Reading:

Unfortunately 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.