Figure 1 shows a simplified model of a continental margin called
‘Birkbeck Margin’. The densities of water, lithosphere and
asthenosphere are 1.03 g/cm3, 2.67 g/cm3 and 3.10 g/cm3
respectively. !
Figure 1 Simplified model of Birkbeck Margin !! f) Explain why the free-air anomaly of the Birkbeck Margin can be
calculated by adding the measured gravity anomalies of the two
half-slabs in d). Why is it not necessary to know the latitude of the
Birkbeck Margin in the calculation? !!g) By using information given in f), calculate the free-air anomaly
across Birkbeck Margin and sketch it in a graph.
!
!h
) Explain how the expected Bouguer anomaly of Birkbeck Margin
(Figure 1) will be different from the free-air anomaly. It is not
necessary to calculate or sketch the Bouguer anomaly in a
graph. !!!!i) By considering the pressure difference at 100 km below the sea
level of Birkbeck Margin (Figure 1), comment on the isostatic
equilibrium of the Birkbeck Margin. Assume that g = 9.8 m/s2 at
all depths. !!!!j)* By considering Equation 1, explain how the free-air anomaly
profile (part g) can be used to determine the isostatic status of
Birkbeck Margin.
Show entire document
‘Birkbeck Margin’. The densities of water, lithosphere and
asthenosphere are 1.03 g/cm3, 2.67 g/cm3 and 3.10 g/cm3
respectively. !
Figure 1 Simplified model of Birkbeck Margin !! f) Explain why the free-air anomaly of the Birkbeck Margin can be
calculated by adding the measured gravity anomalies of the two
half-slabs in d). Why is it not necessary to know the latitude of the
Birkbeck Margin in the calculation? !!g) By using information given in f), calculate the free-air anomaly
across Birkbeck Margin and sketch it in a graph.
!
!h
) Explain how the expected Bouguer anomaly of Birkbeck Margin
(Figure 1) will be different from the free-air anomaly. It is not
necessary to calculate or sketch the Bouguer anomaly in a
graph. !!!!i) By considering the pressure difference at 100 km below the sea
level of Birkbeck Margin (Figure 1), comment on the isostatic
equilibrium of the Birkbeck Margin. Assume that g = 9.8 m/s2 at
all depths. !!!!j)* By considering Equation 1, explain how the free-air anomaly
profile (part g) can be used to determine the isostatic status of
Birkbeck Margin.
Geophysics Assignment 2
Non-seismic geophysical methods and their
geological applications
Dr Vincent C H Tong
Birkbeck, University of London
Instructions
1. The deadline for Assignment 2 is
10 am Monday, 6 January,
2014
(London time).
2.
Work
must
be
submitted
electronically
via
the
link
under
“Assignments” on the module site (moodle.bbk.ac.uk).
No
email or
hard copy submission will be accepted.
3. Please submit your assignment in
one file
in
Microsoft Word
format (.doc or .docx).
The file must be less than 20 Mb in size.
All
text must be typed (i.e. no scanned text), except for figure
annotations
.
All figures must be included in the Microsoft Word file.
Figures may contain scanned images.
Work that does not follow
these rules will not be marked.
4. An acknowledgment email will be sent to you (from Turnitin) to
confirm your submission.
5. Please familiarise yourself with the electronic submission system
well before the deadline by submitting a Microsoft Word file to
“Assignment (Dummy)” on the module Moodle site.
(Note that the
file will not be marked.)
Please allow sufficient time to submit your
work in order to avoid potential last-minute technical difficulties (e.g.,
internet connection problems).
6. This assignment counts for 10% of the total mark for this course.
The first assignment counts for 10%, and the examination will count
for 80%.
7. Feedback on your submitted work will be available electronically
around three weeks after the deadline.
8. Please read the “Academic Honesty and Plagiarism” statements
with regard to submission of assignments, which are available in the
“Assignments” section on the Moodle module website.
There is no
need to include a coursework cover sheet for your work.
Information about plagiarism and how to avoid it is available here:
http://www.bbk.ac.uk/mybirkbeck/get-ahead-stay-ahead/academic-
support/plagiarism/plagiarism-guide-1
9.
Please note that no extension will be granted.
If you have to
submit your work after the deadline (e.g., due to medical or other
reasons),
please complete a mitigating circumstances form with
documentary evidence.
The mitigating circumstances form and
procedures can be found here:
http://www.bbk.ac.uk/mybirkbeck/
services/administration/assessment/exams/mitigating-circumstances
Mitigating circumstances will be considered by the Department.
Unless the mitigating circumstances are approved, late submissions
(within two weeks after the deadline) will receive a maximum mark
of 40%, and no mark will be given to work submitted two weeks after
the deadline.
Advice for completing the assignment
1. Allow
plenty
of
time
to
study
the
course
materials
before
attempting the questions.
Here are some of the topics that are
relevant to this assignment:
l
Gravity anomaly caused by a body with anomalous density
l
Free-air and Bouguer anomalies
l
Gravity modelling
l
Pressure as sum of products of density, gravity and thickness
l
Isostasy and gravity anomalies
2. Here are some mathematical skills needed for completing the
questions.
Please refer to Review of Basic Maths Skills (Session 2).
l
Plot data points in a graph with labelled axes
l
Use correct units
l
Use a scientific calculator (or spreadsheet software as an
alternative)
3.
PART
ONE
consists
of
questions
that
are
relatively
straightforward and lays the foundation for questions in PART
TWO.
4.
PART
TWO
is
the
geological
applications
of
the
theories
investigated in PART ONE.
The asterisked question may require
some more thoughts.
Question
The aim is to apply your knowledge of gravity anomaly and isostasy
to the study of a continental margin.
In
answering
the
following
questions,
you
should
apply
your
geophysical knowledge rather than to describe the details of the
theories behind your explanation.
Write
no more than
three sentences
when answering questions
requiring a description/ explanation.
You may plot your graphs
on graph paper (and scanned them in) or software such as Excel.
Make sure your tables, graphs and figures are all embedded in
your Word file as you will need to submit one file for this
assignment.
PART ONE
(40%)
We have encountered gravity anomalies produced by spheres and
infinite slabs in the course.
Now study the gravity anomalies
produced by half-slabs with different density anomalies (positive and
negative) as well as their variation with the depth of the half-slabs
(Figures 8.12 and 8.18, and the associated explanations in your
course book).
Then answer the following questions:
a)
How does the gravity anomaly of a half-slab vary with its depth?
What happens to the gravity anomaly if the half-slab is a
negative
density
anomaly?
What
happens
to
the
gravity
anomaly if the half-slab is a positive density anomaly?
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