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Geophysics Assignment 2 Non-seismic geophysical methods and their geological applications ! Dr Vincent C H Tong Birkbeck, University of London !...

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.

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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