Electromagnetic fields in boreholes-measurements and theory in three dimensions

A comparison between experimental measurements of the electromagnetic
field in boreholes and a mathematical model is made. The transmitting antenna
is a rectangular wire loop on the ground and the three orthogonal
components of the magnetic field are measured below the ground in the borehole.
In the mathematical model the ground is assumed to be a stratified half
space with a rectangular wire antenna on the ground. Excellent agreement
was found between the experimental results and the theoretical ones in most
cases and where deviations were found, the assumption of a stratified ground
was inaccurate due to present conductive inhomogeneities. The method presented
in this paper is capable of determining an ore body within a radius of
100 m from the borehole. The position (direction and distance) of the target
can be estimated from the measurements in the borehole. The impact on
the economical effectiveness of a drilling programme is considerable when a
prediction of the location of additional boreholes can be made. In the case of
reconnaissance drilling of a geological structure or trend the distance between
adjacent boreholes can be considerably enlarged.