Environmental and Exploration Geophysics I

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

2. Anomaly associated with buried metallic materials Computed magnetic field produced by bedrock Results obtained from inverse modeling Bedrock configuration determined from gravity survey

3. Where are the drums and how many are there?

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

5. Trench boundaries - field data Trench Boundaries - model data Gilkeson et al., 1986

6. Locating abandoned wells

7. Abandoned Wells From Martinek

8. Abandoned Well - raised relief plot of measured magnetic field intensities From Martinek

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

11. Gochioco and Ruev, 2006

13. Magnetic monopoles p1 r12 Fm12Magnetic Force Magnetic Permeability p1and p2pole strengths Coulomb’s Law p2

14. Force Magnetic Field Intensity often written as H pt is an isolated test pole The text uses F instead of H to represent magnetic field intensity, especially when referring to that of the Earth (FE).

15. 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

16. Toxic Waste monopole vs. dipole

17. The earth’s main magnetic field

18. Measuring the Earth’s magnetic field Proton Precession Magnetometers Steve Sheriff’s Environmental Geophysics Course Tom Boyd’s Introduction to Geophysical Exploration Course

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

20. 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 -

21. Magnetic Elements

26. Magnetic north pole: point where field lines point vertically downward Geomagnetic north pole: pole associated with the dipole approximation of the earth’s magnetic field. The compass needle points to the magnetic north pole.

27. Main field intensity in Morgantown

29. Variations of inclination through time

31. Variations of declination W through time

32. Magnetic Elements for your location

33. Today’s Space Weather

34. Magnetic Field Variations

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

36. Magnetic fields like gravitational fields are not constant. Their variations are much more erratic and unpredictable Diurnal variations http://www.earthsci.unimelb.edu.au/ES304 /MODULES/ MAG/NOTES/tempcorrect.html

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

38. Corrections? 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

39. Correcting for Diurnal Variations Reoccupy the base 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.

40. 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.

41. 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.

42. S N + N - F E S + - N Opposites attract + S -

43. Vector Awareness N S

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

45. I=kF Hysterisis Loops I is the intensity of magnetization and FE is the ambient (for example - Earth’s) magnetic field intensity. k is the magnetic susceptibility.

46. The intensity of magnetization is equivalent to the magnetic moment per unit volume or Magnetic dipole moment per unit volume where and also, . Thus yielding and The cgs unit for pole strength is the ups

47. 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,

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

49. Potential versus Force 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?

50. Remember the general power rule for integration Since n is -2, n+1 = -1 so that the potential V (per unit pole) is simply