Frequency-Domain EM

Undoubtedly, helicopter-borne electromagnetics (EM), combined with total field magnetics and often gamma-ray spectrometry, have been one of the most productive and useful of airborne system developments to date, and have accounted for the discovery of billions of dollars-worth of mineral resources, tapped into numerous ground water reservoirs and provided immense volumes of data for environmental site evaluations. These systems are ideally suited for working in rugged, mountainous terrain, or over small claim block-sized properties.

Currently, electromagnetics (EM) combined with a high-sensitivity magnetometer are the techniques of choice for most mining companies worldwide, to locate and define diamondiferous kimberlite pipes and base and precious metal deposits. 

McPHAR’s frequency domain electromagnetic survey system is integrated around the HUMMINGBIRD sensor, which is available in a 5-frequency configuration. The HUMMINGBIRD sensor, which is the heart of this system, can be simply described as a multi-frequency, multi-coil electromagnetic system, which measures the inphase and quadrature responses from a number of coil-pairs installed in a tubular bird, towed beneath a helicopter.

The HUMMINGBIRD system measures the in-phase “I” and quadrature “Q” (sometimes called out-of-phase) components of the total EM field. The amplitude of these components are always given as a value that is relative to the transmitted primary. The ratio of in-phase to quadrature (I/Q) depends mostly on the conductivity of the geology and the operating frequency; the amplitude depends mostly on the depth of the conductor below the sensor.

All components of the HUMMINGBIRD instrumentation are digitally controlled (HUMMINGBIRD is 100% digital from front to back). All digital samples generated are supplied as inphase and quadrature measurements. Data is telemetered on a lightweight serial cable to the data acquisition console onboard the helicopter, where it is displayed on a LCD colour screen and recorded on a removable hard disk.

Pilot guidance and DGPS navigation systems are integrated into the package together with a gammaray spectrometer (optional). Other flight control instruments include radar or laser altimeters and a barometric altimeter.

Typical HUMMINGBIRD system configuration is:

• 5-frequency EM sensor, 880 Hz, 980 Hz, 6.6 kHz, 7 kHz and 34 kHz frequencies
• High-sensitivity cesium magnetometer, 0.001 nT/10 Hz resolution
• Real-time differential GPS navigation system
• PC-based data acquisition system
• Radar altimeter
• Colour digital video imaging system
• Optional gammaray spectrometer and 16.8/4.2 litres sensor