Frequently Asked Questions

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 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 meters.

All GEMs can operate at multiple frequencies, simultaneously. However, the total transmitter energy is limited and it is shared 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, we recommend you limit the number of frequencies to the minimum that you need for your survey.

The recommended number of frequencies is typically 3-5 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.

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 units 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 in 60-Hz areas, 1/25 second in 50-Hz areas) is based on minimizing the powerline noise problem.

For specialists, the GEM-2 allows “passive mode” in which the unit, with the transmitter turned off, collects a “time-series” over one “base period”. The time-series, consisting of 6,400 points, can be Fourier-transformed to obtain a precise environmental noise spectrum of the survey area, which can guide the user to select operating frequencies in spectrally quiet bands.

See the summary entitled “How to use the GEM-2” on the Downloads page for some general guidance on choosing appropriate frequencies for various targets and geological conditions.

There are two complex (i.e., inphase and quadrature) calibration sets, each as a function of frequency. Since they are done at the Geophex factory and stored in your GEM software, you shouldn’t need any further calibration.

The two calibration sets are:

Amplitude calibration – this is done using a “Q coil” with known radius, number of turns, resistance, and inductance. It mainly sets the amplitude scale.

Free-air calibration (or ‘zero’)- this is done by moving away from any conductor, difficult to do in the real world. For airborne sensors, it is done by flying the sensor high, typically 5-10 times the coil spacing. For a GEM-2 ski, we raise the sensor to about 6-10m and call the sensor response there the “free-air values”; these are also stored in the sensor software.

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 there and the calibration affects only the scale and offset. We suggest that you do not change the amplitude calibration.

The offset calibration can be 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 constant from the entire dataset so that it fits the background. Still, the process does not change the map appearance.
A ferrite rod can be used for a quick calibration check since it produces only an inphase response. Theoretically, a ferrite rod produces a constant inphase response and zero quadrature response at all frequencies. The size doesn’t matter since all you want to check is that its inphase response is constant over the bandwidth.

The GEM-2 has two DB-9 serial ports. The male DB-9 connector on the GEM-2 console accepts an RS-232 input from an external device such as a GPS receiver. The following default parameters are used: 8 data bits, no parity none, 1 stop bit, no handshaking at 9600 bps.

Connecting an external GPS:
When an external GPS is connected to the ski, the following conditions apply:

• The GEM-2 uses the 1 PPS timing signal provided by the GPS at pin 8 of the DB9 connector. Serial communications is supported with GPS transmitted data at pin 2 and GPS received data at pin3. Pin 5 is ground. The GEM-2 supplies +12V (500ma maximum) on pin 9. Please confirm compatibility with your particular GPS before using the +12V supplied.
  
  
• The GPS unit must be set to send the NMEA $GPGGA message (time of next PPS pulse) once per second.
  
  
• The GEM-2 will automatically synchronize its internal clock to the GPS time. The $GPGGA strings are stored along with the EM data. When the data file is exported, the GPS position data ( either UTM or Lat/Lon ) is interpolated such that each EM measurement has a unique position associated with it.
  
  
• The Status Bar GPS indicator displays the presence of the GPS signal by displaying (+) when PPS is present, along with the GPS quality/number of satellites.