High Resolution
Frequency-Domain EM Surveys

Sander Geophysics has over fifty 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

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

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 operates the well known and proven Exploranium GR-820 spectrometer as well as the new Radiation Solutions RS-500 spectrometer. Gravity (AIRGrav) and methane detection are also compatible with this system configuration.

Frequency-Domain EM Survey System

Instruments for Simultaneous Multidisciplinary Surveys