School of GeoSciences
EASC08008
Introduction to
Geophysics
2011 - 2012
September 2011
U2089 Introduction to Geophysics
What is Geophysics?
Geophysics draws upon the methods and theory of physics to build up a picture of the interior
of the Earth and the processes which generate its structure and surface features. These
techniques also allow us to sense remotely other planets, as well as the Earth’s oceans and
atmosphere. They form a common foundation for meteorology and other geosciences.
The deepest borehole has not penetrated more than 13 km into the Earth, so that only a thin
layer can be directly sampled and an indirect approach is needed. The properties and
composition of the deep interior are deduced by, for example, studying the way in which
sound waves from earthquakes and large explosions travel through it. Earthquake signals can
still be detectable after passing through the three thousand kilometres of silicate rocks making
up the Earth's crust and mantle, after penetrating the outer core of molten alloys of iron,
crossing the inner core where solidified iron has separated, and re-emerging at the surface on
the other side of the Earth.
One way of identifying deviations from this basic “onion-skin” picture of the Earth – a picture
with spherical shells called crust, mantle, outer core and inner-sore – is by making very
precise measurements of how the acceleration of gravity varies from place to place. Such
observations show, for example, that the layer of lighter rocks making up the crust is much
thinner under oceans than under continents. With very recently increased precision, satellite
gravity missions now allow us to monitor the gravity changes caused by seasonal flooding of
the Amazon Basin, drought in sub-Saharan Africa and melting of the Greenland Ice Cap.
By studying the magnetic fields of the Earth, as well as the Sun and major planets like Jupiter
and Saturn, we deduce that the electrically conducting parts of the interior can act as a
dynamo. Activity that ‘stirs up’ the Earth’s outer molten core generates vast electric currents
that produce the magnetic field pulling a compass needle northwards.
All these techniques have local as well as global applications. The same principles of sound
propagation – this time sound from small artificial explosions rather than an earthquake – can
be applied to detect shallower rock formations likely to trap oil. The same techniques can map
different water masses in the oceans and air masses in the atmosphere, allowing us to monitor
their circulation. Variations in gravity can be used to prospect for ore deposits, or for rock salt
structures associated with oil reservoirs. On a global scale, measurements the Earth’s
magnetic field can detect the drifting apart of continents and the growth of new ocean floor as
a result of plat tectonics, while, very locally, “Time-Team” use magnetic and electrical
measurements to identify the presence of a buried Roman fort.
Learning outcomes
At the end of this course, you should have a comprehensive overview of the physical
processes operating in the solid Earth and its core, and understand the principles of the
geophysical techniques by which this information is derived.
You will have gained practical experience and understanding of geophysical survey
techniques in the field and how observations can be interpreted.
You should be able to write a scientific report including reference to previous work, and
understand the role played by new observations and an assessment of their errors in the
process of scientific inference.
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U02089 Introduction to Geophysics
Load
The course comprises 30 one-hour lectures, and 5 three-hour practicals. You submit three
written reports and have a final degree exam lasting three hours.
Lecturing Staff
Initials
Name
Topic
DS
David Stevenson
David.S.Stevenson@ed.ac.uk
Scientific Method;
Gravity; Heat;
Raddioactivity
AB
Ian Main
Seismology
Ian.Main@ed.ac.uk
WW
Wyn Williams
Wyn.Williams@ed.ac.uk
Geomagnetism,
Palaeomagnetism,
Geoelectricity
Chair of the Board of Examiners
Alasdair Robertson
tel:
0131 650 8546
Room 213
email: Alastair.Robertson@ed.ac.uk
Grant Institute
Course Organiser
Prof. Wyn Williams tel:
Room
0131 650
email: wyn.williams@ed.ac.uk
Grant Institute
Course Secretary
Nikki Muir
tel:
0131 650 4842
Room 332
email: EarthSciUG@ed.ac.uk
Grant Institute
Web site
Course information, lecture notes and practical exercises will appear on WebCT. For some
practical exercises, material may be in the Introduction to Geophysics section of School of
GeoSceinces website. The links to this information will given in practical handout when
needed.
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5
U2089 Introduction to Geophysics
1
Delivery
1.1 Lectures
The lectures are at 1000 - 1050 on Mondays, Wednesdays and Fridays in George Square.
Monday:
George Sq 07, Rm F21.
Wednesday: George Sq 07, Rm F21.
Friday:
George Sq 07, Rm F21.
Please do not feel inhibited from raising points of difficulty after (or during) lectures. You can
also ask for help to deal with problems in practicals.
1.2 Practicals
They take place on either Tuesday or Thursday afternoons – see page 11 for the schedule.
Note that there is no practical class in week 1.
Some are fieldwork – an announcement in lectures and the website will give locations – others
start in various locations at Kings Buildings (see page11).
The list identifying those in each practical group (a, b, c or d) will be posted on the web and
given out in class soon after registration.
For some practicals, you write a report that forms part of your assessment. Practical Reports
should be handed in before the due date to the Teaching Office (Emma Latto), Grant Institute
Room 335. Late submission will be penalised. (See section on assessment.) After marking,
they can be picked up from the Teaching Office, generally about 4 weeks after the hand-in
date.
Some practical work in the field allows you hands-on use of geophysical techniques to
investigate real problems of subsurface structure, often in an urban environment where the
bedrock is not exposured. Before the fieldwork date, the notes describing where to go, what
you will do and some background theory will be handed out in lectures and posted on the
Web. Please make sure that you read these instructions beforehand.
For field exercises, please make sure you dress in warm, waterproof clothing and wear
suitable shoes. Remember that you may be handing cold metal instruments and have to stand
still while making measurements.
2
Assessment
The course is examined by a combination of written examination and continuous assessment
of reports on practical work
Degree examinations
There is a 3 hour Degree examination in May.
The total degree marks are made up of 70% from the May examination and 30% from
practical exercises.
The School is empowered to withdraw your right to sit degree examinations if you have
persistently failed to carry out the work of the course without good reason. Persistent
failure to attend lectures, failure to attend practical classes or to hand in reports on time
will be taken as evidence of inadequate performance unless an acceptable reason is given:
normally a medical certificate is required in case of illness.
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U02089 Introduction to Geophysics
Past Degree Examination papers can be seen on the Library web site, so that you can
familiarise yourself with their format and rubric. It is recommended that you spend the
first hour of the three hour paper on section A, that contains a set of short compulsory
questions covering any aspect of the course. You should spend about 6 minutes on each
of these 10 questions. The remainder of the paper is divided into 4 sections, B – E, each
with a choice of one out of two questions. You should therefore spend about 30 minutes
on each of these longer questions. Mark weighting is the same as the suggested time: one
third for section A and two thirds for your answers to sections B – E.
The course EASC08008 is the same as U02089 Introduction to Geophysics, which was
essentially the same of the former course U00095 Geophysics 2h and very similar to its
predecessor GY0009 Geophysics 2Ah. These degree exam papers therefore give you a
longer perspective to help with your revision this year.
2.2 Other important information
The Teaching Organisation issues a booklet “General Information for Undergraduate
Students” covering what to do
•
•
•
•
if you are ill or have other special circumstances
if you are unable to submit an assessed exercise and consider that you have a
legitimate excuse.
if you hand in assessed work late
to ensure that you are not accused of plagiarism
It also tells you about the criteria associated grading assessments.
3
Writing a Practical Report
3.1 Assessing practical reports
A report describing the aims, background, method, results and conclusions for marked
exercises (see page 13) should be handed in to Heather Hooker in Room 335 in the Grant
Institute as soon as possible after the practical. For each piece of work, there is a deadline for
submission, printed on the instruction sheet. Reports handed in without a satisfactory
explanation after the due date will be subject to an automatic penalty (see sections 2.3, 2.4).
Remember that the marks from the reports contribute 30% towards the degree mark for the
year (the other 70% comes from the May Examination). You should keep your marked reports
in a loose-leaf folder for future reference. It is always possible that the examiners may wish to
look over them in June.
3.2 Report Writing
The objectives are (i) to give you practice at writing reports, an essential skill for careers both
in research and industry, and (ii) to ensure that you have understood the methods and theory
which underpin the exercise.
Reports should not exceed 1000 words of text. Each figure, graph and table should be given a
short title and a number, so that it can be referenced appropriately in the text. Reports may be
presented on loose leaf sheets, but should be securely stapled or clipped together. Each one
should incorporate the instruction sheet distributed with the exercise, and should refer to but
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U2089 Introduction to Geophysics
not duplicate the information it contains. Reports should be presented under section headings
such as those suggested overleaf.
3.3 Report Structure
(i)
Title
Choose the wording of your title carefully. It should be short but contain a clear
statement of what the report is about. "Measurement and interpretation of a gravity
profile across the Pentland Fault" is much more informative than "Geophysics Gravity
Practical".
(ii)
Abstract.
The content of the abstract should be a stand-alone component of your report. Imagine
that someone reads only the abstract - will they understand what you have found out?
The abstract should be brief (5 or 6 lines) and should describe what your results are and
a sufficient summary of the rest to make these results intelligible. Often, it is helpful to
write this section last, when you are clear what the result of your work has been.
(iii) Introduction.
This begins your report proper so it may contain things you have already said in the
abstract. It should describe its context - why you are doing it and what previous
information is known. It can usually be kept short by referring to the instruction sheet
but it should still introduce the general principles involved and summarise the aims of
the practical.
(iv) Method.
This can usually be kept short by referring to the instruction sheet but you should write
just enough to enable the reader to understand what happened without going back to it.
Make sure you do cover those points not described there.
(v)
Results: calculations and errors.
You should see of your numerical results and an assessment of their errors as together
making up you ‘results’. Suppose we carry out a remote sensing experiment to locate the
Pentland Fault under superficial deposits and find that it is 650 m north of the bridge at
Little France. How does a civil engineer know how long a trench he must dig in order to
be sure of finding it? Did our result mean somewhere between 649.5 m and 650.5 m or
somewhere between 600 m and 700 m? Estimating the uncertainty on measurements
must be presented as an integral part of the results of an experiment. Results should be
quoted as for example (650 ± 15) m. According to circumstances, the 15m may mean
the maximum range or it may be probabilistic, for example that 65% of experiments will
give a result in this range. When you can, say what you mean by your error. Note that
the error you quote dictates how many significant figures are needed: (649.3672 ± 15) m
or (649.3672 ± 14.8539) m makes no sense at all. You are never able to estimate errors
to better than 2 or 3 significant figures and your result should be rounded so that you
only quote those significant figures made meaningful by its error: thus (4.76 ± 0.12) m
and (135600 ± 2300) m are given appropriately.
(vi) Conclusions and discussion.
This is where you report and comment on the significance of what you have done – it is
not a summary of the whole report. Your conclusions are things you can be sure about;
but the discussion can be less definitive – arguing different points of view or suggesting
further work.
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U02089 Introduction to Geophysics
(vii) References
A key part of a scientific report is that it builds on previous knowledge. The reader does
not necessarily want you to repeat everything but he does need to know how to be able
to read it for himself. Information must be traceable. Here, any information you need
will be in the instruction sheets but you should follow professional practice by refering
to this information explicitly and precisely. For example, instead of describing the field
procedure in full, you might say “Gravity was measured at 100 m intervals northwards
from the Bridge over Little France Burn [spacing and reference point not given in the
instructions] following the procedure described in section 3.2, page 4, of the instruction
sheet.”
4
Prizes
The Geomagnetism Edinburgh 1981 Prize is awarded annually to the foremost students in the
Introduction to Geophysics class, based upon the results of class work and examinations
The Exxon-Mobil North Sea Prize will be awarded annually to the foremost third year
Honours Geophysics student(s) based on the results of the second and third year classwork
and examination results and/or to the foremost student(s) in the Introduction to Geophysics
class, based on the results of class work and examinations in Geophysics.
5
Where do I get help and how do I complain?
We want you to enjoy the course and be stimulated by it but, if you have any problems with its
structure or content, you should feel free to take them to the Course Organiser
(Wyn.Williams@ed.ac.uk) at any time. Nikki Muir can often help with individual queries. If
you prefer, use your class reps on the Staff-Student Liaison Committee for problems that
you feel are likely to be common to a number of students, or for ideas about how the course
could be improved.
Whatever your difficulties, keep in touch with your Director of Studies – they are ‘on your
side’ and there to help you.
A formal appeals procedure against decisions made by Examiners is described in the
University Examination Regulations and Guidelines. Students should note that appeals against
examination results on the basis of illness will only be considered if the relevant medical
certificate is received before the meeting of the Board of Examiners.
7
Roger.Hipkin
U2089 Introduction to Geophysics: Lecture schedule
SEMESTER 2
All lectures on George Square Campus, 1000 – 1050
Monday:
George Sq 07, Rm F21.
Wednesday:
George Sq 07, Rm F21.
Friday:
George Sq 07, Rm F21.
Geophysics and the scientific method
(Wyn Williams)
Week 1
L1
Mon Jan 16: Introduction to the course and the methodology of science. Paradox
of the inaccessible Earth: surface area – land and oceans; volume – mines and boreholes; time – the written record; How can we really find out about the interior of the
Earth and its past? Observations, codifying them into natural laws (hypotheses);
predictions tested with new observations. The scientific method applied to the age of
the Earth, illustrating how hypotheses are modified as observations change.
Gravity
(David Stevenson)
L2
Wed Jan 18:. Gravitation. The acceleration of gravity g; the mass and mean density
of the Earth; artificial satellites
L3
Fri Jan 20:. Gravity anomalies I. How g varies over the surface of the Earth; effect
of the Earth’s rotation; the Earth’s shape, the geoid and mean sea level; the Reference
Earth model, normal gravity and the International Gravity Formula.
Week 2
L4
Mon Jan 23: Gravity anomalies II. How g varies with height. The free-air correction
and the effect of topographic masses. Rock density.
L5
Wed Jan 25: Absolute and relative gravity measurements. Gravity meters:
corrections and survey procedures.
L6
Fri Jan 27: Interpretation of gravity surveys. Mapping and modelling; examples of
the use of surveys - engineering geology, mineral exploration, sedimentary basins,
diapirs, the continent-ocean margin. Interpretation theory, model fitting and nonuniqueness.
Week 3
L7
Mon Jan 30: Isostasy. The Andes, Schiehallion and the Himalayas; Pratt and Airy
models; the mechanical behaviour of the Earth. Airy, the crust and explosion
seismology; Vening Meinesz, Indonesia and lithospheric plates; Pratt, mid-ocean
ridges and basin subsidence.
L8
Wed Feb 1: Atmospheric geophys
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U2089 Introduction to Geophysics: Lecture schedule
Seismology
(Ian Main)
L9
Fri Feb 3: Introduction to Seismology. Earthquakes and man-made seismic sources.
General uses of seismology, the seismic frequency spectrum.
Week 4
L10
Mon Feb 6: Elementary elastic theory and seismic waves. Hooke's Law and
different types of strain, seismic body waves and their velocities, surface waves and
dispersion.
L11
Wed Feb 8: Seismic refraction and crustal layering. Snell's Laws of reflection and
refraction. The refraction method and application to the layered structure of continental
and oceanic crust.
L12
Fri Feb 10: Seismic reflection. Geometry of reflection profiling, time distance plots,
common mid-point profiling and the seismic stack, seismic sections, seismic
migration.
Week 5
L13
Mon Feb 13: Whole Earth Structure. Body wave phases, travel times and whole
earth structure, application to earthquake location. Layering of the earth's upper
mantle.
L14
Wed Feb 15: Earthquake size. Intensity scales and earthquake damage; earthquake
magnitude and energy; uses of the magnitude scale in nuclear discrimination and
estimating earthquake recurrence.
L15
Fri Feb 17: Earthquake focal mechanisms. Seismic moment and earthquake
faulting. P-wave radiation pattern from a strike-slip earthquake, projections of seismic
stations on to the lower focal hemisphere, normal and reverse faults in plan.
Week 6
L16
Mon Feb 27: Seismotectonics and seismic hazard. Focal mechanisms near oceanic
ridges, transform faults and subduction zones. Estimation of seismic hazard for
building design.
Geomagnetism and Geoelectricity
(Wyn Williams)
L17
Wed Feb 29: Introduction. Magnetic elements; units; isomagnetic lines; global and
regional magnetic charts.
L18
Fri Mar 2: Earth's main magnetic field. Description of the Earth's main field;
Earth's dipole field; magnetospheres of the Earth and planets.
Week 7
L19
Mon Mar 5: The non-dipole field. secular variation; westward drift.
L20
Wed Mar 7: Transient variation of the magnetic field: solar-terrestrial phenomena.
L21
Fri Mar 9: Magnetic survey methods: introduction to interpretation of magnetic
maps, fields of point and line poles, spheres and ellipsoids.
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U2089 Introduction to Geophysics: Lecture schedule
Week 8
L22
Mon Mar 12: Electrical resistivity methods. interpretation of resistivity for
horizontally-layered strata.
L23
Wed Mar 14: Case histories: mineral exploration, geothermal research.
L24
Fri Mar 16: Introduction. methods and techniques; collection of rocks;
magnetisation processes; TRM, CRM, DRM.
Week 9
L25
Mon Mar 19: Reversals. geomagnetic polarity reversals; self-reversals; transitions.
L26
Wed Mar 21: Continental drift: pole positions; apparent polar wander paths;
palaeolatitudes and palaeoclimates.
Radioactivity and Heat
(David Stevenson)
L27
Fri Mar 23: Heat and temperature: warming up and melting. Moving heat about:
conduction, convection and radiation; Temperature and rheology: Hooke's Law and
ductile flow; the lithosphere and the asthenosphere.
Week 10
L28
Mon Mar 26: Heat and time: sills, lava pools and sea ice formation; temperatures in
boreholes – daily, seasonal and glacial cycles.
L29
Wed Mar 28: The Earth’s Heat. Radioactive isotopes in nature; radioametric dating;
heat production in continental and oceanic crust; meteorites and mantle heat
production.
L30
Fri Mar 30: Heat and time: thermal history of the Earth; Kelvin’s age of the Earth;
impact of radioactivity; convection; evolution of oceanic geotherms; temperatures
inside the Earth..
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Geophysics 2h: Practical schedule
SEMESTER 2
Week
Title
17 or19 Jan
No practical classes
Organiser
* 24 or 26 Jan
Pentland Fault Gravity Interpretation (groups a & b)
‡
DS
* 31 or 2 Feb
Pentland Fault Gravity Interpretation (groups c & d) ‡
DS
B 7 or 9 Feb
Seismic Field Work (groups a & b)
IM
B 14 or 16 Feb
Seismic Field Work (groups c & d)
IM
†28 or1 Mar
Seismic Data Interpretation ‡
IM
6 or 8 Mar
Magnetic/ Resistivity Field Work (groups a & b)
WW
13 or 15 Mar
Magnetic/ Resistivity Field Work (groups c & d)
WW
* 20 or 22 Mar
Resistivity data interpretation (groups a & b)‡
WW
* 27 or 29 Mar
Resistivity data interpretation (groups c & d)‡
WW
‡ Reports on these practicals are required. They contribute course work marks to the final
degree mark. Note that, where the report involves the analysis of data collected in the field,
attendance at both the fieldwork and the subsequent interpretation class is essential for
proper understanding of the practical.
* First Floor Computer Lab Annex, Kings Buildings Centre
† SAC – Peter Wilson Building LR-C;
B Peffermill, north end of the main rugby pitch
Other fieldwork locations posted on the web and be given out in class.
11
Reading List
Recommended Text to be bought for the Course
Fundamentals of Geophysics
W. Lowrie; Cambridge University Press, September 1997
£35.00
Alternative textbook for continuing Geophysics students
Frank M. Stacey & Paul M. Davies, Physics of the Earth (2008)
(CUP, 4th edition)
£40.00
Reference Texts for further reading
An Introduction to Geophysical Exploration
P.K. Keary & M. Brooks; Blackwell, 1991 (Third Edition)
£36.00
The Solid Earth: An introduction to Global Geophysics
C.M.R. Fowler; Cambridge University Press.
£39.00
Looking into the Earth
Alan Mussett & Aftab Khan; Cambridge University Press.
£37.00
Earthquakes
Bruce Bolt; Freeman Press 1999 (Fourth edition)
£33.99
Introduction to Seismology, Earthquakes & Earth Structure
Seth Stein & Michel Wysession; Blackwell
£45.00
Introduction to Seismology (suitable for continuing geophysicists)
Peter M. Shearer; Cambridge University Press, 1999
£34.00
All testbooks are available in the reserve collection of the Robertson Library, JCMB.