  Interferometric principles are widely used in precision physics experiments
and/or in advanced laboratory-based phase measurement systems. Phase resolution
of such systems is a few orders of magnitude higher compared to that of
standard mixer-based quadrature demodulators or lock-in technique. The first
attempt of applying interferometric signal processing to
transmitter-target-receiver based electromagnetic (EM) surveying in geophysical
prospecting is described. It is shown that it is possible to build an EM single
carrier surveying system that is, firstly, immune to amplitude variations of
both the primary and the secondary EM fields, and, secondly, can directly
measure phase variations between the primary and secondary EM fields. Its
inherent phase noise floor, if limited by the interferometer itself, can be as
low as tens of nanoradians/\surdHz or below -140 dBc/\surdHz level. A practical
example of an EM gradiometric surveying system based on an interferometric
principle and operating in the Extremely Low Frequency (ELF) range, is
presented. The system has been tested in regional outback Australia, in the
presence of a highly conducting overburden, in the search of a nickel sulphide
deposit.
  Key words: EM gradiometer, interferometric methods, geophysical prospecting
