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 6.5 and 6.9 due today Writing section Essay II final draft due Friday, 18 th by noon Keep reading Chapter 7 on Magnetics. Today we will hand out some introductory magnetics problems (7.1, 2 and 3) - and An abbreviated magnetics lab effort with due date to be announced. We ll spend the next week on magnetic methods. We should wrap that up on Nov. 28 th and Dec 1 st. We may have time for some exam review on the 1 st. Regardless, the entire class Dec. 6 th will be devoted to review. The final is scheduled Wednesday, December 14 th from 11am-1pm in rm 325 Brooks.

3 Any questions about how to analyze these two anomalies and determine the shape of the object producing them?

4 Rescaled and plotted together the distinction is more obvious: horizontal cylinder/sphere Z=10m z s 1.305*x 12 zcyl x 1 2

5 The book covers only the half-maximum relationships If you are sure of the geometry that s all you need 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

58 1/3 max 1/2.76 = 0.36 1/4 max 1/3.72 = 0.

6 Diagnostic positions and multipliers for the vertical cylinder 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)

7 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 or sheet A A A A A A A A The applications of simple geometrical objects extend beyond our brief introduction to the topic

8 Anomaly contours suggest fault/flexure 1mG A A 11mG Half plate or faulted plate 10 mg A A 0 mg

9 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

10 Regional crustal scale features Derived from Gravity Model Studies

11 Thrust belt structure Valley and Ridge thrust sheets

12 To landfill anatomy Determine the thickness of a landfil Roberts, 1990

13 Global scale tectonic regimes Japan Archipelago North American Plate Pacific Plate Philippine Sea Plate Geological Survey of Japan

14 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

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

16 Time lapse showing 4 year decline - India

17 & magnetic fields (sensor kinetics pro iphone app)

18 Magnetic fields measured on the iphone iphone observations Calculated along flight path

19 Magnetic field intensity North America

20 The Earth s magnetic field

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

22 Location of north magnetic pole

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

Additional winds anticipated for the 11 th and 12 th.

24 The Earth s magnetic field is also very dynamic on short time scales Coronal mass ejection hit earth on the 9 th. Additional winds anticipated for the 11 th and 12 th. Current Auroral Oval Charged particles from the sun stream into the earth s magnetic field and crash into the gasses of the upper atmosphere

25 Field surveys

alcohol Steve Sheriff s Environmental Geophysics

26 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

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

28 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

29 Model data and real data: 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

30 Where are the abandoned wells. From Martinek

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

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

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

34 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

35 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 ).

36 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

37 Magnetic fields basic relationships The dipole field, for example, approximates the field of a buried drum monopole vs. Toxic Waste dipole

38 The Earth s main magnetic field

39 Magnetic Elements (see section 7.2.1)

40 Magnetic Elements

41 Magnetic Elements

42 Magnetic Elements

43 A problem to consider (for discussion next time) At this point start reading through Chapter 7 to get familiar with topics of discussion related to magnetic field components and problems 7.1-3

associated with the dipole approximation of the earth s

44 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.

45 From ~1900 to 2000

46 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

47 Magnetic Inclination

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

49 Magnetic declination Currently the compass point west of true north

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

51 Magnetic Elements for your location

52 Magnetic elements and their change

53 Magnetics lab is posted and we ll take a quick look at the analysis of magnetic data Anomaly associated with buried metallic materials Computed magnetic field produced by bedrock Results obtained from inverse modeling Bedrock configuration determined from gravity survey

54 We ll get started on this next time Where are the drums and how many are there?

55 Items on the list 6.5 and 6.9 due today please turn in before leaving Writing section Essay II final draft due Friday, 18 th by noon Keep reading Chapter 7 on Magnetics. Today we will hand out some introductory magnetics problems (7.1, 2 and 3, due Nov. 17th) - and An abbreviated magnetics lab effort with due date to be announced. Get started on the problems and lab effort so we can have a productive session next Tuesday! We ll spend the next week on magnetic methods. We should wrap that up on Nov. 28 th and Dec 1 st. We may have time for some exam review on the 1 st. Regardless, the entire class Dec. 6 th will be devoted to review. The final is scheduled Wednesday, December 14 th from 11am-1pm in rm 325 Brooks.

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 Gravity