GEM-2 - How to survey

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

Setting up the GEM-2 parameters for a survey

Although the GEM-2 is a sophisticarted digital sensor, once it is set up, it is simple to use in the field. The data logger stores data for about two hours (50,000 data points) before it requires downloading.

Before the start of the survey, the user can specify GEM-2 operating parameters, such as a set of frequencies to be used for a particular survey. Although up to 10 simultaneous frequencies are possible, a survey will typically use 3 to 5 frequencies in order to concentrate the transmit power and obtain cleaner data.

To minimize the power-line noise, use only odd harmonics of 30Hz (e.g., 330, 390, etc.) in 60-Hz supply countries, or odd harmonics of 25Hz (e.g., 425, 475, etc.) in 50-Hz supply countries.

Using the main software called WinGEM2 in a Windows environment, a PC connected to the GEM-2 can upload the operating parameters to the GEM-2 or download the data after the survey. The WinGem2 software will help you to avoid inappropriate frequencies choices. For more information on WinGEM2, please check the Operator's Manual.

Typical Fieldwork Procedure

This is an overview, for detailed descriptions, please review the GEM-2 Manual.

The most popular and simple surveys are done by dead reckoning, following predetermined survey paths. Locating the data points on a map is always a big part of a survey. 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. Start data collection at the beginning of the line, then walk steadily along the line, and stop the data collection at the end of the line. The WinGEM software 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. Use the provided EMExport software to convert the stored data to a .CSV file, then transfer the .CSV file to your PC for plotting and analysis. Many data examples shown here are plotted as contours using the commercially available SURFER software.

Hints and tips

Visual markers, such as a traffic cone at ends of each line, help the surveyor maintain a straight walking path. Once the site has been prepared, the GEM-2 survey can be 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 about 10cm. There is a "pause" function when the operator wishes to pause in the middle of a line, for instance to avoid obstacles.

If you made a mistake on the current line, you may make a note of it and delete a particular line once the data have been downloaded. Don't panic if you made mistakes, it is relatively easy to correct them.

Realtime PC Connection for Large Surveys

A PC may be connected to the GEM-2 via an RS-232 cable 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 vehicle.

Interfacing GPS and Other Instruments