Title: Introducing Geonics EM38-4 for environmental studies and geophysical explorations
Date: Tuesday February 14th at 4:05 PM
Speaker: Farzad Shirzaditabar, School of Environmental Sciences (SES), University of Guelph, Guelph, ON
Location: Virtual, register here
Geonics Ltd. has recently designed and manufactured EM38-4; a new version of the most popular handheld electromagnetic (EM) instrument EM38. This new instrument has one transmitter and four receiver coils at distances of 0.5, 0.75, 0.88 and 1 m apart from transmitter. Transmitter coil is energized by an alternating electric current of 14.5 kHz and produces an alternating primary electromagnetic field. The magnetic component of this EM field penetrates to the subsurface and induces electric currents in it. These currents are proportional to electrical conductivity (EC) and magnetic susceptibility (MS) of the subsurface materials. All receivers have the ability to sense, simultaneously, in-phase and out of phase components of secondary magnetic field created by the induced currents. The instrument is connected to a tablet, which works in windows operating system and has a built-in GPS, via a cable where the data and position of each data point are recorded together. Although there are manual adjusting knobs on the instrument, calibration of all receivers can be automatically done using an appropriate software when the instrument is placed on a monopod 1.5 m above surface. Since the transmitter-receiver separations in EM38-4 are much less than skin depth of the electromagnetic wave of the transmitter, the in-phase and out of phase components are directly proportional to MS and EC of subsurface materials. Each receiver has also its own depth of penetration. So, the instrument can reveal apparent MS and EC of the ground under the measuring point at four different depths. This feature makes the instrument a proper and efficient tool for environmental and agricultural studies as well as archaeological and even near surface geophysical explorations. Inversion of recorded data are also possible where details of the subsurface are required.
We had the chance to borrow the instrument from Geonics Ltd., in September 2022 and test it over an agricultural land, located at Haldimand County in southern Ontario, to map variations of both MS and EC of soil by depth. Plotting the maps of measured MS and EC revealed a depressional area having high EC and low MS at all receivers. We also inverted the data using nonlinear Occam’s inversion method to calculate EC and MS variations with depth. Results of inverting the data showed that EM38-4 is more sensitive to EC than MS of materials under the instrument.