Standard Dighem^V helicopter electromagnetic (HEM) systems include both vertical coaxial (the coil plane is vertical, not the axis) and horizontal coplanar EM transmitters and receivers. These are designed to provide the maximum amount of information possible over all geological conditions surveyed.

The coaxial coils (CX) are designed to provide the best electromagnetic coupling with steeply-dipping planar conductors in the earth. These conductors would be typified by massive sulphide mineralisation hosted in vertically-dipping geology, or thin shear zones. The response of a vertical coaxial system to vertical conductors is twice that of a horizontal coplanar system with the same frequency, power and coil separation.

Coplanar coils (CP) are designed to couple best with flat-lying conductors, including layers in horizontally bedded geology, and the half-space formed by a homogeneous earth. The response of a coplanar coil to horizontal or half-space features is twice that of coaxial coils with the same frequency and coil separation.

A Dighem^V HEM system is designed with both CX and CP coils, at frequencies selected to provide the best sensitivity to as many geological models and conductor types as possible with one HEM system. There are three CP coil sets, at 900 Hz, 7200 Hz and 56,000 Hz. These provide maximum sensitivity to both layered and half-space geology for mapping of host rock resistivity and detecting changes in layers which may reflect changing geology, mineralisation or ground water. The lowest frequency can be reduced to 380 Hz, to expand the range over which the system is most sensitive.

The CX coil pairs are at 900 Hz and 5500 Hz. The frequencies are chosen to be close to the CP frequencies, so that conductor anomalies can be quantitatively compared between the two coil orientations. This provides for the best possible interpretation of the geometry of the conductive source generating the anomaly, to determine the dip, depth and the best geological model to describe it.

¹In comparing response amplitudes, we are referring to the geophysical response to a transmitter of the same power, in either orientation. Normalisation of the secondary relative to the primary field measured at the receiver for each orientation can affect the apparent ratio of CX to CP, but does not truly affect the response of the ground, nor the sensitivity of the system. The Fugro Technical Note Coplanar Changes provides more information on normalisation.