1. Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics I tom.h.wilson tom.wilson@mail.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV Magnetic Methods (II)

2. Tom Wilson, Department of Geology and Geography Long term drift in magnetic declination and inclination Magnetic field variations are generally of non-geologic origin

3. Tom Wilson, Department of Geology and Geography Magnetic Field Variations – annual drift of the magnetic pole

4. Tom Wilson, Department of Geology and Geography Diurnal variations in the Earth’s Magnetic field

5. Tom Wilson, Department of Geology and Geography Magnetic fields like gravitational fields are not constant. However, magnetic field variations are much more erratic and unpredictable http://www.earthsci.unimelb.edu.au/ES304 /MODULES/ MAG/NOTES/tempcorrect.html Diurnal variations

6. Tom Wilson, Department of Geology and Geography Solar activity and sunspot cycles

7. Tom Wilson, Department of Geology and Geography Today’s Space Weather http://www.swpc.noaa.gov/today.html Real Time Magnetic field data Real Time Magnetic field data http://www.swpc.noaa.gov/ace/ace_rtsw_data.html

8. Tom Wilson, Department of Geology and Geography http://www.swpc.noaa.gov/ace/ace_rtsw_data.html From the Advanced Composition Explorer Satellite

9. Tom Wilson, Department of Geology and Geography 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 north to south across the site. International geomagnetic reference formula

10. Tom Wilson, Department of Geology and Geography 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.

11. Tom Wilson, Department of Geology and Geography 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.

12. Tom Wilson, Department of Geology and Geography 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.

13. Tom Wilson, Department of Geology and Geography The Earth’s main field S N The induced magnetic field of a metallic drum

14. Tom Wilson, Department of Geology and Geography SN

15. 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 + - l n turns Cross sectional area A pl is the dipole moment

16. Tom Wilson, Department of Geology and Geography 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.

17. Tom Wilson, Department of Geology and Geography 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 The cgs unit for pole strength is the ups

18. Tom Wilson, Department of Geology and Geography 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,

19. Tom Wilson, Department of Geology and Geography What does this tell us about units of these different quantities? We refer to the magnetic field intensity as H (or as Burger et al. do, F)

20. Tom Wilson, Department of Geology and Geography Basic Magnetic Units and Vector Concepts

21. Tom Wilson, Department of Geology and Geography x and y components of field associated with each pole

22. Tom Wilson, Department of Geology and Geography Sum x and y components to get the resultant field

23. Tom Wilson, Department of Geology and Geography Problem - At a point 20 cm from the center of a thin magnetized rod 40 cm long and equidistant from its ends, the magnetic field is 500 nT. What is the pole strength in Oersted-cm 2 ?

24. Tom Wilson, Department of Geology and Geography Sign conventions assume that the test pole is positive.

25. Tom Wilson, Department of Geology and Geography H R =2H x =500nT Resultant x and y components

26. Tom Wilson, Department of Geology and Geography The different ways of expressing magnetic field intensity lead to different units; ups/cm 2, Oersteds & nanoTeslas

27. Tom Wilson, Department of Geology and Geography 10 5 Some units interrelationships

28. Tom Wilson, Department of Geology and Geography H RX = 500nT

29. Tom Wilson, Department of Geology and Geography Then, what is H + or H - ? Once we know this, we can then determine the pole strength. H = p/r 2 so p = Hr 2

30. Tom Wilson, Department of Geology and Geography

31. Bring questions to class this Thursday Problems 1 & 2 will be due on December 2 nd

32. Tom Wilson, Department of Geology and Geography Since the bedrock is magnetic, we have no way of differentiating between anomalies produced by bedrock and those produced by buried storage drums.

33. Tom Wilson, Department of Geology and Geography Acquisition of gravity data allows us to estimate variations in bedrock depth across the profile. With this knowledge, we can directly calculate the contribution of bedrock to the magnetic field observed across the profile.

34. Tom Wilson, Department of Geology and Geography How many drums are represented by the triangular-shaped object you entered into your model? Use the magic eye to get the coordinates of the polygon defining the drums Plot the corner coordinates for the triangular shaped object you derived at 1:1 scale and compute the area.

35. Tom Wilson, Department of Geology and Geography How many drums? 4 square feet Area of one drum ~ We’ll talk more about the last bullet (1/r 3 ) on the results-to- be-discussed list a little later. What’s wrong with the format of this plot?

36. Tom Wilson, Department of Geology and Geography Anomaly associated with buried metallic materials Bedrock configuration determined from gravity survey Results obtained from inverse modeling Computed magnetic field produced by bedrock Introduction to the magnetics computer lab

37. Tom Wilson, Department of Geology and Geography Where are the drums and how many are there? Let’s continue with the second part of the magnetics lab

38. Tom Wilson, Department of Geology and Geography Turn in Part 2 of second gravity problem set Magnetics paper summaries are due on Thursday, December 4th Due dates