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Tom Wilson, Department of Geology and Geography
Environmental and Exploration Geophysics I
tom.h.wilson
Department of Geology and Geography
West Virginia University
Morgantown, WV
Magnetic Methods (V) and
Exam Review session 1
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On tap for the day
Tom Wilson, Department of Geology and Geography
• Solutions to group problems with discussion.
• Additional review materials
• Determining pole strength
• Example test questions for take-home study …
look over and bring questions to class for next Tuesday –
the last class
• Turn in problems 4 & 5 before leaving
• You have been given an extra 5 days to do the magnetics
lab if needed. Hand in no later than Dec 5th in class.
• Final is from 11:00am to 1 pm Wednesday, December 14th.
In room 325 Brooks
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Breakout problems > Group 1
Tom Wilson, Department of Geology and Geography
Questions about problems for groups 1-3?
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Each group had one of these source geometry
evaluation problems
Tom Wilson, Department of Geology and Geography
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In each case, you start by noting diagnostic
positions along the anomaly profile
Tom Wilson, Department of Geology and Geography
Group 1 anomaly
X3/4 =1.25X1/2 =2.05X1/4 =2.9
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For group 1
Tom Wilson, Department of Geology and Geography
1.252.052.9
Sphere or vertical cylinder? Depth?
44.14
2.712.692.4
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For groups 2 and 3
Tom Wilson, Department of Geology and Geography
Group 2: cylinder – 10kmGroup 3: cylinder – 10km
That scale should have been in meters but you could think of it as a large igneous pipe or pluton. The important issue here is that real data doesn’t always have a zero coordinate at the peak.
X3/4 =4.4X1/2 =7.7X1/4 =12.3
X3/4 =4.6X1/2 =7.7X1/4 =12.5
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Group 2 – problem 1
Tom Wilson, Department of Geology and Geography
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Problem 1
Tom Wilson, Department of Geology and Geography
In the class problem set handout, the depth z was set equal to 1.9m. Think
about the rationale involved in deciding on the depth to the center of the
cylindrical approximation of the wall. Using 1.9m we get Zmax=4.3nT.
Using 1.75m, we get 5.46 nT. Although a minor difference, which
do you think yields the better approximation? Neither anomaly
would be detectable above a background noise level of ±5nT.
3
max 3 3
8
8.38 0.064 55 (0.536)553 4.31.9 6.86
ER kFx x
Z nTz
Actually worked for you in the problem set handout.
z in either case represents depth to center!
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If you double the depth, the anomaly amplitude
drops by …
Tom Wilson, Department of Geology and Geography
At a depth of 1 meter Zmax is 29.26.At a depth of 2 meters Zmax is
3
max 3
8
3ER kF
Zz
3.66 (1/8th or 1/23).
Zmax varies inversely as the cube of the depth. If the depth is doubled, Zmax will decrease to one-eighth of its former value.
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Review group 3 problem 1
Tom Wilson, Department of Geology and Geography
Could a total field magnetic survey detect the burial chamber shown below given ZE=55,000nT and k=0.0001. Assume that background noise in the area is 3nT
Z=3m
R=1m
Burial chamber
Relative susceptibility contrast, k = -0.0001 emu.
3
max 3
8
3ER kF
Zz
Again, the more accurate estimate would be made using equations 7.36 and 7.37 and Burger’s Table 7.3
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Compare result you obtained to the one
obtained using excel calculation: ~-1nT
Tom Wilson, Department of Geology and Geography
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Group 3 – problem 1
Tom Wilson, Department of Geology and Geography
The first question usually asked before beginning a geophysical investigation of any kind is “Will this method provide the information I want?” That usually means
“Can I detect the object(s) I want to find?”
R=1m
Z=3m
Burial Chamber
Relative susceptibility contrast =-0.0001emu.
Surface
We’ve always assumed that the magnetometer is sitting on the ground; however, in most cases it is located above ground and Z, in this problem, would no longer be 3 meters
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This problem and all other problems would change.
The location of the magnetometer matters!
Tom Wilson, Department of Geology and Geography
Have to add that additional 2m into the depth z
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What is the reference point for measuring
distances to diagnostic positions ?
Tom Wilson, Department of Geology and Geography
Just keep in mind that anomalies don’t always come with 0 x-coordinates at the peak!
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Problem 2 – groups 2 and 3
Tom Wilson, Department of Geology and Geography
Group 2: cylinder – 10kmGroup 3: cylinder – 10km
X3/4 =4.4X1/2 =7.7X1/4 =12.3
X3/4 =4.6X1/2 =7.7X1/4 =12.5
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Question posed last time: what is the pole
strength?
See today’s handout
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For now, let’s get started on three group
problems – Problem 1:
Tom Wilson, Department of Geology and Geography
Groups focus on their problem initially, but try to do all three.
Take about 15 minutes now to get going on these problems.
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Problem 2: Underground storage tank or
missile silo. See diagnostic relationships
Tom Wilson, Department of Geology and Geography
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Another – gravity vs mag
Tom Wilson, Department of Geology and Geography
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Tables of depth index multipliers will be
included in the exam equation reference sheet
Tom Wilson, Department of Geology and Geography
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Practice final exam – see handout
Tom Wilson, Department of Geology and Geography
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Work over practice problems – review gravity
and magnetic homework problems AND
Tom Wilson, Department of Geology and Geography
bring questions to class next time!
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What to bring for the exam
Bring a pencil, ruler, eraser and calculator
Show all your work!
Tom Wilson, Department of Geology and Geography
I will hand out an equation list posted on the class page.
See
http://pages.geo.wvu.edu/~wilson/geol454/equationlist.pdf
Or access from link on topic 30 – class page.
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Reminders
Tom Wilson, Department of Geology and Geography
• Turn in problems 4 & 5 before leaving
• If needed, you’ve got an extra 5 days to get the
magnetics lab in. Final due date next Tuesday – in
class.
• Next Tuesday’s session will be devoted to exam
review
• Go over practice final and other materials on gravity
and magnetics including homework and handouts
• Bring questions to class next time!
• Final is from 11:00am-1 pm on Wednesday, December
14th.
In room 325 Brooks