Note to the readers: Although this FAQ is written for GEM-2, most questions and answers equally apply well to all other GEM sensors including GEM-2A, GEM-3, and GEM-5 Array.

How many frequencies may I use for a  typical survey?

All GEMs operate at multiple frequencies, simultaneously. However, the total transmitter energy is limited and it is shared equally by each frequency. This means that the energy for each frequency will decrease with an increasing number of frequencies. This also means that the signal-to-noise ratio (SNR) will degrade, as the number of frequencies increases. Therefore, please restrict the number to what you need for your survey. Recommended number of frequencies is typically 3 for the GEM-2. If you really need more than that, consider running the survey more than once with a different set of frequencies each time. For the GEM-3, when the sensor is very close to the target, you may use as many as 10.

How do you select frequencies?

Environmental noise, generally high in urban and low in rural areas, governs the frequency selection particularly at the lower end of the GEM spectrum. The GEM data stream includes the amplitude of a specified powerline frequency (60 Hz or 50 Hz) in a unit of milli-Gauss.To minimize noise induced by powerline harmonics, the GEM-2 uses frequencies that are "odd multiples of half the powerline frequency." In 60-Hz areas (U.S., Canada), these are 90, 150, 210, 270 Hz, and so on. In a 50-Hz areas (Europe, Japan, Australia), these are 75, 125, 175, 225Hz, and so on. These frequencies have minimal powerline interferences. In fact, the concept of "base period" of operation (1/30 second at 60Hz areas, 1/25 second in 50-Hz areas; for more, read the "How it works" page) is based on minimizing the powerline noise problem.

As a general guide, first select a frequency below 1 kHz., say 750 Hz, assuming that you are in a 60-Hz area. For the rest, multiply an odd number, say 5, each time: 750Hz, 3,750Hz, and 18,750Hz. Try not to use more than 3-4 frequencies at a time because the total transmitter energy is shared equally among all frequencies and, therefore, you may encounter reduced signal-to-noise ratio as you add more frequencies.

For specialists, the GEM-2 allows a "passive mode" in which the unit, with the transmitter turned off, collects a "time-series" over one "base period" (1/30 second at 60Hz areas, 1/25 second in 50-Hz areas). The time-series, consisted of several thousand points, can be Fourier-transformed to obtain a precise environmental noise spectrum of the survey area, which may guide the user to select operating frequencies only in spectrally quiet bands.

What is the Depth of Exploration for GEM-2?

This is a very complex question because the answer depends on many factors, particularly on ground conductivity, target volume, and ambient electromagnetic noise. Based on many analyses and field data, we estimate the GEM-2 should be able to see about 20-30 m in resistive areas (>1000ohm-m) and about 10-20 m in conductive areas (<100 ohm-m). This figure assumes an ambient noise level of 5 ppm. The noise level is generally high in urban areas and low in rural areas. For typical applications, we do not recommend the GEM-2 for depths deeper than 30 m. For more discussion on this subject, consult the Skin-depth Nomogram in GEM-2 Principle of Operation on our webpage.

Depth of exploration for a given earth medium is determined by the operating frequency. Therefore, measuring the earth response at multiple frequencies is equivalent to measuring the earth response from multiple depths. Hence, such data can be used to image a 3-D distribution of subsurface objects.  Results from several environmental sites indicate that the multi-frequency data from GEM-2 is far superior in characterizing buried, metallic and non-metallic targets to data from conventional single-frequency sensors. For handheld sensors, the transmitter-receiver coil separation is inherently small and has little to do with the depth of exploration.

For detailed technical discussions, please read an article in GEOPHYSICS by Huang, Depth of investigation for small broadband electromagnetic sensors, Geophysics, v. 70, n. 6, pp. G135-G142, 2005. Very simply speaking, the "practical" depth of exploration for a GEM-2 type sensor is approximately the square-root of the skin depth expressed in meter.