Intro to magnetic methods

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1 Environmental and Exploration Geophysics I Intro to magnetic methods tom.h.wilson tom.wilson@mail.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV

2 Items on the list Gravity lab due today, November 18 th Writing section Essay II final draft due today Problems 6.5 and 6.9 will be due on Thursday, Nov. 20 th Keep reading Chapter 7 on Magnetics. We ll get into magnetics next Tuesday and Thursday. Following Thanksgiving break we will continue with magnetic methods on December 2 nd and 4 th and begin some final review on the 4 th with a wrap-up review session on the 9 th. There s just under 4 weeks till the final so there is more to do! Good news no magnetics lab and no magnetics paper summary

3 Finish up and hand in

4 To be handed in this Thursday: Problems 6.5 and 6.9 Problem 6.5 What is the radius of the smallest equidimensional void (such as a chamber in a cave & think of it more simply as an isolated spherical void) that can be detected by a gravity survey for which the Bouguer gravity values have an accuracy of 0.05 mg? Assume the voids are in limestone and are air-filled (i.e. density contrast,, = 2.7gm/cm 3 ) and that the void centers are never closer to the surface than 100m. i.e. z 100m

5 For problem 6.5: You are given Z and, solve for R 3 G (4 / 3 R ) gmax 2 Z 3 R for meters 2 Z 3 R for feet 2 Z 2 1/3 gmax Z R (4 / 3) G 1/3 2 gmax Z R (feet) gmaxz (feet) 3 (4 / 3 ) GR 2 gmaxz (feet) R These constants (i.e or ) assume that depths and radii are in the specified units (feet or meters), and that density is always in gm/cm 3.

6 Problem 6.9 In a problem similar to problem 6.9 (Burger et al.) you re given three anomalies. These anomalies are assumed to be associated with three buried spheres. Determine their depths using the diagnostic position and depth index multiplier as discussed in class. Carefully consider where the anomaly drops to one-half of its maximum value. Assume a minimum value of Bouguer Anomaly (mgals) A. B. C Distance from peak (m)

7 For this problem you only need one diagnostic position Diagnostic Position Depth Index Multiplier (g/gmax) 3/4 max 1/0.46 = /3 max 1/0.56 = /2 max 1/0.77 = /3 max 1/1.04 = /4 max 1/1.24 = 0.81 But keep in mind that regardless of which diagnostic position you use, you should get the same value of Z. Each depth index multiplier converts a specific reference X location distance to depth. Z (depth index multiplier) times X at the diagnostic position

8 For example, you ve got two anomalies. One is broader than the other. Depth index multiplier for X 1/2 is *1.305= Depth index multiplier for X 3/4 is *2.17 = What depth do you get? ~750 ~450

9 The book covers only the half-maximum relationships Diagnostic Position Depth Index Multiplier (g/gmax) 3/4 max 1/0.46 = /3 max 1/0.56 = /2 max 1/0.77 = /3 max 1/1.04 = /4 max 1/1.24 = 0.81

72= 0.58 1/3 max 1/2.76 = 0.36 1/4 max 1/3.72 = 0.

10 Just note that this approach has been developed for a number of simple geometrical shapes Diagnostic Position Depth Index Multiplier 3/4 max 1/0.86 = /3 max 1/1.1 = /2 max 1/1.72= /3 max 1/2.76 = /4 max 1/3.72 = 0.27 g G R z x z L x 2 2 1/ 2 2 1/ R for meters Z 1 2 R for feet Z gmaxz1 R gmaxz R 1 1/ 2 (feet) (feet)

11 For a given anomaly certain simple geometries can be assumed and tested Sphere or vertical cylinder Calculate Z using depth index multipliers for each object and compare. Horizontal cylinder or vertical dyke A A A A A A A A

12 How about the anomaly below? 1mG 11mG Half plate or faulted plate 10 mg 0 mg

13 High angle fault: normal or reverse Fault is located at the anomaly inflection point Half plate or faulted plate 10 mg 0 mg Low High We ll be carrying these ideas over into the analysis of magnetic anomalies

14 Consider another in-class problemhand in before leaving 12 sectors with R i =1100 and R o =2200 Ring The butte fits into one sector Butte

15 Gravity model studies help us estimate the possible configuration of the continental crust across the region Derived from Gravity Model Studies

16 Use it to model complex structures in the upper crust Unveil large thrust sheets

17 Gravity applications span a variety of scales Determine the thickness of a landfil Roberts, 1990

18 Topographic extremes Japan Archipelago North American Plate Pacific Plate Philippine Sea Plate Geological Survey of Japan

19 The Earth s gravitational field In the red areas you weigh more and in the blue areas you weigh less. g ~0.6 cm/sec 2 North American Plate Pacific Plate Philippine Sea Plate Geological Survey of Japan

20 In addition, iphone technology brings a lot of this to you for $0.99 iphone app

21 The accelerometer basically provides acceleration in three directions

22 Accelerometer and gravity sensors

23 magnetometer

24 Export data for display/analysis

25 Accelerations at landing and takeoff

26 Magnetic fields measured on the iphone

27 Using satellites to map groundwater depletion GRACE: gravity recovery and climate experiment California s central Valley See

28 4 year decline - India

29 A global problem

30 Gravity methods have applications over a wide range of scales

31 The Earth s magnetic field

32 Magnetic polarity reversals occur on average about 4 to 5 times per million years Magnetic field applications, likewise, cover the cosmic to backyard scale.

33 Location of north magnetic pole

34 Protons and electrons in the solar wind crash into earth s magnetosphere.

35 The Earth s magnetic field is also very dynamic on short time scales Charged particles from the sun stream into the earth s magnetic field and crash into the gasses of the upper atmosphere

36 The solar wind at work on a comet s tail

37 Field surveys

alcohol Steve Sheriff s Environmental Geophysics

38 Making measurements of the Earth s magnetic field Proton Precession Magnetometers Water, kerosene & alcohol Steve Sheriff s Environmental Geophysics Course Tom Boyd s Introduction to Geophysical Exploration Course

39 Magnetic Fields Basic Relationships Source of Protons and DC current source Proton precession generates an alternating current in the surrounding coil

40 M GF f F 2 L 2 Proton precession frequency (f) is directly proportional to the main magnetic field intensity F and magnetic dipole moment of the proton (M). L is the angular momentum of the proton and G is the gyromagnetic ratio which is a constant for all protons (G = M/L = sec). Hence - F f

41 Locate trench boundaries and map distributions of metallic debris Theoretical model Examination of trench for internal magnetic anomalies. actual field data. Gilkeson et al., 1986

42 Where are the abandoned wells. From Martinek

43 We are also interested in local induced magnetic fields produced by metallic objects Where are the abandoned wells? Gochioco and Ruev, 2006

44 Anomalous fields from abandoned wells often produce large distinctive anomalies Locating abandoned wells

45 Raised relief display raised relief plot of measured magnetic field intensities From Martinek

46 Measurement of different field components: total, vertical and horizontal gradients

47 Test site

48 Results of field tests

49 Magnetic Fields Basic Relationships Magnetic monopoles F 1 pp p m r12 F m12 Magnetic Force Magnetic Permeability p 1 and p 2 pole strengths p 2 r 12 Coulomb s Law

50 Magnetic Fields Basic Relationships H o F 1 pp 1 2 m r12 F p o t " F " E 1 p 4 r F p t o 2 Force Magnetic Field Intensity often written as H p t is an isolated test pole 1 p 4 r E 2 The text uses F instead of H to represent magnetic field intensity, especially when referring to that of the Earth (F E ).

51 Magnetic fields basic relationships 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

52 Magnetic fields basic relationships monopole vs. Toxic Waste dipole

53 The Earth s main magnetic field

54 Magnetic Elements

55 Magnetic Elements

56 Magnetic Elements

57 Magnetic Elements

associated with the dipole approximation of the earth s

58 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.

59 From ~1900 to 2000

60 Magnetic field intensity in Morgantown, WV F (nanoteslas or gammas) The drop is associated with the westerly rotation of the earth s magnetic field Date

61 Magnetic Inclination

62 Magnetic inclination in the Morgantown area 72 Inclination (degrees) westerly rotation of the earth s magnetic field produces the current decrease in inclination Date

63 Magnetic declination Currently the compass point west of true north

64 Magnetic declination and its variation through time West declination is negative -9 declination (degrees west) Date

65 Magnetic Elements for your location

66 Magnetic elements and their change

67 Magnetics lab is posted and you may find it of interest to look over. Anomaly associated with buried metallic materials Computed magnetic field produced by bedrock Results obtained from inverse modeling Bedrock configuration determined from gravity survey

68 Let s get started Where are the drums and how many are there?

69 Items on the list Gravity lab due today, November 18 th Writing section Essay II final draft due today Problems 6.5 and 6.9 will be due on Thursday, Nov. 20 th Keep reading Chapter 7 on Magnetics. We ll get into magnetics next Tuesday and Thursday. Following Thanksgiving break we will continue with magnetic methods on December 2 nd and 4 th and begin some final review on the 4 th with a wrap-up review session on the 9 th. There s just under 4 weeks till the final so there is more to do! Good news no magnetics lab and no magnetics paper summary

Intro to magnetic methods

Environmental and Exploration Geophysics I Intro to magnetic methods tom.h.wilson tom.wilson@mail.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV Items on the list 6.5