DIGHEM System Specifications
Fugro Airborne Surveys helicopter systems include:
| DIGHEMV |
The standard DIGHEM 5 frequency EM/Mag system |
| DIGHEMVRES |
Designed to map layered earth resistivities |
DIGHEMV
The DIGHEMV system is mounted in a 9 metre cylindrical "bird", which is carried beneath a helicopter. The altitude of the bird above ground is kept at about 30m, and the airspeed is about 120 km/h. The system records the five EM frequencies at 10 samples per second, resulting in about 1 sample for each 3m along the flight line. The separation between survey lines varies, depending on the size of the target, and the detail desired in the resistivity mapping. Typical line separations are between 50m and 200m.
The DIGHEMV system employs 5 pairs of transmitting and receiving coils, three horizontal coplanar coil pairs at frequencies of 900 Hz (or 380 Hz), 7200 Hz, and 56 000 Hz, and two vertical coaxial coil pairs at 900 Hz and 5500 Hz. The coil separation is 8 m on all frequencies except 56 000 Hz, which has a coil separation of 6.3 m due to its high signal strength.
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.
The penetration depth of the survey increases as the frequency is reduced, rendering a three dimensional data set.
Fugro Airborne Surveys operates multiple DIGHEMV systems world-wide for mineral exploration and environmental and engineering applications.
DIGHEMVRES
The DIGHEMVRES is a 5-coplanar system with frequencies of 380 Hz, 1400 Hz, 6200 Hz, 25,000 Hz, and 101 000 Hz (the highest HEM frequency commercially available!). It is a system designed to provide the optimum conductivity mapping range and resolution for horizontally layered geology. With a full 7.9 m coil separation on all frequencies it is the optimum system for geologic mapping and many environmental and engineering type problems.
The "footprint", or width of ground that the DIGHEM systems are sensitive to is between 50 m and 100 m in diameter, depending on the ground conditions. The depth of exploration varies widely, depending on the nature of the target and the host geology.
The actual amount of ground surveyed in a normal day of operation depends on the line separation, the shape of the survey area, the weather and the amount of daylight among other factors. Coverage of 200 to 500 line-km per day is common, which would be 10 km2 to 100 km2 per day, depending on the flight line spacing.
Inside the HEM bird, there is also a sensitive magnetometer, to collect magnetic data simultaneously with the EM data. Gamma-ray radiometric data can also be collected, typically by a 16 litre crystal pack in the helicopter.
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