Chap 1 - Introduction.pdf

7/30/2019 Chap 1 - Introduction.pdf

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SOEE3350 / SOEE5141

Near Surface Geophysics Geoelectrics

Course Notes

Jon MoundSchool of Earth and Environment

University of Leeds

2010


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Disclaimer and Acknowledgments

These notes are intended as a supplement for the Geoelectrics module. They are not a replacement for the

recommended texts listed in the syllabus or for lecture attendance.

I have attempted to include references to the original sources of any borrowed figures - full rights remain

with the copyright holders.

This document is based upon material provided by Manuel Matias. The EM section also benefited from

material provided by Greg Houseman. Credit for the depth and scope of this work go to them. Responsibility for

errors and oversights rests with me. This is an evolving piece of work; please let me know of any shortcomings,

typos, confusions or mistakes you encounter.

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Module Information

Module Objectives

These lectures comprise the whole of the undergraduate module SOEE3350 and a part of the masters module

SOEE5141. To a great extent the objectives of these two modules overlap, to some extent they differ. The

objectives that are particularly related to the masters module are indicate in bold type.

The objectives of this module are:

To introduce you to and train you in some of the principle geoelectrical methods used to study near

surface layers.

To give you a working knowledge of the theoretical and practical basis of self-potential, resistivity,

induced polarisation, ground-penetrating radar and electromagnetic methods.

To enable you to make qualitative and quantitative interpretation of data produced by the above

methods.

To provide field experience in these techniques, and training in reporting/documenting

geophysical surveys.

Assessment

Unseen Examination: 70%

Mainly essay style questions with some numerical elements. Separate exams for 3350 and 5141. 2 hours.

Problem Sets: 2 x 15%

Solution of theoretical problems and analysis/interpretation of presented data. Assignments will containquestions common to both groups plus additional questions highlighting interpretation for the students in 5141.

Contact

Jon Mound

Office: 8.136 Earth & Environment

Phone: 343-5216

Email: [email protected]

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Syllabus

1. Fundamentals. General terminology. Resistivity in the Earth. Types of conduction. Dielectric proper-

ties. Anisotropy.

2. Self Potential Methods. Field work techniques, electrodes, modelling.

3. Resistivity Methods. Potential at a point P on the surface of a medium. Sources and sinks. Profiling,

mapping, sounding, pseudo sectioning. Sounding sampling. 1D interpretation. Ambiguity, equivalence

and suppression. Meaning and importance of transverse resistance and horizontal conductance in Hydro-

geology. Curve matching- logarithmic scales. 2D interpretation.

4. IP methods. Time and frequency domains. Analogue circuits. Pseudo sectioning.

5. GPR methods. Electrical properties and GPR methods. Basic Physics. Air, ground and reflected

waves. Field parameters: frequency (central frequency), spacing, time window, station spacing, sampling

interval. GPR range equation. Data processing.

6. Electromagnetic methods. Primary, secondary fields, eddy currents. Basic physics. In phase and

out of phase components. Low induction numbers and skin depths concepts. Near field and far field

measurements. Fixed source and moving source methods. Principles of time domain measurements.

Importance and applications Principles of MT measurements. Importance and applications.

Recommended Texts

These texts provide much useful and necessary information; these notes are merely a supplemental guide.

Telford, Geldart & Sheriff, Applied Geophysics, 2nd Ed., Cambridge University Press

An excellent, if somewhat old, treatment of the majority of exploration methods. This book has a partic-

ularly thorough mathematical development.

Reynolds, An Introduction to Applied and Environmental Geophysics, John Wiley & Sons

A comprehensive and relatively modern overview of exploration methods. Much description with relatively

little math.

Keller & Frischknecht, Electrical Methods in Geophysical Prospecting, Pergamon Press.

A classic text in the field, now somewhat dated.

Keary, Brooks & Hill, An Introduction to Geophysical Exploration, 3rd Ed., Blackwell Publishing.

A short, descriptive, basic introduction to exploration methods.

Zhdanov & Keller, Geotechnical Methods in Geophysical Exploration, Elsevier

An overview of both electrical properties and methods, includes a number of case studies.

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Contents

Disclaimer ii

Module Information iii

List of Tables viii

List of Figures ix

1 General Principles 1

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Electrical Properties of the Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.1 Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.2 Dielectric Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.2.3 A Geoelectrical Section Anisotropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2 Self Potential/Spontaneous Potential 10

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2 SP in the Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2.1 Field measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2.2 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3 SP Anomaly Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3.1 A Linear Conductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3.2 Current Point Source - Infinite Medium . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.3.3 Point Source in a Half Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.3.4 Multiple Poles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.3.5 Potential of an Ore Body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.3.6 Depth Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3 Resistivity Methods 213.1 Equipotential Line Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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3.2 Fixed Source Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.3 Moving Source Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.4 Electrical Sounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.4.1 Image Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.4.2 Potential Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4 Induced Polarization 39

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4.1.1 Sources of IP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.1.2 Equivalent Electrical Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.2 Field Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.2.1 Time Domain Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.2.2 Frequency Domain Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.2.3 Spectral IP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5 Ground Penetrating Radar 46

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

5.1.1 Propagation of Radio Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.1.2 Principles of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.2 Survey Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.3 Data Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

6 Electromagnetic Methods 54

6.1 Principles of EM Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

6.1.1 Steady Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

6.1.2 Time Varying Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

6.1.3 Inductive coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

6.1.4 Induction Amplitude and Phase Lag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

6.1.5 Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

6.1.6 Depth of Penetration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

6.2 Field Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

6.2.1 Tilt Angle Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

6.2.2 Fixed Source Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

6.2.3 Moving source Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

6.2.4 Time Domain EM (TDEM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

6.2.5 Magneto-Telluric (MT) Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

6.2.6 Controlled Source and Multi-Transient EM . . . . . . . . . . . . . . . . . . . . . . . . . . 70

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Bibliography 74

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List of Tables

1.1 The resistivity of some materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2 The relative dielectric constant of some materials . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4.1 The metal factor of some materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

5.1 Some typical GPR Relevant Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

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List of Figures

1.1 Inductive and Galvanic responses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Different exploration target frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 A simple current path through a rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.4 Parallel plate capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.5 A geoelectric section. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1 A mechanism for mineral self-potential anomalies . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2 SP equipment details and set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.3 Fixed spacing SP deployments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.4 Fixed electrode SP deployment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.5 Current through a linear conductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.6 An electric current point source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.7 An electric current point source in a half space . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.8 The method of images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.9 Current addition for a source and sink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.10 O re body geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.11 O re body SP anomaly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.12 Depth calculation to an infinite body from the SP anomaly . . . . . . . . . . . . . . . . . . . . . 18

2.13 Depth calculation by the ratio method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.14 Depth calculation to a vertical, finite body from the SP anomaly . . . . . . . . . . . . . . . . . . 20

3.1 Geometry for the equipotential line method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.2 Current paths and equipotential lines for a homogenous half-space . . . . . . . . . . . . . . . . . 22

3.3 Current paths and equipotential lines near a conductor and insulator . . . . . . . . . . . . . . . . 22

3.4 Geometry of the fixed source method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.5 Geometry of the Wenner array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.6 Geometry of the Schlumberger and dipole-dipole arrays . . . . . . . . . . . . . . . . . . . . . . . 24

3.7 Examples of resistivity profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.8 Example of resistivity mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.9 Schematic of electrical sounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

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3.10 P seudosection example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.11 3 D resistivity example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.12 Method of images for a perfect insulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.13 Method of images for two conductive media, near side . . . . . . . . . . . . . . . . . . . . . . . . 31

3.14 Method of images for two conductive media, far side . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.15 Method of images at the boundary of two conductive media . . . . . . . . . . . . . . . . . . . . . 32

3.16 Method of images for two subsurface layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.17 Two layer master curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.18 Geometry of a three layer Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.19 Schematic three layer master curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.1 Time delayed response in measured potential (right) to abrupt changes in applied current (left). 39

4.2 Membrane and electrode polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4.3 Equivalent Earth circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.4 Dipole-dipole spread and pseudo-section point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.5 Time domain IP measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.6 a and PFE pseudo-sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.7 Typical spectral IP response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

4.8 Characteristic phase-angle plots for various mineralisation types . . . . . . . . . . . . . . . . . . . 45

5.1 Schematic illustration of GPR surveying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

5.2 Example radargram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5.3 Idealized GPR ray paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5.4 Flowchart of Radar Range Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.5 Example GPR profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.6 Example GPR depth slices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

6.1 Schematic illustration of EM surveying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

6.2 Magnetic field about a line current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

6.3 Inductively coupled loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

6.4 Arbitrary loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

6.5 Plot of the current ratio in the complex plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

6.6 RLC series circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

6.7 Fields through time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

6.8 Field vectors plotted in Q-IP space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

6.9 The ellipse of polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

6.10 T he tilt angle method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

6.11 S undberg and Turam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

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6.12 HLEM profile over a vertical conducting sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

6.13 Ground conductivity meters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

6.14 TDEM set-up and induced currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

6.15 TDEM quantities plotted as functions of time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

6.16 MT resistivity cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

6.17 M TEM and CSEM sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

6.18 Sketch of MTEM survey principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

6.19 A comparison of MTEM and seismic data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

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