Final Review session 2

Transcription

1 Environmental and Exploration Geophysics I Final Review session 2 tom.h.wilson tom.wilson@mail.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV

2 Reminders Turn in magnetics lab before leaving today Final is from 11:00am-1 pm on Wednesday, December 14th. In room 325 Brooks

3 Continue exam review On tap for the day Brief answers to review problems from last time with review/questions on practice final and other problems from the homework Hand in magnetics lab before leaving. Final is from 11:00am to 1pm on Wednesday, December 14 th rm 325 Brooks

4 Final exam Wednesday, 11am-1pm December 14 th 325 Brooks Hall Spread out in the exam room. Sit in back of the person in front of you. Bring a pencil, ruler, eraser and calculator Show all your work! I will hand out an equation list following the earlier format See See links in sessions 29 and 30, class web page for review materials I m generally in my office. If you have questions drop by but me to ensure I ll be around.

5 Exam Schedule: Rm 325 REMEMBER! Exam will be held in rm 325 Brooks Hall No early or make-up exams offered!

6 Last week s review questions 1. Devil s tower decreases the acceleration of gravity at the station by 0.3milliGals. The topographic effect is -0.3mG; the topographic correction, 0.3mG. 2. The anomaly is associated with a silo (vertical cylinder) with depth-to-top of ~10m 3. The anomaly is produced by a horizontal cylinder with depth to center of ~500m. The radius of the cylinder is ~125m.

7 Just a reminder about magnetometer position The main idea behind these kinds of problems is to evaluate detectability, you have to accurately represent the position of the magnetometer! If the magnetometer is 2m above ground, then the distance to the center of the void (z) is 5m and Z max 8 R 3 z 3 3 kf E Burial Chamber Z=3m R=1m Surface Relative susceptibility in this example = emu.

8 The added 2 meters causes the field to drop With the magnetometer on the ground, Z max is nT; at 2 meters above ground the field drops to -3.67nT a drop of about 79% Z max 8 R 3 z 3 3 kf With z 3 in the denominator, the field drops rapidly as z increases. E

9 This problem and all other problems would change. The location of the magnetometer matters! Z max 2 R z 2 2 I If the magnetometer is located on the 2m aluminum staff, z (the depth to center) would have to be increased by 2m. If z is 3.75 m instead of 1.75m, will the wall still be detectable? The added 2 meters drops the anomaly from approximately 18nT to 3.9nT. With 5nT noise, we would conclude that the wall would not be detectable, BUT we should also consider reducing the magnetometer elevation before giving up!

10 A final point remnant magnetization Z 2 2 R 2 z In this formula for the horizontal cylinder max we have assumed I is magnetization induced in magnetic objects by the Earth s main field. However, in the case of basaltic lava flows, for example, their magnetic fields were formed when the melt fell below the Curie temperature and their induced magnetic field intensity might be given without reference to the present day field. I The remnant field intensity might simply be given, in such cases, as the field intensity of the lava flow at that location.

11 Also note that magnetic anomalies cross the 0nT value into the negative Example: the sphere For the dipole or spherical distribution the anomaly falls to 0 at x o /z=1.41

12 Magnetic anomalies cross the 0nT value example: the horizontal cylinder For the horizontal cylinder the anomaly falls to 0 at x o /z=1 so x o =z. What is z in this case? Remember x=0 can be at arbitrary points along a profile line!

13 In-class problem from last time: what is the pole strength? Last time we found that the vector field intensity of a single pole was nt (or ). Did you complete this problem? What is the pole strength in ups?

14 Practice final review Problem 1

15 Practice final review Problem 2

16 Practice final review Problem 3

17 Practice final review - Problem 4

18 Practice final review Problems 5-8.

19 Practice final review Problem 9

20 Practice final review Problem 10

21 Review problem 10 (continued)

22 Practice final review problem 11

23 Practice final review problem 12

24 Practice final review problems 13 &14

25 Tables will be provided in the equation list Looks like it might be a spherical void or void in the form of a horizontal cylinder.

26 Remember to use appropriate tables gravity or magnetics

27 For pre-exam reference, the equation list is linked on the class page with some minor modifications from mid-term.

28 Exam Schedule: REMEMBER! Exam will be held in rm 325 Brooks Hall No early or make-up exams offered!

29 Bring a pencil, ruler, eraser and calculator Show your work! I will hand out an equation list. It will look like that posted on the class page. See Or access from link on session 30.

30 Reminders On line course evaluation has been accessible since Nov. 17 th. Please provide your feedback. Turn in magnetics lab before leaving Final is from 11:00am-1 pm on Wednesday, December 14th. Final exam given in Room 325 Brooks