1. GG 450 February 19, 2008
Magnetic Anomalies

2. Magnetic Field Methods
Field Procedures:
1. As always, fit the station spacing to the suspected depth of the source.
2. Be prepared to decrease the spacing in areas of particular interest.
3. Take hourly base station measurements in the field area to correct for diurnal variations - as you would correct for drift in a gravity meter. However, variations in magnetism are real - not instrumental, so your correction curve will probably not be linear.
4. No need for elevation correction.
5. Be particularly dense along N-S profiles, since induced anomalies viewed along N-S profiles are the most diagnostic.

3. 6. On arrival in the field area, mark off a magnetic N-S line covering the feature of interest. Decide on a “0” location, mark it, and describe it as precisely as possible.
7. take reconnaissance measurements (about 10 along the line) at equal intervals along a non-magnetic tape. Record the data and location.
8. If an anomaly of interest is found, take enough measurements along this line to define its limits and features.
9. Move roughly half a wavelength east or west and run another line over the anomalous area. Again, take care in recording locations.

4. Place sensor head on pole supplied
As always, take good notes! You should be able to re-occupy a station within a foot.
12. As time permits, take readings at two different heights above ground to get the VERTICAL GRADIENT. This may help to define the depth of the anomalous bodies.
[Use the gradient option in GMSys when modeling]

5. 13. If data are erratic and can’t be repeated, check batteries
13. If data are erratic and can’t be repeated, check batteries. If OK, - quit. Probably a magnetic storm.
14. Get rid of all metallic objects on the observer
15. Watch out for pipes, fences, power lines - anything metallic. The electric currents generate magnetic fields that can overwhelm anomalies.
16. Set the display unit on the ground while taking readings. (This is only true for OUR [old] meter.

6. Reduction:
1. Remove any time drifts and regional trends.
Input the data into a magnetic modeling program, such as GMSys, for analysis of profiles. If multiple profiles are made and “grids” of data, make:
MAGNETIC MAPS: Magnetic maps are often filled with large and striking anomalies of many wavelengths. Their interpretation in terms of geology is difficult, at best. It’s a good idea to plot up some model anomalies for your latitude so you know what you might expect.

7. Magnetic dipole
field showing equipotentials and lines of force using magpot.m (same as gravpot.m with a negative mass for one point mass). Think of the circle as the surface of the earth.

8. How does a tape recorder work?
Hints for determining the general shape of an induced magnetic anomaly from models:
1). Determine the direction of the earth's field. The induced poles will be aligned such that an arrow in the direction of the earth's field will go from the - pole to the + pole.

9. 2) Sketch the induced field by itself, paying particular attention to where the field intersects the earth's surface (or on the surface where the magnetic measurements will be made). Use arrows to show the direction of the field.

10. 3) Wherever the induced field is in a direction orthogonal to the earth's field along the profile, the total magnetic anomaly will be zero. Put big dots at these points.

11. 4) Wherever the anomalous field is in the same direction as the earth's field, the anomaly will be POSITIVE. Wherever the anomalous field is in the opposite direction, the anomaly will be NEGATIVE.

12. 5) The anomaly will be largest near the magnetic poles, where the magnetic lines of force are closest together.

13. What shape of anomalies might you expect in the field?
First, most magnetic anomalies are INDUCED, so the induced field results in magnetic anomalies having negative anomalies north of the positive anomalies (assuming today’s magnetic field).
In the northern hemisphere will have negative poles at shallower depths than positive poles, and near the equator, positive and negative poles will be at the same depth:

14. In the figures below, North is to the right (assuming today’s field).
North of the equator, negative anomaly to the north, positive anomaly larger:

15. At the magnetic equator:

16. As the poles spread farther apart, the anomaly between them gets smaller, since the lines of force spread out away from the poles.
What about a sphere?

17. The anomaly caused by ANY body can be estimated by summing the potentials of poles along its edges in the direction of the earth’s field.
What would the anomaly above look like in MAP view?

18. What about a flat sheet of magnetic material
What about a flat sheet of magnetic material? If the flat sheet extends out in all directions, we won’t see it, since the anomaly generated will be flat with no structure. But what happens to the anomaly at the EDGE of a sheet?

19. HOMEWORK: Due Thursday, Feb 21
Use GmSys to generate magnetic anomalies for the following models. All declinations are 0°, N-S profiles, susceptibility of surroundings: 0. Print the results with appropriate scales to display the models and anomalies.
1) Buried cylinder: Fe inclination -90°, susceptibility of cylinder 0.001, cylinder long axis is horizontal, diameter is 10m, depth to center is 50 m.
2) Same as 1) with Fe inclination 0°.
3) Edge of magnetic plate: Fe inclination: -45°, plate susceptibility: 0.001, plate extends to infinity in E-W direction and to south, edge of plate is at 0 m on the profile. depth to top of plate: 50 m, thickness of plate: 10 m.
4) Same as 3) with Fe inclination 0°.