Setting up the GEM-2 parameters for a survey

The GEM-2 has been in commercial use since 1994. The sensor has worked at hundreds of project sites in different countries, weathers, and settings. It is basically a handheld instrument although it can also be carried by other platforms.

Internally, the GEM-2 is a complex digital sensor. Externally, it is simple to use. There are only three buttons: just follow the menu on the console display and push the button according to the menu directions. The operator can specify GEM-2 operating parameters, such as a set of frequencies to be used for a particular survey. A typical survey may specify 3-5 frequencies; the operating software can take up to 10 frequencies. To minimize the power-line noise, we recommend using only odd harmonics of 30Hz (e.g., 330, 390, etc.) in 60-Hz supply countries. In 50-Hz supply countries, use odd harmonics of 25Hz (e.g., 425, 475, etc.). The console stores data for about two hours (50,000 data points) before it requires downloading.

Using the main software called WinGEM in a Windows environment, a PC connected to the GEM-2 may upload the operating parameters to the GEM-2 or download the data after the survey. For more information on WinGEM, please check the Operator's Manual.

Fieldwork with the GEM-2 is fairly simple. For detailed descriptions, please review the
pdf">GEM-2 Manual. Here, we explain some basic features. Locating the data points on a map is always a big part of a survey. The most popular and simple surveys are done by dead reckoning, following predetermined survey paths. A general guide for such a survey follows:

Step 1. Divide the survey site into rectangles. Determine the line spacing and mark the endpoints using spray paint or other markers.

Step 2. Begin the survey along Line 0. Press Button 1 at the beginning of a line to start the GEM-2. Walk steadily along the line. Press Button 3 at the end of the line to stop the GEM-2. WinGEM places the entire data along this line at an equal interval.

Step 3. Make a U-turn at the end of Line 0 and repeat the same along Line 1. Continue the survey to finish.

Step 4. Download the data and plot raw data. WinGEM provides some plotting support. Many data examples shown here are plotted using the commercially available SURFER software.

The following hints and reminders may be useful to remember:

Visual markers, such as a traffic cone at each end, helps in maintaining a straight walk path for the walker. Once the site has been prepared, the GEM-2 survey is routinely a one-person operation.

Typically, the GEM-2 collects about 10 multi-frequency data points per second. For regular walking speed, this results in a data interval of 15 to 30cm. Depending on specified parameters, GEM-2 can collect data for one to two hours before it runs out of its memory. There is a "pause" function when the operator wishes to pause in the middle of a line, for instance, to avoid obstacles.

Turning off the GEM-2 power switch does not erase any data! Turning the power back on returns the system to where you left it. If you would like to take a break, turn off the GEM-2 until you're ready to go back. On the other hand, you may leave the power on since the power consumption is minimal when the GEM-2 is not collecting data.

The operator may at any time turn on the "status" menu to see the available memory. GEM-2 produces a sound signal automatically when it fills its memory. At this point, the data must be downloaded.

If you made a mistake on the current line, you can delete the current line and redo it. Alternatively, you may make a note of it and delete a particular line once the data have been downloaded. The main point: don't panic if you made mistakes; it is relatively easy to correct them.

A PC may be connected to the GEM-2 during a survey for continuous downloading to the PC in realtime. This mode is suitable for a long duration survey, such as when the sensor is towed by a car. The WinGEM software allows "Remote Operation" mode, where a PC takes over the GEM-2 console's role of operating and storing the data.

The GEM-2 outputs ASCII format data using an RS232 connector. A multi-port PC may be used to interface with a GPS or other devices (a magnetometer, for instance). Geophex provides custom technical services for designing, manufacturing, and operating specialized Data Acquisition Systems. Please contact Geophex personnel for this service.