TN-35 Archaeological Mapping Using the Geonics EM38B
to Map Terrain Magnetic Susceptibility
is a technical note listed below as TN-35 Archaeological Mapping Using the Geonics EM38B to Map Terrain Susceptibility (With Selected Case Histories) released by Geonics, January 2013 and authored by J. Duncan McNeill. This paper is a great reference, for more than the title suggests. While the geophysical case histories are directed towards archeological applications, the technical note presents the geophysical theory and methods utilized to yield good results with a electromagnetic terrain conductivity meter, which also measures magnetic susceptibility. Over the years, much discussion has been provided by many geophysicists, institutions, and businesses about the uses for electromagnetic terrain conductivities. With the exception of locating buried metal, geophysical applications and methods for terrain magnetic susceptibility surveys seem to have been more limited. These case histories are worth reading. The theory presented in the technical note not only pertains to the EM-38 but is relevant to the EM-31 and somewhat to the EM-34. Historically, magnetic susceptibility is often called the inphase response. The inphase response, at times, seemed to be a dimensionless quantity measured by an electromagnetic conductivity meter. This geophysical paper removes much of the mystery behind what was once called the inphase response.
Based on the orientation of the instrument,
the EM-38 offers two average depths of penetration
for measuring magnetic susceptibility. Understanding the response curves of the two orientations can provide additional clues of what is at depth. This flexibility leads to survey techniques that may not be considered otherwise. The paper, supported by examples, demonstrates that past human interaction observed at archeological sites leads to alterations of the soils. These alterations change the magnetic susceptibility of the soils. In TN-35 a list of five factors are quoted from Aspinall et al. (2008) book. These alterations, as well as various types of artifacts, seem to consistently yield a magnetic susceptibility many times greater than the geologic background response of the soils. Below is an image of the cover page to TN-35.
