PHYS/GEOL 406 Homework
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   Homework #3: Do not turn this in since there is a test next week. I will post my solutions so that you can check your own work. Note: It's never a good idea not do the work and to just look at my solutions and (most likely) agree with them. You always need to do these things yourself first and then "grade" your own work.


    Our text does not cover this "mass excess" concept. It's covered briefly in "An introduction to geophysical exploration" by P. Keary, M. Brooks, and I. Hill, but without derivation. For the gory details, you can check out the source of all things geophysical called "Applied Geophysics" by Telford, Geldart and Sheriff. Be aware that you may go blind when you look at this book. However, it's all in that book!
  1. The image below shows a gravity map from the Camaguey district in Cuba from a survey in WWII. This shows a chromite ore body, ρ=4,000 kg/m3 in background with density 2,550 kg/m3. The gravity contours are listed in units of mgal.
    Since you don't care about the background I have linked the image above to a blown up version showing only the relevant part of this for you. You should use the 0mgal as your zero line.
    Use the large image to calculate
       (a) the mass excess (is it positive or negative?), and
       (b) the total mass of the chromite ore body.
    Note: You should choose your grid to get enough detail but don't go crazy. If you have questions about grid size come ask me.
  2. Find the mass of the body indicated by the gravity contours below. Note that there is regional drift, and that you will definitely need to add your own background gravity lines to this since there are very few contour lines. In other words, you should add contours such as -36, -37, -38, etc. to this image. Assume the background density is 2,550 kg/m3 and the magnitude of the density contrast (yes, you have to intuit the sign) is 330 kg/m3.

   That's all for homework #3.