west virginia universitypages.geo.wvu.edu/~wilson/geol454/lect13/mag5.pdf · title: slide 1...

Tom Wilson, Department of Geology and Geography

Environmental and Exploration Geophysics I

tom.h.wilson

[email protected]

Department of Geology and Geography

West Virginia University

Morgantown, WV

Magnetic Methods (V) and

Exam Review session 1

On tap for the day


• 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

Breakout problems > Group 1


Questions about problems for groups 1-3?

Each group had one of these source geometry

evaluation problems


In each case, you start by noting diagnostic

positions along the anomaly profile


Group 1 anomaly

X3/4 =1.25X1/2 =2.05X1/4 =2.9

For group 1


1.252.052.9

Sphere or vertical cylinder? Depth?

44.14

2.712.692.4

For groups 2 and 3


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

Group 2 – problem 1


Problem 1


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!

If you double the depth, the anomaly amplitude

drops by …


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.

Review group 3 problem 1


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

Compare result you obtained to the one

obtained using excel calculation: ~-1nT


Group 3 – problem 1


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

This problem and all other problems would change.

The location of the magnetometer matters!


Have to add that additional 2m into the depth z

What is the reference point for measuring

distances to diagnostic positions ?


Just keep in mind that anomalies don’t always come with 0 x-coordinates at the peak!

Problem 2 – groups 2 and 3


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

Question posed last time: what is the pole

strength?

See today’s handout

For now, let’s get started on three group

problems – Problem 1:


Groups focus on their problem initially, but try to do all three.

Take about 15 minutes now to get going on these problems.

Problem 2: Underground storage tank or

missile silo. See diagnostic relationships


Another – gravity vs mag


Tables of depth index multipliers will be

included in the exam equation reference sheet


Practice final exam – see handout


Work over practice problems – review gravity

and magnetic homework problems AND


bring questions to class next time!

What to bring for the exam

Bring a pencil, ruler, eraser and calculator

Show all your work!


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.

Reminders


• 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

west virginia universitypages.geo.wvu.edu/~wilson/geol454/lect13/mag5.pdf · title: slide 1...

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