1. Environmental and Exploration Geophysics I tom.h.wilson wilson@geo.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV Magnetic Methods (I) http://www.geo.wvu.edu/~wilson/geo252/lect12/mag2.pdf

2. Locating Trench Boundaries Theoretical model Examination of trench for internal magnetic anomalies. actual field data Gilkeson et al., 1986

3. Locating abandoned wells

4. From Martinek Abandoned Wells

5. Falls Run Coal Mine Refuse Pile Magnetic Intensity Wire Frame

6. Magnetic monopoles p1p1 p2p2 r 12 F m12 Magnetic Force  Magnetic Permeability p 1 and p 2 pole strengths Coulomb’s Law

7. Force Magnetic Field Intensity often written as H p t is an isolated test pole We will use F instead of H to represent magnetic field intensity, especially when referring to that of the Earth (F E ).

8. The fundamental magnetic element is a dipole or combination of one positive and one negative magnetic monopole. The characteristics of the magnetic field are derived from the combined effects of non-existent monopoles. Dipole Field

9. The earth’s main magnetic field

10. Source of Protons and DC current source Proton precession generates an alternating current in the surrounding coil

11. Proton precession frequency (f) is directly proportional to the main magnetic field intensity F. L is the angular momentum of the proton and G is the gyromagnetic ratio which is a constant for all protons (G = 0.267513/  sec). Hence -

12. Magnetic north pole: point where field lines point vertically downward Geomagnetic north pole: pole associated with the dipole approximation of the earth magnetic field. Compasses point to the magnetic north pole.

17. W

18. Magnetic Elements for your location

19. Magnetic Field Variations

20. Long term drift in magnetic declination and inclination Magnetic field variations generally of non-geologic origin

21. Magnetic fields like gravitational fields are not constant. Their variations are much more erratic and unpredictable

22. Today’s Space Weather Real Time Magnetic field data

23. In general there are few corrections to apply to magnetic data. The largest non-geological variations in the earth’s magnetic field are those associated with diurnal variations, micropulsations and magnetic storms. The vertical gradient of the vertical component of the earth’s magnetic field at this latitude is approximately 0.025nT/m. This translates into 1nT per 40 meters. The magnetometer we have been using in the field reads to a sensitivity of 1nT and the anomalies we observed at the Falls Run site are of the order of 200 nT or more. Hence, elevation corrections are generally not needed. Variations of total field intensity as a function of latitude are also relatively small (0.00578nT/m). The effect at Falls Run would have been about 1/2 nT from one end of the site to the other. International geomagnetic reference formula

24. The single most important correction to make is one that compensates for diurnal variations, micropulsations and magnetic storms. This is usually done by reoccupying a base station periodically throughout the duration of a survey to determine how total field intensity varies with time and to eliminate these variations in much the same way that tidal and instrument drift effects were eliminated from gravity observations.

25. Anomalies - Total Field and Residual The regional field can be removed by surface fitting and line fitting procedures identical to those used in the analysis of gravity data.

26. Magnetic susceptibility is a key parameter, however, it is so highly variable for any given lithology that estimates of k obtained through inverse modeling do not necessarily indicate that an anomaly is due to any one specific rock type.

27. Opposites attract S N S N

28. SN

29. Magnetic fields are associated fundamentally with circulating electric currents, so that we can also formalize concepts like pole strength, dipole moment, etc. in terms of current flow relationships. pl = n iA + - l n turns Cross sectional area A pl is the dipole moment

30. I=kF I is the intensity of magnetization and F E is the ambient (for example - Earth’s) magnetic field intensity. k is the magnetic susceptibility.

31. The intensity of magnetization is equivalent to the magnetic moment per unit volume or and also,. Thus and yielding Magnetic dipole moment per unit volume where

32. Recall from our earlier discussions that magnetic field intensity so that Thus providing additional relationships that may prove useful in problem solving exercises. For example,

33. What does this tell us about units of these different quantities? We refer to the magnetic field intensity as H or ambiguously by some as F

34. Force varies inversely as the square of the distance between charges, masses or poles. It has the general form Potential on the other hand refers to the energy available to do work and is the integral of the force times displacement. What is this integral?

35. Remember the general power rule for integration Since n is -2, n+1 = -1 so that the potential V is simply

36. As we have done repeatedly with the force, we express it in terms of force per unit mass, charge or pole to obtain where F is acceleration, electric or magnetic field intensity. We can do the same with the potential writing it as the potential per unit pole strength, or just Note that working with potentials may offer us some simplification since the denominator is in r and not r 2.