GEOSPHERE INC -- Ground Penetrating Radar: Ground Penetrating Radar

The radar method determines subsurface conditions by sending pulses of high frequency electromagnetic waves into the ground from a transmitter antenna located on the surface. Subsurface structures cause some of the wave energy to be reflected back to the surface, while the rest of the energy continues to penetrate deeper. The reflected wave energy is picked up by a receiver antenna on the surface. These signals are then processed and plotted in a distance-versus-time display. Thus, as the radar antenna is slowly towed across the surface, a continuous cross-sectional "picture" of subsurface conditions is generated. The radar reflections are caused by wave responses at interfaces of materials having different electrical properties. These interfaces include many natural conditions such as bedding, cementation, changes in moisture and clay content, voids, fractures, and intrusions as well as man-made objects.

Depth of penetration is dependent on conditions found at each site. Radar waves are attenuated (absorbed or scattered) by certain properties of the site's soil, the most important of which is the electrical conductivity of the material. Generally, better overall penetration is achieved in dry sandy soils; reduced penetration is achieved in moist, clayey or conductive soils. Considerable depth may be attained in saturated sands or through lake water if the specific conductance of the water is low. Radar penetration is excellent in massive dry materials such as granite, limestone and concrete.

Resolution of radar reflections can be increased by increasing the frequency of the radar waves transmitted into the ground. This is done by using one of several different antennas available for the radar system. However, there is a trade-off between increased resolution and depth of penetration. In good conditions, the higher frequency (900 and 500 megahertz (MHz)) antennas are capable of penetrating 5 to 15 feet of soil with resolution on the order of 0.5-2 inches, whereas the lower frequency (300 and 80 MHz) antennas may attain 30 to 80 feet depths with resolution on the order of 0.5-3 feet.

Depths to buried layers or objects is proportional to the time it takes the radar pulse to travel from the surface antenna to the target and back again to the receiver antenna. This time is called two-way travel time and is dependent on the dielectric properties of the media through which the radar pulse travels. These dielectric properties are in turn a complex function of the composition and moisture content of the soil and rock. In almost all cases, the moisture content has the greatest influence because water has a very high dielectric value compared to common soils and rock. The greater the amount of water saturation, the lower the radar velocity and the lower the object will appear in the radar profile.