| Ground penetrating radar (GPR) is a tool for indirectly
looking at underground objects (such as graves), gravel and sand layers, and other
underground structures. The information or data received by GPR is like an x-ray or map of
the underground. In fact, GPR uses electromagnetic (EM) waves, as x-ray machines do, but
GPR uses radio waves, which have a longer wavelength (see Figure 1). The wavelength, or
the length of one wave, is the fundamental difference between the forms of electromagnetic
energy. For example, the wavelength of x-rays range from about 10 billionths of a meter to
about 10 trillionths of a meter, whereas radio waves can be a few meters long. GPR antennae To collect GPR data, two antennae are repeatedly placed along the ground surface at a constant interval. Unlike a radio or TV antenna, GPR antennae usually look like two skis that vary in length from less than ˝ meter long to 4 or more meters long as shown in Figure 2. Different length antennae send different frequency waves into the ground. Frequency is the number of wavelengths per second, measured in Hertz; radio waves are measured in millions of wavelengths per second, which is called megahertz (MHz). The wavelength for each antenna is approximately equal to the antenna length in most underground material. Figure 3 compares different frequency antennae and the waves they produce. The 200 MHz wave completes four wavelengths in the same amount of time that the 100 MHz wave completes two wavelengths, which is why 200 MHz has been designated as higher frequency. The higher frequency enables 200 MHz waves to pick up more layers, yielding higher resolution data. |
Figure 1. The electromagnetic spectrum, and the definition of a wavelength. Wavelengths become shorter as the spectrum moves from radio waves to gamma rays. (Modified on 9/21/98 from http://observe.ivv.nasa.gov/nasa/education/reference/emspec/emspectrum.html) 
Figure 2. GPR antennae with frequencies and length
labeled. Handles, transmitter, and receiver are on the 25 MHz antennae. (Photo by Brian
Thayer.)
Figure 3. Higher frequency waves will
complete wavelengths more often than lower frequency. |
