheila Kirkwood is also thanked
for proof-reading the typescript of this thesis.
I an pleased to extend warm thanks to Dr. Ian Gough for his
invitation to process the GDS Array data initially at the Department
of Physics of the University of Alberta, Edmonton, Canada. During my
two-month visit to that Department, Dr. Remi Alabi gave up much of
his time both to train me in the data reduction procedure and to
entertain me during the weekends - for which I am very grateful.
F
I wish to thank most sincerely all the people on whose land the
instruments were located, many of whom gave up garages, outbuildings
and even houses to provide shelter for the MT filtering and recording
equipment. Their overwhelming generosity and hospitality made the
fieldwork for the MT project very enjoyable.
I should like to thank all the staff of the Edinburgh Regional
Computing Centre who have helped me at the data processing stage of my
research. I express appreciation especially to Ms. Beth Egan for
digitising some of the MT data sections.
My financial support for this research was by a Natural Environment
Research Council studentship, for the initial two years, and a University
of Edinburgh studentship for the final 18 months. The Royal Society of
London provided.the funds for the purchase and construction of the MT
equipment. I express gratitude to all three bodies.
Finally, I wish to thank,Ms. Fiona Train for aiding me in inumerable
ways, and Ms. Kathy Dodds who gave up many hours to type this thesis -
an achievement for which I mn indebted to her.
CONTENTS
Page
CHAPTER I INTRODUCTION
1.1 General Comments 1
1.2 Natural Source Field Characteristics 3
• (cid:9)
1.3 Regional Magneto-Telluric Studies 5
(cid:9)
1.4 Geophysical Significance of Conductivity
•
Measurements (cid:9) 17
.
1.5 Geology and Geophysics of the Southern
Uplands Region (cid:9) 21
••.
1.5.1 (cid:9) Geology (cid:9) .• 21
1.5.2 (cid:9) Geophysics (cid:9) . 45
1.6 The Purpose of the Induction Studies 30
CHAPTER 2 THEORY
2.1 Brief Historical Review 32
2.2 Relationships from Maxwell's Equations 37
2.3 General Geomagnetic Induction over One-
Dimensional Structures (cid:9) •. 140
2.3.1 (cid:9) Homogeneous half-space (cid:9) . 140
2.3.2 (cid:9) N-layered isotropic half-space 142
2.14 General Geomagnetic Induction over Two-
Dimensional Structures 145
2.14.1 (cid:9) General considerations 145
•
-
2.14.2 (cid:9) Typical two-dimensional models 46
a. (cid:9) Fault 146
b. (cid:9) Dyke 148
General body (cid:9) . 148
C. (cid:9)
2.4..3 (cid:9) Dimensionality and directionality
indicators (cid:9) . 53
Page
CHAPTER 3 (cid:9) DATA ACQUISITION
3.1 The GDS Array 61
3.2 The MT Project 64
3.3 MT Instrumentation 68
The telluric systems 68
The magnetic systems 71
C. The recorders lit
d. Calibration 75.
3.4 Discussion of MT Data Collected 77
Data quality and quantity 77
Type and. form of activity recorded 17
C. Site problems 79
CHAPTER 4 (cid:9) DATA PROCESSING
4.1 General Considerations
4.2 Processing of Fourier Transient Events from
the GDS Array
4.2.1 Data selecticn
4.2.2 Data reduction
4.2.3 Time domain analysis (cid:9) - ..
4.2.4 Frequency domain analysis
4.3 Processing of Fourier Intransient Record
Sections for Single Station MT and
GDS Data
4.3.1 Data selection
4.3.2 Data reduction
4.3.3 (cid:9) Data conditioning
4.3.4 Spectral estimation and band averaging
Page
4.3.5 Correction for instrument response 100
14.3.6 Digital filtering 1014
4.3.7 Frequency-time analysis io6
4.3.8 Polarisation analysis 109
14•14 Linear System Parameter Evaluation 110
14.4.1 Single input/single output system 111
4.4.2 Two input/single output system 116
4.14.3 Bias associated with coherence function
estimation 119
4.4.4 Normalised transformed coherence
-
functions 121
•
4.5 Magneto-Telluric Impedance Estimation 128
14.5.1 Cagniard impedance estimation 128
• •
4.5.2 Tensor impedance estimation 129
4.5.3 Rotation of the tensor impedances 131
•
4.5.14 Minimum-maximum analysis 133
4.6 GDS Transfer Function Estimation (cid:9) • 1314
4.7 Station Data' Averaging 134
4..1 Acceptance criteria and averaging of
algorithms for GDS data (cid:9) • 135
Acceptance criteria (cid:9) • 135
Averaging algorithms 136
14.7.2 Acceptance criteria and averaging
algorithms for MT data 138
a. (cid:9) Acceptance criteria 138
b. (cid:9) Averaging algorithms
139
14.8 Confidence Limit Estimation 1141
14.8.1 For GDS data 142
Page
(cid:9)
5.8.2 For MT data 155
(cid:9)
5.8.3 For azimuthal data 152
(cid:9)
4.8.5 For skew data 155
CHAPTER 5 (cid:9) RESULTS
(cid:9)
5.1 GDS Array Data 155
(cid:9)
5.2 Synopsis of MT and GDS Single-Station Data 157
(cid:9)
5.3 Midland Valley Response 160
(cid:9)
5.4 Southern Uplands Response 161
(cid:9)
5.5 Northern England Response 166
(cid:9)
5.6 Unreliable and Anomalous Responses 167
(cid:9)
CHAPTER 6 AND CONCLUSIONS
6.1 Introduction 169
6.2 One-Dimensional Interpretation Techniques 171
6.2.1 Validity 171
6.2.2 First approximation inversion schemes 176
6.2.3 Curve, fitting procedures 179
6.2.5. Monte-Carlo inversion schemes 181
6.2.5 Heuristic linear inversion methods 190
6.3 Monte-Carlo Inversion of the MT Results 193
6.3.1 Midland Valley results - FTH and SAL 193
6.3.2 Southern Uplands results - BOR, NEW
and PRE 196
6.3.3 Northern England result - TOW 200
6.5 Geophysical and Geological Interpretation 201
6.5.1 a. (cid:9) Geophysical interpretation -
qualitative 201