EOSC 454 Lab #3. 3D Magnetics. Date: Feb 3, Due: Feb 10, 2009.

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Adele Gibbs
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1 EOSC 454 Lab #3 3D Magnetics Date: Feb 3, Due: Feb 10, Introduction In this exercise you will perform both forward models and inversions of magnetic data. You will compare the response of a dipole with that of a small susceptible prism. You will be using Meshtools as well as the UBC-GIF Magnetic Dipole Applet, which is located at: First experiment with the different parameters to get a feel for how the applet is used. Use the following options when using the applet for this tutorial: magnetic moment of 100 Am 2, inclination of 0, declination of 0, data spacing of 1 and a length of 25. The applet places a at dipole a specified depth below the surface and calculates the response at an elevation of 1m above the surface. Use the B t component for comparison with Meshtools. Note: Data in the applet is calculated at an elevation of 1m above the surface of the Earth. Meshtools Setup: Using Meshtools, create a mesh with the following properties: top south-west corner (-11, -11,0), (dx,dy) of (1,1) and length of (22,22). This will create cells of dimension (1, 1, 0.5). When performing forward model, use field strength of 40,000nT, inclination and declination of (0,0) and take the survey at a constant elevation of 1.0m above the mesh surface. You will need to use a data spacing of (2,2) to ensure that you do not generate more than 200 data points. Also, use a data area of x0 = -11.5, xn = 11.5, y0 = -11.5, yn = 11.5 to compute the data in the magnetics forward modelling dialog box. This ensures that you have a data point directly above the center of the mesh. 1

2 Questions: 1. Determine the susceptibility κ that will give rise to a magnetic moment of 100 Am 2 for a 1mx1mx1m prism. 2. Using Meshtools, create a 1mx1mx1m prism with at or near the center of the mesh. Use a value of susceptibility you just calculated. Place the center of the prism at a depth below the surface such that the top is at z=-5.5m and the bottom is at z=-6.5m (the center of the prism is now at z=-6m). Calculate the response at an elevation of 1m above the surface. What are the maximum and minimum values of the data? 3. Using the applet, generate the data from a dipole with a magnetic moment of 100 Am 2 at a depth of 6m below the surface. Compare this result with the response generated from the susceptible prism. Does a dipole appear to be a good approximation for a small prism at this distance from the observation point? Make your comparison by comparing the maximum and minimum values and the shape of the anomaly by generating profiles along the surface. 4. Calculate the max/min values of the response for both the prism and the dipole. Evaluate how well the dipole does in estimating the response of a prism by calculating the forward model at a variety of heights. Use an elevation above the surface of the mesh of 1-12 meters with 1 meter increments. (Note the dipole applet is distance to dipole, not height above the surface, and measurements in the dipole are 1m above the surface). Plot the difference between the max values computed using mag3d and with the dipole applet using d prism d dipole d prism (1) where d prism and d dipole are the maximum values computed at a particular elevation using meshtools, and the dipole applet. Using the maximum response at each depth as the data. At what depth does the dipole fail to be a good approximation to the prism? Support your findings with sketches of the profiles over the targets. Printed images of the results are optional. 2 Applet Questions To determine whether a magnetic survey might detect a buried object it is important to estimate the amplitude of the expected magnetic anomaly. Instruments may be accurate to about 1nT but other factors, like inaccurate removal of the time varying fields, geologic noise, etc. may require that the magnetic amplitude from the anomalous body is significantly larger than the 1nT threshold. 2

3 We will now carry out an analysis of a dipole buried at Vancouver. The first step is to determine the strength and geometry of the inducing field at Vancouver. Using the website: Determine the parameters for the inducing field at Vancouver. Use coordinates of 49 o N, 123 o W, altitude of 200m and today s date. From the output, obtain the inclination declination and field strength at Vancouver. 1. What are the Earth s magnetic field inclination, declination and total strength for Feb 3, 2009 at Vancouver? 2. Consider a partly rusted iron sphere that has a diameter of 60 cm and an effective susceptibility κ = 3 and is buried at a local site in Vancouver. What is the magnetic moment of this sphere when buried at Vancouver? 3. Plot a graph of the maximum value of the magnetic anomaly when the sphere is buried within the range 1-30 meters beneath the surface? To evaluate the amplitudes, use the Magnetic Dipole Applet. Plot amplitude on the vertical axis and the horizontal axis should be depth below the sensor. (Remember the receiver in the Applet demo is at a height of 1 m above the earth s surface. ) 4. Assuming a 1nT noise estimate, what is the maximum depth of burial of the sphere so that it can still be detected with the magnetic survey? 5. Plot a graph of the half-width of the anomaly as a function of depth of burial. Again, make the horizontal axis the depth of the dipole below the sensor. A magnetic survey was conducted directly over the body (that s fortunate!) and along a direction of N22E. The half-width of the curve was found to be 8 meters. Use the graph you constructed to estimate how deep you need to trench before you hit the buried sphere. 6. Suppose you know that the sphere lies between 4-8 meters depth. What should your line spacing and data spacing be? How do you arrive at your answer? 7. It is not only the peak amplitude of the anomaly which is important but also the shape of the anomalous field. By the time we have finished magnetics I want you to be able to visualize magnetic fields that would result from simple buried objects and then to be able to sketch the individual components of the field in the x- or z- directions or that component in the direction of the earth s field. The latter is the total field anomaly as measured with a proton precession magnetometer. To provide practise, consider the following: When a small spherically shaped object is placed in a magnetic field it produces a magnetic field like that due to a dipole which is orientated in the direction of the inducing field. Establish a coordinate system with x positive in the north magnetic direction, y positive east, and z positive down. The relationship between inclination of the earth s field and latitude is given by tan(i) = 2/tan(θ) 3

4 Sketch B z, along a traverse in the x direction at y = 0. but with latitudes 90 o, 45 o, 0 o. Sketch B T, along a traverse in the x direction at y=0. but with latitudes 90 o, 45 o, 0 o. Of course you can simply use the Applet profile tool and so everybody should get the right answer. That is why there won t be many marks for this question. However, the learning portion of this problem arises when you try to make your sketches before you view the answer. To make your sketches it is easiest to begin with the fields from a dipole (use one of the pictures handed out in class) and draw a straight line through the fields which represents the observation plane. Then make your inferences about sign and magnitude of the field components to complete your sketch. Check to see that you agree with the profiles provided in the Applet demo. 3 Synthetic inversion of magnetic data You will now explore the 3D magnetics inversion code through a synthetic example. 1. Using MeshTools3D create a mesh for the San Nicolas region, using the following parameters: (dx,dy) = (150,150), (length x,length y) = (3450,3450), top south-west corner = (-3400,-1800,0) 2. Create a prism to approximate the sulphide deposit of San Nicolas. Use parameters of 0.01 SI susceptibility with the following geometry: x = (-1750,-1450), y = (-300,0), z = (-225,-375). 3. Forward model the magnetics response of this prism. On the dialogue window, use parameters of Date area (x0 = -2575, xn = -525, y0 = -975, yn = 825) and Data spacing (X = 140, Y = 140). Using a constant height of 1 meter. The inclination at San Nicolas is degrees, declination is degrees and field strength of 44,000 nt. 4. What are the minimum and maximum values of the computed magnetic field 5. Assign a standard deviation of 5% with a floor or 0.8. Save this data file. 6. Load the data file into the mag3d-gui. Set the wavelet compression to none and invert the synthetic magnetic data. 7. What is the size of the recovered anomaly? What is the maximum and minimum recovered susceptibilities? 4

5 4 Inversion of San Nicolas Magnetic Data The magnetics data have been provided to you in the SN mag.dat and these data have already had the regional field removed. Load the data file into the Magnetics inversion GUI. View the data. Assign a standard deviation of 5% plus a floor of 2. Click the create-mesh button. Use a cell size of 50 and an elevation of 0. Disable wavelet compression, save the.inp file, and run the inversion. View the inversion log and view the predicted data. Does the inversion converge to the desired misfit? Does the misfit map show any correlations? After viewing the inversion model, it appears that the default mesh for this model is not deep enough. Open the mesh file in note, and add 5 more 25 meter cells to the z-line of the file, note you will need to increase the number of z cells on the first line of the file. Re-invert the San Nicolas magnetics data using this new mesh. What is the maximum susceptibility in the recovered model? What are the dimensions of the anomaly? 4.1 Discrimination of massive sulphide You now have two separate models of the San Nicolas deposit: a density model and a susceptibility model. You are going to delineate the San Nicholas deposit by isolating model cells which are both density and susceptible. Since the gravity and magnetics models are on two different meshes, the first step is to interpolate the magnetics model onto the gravity. A matlab utility is provided to perform the interpolation. Run the util remesh.m Matlab file. The, select the magnetics mesh, then select the gravity inversion mesh. Then select the magnetic inversion model maginv3d.sus. Select an output file to output the magnetics model now using the gravity inversion mesh. Using Matlab s importdata command, load both the density and susceptibility models, i.e. rho = importdata( gzinv3d.den ); sus = importdata( maginv3d.sus ); 5

6 Write a matlab script that generates a model file that contains 1 for each cell that is both susceptible and dense, and 0 otherwise. You will need to view the gravity and susceptible models to estimate cutoff values. You can use the Matlab command save to output the model save( output_filename, model_variable, -ascii ); where output filename is the filename that will be output, model variable is the Matlab variable containing your San Nicolas model containing 0 or 1. View your San Nicolas deposit estimate in MeshTools. What cut-off values did you select for the density and susceptibility? What are the dimensions of your estimated San Nicolas model? How does your estimated deposit model compare to the estimated San Nicolas cross-section provided in lab 1? Contact Elliot Holtham at eholtham@eos.ubc.ca, or drop by Room 332 in the Geophysics building for help regarding the exercises. 6

Question: What are the origins of the forces of magnetism (how are they produced/ generated)?

Question: What are the origins of the forces of magnetism (how are they produced/ generated)? This is an additional material to the one in the internet and may help you to develop interest with the method. You should try to integrate some of the discussions here while you are trying to answer the