What is Ground Penetrating Radar?

• Uses electromagnetic radiation to detect subsurface features.

• Operates generally with frequencies between 10-1000 MHz.

• Lower frequency = greater depth, low resolution.

• High frequency = less penetration, high resolution.

• Depth of penetration vary between less than a meter to a hundred meters.

• Depth of penetration is related to the type of material and not the precision of the instrument.

• Velocity is dependent on the electrical conductivity and dielectric constant.

• Velocity increases with an increase in porosity. It decreases with addition of water in material.

• Fractures in rocks have a limited effect on propagation of electromagnetic energy.

• Reducing the frequency too low, may cause the waves to diffuse.

                                                           The electromagnetic spectrum

    All waves in the electromagnetic spectrum travel at the speed of light (c = 3.0 x 10⁸ m/s), when in a vacuum. They do not require a medium to pass through (as sound and water waves do).  Their wavelengths differ from being long on the left side of the spectrum (radio, microwave) to short on the right side (X rays and gamma rays).  The change in wavelength effects the frequency of the wave. Longer wavelengths have low frequencies and shorter waves have high frequencies. The frequency of a wave measures the amount of times a motion repeats itself in a given time interval. Frequency (f) is measured in Hz (cycles)/sec.  The wavelength is measured in nm from crest to crest of the wave.

Ground penetrating radar utilizes radio frequency waves to detect subsurface features. An antenna, which is pulled across the surface of the ground, transmits a signal that penetrates the subsurface. As this signal passes through various layers of sediment, it is reflected back up and received by the antenna. The "signals" that are picked up are displayed graphically on a computer.

GPR’s operate at frequencies between 20 and 1000 MHz. The lower frequencies give lower resolution, penetrating approximately 30 m. Higher frequencies show better resolution, but do not penetrate as far.

Waves reflect when they encounter a change in the electrical properties of sediment. Attenuation (loss of energy) occurs when the wave penetrates a soil with a high electric conductivity (such as clay). Thick layers of clay can limit the penetration of the waves to less than a meter. Salt water, which also has a high electric conductivity, will severely limit penetration. Dry sands and gravel have a low electric conductivity and therefore waves can pass through easily.

back to Home Page