The analysis of magnetic data commonly aids geologic investigations of Earth's upper crust. In particular, Precambrian crystalline rocks and igneous rocks generally contain sufficient magnetic minerals to cause variations in the planet's magnetic field. The predominant magnetic mineral in these rocks is magnetite. Sedimentary rocks, however, are generally nonmagnetic and produce little or no change in the magnetic field. Thus a magnetic anomaly map sees through the sedimentary-rock cover and conveys much information on lithologic contrasts and structural trends related to Precambrian crystalline basement and buried igneous bodies (Hildenbrand and Kucks, 1984).
Thomas G. Hildenbrand and Robert P. Kucks of the U.S. Geological Survey have combined data from several sources to produce a series of magnetic anomaly maps for Ohio. The maps have been published in two releases (see below), which are available for purchase from the Ohio Geological Survey. We, in turn, are making the digital data available to the public via our FTP Website.
The data files have all been compressed into one self-expanding executable file (MAGDATA.EXE), which itself is over 7 MB in size. The size of this digital file may make it very costly for some to download from this FTP site. If you need this file loaded on CD-ROM, contact Tim Leftwich (614) 265-6459 to make arrangements; cost is $25 (plus tax and shipping). When uncompressed, you will find six files totaling over 24 MB of data. These files contain latitude, longitude, and magnetic values for gridding points and thus are suitable for use in contouring systems. The files and their content are listed below:
||East-central U.S. magnetics
||Ohio magnetic data - reduced to North Pole
||Ohio magnetic data - first vertical derivative
||Ohio magnetic data - second vertival derivative
||Ohio magnetic data - magnitude of gradient of pseudo-gravity
||text file containing file format and some parameters.
All users of these data sets should consult the following to understand the data sources and limitations: