High Resolution
Frequency-Domain EM Surveys

Sander Geophysics has sixty years of worldwide experience in high resolution airborne surveys and flew its first electromagnetic (EM) survey in 1958.  SGL offers frequency-domain electromagnetic (FEM) surveys using a fixed-wing aircraft.  FEM systems generate electromagnetic fields (primary field) which in turn induce secondary EM fields that vary in amplitude and phase depending on the electrical conductivity of the medium.  At SGL, this secondary field is measured by the receiver coils and then separated into two components:  the in-phase (or real) component which is in phase with the primary field and the quadrature (or imaginary) component which is 90° out-of-phase with the primary field.  The relationship between the signal strengths of the two components is then used to estimate the conductivity and depth of measured anomalies.  This system can be used for both mineral exploration and environmental programs to:

  • map lateral changes in geological conditions such as changes in lithology and structure, as well as presence of ore bodies;
  • perform groundwater investigations;
  • detect large buried metal objects.

Airborne Frequency-Domain EM Surveys at SGL

SGL's de Havilland DHC-6 Twin Otter can be configured with a four-frequency wingtip mounted EM system (SGFEM). This configuration results in a large transmitter-receiver coil separation which improves the signal to noise ratio. This allows surveys to be flown higher thus making the system a viable option for surveying in areas where local regulations restrict minimal flying altitudes.

SGL´s de Havilland Twin Otter on survey in Ireland

SGL´s de Havilland Twin Otter on survey in Ireland

The transmitter-receiver coil pairs are mounted in a vertical-coplanar orientation which reduces noise by minimizing coupling with the wingtip surface. Additionally, the coils in any one set (transmitter or receiver) are axially offset and are kept adequately separated from each other. The system also comes equipped with a 50/60 Hz power line monitor which becomes particularly useful in identifying cultural interference when surveying in urban settings. Other ancillary equipment includes radar and laser altimeters and a real-time digital video recording system.

Apparent conductivity calculated from 25 kHz EM data

Test line over the Reid-Mahaffy test site in Ontario showcasing all electromagnetic components in profile plus CDI generated from all SGFEM frequencies. The targets of interest can be seen in the centre of the CDI along with the overburden across the entire length of the line.

Other geophysical survey methods may also be employed simultaneously with EM data acquisition. High resolution aeromagnetic data can be gathered in conjunction with this EM system. The cesium magnetometer sensor is mounted in the receiver side (left) wingtip of the aircraft. Acquiring horizontal magnetic gradient data is also possible by housing an extra magnetometer in our custom built nose stinger. The Twin Otter can also be fitted with gamma ray detectors for radiometric surveying with a detector volume tailored to fit the survey program objectives. SGL uses SGSpec and Radiation Solutions Inc. (RSI) spectrometers, which are recognized as the most advanced airborne spectrometer systems available. Gravity (AIRGrav) and methane detection are also compatible with this system configuration.

Frequency-Domain EM Survey System

Airborne Instruments
Electromagnetic System Frequencies (Hz) 925 3,005 11,962 27,933
Tx-Rx coil spacing (m) 21.35 21.35 21.38 21.38
Supplementary Systems Power line monitor (50/60 Hz)
Sferics monitor
Data Acquisition System Sander Geophysics — SGDAS airborne computer
Capable of recording unlimited number of channels at variable intervals, and digital scrolling chart display of the data.  Data are recorded on a vibration tolerant removable drive.  The system clock is a quartz time standard automatically synchronized to UTC by the GPS signal to an accuracy of 1 millisecond.
Video Imaging System Sander Geophysics — SGDIS digital video
    Resolution (m) Calibrated to Range
Laser Altimeter Riegl LD90-3300VHS-FLP 0.25 <1% 0 to 10,000
(33,000 ft)
Radar Altimeter Collins 0.5 1% 0 to 760 (2,500 ft)
Barometric Altimeter Honeywell TJE 2.0 ±4 m 0 to 10,000
(30,000 ft)

Instruments for Simultaneous Multidisciplinary Surveys

Sensor Geometrics
   Strap-down, optically pumped, cesium split beam
   Sensitivity:  0.005 nT
   Sensor noise level:  < 0.02 nT
   Sampling rate:  10 Hz
Compensator Sander Geophysics — AIRComp real-time digital compensation
   Range:  20,000 to 200,000 nT
   Resolution:  0.001 nT
   Sampling rate:  160 Hz
Spectrometer SGSpec or Radiation Solutions RS-500
Detector Volume 16.8 to 50.4 litres of downward-looking crystals
4.2 to 12.6 litres of upward-looking crystals
Gravimeter Sander Geophysics — AIRGrav, Airborne Inertially Referenced Gravimeter
Analyzer Sander Geophysics — SGMethane, methane gas sensing system