Permafrost mapping
Survey

The three surveys performed were apparent conductivity mapping using frequency domain EM, resistivity profiling using an automated multi-electrode - both techniques were interpreted using formal inversion methods - and ground penetrating radar. There were six lines involved in the geophysical investigation, with stations staked at 25 metre intervals. Significant surface features were noted carefully for later inclusion on base maps.

Apparent conductivity mapping

Surveys were performed using a Geonics EM-34™ terrain conductivity meter. This is a fixed frequency system operated in this case by two people with horizontal transmitter and receiver coils at a 20 metre spacing. See the Geonics web site for a short summary (and photograph) of EM-34 operation.

Apparent conductivity was measured along all six lines at 10 m (32.8 ft) intervals using both the horizontal and vertical dipole modes. Data were recorded manually into a field book.

Electrical imaging

Also known as 2D resistivity profiling, this survey was carried out using the Sting™ R1 Earth Resistivity Meter (Advanced Geosciences, Inc), combined with the Swift™ automatic multi-electrode data acquisition system. This geophysical survey method involved placing 28 electrodes on the surface, connecting them all to an automated data acquisition system, injecting a current source at one pair of electrodes, and recording resulting voltages at a second pair of electrodes.

Three lines were surveyed by this electrical imaging (EI) technique. Each line-array consisted of 28 electrodes spaced 10 metres apart. To extend lines beyond the 280 m limit, the array was effectively advanced 140 m along the line, resulting in overlap of one-half spread length. Data were collected using Schlumberger, Wenner, and dipole-dipole configurations. This could be done efficiently because, once electrodes were in place, data acquisition was automated by the instrument.

One difficulty at sites such as this can be obtaining reliable electrical contacts with the frozen gravelly ground. To help alleviate this problem, electrodes were planted in holes that were pre-drilled using a hand-held hammer drill. Electrodes were then wetted with a saline solution to improve the electrical coupling with the ground. The transmitter was capable of injecting a current of between 5 and 20 milliamperes, at a maximum voltage of 400 volts.

Ground penetrating radar (GPR)

GPR surveys provide images of interfaces within the ground that reflect radio frequency electromagnetic waves. Signals respond primarily to changes in electrical and dielectric properties, rather than to bulk or average properties such as resistivity or permittivity. For a short summary of how GPR works, see the tutorial on the GeoRadar Inc. website.

The surveys described here were performed using a Pulse Ekko 1000 system with 25 or 50 MHz antennas, from Sensors and Software Inc., of Mississauga, Ont. Data were gathered at 1 metre spacing along portions of all six lines. The 25 and 50 MHz antennas used were 2 and 4 metres long, so there were sections of line in the bush that were not wide enough to accommodate these antennas.

© UBC-GIF  January 9, 2007

preproc.