1. Magnetic Methods (V) Environmental and Exploration Geophysics I
tom.h.wilson
Department of Geology and Geography
West Virginia University
Morgantown, WV
Tom Wilson, Department of Geology and Geography

2. Magnetics Lab – final overview
GPR
Local soil disturbance
We are working in the context of what we have
Where are the drums?
Tom Wilson, Department of Geology and Geography

3. Tom Wilson, Department of Geology and Geography
Having trouble structuring your magnetics lab summary? See link on the class page
Tom Wilson, Department of Geology and Geography

4. Why is the gravity useful in this case?
Tom Wilson, Department of Geology and Geography

5. Tom Wilson, Department of Geology and Geography
The gravitational field over the area helps define bedrock configuration
Now that it’s known …
Tom Wilson, Department of Geology and Geography

6. We know the contribution of bedrock to the magnetic field
Bedrock related magnetic anomaly
Tom Wilson, Department of Geology and Geography

7. Magnetic anomaly not explained by bedrock
Where are the drums?
Magnetic anomaly not explained by bedrock
From the bedrock
Tom Wilson, Department of Geology and Geography

8. Inversion will give you a solution, but it may be,
and most likely will be, an incorrect representation of
region enclosing the buried drums
Pancake solution
Tom Wilson, Department of Geology and Geography

9. non-uniqueness and the drum cluster
anomaly
Tom Wilson, Department of Geology and Geography

10. The x and y scaling has to be 1:1!
How many drums?
Area of one drum ~
4 square
feet
What’s wrong with the format of this plot?
The x and y scaling has to be 1:1!
Tom Wilson, Department of Geology and Geography

11. Consider one of the Lab Questions
…. compare the field of the magnetic dipole to that of the gravitational monopole
Gravity:500, 1000, 2000m
A more rapid decay
Increase r by a factor of 4 reduces g by a factor of 16
Tom Wilson, Department of Geology and Geography

12. A 4 fold increase in distance
For the dipole field, an increase in depth (r) from 4 meters to 16 meters produces a 64 fold decrease in anomaly magnitude
Thus the 7.2 nT anomaly (below left) produced by an object at 4 meter depths disappears into the background noise at 16 meters.
0.113 nT
7.2 nT
Tom Wilson, Department of Geology and Geography

13. Carefully read over the Magnetics lab manual
Incorporate answers to the questions on the handout into your summary/conclusions
Tom Wilson, Department of Geology and Geography

14. Consider some of the following points (see lab guide)
1. Why are both gravity and magnetic surveys needed to accurately locate the buried drums in this area.
2. Using the triangular outline provided in the graph. Estimate the number
of drums contained within the triangle.
The drums at the bottom of the heap are about twice as far from the surface as the drums near the top of the pile, so their contribution will be about 1/8th that of drums in the near-surface
3. Assume that the magnetic anomaly produced by a single drum located 10 feet beneath the surface is 10 nT: what anomaly will it produce if buried 20 feet beneath the surface?
Tom Wilson, Department of Geology and Geography

15. Tom Wilson, Department of Geology and Geography
The triangular area shown below shows the possible location and distribution of the drum cluster
What is the base?
What is the height?
Tom Wilson, Department of Geology and Geography

16. Exam review part 1 & problems we’ve been working on …
See problem 7.2
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17. Tom Wilson, Department of Geology and Geography
Problems from chapter 7
The first problem relates to our discussions of the dipole field and their derivatives.
7.1. What is the horizontal gradient in nT/m of the Earth’s vertical field (ZE) in an area where the horizontal field (HE) equals 20,000 nT and the Earth’s radius is 6.3 x 108 cm.
Tom Wilson, Department of Geology and Geography

18. Evaluate the horizontal gradient
Since  is co-latitude, the direction of increasing  is southward (in the northern hemisphere). As we travel from pole to equator ZE decreases, thus the gradient is negative.
Tom Wilson, Department of Geology and Geography

19. Can you find it? What is z? What is I?
4. A buried stone wall constructed from volcanic rocks has a susceptibility contrast of 0.001cgs emu with its enclosing sediments. The main field intensity at the site is 55,000nT. Determine the wall's detectability with a typical proton precession magnetometer. Assume the magnetic field produced by the wall can be approximated by a vertically polarized horizontal cylinder. Refer to figure below, and see following formula for Zmax.
What is z?
What is I?
Background noise at the site is roughly 5nT.
Tom Wilson, Department of Geology and Geography

20. Tom Wilson, Department of Geology and Geography
is a function of the unit-less variable x/z
Dipole/sphere
Vertical cylinder
Horizontal cylinder
The vertical field is often used to make a quick estimate of the magnitude of an object. This is fairly accurate as long as i is 60o or greater
Tom Wilson, Department of Geology and Geography

21. vertical polarization
Zmax
vertical polarization
Vertically polarized sphere or dipole
Vertically polarized horizontal cylinder
Tom Wilson, Department of Geology and Geography

22. Considerable difference in magnitude of
As discussed in class, it’s probably best to center the cylinder at 1.75 meters depth.
For the dipole
=(6.28*0.16*55)/1.75squared = 55.26/3 = 18 nT
For the horizontal cylinder
Tom Wilson, Department of Geology and Geography

23. Tom Wilson, Department of Geology and Geography
Sample rate is an important decision in any survey. The sample rate you use will depend on the minimum depth of the objects you wish to find.
Your sample interval should be no greater than X1/2.
But don’t forget that equivalent solutions with shallower origins do exist!
Tom Wilson, Department of Geology and Geography

24. Tom Wilson, Department of Geology and Geography
Sampling
X1/2=Z/2
How often would you have to sample to detect this drum?
Tom Wilson, Department of Geology and Geography

25. oops! …. how about this one?
The anomaly of the drum drops to ½ at a distance = ½ the depth.
Tom Wilson, Department of Geology and Geography

26. Tom Wilson, Department of Geology and Geography
Sampling issues – file for future reference!
You are asked to run a magnetic survey to detect a buried drum. What spacing do you use between observation points? $$
Reliability
If the depth to the center of the drum is 3 meters then we’d want to have a minimum sample interval of 1.5 meters.Why?
An important point that you’ve run across in the literature.
Tom Wilson, Department of Geology and Geography

27. Other questions we can also ask:
Is the anomaly associated with a sphere?
i. e. buried magnetic dipole
You can usually make quick work of it an use only three diagnostic positions (red above)
Tom Wilson, Department of Geology and Geography

28. Question 4 or vertical cylinder?
Again, we can get by with only three diagnostic positions (red above)
Tom Wilson, Department of Geology and Geography

29. Tom Wilson, Department of Geology and Geography
Let’s go through it again: Determine depths (z) assuming a sphere or a cylinder and see which assumption yields consistent estimates.
It’s all about using diagnostic positions and the depth index multipliers for each geometry.
Tom Wilson, Department of Geology and Geography

30. Tom Wilson, Department of Geology and Geography
0.9
X3/4
1.55
X1/2
2.45
X1/4
diagnostic distance
Sphere vs. Vertical Cylinder; z = __________
The depth
Diagnostic positions
Multipliers
Sphere
ZSphere
Multipliers Cylinder
ZCylinder
X3/4 =
X1/2 =
X1/4 =
3.18 2
1.37
2.86
3.1
3.35
2.86
3.1
3.35
2.17
1.31
0.81
1.95
2.03
2.00
1.95
2.03
2.00
Tom Wilson, Department of Geology and Geography

31. Tom Wilson, Department of Geology and Geography
gmax
g3/4
g1/2
g1/4
Sphere or cylinder?
Diagnostic positions
Multipliers
Sphere
ZSphere
Multipliers Cylinder
ZCylinder
X3/4 = 1.6 meters
3.18
2.17
X1/2 = 2.5 meters 2
1.31
X1/4 = 3.7 meters
1.37
0.81
5.01
5.0
5.07
5.08 5
5.1
3.47
3.28
3.00
Tom Wilson, Department of Geology and Geography

32. Tom Wilson, Department of Geology and Geography
What is R?
Algebraic manipulation
5. Given that derive an expression for the radius,
where I = kHE. Compute the depth to the top of the casing for the anomaly shown below, and then estimate the radius of the casing assuming k = 0.1 and HE =55000nT. Zmax (62.2nT from graph below) is the maximum vertical component of the anomalous field produced by the vertical casing.
Tom Wilson, Department of Geology and Geography

33. The end of the tunnel is in sight ! - Onward ...
Hand in Chapter 7 problems before leaving today.
More review coming this Thursday
Look over today’s questions, the practice final questions, review your notes and come prepared to ask questions this Thursday.
If you want an additional review session, let me know. You can always drop by my office if you have questions
Prepare magnetics lab summary and submit in class this Thursday, December 6th
Magnetics paper summaries are also due this Thursday (December 6th)
Exam, Friday December 14th; 3-5pm
Questions?
Tom Wilson, Department of Geology and Geography