Variables Affecting GPR Resolution and Energy Penetration  

There are numerous factors that must be considered before conducting a GPR survey.

Michele Koons and Sara Gale with a SIR-2000 system in Utah.

The success of GPR surveys in archaeology is to a great extent dependent on soil and sediment mineralogy, clay content, ground moisture, depth of burial, surface topography and vegetation. It is not a geophysical method that can be immediately applied to all geographic or archaeological settings, although with thoughtful modifications in acquisition and data processing methodology, GPR can be adapted to a great variety of site conditions.

The maximum effective depth of penetration of GPR waves is a function of two main factors:

1) the frequency of the waves that are propagated into the ground; and

2) the physical characteristics of the material through which they pass.

The physical properties that affect the radar waves as they travel through a medium are electrical conductivity and the magnetic permeability. Soils, sediment or rocks that are dielectric will permit the passage of a great deal of electromagnetic energy without dissipating it. The more electrically conductive a material, the less dielectric it is, and energy will attenuate at a much shallower depth. In a highly conductive medium the electrical component of the propagating electromagnetic wave is being conducted away in the ground, and when this happens the wave as a whole dissipates. This occurs because for propagation to occur the electrical and magnetic waves must constantly "feed" on each other during transmission.

Highly electrically conductive media include those that contain salt water and some that have certain types of electrically conductive clay, especially if that clay is wet. Any soil or sediment that contains soluble salts or there are electrolytes in the ground water will create a medium with a high electrical conductivity. Often desert soils, even if they appear to be extremely dry and therefore should readily allow radar transmission, contain hydrous salts in their interstices, which conduct electricity. In these types of soils radar energy will become attenuated at a shallow depth.

Other minerals in the ground, especially those that can dissolve in water, will create free ions, which allow for greater electrical conductivity. Sulfates, carbonate minerals, iron, salts of all sorts and any charged elemental species of mineral will create a highly conductive ground and readily attenuate radar energy at shallow depths. Under the very unfavorable conditions of wet (with slightly saline water), a calcareous sediment or soils that contain certain clay-rich minerals, the maximum depth of GPR penetration in the ground can be much less than a meter, no matter what frequency of antenna is used.

Magnetic permeability also affects radar penetration in a medium. It is a measure of the ability of a medium to become magnetized when an electromagnetic field is imposed upon it. Most soils and sediments are only slightly magnetic and therefore usually have a low magnetic permeability. The higher the magnetic permeability, the more electromagnetic energy will be attenuated during its transmission, and when this occurs the magnetic portion of the EM wave is destroyed, just as with increased electrical conductivity the electrical component is lost. Media that contain magnetite minerals, iron oxide cement or iron-rich soils can all have a high magnetic permeability and therefore transmit radar energy poorly.

Radar energy will not penetrate metal. A metal object will reflect all of the radar energy that strikes it and will “shadow” anything directly underneath. Buried metal objects are quite easy to see in GPR reflection profiles because they usually create multiple reflections stacked on top of one another below the metal object.

One of the most important variables in GPR surveys is the selection of antennas with the correct operating frequency for the depth necessary and the resolution of the features of interest.
General purpose GPR systems use dipole antennas that typically have a two octave band width, meaning that the frequencies vary between one half and two times the dominant frequency. For example a 300 MHz center frequency antenna generates radar energy with wavelengths ranging from about 150 to 600 MHz.

Some of the important factors that must be considered in choosing an antenna frequency are: 1) electrical properties of the ground at the site, 2) depth of radar energy transmission necessary to study the buried features of interest, 3) size and dimensions of the archaeological features that must be resolved, 4) site access, and 5) presence of possible external electrical interference within the frequency spectrum of the antenna contemplated for use.