How Do You Convert the GEM-2 Data to Apparent Conductivity and Susceptibility?

This is a standard feature for the GEM-2 in the ?locate? menu in WinGEM operating system. On WinGEM, the GEM-2 ppm data can, at a mouse stroke, be converted to apparent conductivity at each frequency and susceptibility at the lowest frequency used for the survey. For theory and practice, see the article by Huang and Won (2001), entitled ?Conductivity and Susceptibility Mapping Using Broadband Electromagnetic Sensors,? published in Journal of Environmental and Engineering Geophysics. The article may be downloaded from the Aeroquestsensortech website.

How Do You Calibrate the GEM-2?

There are two complex (i.e., inphase and quadrature) calibration sets, each as a function of frequency. Since they are done at Aeroquestsensortech and stored inyour GEM software, you shouldn't need any further calibration. The two calibration sets are:

Amplitude calibration - this is done using a "Q coil" with knownradius, number of turns, resistance, and inductance.It mainly sets the amplitude scale.

Free-air calibration - The sensor output must approach zerowhen youmove it away from any conductor, which is of course hard to do on earth. For airborne sensors, it is done by flying the sensor high. Theneeded height istypically 5-10 times of the coil spacing. For GEM-2, we raise the sensorto about 6-10m - we pull it up to a pine tree in our backyard - and we call the sensorresponse there the "free-air values" that are also stored in the sensor software. As you notice, this calibration does depend on the ground conductivity, but is simply a DC offset. A ferrite rod - permeable but not conductive - cannot be used for either of the two calibrations described above. We sometimes use it, but not necessary, for a quick calibration check since it produces only the inphase response. Theoretically, a ferrite rod produces a constant inphase and zero quadrature at all frequencies. Our older manualdescribed it for users to try, but we deleted it since we decided that it is not necessary.

One can buyferrite rods locally at any "radio shacks" where they sell electronic components, since it is commonly used in circuits to enhance coil inductance. Most radios have them in their antenna tuning coils. Usually, they are about 1cm or less in diameter and 2-3cm long. The size doesn't matter since all you want to check is that its inphase response is constant over the bandwidth.

Both calibration factors are either a constant multiplier - for amplitudes - or a DC offset - for the free air. In other words, they do not affect the appearance of your data in a map or profile. The "bumps" are always thereand the calibration affects only the scale and offset. We suggest that you do not change the amplitude calibration.

The offset calibration canbe a problem over a very conductive area, where one wants an absolute conductivity map. In this case, if you know the background conductivity from other measurements (DC resistivity, for instance), you can simply add or subtract a constantfrom the entire dataset so that it fits the background. Still, the process does not change the map appearance.

How Do You Invert GEM-2 Multi-frequency Data to a Layered Earth?

The program is available for the time being as Beta Version free of charge to all GEM owners. It is downloadable from our website. We request users to work with our Aeroquestsensortech scientists to improve it. The primary author is Haoping Huang at Aeroquestsensortech. The code makes a 2-D cross-section based on continuous 1-D interpretation. Also there are several commercial software packages that invert the GEM data into a multi-layered model.

How do I Learn More about the GEM Sensors?

Aeroquestsensortech website, www.aeroquestsensortech.com, provides ample information about the GEM sensors, design and operating principles, survey tips, manuals, and down-loadable operating software. The site also carries many, many journal articles related to the GEM sensors, which can be downloaded or printed. For more information and questions, please contact Aeroquestsensortech.