Earth Magnetism: A Tool for Subsurface Studies (GC-04)
Most geologic exploration is restricted to surface observations except for isolated glimpses below the surface in caves, mines and drill holes. Because such features are widely scattered, geophysicists at the Indiana Geological Survey must use indirect methods to study materials far below the surface of Indiana. The earth's magnetic field at Bloomington points almost true north and downward at an angle of about 70 degrees. If a magnetic body lies buried within the earth, it distorts this magnetic field very slightly. Sensitive magnetometers can detect and measure these changed in the earth's magnetic field. Analysis of these measurements can tell us something about the deeply buried bodies of rock or minerals that caused them. Most variations in the earth's magnetic field in Indiana are due to differences in the iron content and depth of basement rocks hundreds of millions of years old.

Our Hoosier State Beneath Us: Geochemistry/Geophysics