Applied Geophysics - Magnetics Problem Set - Modeling

Due March 9, 2000

For this problem set you can use the modeling equations from the class notes that you set up on a spreadsheet; the magnetics and gravity modelling program GM-SYS and the data files on a PC; or the MagModel program from Burger's text on a Mac.

For simplicity, I suggest you use Burger's MagModel on your Mac or the PowerPC in LME 320. See III below:

  1. The full, latest version of GM-SYS is supposed to be installed and running on the Sun and PCs in the MMV Lab. Click here for instructions on using GM-SYS in the MMV lab.

    To use a student version of GM-SYS on your PC, first acquire the gmsys.zip package (click here). Make a new folder on your hard drive and put the gmsys.zip file in it. Then when you double-click on the .zip file a DOS window should open and you will see the names of files expanded out of the archive. You should see a file called ``cnfgmsys.exe'' (hopefully with a DOS icon); double-click on it. You will have to change the display driver to something your Windows95-98 screen can emulate. You may have to change your Windows display settings as well (under Start->Settings->Control Panel->Display, Settings tab; primitive, eh?). If this works you should be able to start GM-SYS by double-clicking on the ``gm.exe'' DOS icon.

    For problems 1-4, you can use the TEST or BLANK model in the GM-SYS folder as a template to set up the problems. Please see Karlin if you want to look at the manual. The program will print out to an HP laserjet printer or to a postscript printer. Since the program prints only one plot at a time, we would recommend tracing the plots for each problem onto a single plot in different colors. Remember to label each plot.

  2. For your PC we also have the free ``magnetic'' program from the USGS. Download the Zip pakage here. This program may not require so much configuration to run. Double-click on ``Magnetic.exe'' to start it. Then you see a horizontal menu that you use the arrows to move across, and hit return to select an item. Select Polygon, then in the Polygon menu select Read, and in the table of file names below arrow over to ``DIKE.PLY'' and hit return. Then arrow over the Polygon Menu to Observed and Read, and selecting ``MAGTEST.OBS''. Arrow over to Polygon and then to Edit. Hit the ``C'' key to calculate the synthetic magnetic profile. Note the ``F2'' key moves the cursor to the next polygon vertex, and that you can use the arrows in this screen to move the vertices within the section. ``F9'' shows you the available key commands. ``D'' lets you change the susceptibility. I could not figure out how to change the line orientation, declination, inclination, or total field. The ``Esc'' key gets you back to the menus.

  3. The MagModel Mac application is from Burger's book. If you do not own the book, the instructor will loan you a copy of the program. You can also go into LME 320 and find MagModel on the SeismoPPC, in the MagModel folder on the desktop.

    With MagModel you first Open the ``zero magnetics data - 2 km'' file from the File menu, and then Load the ``shallow vertical prism'' model from the Model menu. The Polygon menu options allow you to adjust the location and shape of the slab. With MagModel you do not have to exactly match the depths and slab sizes indicated below. But you should make your models proportionally correct in terms of the ratios of slab depth, thickness, and width. To print you may have to select the available printer in the Chooser application under the Apple menu. MagModel also prints only one plot at a time. See Louie if you have problems with the set up or printing.

    You can view some of the printed results from MagModel by downloading the Adobe Acrobat PDF files linked below. Applications for viewing PDF files are available online free from Adobe Systems Inc.

Assignment

Problem 1. Calculate and plot the total magnetic intensity anomaly south-north profiles over a two-dimensional slab as a function of depth to the top: 10 ft., 20 ft., 50 ft., and 100 ft. The thickness of the slab is 100 ft and the width of the slab is 10 ft. The susceptibility contrast is 0.0025 emu/cc in an inducing field of 58000 nT inclined 60 degrees below the horizontal. What do you conclude regarding the effect of increasing depth to target ?

MagModel printouts in PDF format for depths proportional to:

Problem 2. Calculate and plot the total magnetic intensity anomaly south-north profiles over a two-dimensional slab with depth to top of the slab of 10 ft. and slab thickness of 50 ft., 20 ft., 10 ft., and 1 ft. The width of the slab is 10 ft. The susceptibility contrast is 0.0025 emu/cc in an inducing field of 58000 nT inclined 60 degrees below the horizontal. What do you conclude regarding the effect of thickness on the magnetic signature of the target?

MagModel printouts in PDF format for thicknesses proportional to:

Problem 3. Calculate and plot the total magnetic intensity anomaly south-north profiles over a two-dimensional slab with depth to top of the slab of 10 ft. , a thickness of 10 ft. and a width of 100 ft., 50, 10 ft., and 1 ft. The susceptibility contrast is 0.0025 emu/cc in an inducing field of 58000 nT inclined 60 degrees below the horizontal. What do you conclude regarding the effect of width on the magnetic signature of the target?
Problem 4. Calculate and plot the total magnetic intensity anomaly south-north, N45E, and east-west profiles over a two-dimensional slab with thickness and width of 10 ft. and depth to top of the slab of 10 ft. The susceptibility contrast is 0.0025 emu/cc in an inducing field of 58000 nT inclined 60 degrees below the horizontal. What do you conclude regarding the effect of profile orientation on the magnetic signature of the target?