September 16, 2010 Magnetic surveying

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Bryan Freeman
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the pattern. Explain the relation between dipoles and real targets.

1 September 16, 2010 Magnetic surveying After today, you will be able to Sketch anomalies over objects at any location, and explain how you derived the pattern. Explain the relation between dipoles and real targets. Outline the needs of a field survey. Outline necessary & some optional processing. Slide 1

Basics of Magnetics Surveying 1. Earth s magnetic field is the source: 2. Materials in the earth become magnetized (M = κh where H = B/0) (dipole moment per unit volume) 3.

2 Basics of Magnetics Surveying 1. Earth s magnetic field is the source: 2. Materials in the earth become magnetized (M = κh where H = B/0) (dipole moment per unit volume) 3. Magnetized material creates anomalous field (applet) magnetic moment m = M x (volume) 4. Magnetometer measures the total field 5. Anomalous field obtained by subtracting off the earth s field

3 Magnetics Earth s field Web Notes, GPG Ch.3.d.7. How is the field described anywhere? X, Y, Z Inclination, Declination, Magnitude Compass? Inclination? Declination? Earth s field strength vs anomalies. Slide 3

4 Earth s magnetic field: Strength B Inclination I Declination D Total Field Strength (nt) Declination (degrees) Inclination (degrees) B max = 70,000 nt H max = 55.7A/m B min = 20,000 nt H min = 15.9A/m

5 Magnetic field due to a dipole A dipole is the basic quantity in magnetostatics

6 Drawing the field due to burried dipoles Use the java applet on the website: Via GPG 8.c Start the applet alone here Or via: rcises/meth_3a/dipoleapp.html (there will be a link on the main EOSC 350 page)

7 Examples

8 Basics of Magnetics Surveying 1. Earth s magnetic field is the source: 2. Materials in the earth become magnetized (M = κh where H = B/0) (dipole moment per unit volume) 3. Magnetized material creates anomalous field (applet) magnetic moment m = M x (volume) 4. Magnetometer measures the total field 5. Anomalous field obtained by subtracting off the earth s field

9 The Anomalous field B 0 B A Measured field B = B 0 + B A Therefore components are: B AX = B X - B 0X, B AY = B Y - B 0Y, B AZ = B Z - B 0Z The total field anomaly: B = B - B 0 If B A << B 0 then B (BA Bˆ 0 ) That is, total field anomaly B is the projection of the anomalous field onto the direction of the inducing field.

10 Details on Anomalous Field See GPG d.8 Principles (anomalous fields)

Magnetics Data What exactly is measured? GPG Ch.3.d.1. The total field magnetic anomaly. Consequently, what pattern of response can be expected for buried targets?

11 Magnetics Data What exactly is measured? GPG Ch.3.d.1. The total field magnetic anomaly. Consequently, what pattern of response can be expected for buried targets? (sketch on BB) Start simple buried dipole. GPG Ch.3.d.2. Response to shallow or deep dipole targets. Response near magnetic poles near magnetic equator. Slide 11

12 Examples

13 Homework and Readings Magnetics GPG 3.d.0 3.d.13 Use the Java Applet to investigate the field due to burried GPG 8.c nt/exercises/meth_3a/dipoleapp.html Slide 13

14 Processing Magnetic Field data Account for time variations (need a base station) Remove regional

15 Magnetics Earth s field Slide 15

16 Time Variations of the Earth s Field External sources Solar wind (micro-seconds, minutes, hours) Solar storms (hours, days, months) Man made sources Power lines (50/60 Hz plus harmonics) DC Motors, generators All electronic equipment Internal sources Fluctuations in core (days millions of years)

17 Field procedures Earth s magnetic field varies as a function of time Necessary to record the magnetic field at a fixed location to determine the Earth s field

18 Base station correction Set out another magnetometer (base station) Assume time variations at the base stations are the same as at the observation location Synchronize the times Perform a correction by subtraction

19 Anomalous field We measure the field at the Earth s surface but we are interested in the anomalous features Regional removal

Some geological structural information is shown as black lines.

20 Example of regional field removal Airborne magnetic data gathered over a 25 square km area around a mineral deposit in central British Columbia. Some geological structural information is shown as black lines. The monzonite stock in the centre of the boxed region is a magnetic body, but this is not very clear in the data before removing the regional trend.

21 Summary: Data Collection Now we know what is measured. How to make measurements? Requirements: Base station. Positioning & time tied to each measurement. Measurement while moving. Identify potential noise sources. Slide 21

22 Possible routes to extracting information Plot the data in various map displays Interpret with a dipole Interpret with simple bodies of uniform magnetization Interpret as complex bodies (inversion)

23 Aeromagnetic map construction

24 Essential airborne data preparation (processing) Levelling data maps adjustingline locations decorregation drapping What can be learned directly? Trends Contacts Geologic settings Slide 24

25 Positioning errors of flight lines (old data)

Essential airborne data preparation (processing) Recall Drapping De-corrugation and Levelling Decorrugation is a frequency domain procedure based on a directional

26 Essential airborne data preparation (processing) Recall Drapping De-corrugation and Levelling Decorrugation is a frequency domain procedure based on a directional cosine filter. This filter retains anomalies, from gridded data, in the flight line direction only. Source: Slide 26

27 Essential airborne data preparation (processing) Plotting Editing Drapping Source: Slide 27

28 The entire data set is available as a 200 m grid which is updated annually to reflect recent data acquisition. $45 from the Canadian government

29 Essential airborne data preparation (processing) Residual total magnetic field Reduced to pole What can be learned directly? Trends Contacts Geologic settings Slide 29

30 Processing needed before interpreting Required Temporal correction Remove regional if present. Noise suppression Optional Filtering to emphasize edges, other features. GPG Ch.3.d.6. Mt Milligan, BC. GPG Ch.6.d.1 Slide 30

31 Reduction to Pole Same object buried at different locations on the earth yields different total field anomalies. Inclination=0 Inclination=45 Inclination=90

32 Reduction to Pole Filter the data to emulate the response as if the survey was taken at the pole. (Earth s field is vertical; measure vertical component of the anomalous field) 2D Fourier Filter This simplifies interpretation. Causative body lies beneath the peak.

33 Possible routes to extracting information Plot the data in various map displays Interpret with a dipole Interpret with simple bodies of uniform magnetization Interpret as complex bodies (inversion)

34 Environmental: How do we find UXO??

35 Northing (m) Environmental : Magnetic Survey 50 nt Ferrex Easting (m) -200 TM4

Eg of dipoles UXO data http://www.eos.ubc.

36 Eg of dipoles UXO data Slide 36

37 Possible routes to extracting information Plot the data in various map displays Interpret with a dipole Interpret with simple bodies of uniform magnetization Interpret as complex bodies (inversion)

38 Magnetic Charges (or Poles) (GPG d3 & d8) A magnetic charge creates a magnetic field H

39 Magnetic Charges (or Poles) In nature: magnetic poles always appear in pairs with a positive and negative pole yielding a dipole. Magnetic moment Magnetic field of dipole Magnetic field of dipole is identical to field from a loop of current

40 Beyond dipoles real targets When is a buried feature like a simple dipole? When it s diameter is much less than depth to it s centre. GPG Ch.3.d.3. Ch.3.d.4. Ch.3.d.5. Fields from some buried bodies, (cylinders, dykes) can be estimated by using charge concepts. Charge strength = M nˆ Slide 40

41 Magnetics Interpretation 2D modelling Forward modelling Line profiles might indicate 2D structure First identify the feature of interest Analyse data perpendicular to the structure Forward model in 2D Slide 41

42 Interpetation using Forward modelling: Response depends on K and size / shape. Solution is non-unique EOSC Slide 42

43 Possible routes to extracting information Plot the data in various map displays Interpret with a dipole Interpret with simple bodies of uniform magnetization Interpret as complex bodies (inversion)

44 Superposition for Magnetics Data (GPG d5) Magnetic field for one prism Magnetic field for 5 prisms

45 Earth can be complicated A complicated earth model Magnetic data for a complicated earth model. To interpret field data from a complicated earth we need to have formal inversion procedures that recognize non-uniqueness.

46 Example: Raglan aeromagnetic data Select a region of interest. Keep data set size within reason. Digitized the Earth up to 10 6 cells. Slide 46

47 Inversion: Finding an earth model that generated the data? Inversion Divide the earth into many cells of constant but unknown susceptibility Solve the large inverse problem to estimate the value of each cell Slide 47

Compare predictions to these measurements.

48 Misfit: comparing predictions to measurements Once a model is estimated Calculate data caused by that model. Compare predictions to these measurements. Is comparison within errors? YES Compare NO Modify model and try again Proceed to check for acceptibility Slide 48

Raglan aeromagnetic data Estimate a model for the distribution of subsurface magnetic material. Model will be smooth, and close to pre-defined reference.

49 Raglan aeromagnetic data Estimate a model for the distribution of subsurface magnetic material. Model will be smooth, and close to pre-defined reference. Display result as cross sections and as isosurfaces.?? Are sills connected at depth? Inversion result supports this idea. It helped justify a 1050m drill hole. 330m of peridotite intersected at 650m 10m were ore grade. Image shows all material which has k > 0.04 SI. Slide 49

50 Summary: Magnetics interpretation 1. Qualitative: Correlate magnetic patterns to geology 2. Quantitative interpretation Determine shapes, volumes, contacts, materials 3. Direct interpretation of patterns 4. Forward modelling 1. Guess geology 2. Calculate result - Compare to data 3. Iterate. 5. Inversion: Given data, estimate possible configurations of susceptible material that could cause those data. Slide 50

51 Homework and Readings Magnetics GPG 3.d.0 3.d.13 Use the Java Applet to investigate the field due to buried GPG 8.c Quiz/Teamwork exercise Wednesday Sept 21 Slide 51

52 What s missing: Estimates of depth of burial for various objects (sphere, cylinder) EOSC Slide 52

What have we learned from the Case Histories

What have we learned from the Case Histories Earth materials have a range of physical properties. Application of geophysics is carried out in a 7 Step process. Physical property of the target must be different