Fugro Airborne Surveys’ helicopter-borne frequency domain electromagnetic (HEM) systems are the product of more than 40 years of experience, the acquisition of over 2 million line kilometres of geophysical data, and sustained research and development programs.
The DIGHEM frequency domain EM system is configured in a cylindrical “bird” which is carried beneath a helicopter. DIGHEM employs 5 pairs of transmitting and receiving coils; three horizontal coplanar coil pairs at frequencies of nominally 900, 7200 and 56000 Hz, and two vertical coaxial coil pairs at approximately 1000 and 5500 Hz. The coil separation is 8 metres on all frequencies except 56000 Hz, which has a coil separation of 6.3 metres due to its high signal strength.
The DIGHEM system uses multiple frequency electromagnetic fields to measure and map the electrical conductivity of the earth in three dimensions in the top 150 metres of the earth. The coplanar coil pairs provide greatest sensitivity to the conductivity of the earth, to layers and to subtle changes in conductivity. They are also least sensitive to the direction of the survey. The coaxial coil pairs produce the strongest anomalies from vertical conductors, such as steeply dipping massive sulphides and cultural or human-made conductors.
Changes in the conductivity of the earth are caused by changes in geology, such as the presence of base-metal ore deposits, kimberlite pipes, geologic alteration zones or conductive sediment layers; by changes in groundwater location or salinity; or by changes in overburden type or depth. The HEM systems simultaneously record magnetic data to measure the earth's magnetic field, which can be interpreted to show changes in bedrock geology to great depth.