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

Tom Wilson, Department of Geology and Geography 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 Gravity reminders: some of those in-class problems

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

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

Tom Wilson, Department of Geology and Geography In this in-class/take home problem determine whether the anomaly below is produced by a sphere of a cylinder Given:  = 1 gm/cm 3 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 X 3/4 =280m X 1/2 =500m X 1/4 =890m g 3/4 = 0.98mG g 1/2 = 0.65mG g 1/4 = 0.33mG Analyzing the data

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

Tom Wilson, Department of Geology and Geography 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? Making the terrain correction

Tom Wilson, Department of Geology and Geography (0.03mG)0.0279mG 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/cm 3 is used instead of 2 gm/cm 3 and the result obtained using the ring equation. What did you get?

Tom Wilson, Department of Geology and Geography Equation 6-30 What would the answer be if the replacement density were 2.67gm/cm 3 Making the terrain correction

Tom Wilson, Department of Geology and Geography Remember that the Hammer tables assume a density of 2 gm/cm 3. So the result must be adjusted to the local density. In the example below, we assume the local density is 2.67 gm/cm x 0.64 = 0.85mG 1.34 x 0.61 = 0.81mG

Tom Wilson, Department of Geology and Geography 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 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 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 Max = 0 Min ~-1.8 negative What is the depth? X 1/2 ~6000’ d center ~7800’ for the sphere

Tom Wilson, Department of Geology and Geography If the anomaly was due to a vertical cylinder X 1/2 ~6000’ d top = 6000’x 0.58 ~ 4800’

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

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

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

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

Tom Wilson, Department of Geology and Geography Locating abandoned wells

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

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

Tom Wilson, Department of Geology and Geography Data Acquisition

Tom Wilson, Department of Geology and Geography Magnetic monopoles p1p1 p2p2 r 12 F m12 Magnetic Force  Magnetic Permeability p 1 and p 2 pole strengths Coulomb’s Law Magnetic Fields – Basic Relationships

Tom Wilson, Department of Geology and Geography Force Magnetic Field Intensity often written as H p t 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 (F E ). Magnetic Fields – Basic Relationships

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

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

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

Tom Wilson, Department of Geology and Geography Steve Sheriff’s Environmental Geophysics CourseEnvironmental Geophysics Proton Precession Magnetometers Tom Boyd’s Introduction to Geophysical Exploration CourseIntroduction to Geophysical Exploration Measuring the Earth’s magnetic field water kerosene & alcohol

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

Tom Wilson, Department of Geology and Geography 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 = /  sec). Hence -

Tom Wilson, Department of Geology and Geography Magnetic Elements

Tom Wilson, Department of Geology and Geography Magnetic Elements

Tom Wilson, Department of Geology and Geography Magnetic Elements

Tom Wilson, Department of Geology and Geography Magnetic Elements

Tom Wilson, Department of Geology and Geography 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 Magnetic Intensity

Tom Wilson, Department of Geology and Geography Magnetic Inclination

Tom Wilson, Department of Geology and Geography Magnetic Inclination

Tom Wilson, Department of Geology and Geography Magnetic Declination

Tom Wilson, Department of Geology and Geography W Magnetic Declination

Tom Wilson, Department of Geology and Geography Magnetic Elements for your location

Tom Wilson, Department of Geology and Geography Today’s Space Weather Magnetic Elements

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

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

Tom Wilson, Department of Geology and Geography 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 Looking ahead