1 / 46

Environmental and Exploration Geophysics I

Environmental and Exploration Geophysics I. Gravity Wrap up > Magnetic Methods (I). tom.h.wilson tom.wilson@mail.wvu.edu. Department of Geology and Geography West Virginia University Morgantown, WV. Gravity reminders: some of those in-class problems.

sol
Download Presentation

Environmental and Exploration Geophysics I

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

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

  2. Gravity reminders: some of those in-class problems 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/cm3) and that the void centers are never closer to the surface than 100m. i.e. z ≥ 100m Tom Wilson, Department of Geology and Geography

  3. Let gmax = 0.1 Basic formula are available for the simple geometrical objects. In this case we use those for the sphere. We reasoned that ganom shouldbe at least 0.1 mGal; that Z would be at least 100m, and  = 2.7 2.7gm/cm3 or 1.7gm/cm3 R ~ 24m Tom Wilson, Department of Geology and Geography

  4. A. C. B. Anomalies associated with buried equidimensional objects - • Determine their depths • Use a diagnostic position • Assume a minimum value of 0 • 500m • 1000m • 2000m Tom Wilson, Department of Geology and Geography

  5. In this in-class/take home problem determine whether the anomaly below is produced by a sphere of a cylinder Given:  = 1 gm/cm3 Is the anomaly observed along this profile produced by an equidimensional shaped object (sphere) or horizontal cylinder? What is the cross sectional radius of the object? Tom Wilson, Department of Geology and Geography

  6. X1/4=890m X3/4=280m X1/2=500m Analyzing the data g3/4 = 0.98mG g1/2 = 0.65mG g1/4 = 0.33mG Tom Wilson, Department of Geology and Geography

  7. Last item would be to estimate R Cylinder Sphere Tom Wilson, Department of Geology and Geography

  8. Making the terrain correction What’s the station elevation? What’s the average elevation in Sector 1? What’s the relative difference between the station elevation and the average elevation of sector 1? 200 520 2840 520 280 Tom Wilson, Department of Geology and Geography

  9. 2640 200 3 (0.03mG) 0.0279mG What did you get? Determine the average elevation, relative elevation and T for all 8 sectors in the ring. Add these contributions to determine the total contribution of the F-ring to the terrain correction at this location. We will also consider the F-ring contribution if the replacement density of 2.67 gm/cm3 is used instead of 2 gm/cm3 and the result obtained using the ring equation. Tom Wilson, Department of Geology and Geography

  10. Making the terrain correction Equation 6-30 What would the answer be if the replacement density were 2.67gm/cm3 Tom Wilson, Department of Geology and Geography

  11. Remember that the Hammer tables assume a density of 2 gm/cm3. So the result must be adjusted to the local density. In the example below, we assume the local density is 2.67 gm/cm3. 1.34 x 0.64 = 0.85mG 1.34 x 0.61 = 0.81mG Tom Wilson, Department of Geology and Geography

  12. Graphical Separation of Residual Examine the map at right. Note the regional and residual (or local) variations in the gravity field through the area. The graphical separation method involves drawing lines through the data that follow the regional trend. The green lines at right extend through the residual feature and reveal what would be the gradual drop in the anomaly across the area if the local feature were not present. Tom Wilson, Department of Geology and Geography

  13. ? -1 Graphical Separation of the Residual The residual anomaly is identified by marking the intersections of the extended regional field with the actual anomaly and labeling them with the value of the actual anomaly relative to the extended regional field. 0 -0.5 -1 -0.5 -0.5 After labeling all intersections with the relative (or residual ) values, you can contour these values to obtain a map of the residual feature. Tom Wilson, Department of Geology and Geography

  14. Zero Max = 0 Min ~-1.8 negative What is the depth? X1/2~6000’ dcenter~7800’ for the sphere Tom Wilson, Department of Geology and Geography

  15. If the anomaly was due to a vertical cylinder ? A A' X1/2~6000’ dtop= 6000’x 0.58 ~ 4800’ Tom Wilson, Department of Geology and Geography

  16. Magnetic Fields Locating Trench Boundaries Theoretical model Examination of trench for internal magnetic anomalies. actual field data Tom Wilson, Department of Geology and Geography Gilkeson et al., 1986

  17. Trench boundaries - field data Trench Boundaries - model data Tom Wilson, Department of Geology and Geography Gilkeson et al., 1986

  18. Abandoned Wells From Martinek Tom Wilson, Department of Geology and Geography

  19. Abandoned Well - raised relief plot of measured magnetic field intensities From Martinek Tom Wilson, Department of Geology and Geography

  20. Locating abandoned wells Tom Wilson, Department of Geology and Geography

  21. Using the GEM 2 to locate abandoned wells Gochioco and Ruev, 2006 Tom Wilson, Department of Geology and Geography

  22. Falls Run Coal Mine Refuse Pile Magnetic Intensity Wire Frame Tom Wilson, Department of Geology and Geography

  23. Data Acquisition Tom Wilson, Department of Geology and Geography

  24. Magnetic Fields – Basic Relationships Magnetic monopoles p1 r12 Fm12Magnetic Force Magnetic Permeability p1and p2pole strengths Coulomb’s Law p2 Tom Wilson, Department of Geology and Geography

  25. Magnetic Fields – Basic Relationships 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). Tom Wilson, Department of Geology and Geography

  26. 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 Tom Wilson, Department of Geology and Geography

  27. Magnetic Fields – Basic Relationships monopole vs. Toxic Waste dipole Tom Wilson, Department of Geology and Geography

  28. The earth’s main magnetic field Tom Wilson, Department of Geology and Geography

  29. Measuring 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 Tom Wilson, Department of Geology and Geography

  30. Magnetic Fields – Basic Relationships Source of Protons and DC current source Proton precession generates an alternating current in the surrounding coil Tom Wilson, Department of Geology and Geography

  31. Proton precession frequency (f) is directly proportional to the main magnetic field intensity F and magnetic moment of the proton. L is the angular momentum of the proton and G is the gyromagnetic ratio which is a constant for all protons (G = M/L = 0.267513/  sec). Hence - Tom Wilson, Department of Geology and Geography

  32. Magnetic Elements Tom Wilson, Department of Geology and Geography

  33. Magnetic Elements Tom Wilson, Department of Geology and Geography

  34. Magnetic Elements Tom Wilson, Department of Geology and Geography

  35. Magnetic Elements Tom Wilson, Department of Geology and Geography

  36. 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. Tom Wilson, Department of Geology and Geography

  37. Main field intensity Magnetic Intensity in Morgantown Tom Wilson, Department of Geology and Geography

  38. Magnetic Inclination Tom Wilson, Department of Geology and Geography

  39. Magnetic Inclination Variations of inclination through time Tom Wilson, Department of Geology and Geography

  40. Magnetic Declination Tom Wilson, Department of Geology and Geography

  41. Variations of declination Magnetic Declination through time W Tom Wilson, Department of Geology and Geography

  42. Magnetic Elements for your location http://www.ngdc.noaa.gov/geomagmodels/struts/calcPointIGRF Tom Wilson, Department of Geology and Geography

  43. Magnetic Elements http://www.ngdc.noaa.gov/geomag/magfield.shtml Today’s Space Weather http://www.swpc.noaa.gov/today.html Tom Wilson, Department of Geology and Geography

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

  45. Where are the drums and how many are there? Tom Wilson, Department of Geology and Geography

  46. Looking ahead • Begin preparing your magnetics paper summaries • Look over the initial gravity modeling effort that is combined with the magnetics lab. • Look over the magnetic problems handed out in class today (problems 1 and 2). • Read chapter 7. • Consider problems 7.1 and 7.3. We’ll discuss after Thanksgiving break Tom Wilson, Department of Geology and Geography

More Related