Note to the readers: Although this FAQ is most applicable to GEM-2, most questions and answers equally apply well to all other GEM sensors including GEM-3 and GEM-5 and their variations.
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.
See the attached summary entitled �How to use the GEM-2�for some general guidance on choosing appropriate frequencies for various targets and geological conditions. 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.
What is the Quickest Way of Plotting the GEM Data? Most simple GEM surveys (geological, geotechnical, and environmental) do not require navigational aids, such as GPS. The operator steadily walks across a survey area between two opposite rectangular boundaries in a zigzag fashion at a predetermined line spacing. We call it a ?dead-reckoning? survey. Please see the manual or visit our website for graphic description. For dead-reckoning data, WinGEM assigns coordinates to each data point and generates Excel (csv) files that contains X, Y, I1, Q1, I2, Q2, etc.,where X and Y are the data coordinates and the rest are the inphase and quadrature data for frequency 1, 2, 3, and so forth. To make 2-D maps, the user must provide other (commercial) software such as SURFER, Geosoft, etc. SURFER is the most commonly available 2-D plotter. For such a dead-reckoning survey, the operator can view the whole data on contoured maps within minutes after the survey. How can I connect a GPS to the GEM-2 and how do I simultaneously acquire GPS and EM data? The male DB-9 connector on the GEM-2 provides for RS-232 input from an external device such as a GPS receiver. The following communications parameters are used: Baud rate Selectable on the router port 9600 or 4800 Data bits 8 Parity none Stop bits 1 Handshaking none
Connecting an external GPS: When an external GPS is connected to the ski, the following conditions apply:
1. The GEM-2 uses the 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. 2. The GPS unit must be set to send the $GPGGA (time of next PPS pulse) once per second. 3. The GEM-2 will automatically synchronize its internal clock to the GPS time. The $GPGGA strings are stored along with the EM data. 4. 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. The GEM-2 has two DB-9 serial ports Does the GEM Drift in Time? The GEM-2 is designed to minimize the temporal drift in a quite different way from all other instruments. The GEM-2 ski contains three coils that are precisely maintained in their relative separations amongst each other. Any small changes in the relative separations can cause a shift (or drift) in the signal level. The GEM-2 coils are permanently entombed in a ? ski? structure made of synthetic materials that has a low thermal expansion coefficient and, therefore, their relative locations are firmly fixed. The sensor is designed to ?linearly? expand or contract following the ambient temperature. This linear expansion precisely maintains the relative positions and, therefore, the bucking condition. This feature is quite unique for this design. Owing to this careful design and manufacturing, the GEM-2 has not shown any appreciable drift, as would be testified by numerous users in the last five years of its usage. The other important feature is the way the console (which is a big chunk of metal) is designed and mounted on the ski. The location of the console is where the field gradient is minimum so that its slight displacement (due to not tightening the mounting screw, for instance) would cause little shift in the signal. There are no moving parts in the console; for instance, the battery is internal and not removable. If you think the sensor maybe drifting, tighten the screws that mount the console to the ski; please do not over tighten. Diurnal temperature changes should not be a significant cause. How Do You Plot the Data on a Rough Terrain? The GEM as a handheld unit can go wherever the operator can go. For data-plotting purposes, if the surface grade isn't steep or the slope is more or less constant, the interpreted results may be vertically shifted in parallel to the surface. For unusual surfaces (e.g., across a cliff), the user must come up with his or her own codes. Note that the primary purpose of the GEM is to find anomalies first, and then worry about what they mean. Most of the time, the answers are obvious to the people who are familiar with the site.
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 Geophex 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 Geophex 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 - The sensor output must approach zero when you move 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. The needed height is typically 5-10 times of the coil spacing. For GEM-2, we raise the sensor to about 6-10m - we pull it up to a pine tree in our backyard - and we call the sensor response 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 manual described it for users to try, but we deleted it since we decided that it is not necessary. One can buy ferrite 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 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. How Do You Invert GEM-2 Multi-frequency Data to a Layered Earth? The utility program EMInvertor 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 Geophex scientists to improve it. The primary author is Haoping Huang at Geophex. 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. (Interpex, etc) The Interpex website has a 1D inversion program available as shareware, but the most valuable asset there is their tutorial on layered earth inversion as it is based on GEM-2 data. Lastly, the University of British Columbia Geophysical Inversion Facility also offers some tutorial information as well as links to related scientific papers and background material on inversion techniques. Alas, our own website (www.geophex.com) has relatively little information related to layered earth inversion with the exception of some journal articles by Haoping and I.J. Won that may be of value. http://www.interpex.com/ - Interpex offers several inversion programs as well as an excellent tutorial that addresses GEM-2 data.
http://www.eos.ubc.ca/ubcgif/ - this is an excellent source for anything concerning geophysical inversion.
http://www.geo-em.com/ - website of Dr. Haoping Huang, author of several programs for geophysical inversion
sean@rockpointgeo.com/ - website of Sean Walker, alumnus of UBC and consultant in geophysical data analysis.
The inversion software that we distribute was designed to be used with both the GEM2 and GEM3. I have attached what I believe to be the latest version as a zipped file. When you select the GEM sensor type (GEM 2,3 or 5), the program will prompt for the coil spacing and will default to the value of 1.6m which is appropriate for the GEM2. If the data are from a GEM3, I have been told that the coil spacing value should actually be the sensor diameter. I am afraid that we have little in the way of documentation on this program, and most of the program options are only accessible by editing the file invertor.ini (you can use a text editor to change most of the initial parameters). If you have more detailed questions regarding application or interpretation, I would suggest that you contact the author directly (Dr. Haoping Huang, www.geo-em.com/).
How do I Learn More about the GEM Sensors? Geophex website, www.geophex.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 Geophex. I want to use an EM sensor for detecting buried objects. Which EM sensor should I choose? My IPAQ (or similar PDA) data logger will not connect to the GEM-2. What can I do to reestablish a connection? We understand that you can not get the GEM to connect either to the IPAQ by Bluetooth nor by direct cable connection. Apparently, the system has worked properly up until this time, so that indicates that a hardware failure is unlikely, and that reconnection should be possible. Please note that the Bluetooth setup requires no password - we set them up for open discovery. First, let's establish a connection through the direct cable connection. I will assume that you have a PC available with a standard RS-232 serial port and that you have the PC version of WinGEM installed. Be sure that you know which serial port you are dealing with (comm1, comm2, etc.). If you are unusure, right click on MY COMPUTER, select PROPERTIES, HARDWARE and DEVICE MANAGER. Scroll down to PORTS and click on that to see what comm ports are available. Now, the GEM2 electronics console has a reset button on the bottom of the console that, when pushed, will always set it to communicate over the RS-232 comm port. To access the reset button, remove the four screws loacted at each corner of the electronics console and gently pull up on the console to free it from the connectors and the sensor ski. Connect the serial cable from the PC to the electronics console serial port and turn on power to the console. Using a small pointed object (for example a paper clip), momentarily press the reset switch on the bottom of the console. Now, start WinGEM on the PC and select the serial port (commX) found earlier. WinGEM should connect to the console and allow you to activate all functions. If all works well at this point, you can then use the SETUP menu to select PORTS, GEMPORT, then SWITCH GEMPORT TO BLUETOOTH. Doing this will insure that the Bluetooth port is subsequently the primary communications port and will immediately exit from WinGEM. You can now turn off the power to the electronics console. If you have now established that the GEM console will communicate via the serial cable connection and have successfully switched to Bluetooth, the next step is to replace the console onto the GEM ski, being careful to mate the connectors gently so as not to bend any pins. After mating the console and ski, replace the four retaining screws; tighten them gently. You now have the electronics console mounted on the ski and configured for Bluetooth operation. Now let's connect using Bluetooth. Turn on the GEM-2 console and turn on the IPAQ. Be sure that the Bluetooth radio on the IPAQ is turned ON. Bluetooth manager should see the GEM Bluetooth module. Look at the attached screen photos for guidance in checking that the Bluetooth connection is properly configured (see Bluetooth turn on and setup.pdf). Note which outbound serial port is indicated (usually Com6) and that the service is enabled. Now start WInGEM on the IPAQ. If the connection is not established right away, refer to the attached WINCEGEM BT comm port setup.pdf file for guidance. Select SETUP from the menu, then PORTS, then GEMPORT and check that the proper comm port is selected and that the default baud rate is 115,200 baud. AT this point, the IPAQ should connect to the GEM. If not, review the attached screen shots and check all entries. Most of the time when a GEM fails to connect via Bluetooth, it is due to one of the following:
1. the GEM console is set for communication over the RS-232 serial cable.
2. the Bluetooth radio in the IPAQ is turned off,
3. a Bluetooth configuration setting has been inadvertently changed (frequently the comm port selection).
Usually the GEM2 comes calibrated down to a frequency of 300 to 450 Hz, and will not allow you to select frequencies outside of the range of calibration frequencies. Regarding the use of very low frequencies, there are two considerations; first, even though the skin depth is greater at lower frequencies, there is the offsetting problem of signal attenuation. You still have to have sufficient signal-to-noise ratio at the chosen frequency(s) to get a reasonable response. The second problem is that, at very low frequencies, motion induced noise becomes substantial, hence the signal-to-noise ratio is further degraded. Experience with the instrument and with various near-surface geological conditions is the best guide for choosing the optimum frequency, just keeping in mind that lower frequencies see deeper targets and higher frequencies see targets closer to the surface. We generally recommend the initial use of three or four frequencies in swept mode, for example roughly 1kHz, 5kHz, 15kHz and 40kHz. Step mode puts the maximum signal strength out at each frequency, but at the expense of more time to collect the data. The maximum current setting was chosen on the basis of component capabilities and, while it is somewhat conservative, it is not a good idea to exceed the 10A current limit setting. My GEM-2 has been working fine, but is now erratic even though I have made no changes to the configuration. What can I do? Erratic operation that starts suddenly is frequently an indication that the battery is low on charge or that the battery is faulty. Charge the battery or replace it with a freshly-charged battery. If erratic operation continues, reload the configuration file (for example, GEM2-425.gem) that came with your GEM. This will restore all of the original configuration features and calibration data. If erratic operation continues after these steps, contact Geophex for additional diagnostic procedures.
Is the GEM-2 available for rent?
Geophex has a limited number of GEM-2s available for rent on a first-cone, first-served basis. We only rent within the US and the minimum rental period is one week. Contact Geophex for pricing and availability. Users who have not used similar instruments before should plan on at least one day of familiarization prior to undertaking any sizable survey.
In what format does the GEM-2 store the EM data? How do I access, examine and analyze the data?
For instruments that use the IPAQ or similar PDAs for logging data, data files are stored in internal memory as well as on a removable memory card (typically an SD card). Raw data are stored in binary (.gbf) files. The utility program EMExport (link to this) (available on both the PDA and in a PC version) is subsequently used to convert the binary files to comma separated variable (.csv) files that can be examined using a text editor or (more commonly) a spreadsheet program. The spreadsheet approach is quite versatile as it allows for addition of comments and notes to the data file as well as plotting the data for visual analysis. More complex analysis such as contour plots and/or image plots can be created using commercially available programs such as Surfer or Geosoft. These analysis techniques can be used to advantage for both quality assurance of the data and final presentation of the data. (add links to sample Excel plot and Surfer plot) What is included with the GEM-2 Electromagnetic Sensor System?
1 each sensor ski with electronics console 1 each aluminum shipping/storage case 1 each data logger with accessories 2 each DR36AAS smart batteries 1 each battery charger with power adapter, cord, reference card 1 each RS232 cable 1 each RS232 to USB adapter 2 each calibration ferrite 1 each software CD 1 each (optional) Garmin GPS18X GPS receiver with external power adapter
