What is Ground Penetrating Radar?
Ground penetrating radar (GPR) is a non-destructive geophysical method that produces a continuous cross-sectional profile or record of subsurface features, without drilling, probing, or digging. The GPR cross-section shows the ground surface at the top of the profile, and the reflections of subsurface geologic units and objects to a certain depth at the bottom.
Ground penetrating radar (GPR) profiles are used for evaluating the location and depth of buried objects and to investigate the presence and continuity of natural subsurface conditions and features.
How GPR Works
Ground penetrating radar operates by transmitting pulses of ultra high frequency radio waves (microwave electromagnetic energy) down into the ground through a transducer (also called an antenna). The ground penetrating radar antenna (transducer) is pulled along the ground by hand or behind an ATV or a vehicle. The transmitted energy is reflected from various buried objects or distinct contacts between different earth materials. The antenna then receives the reflected waves and stores them in the digital control unit. The control unit registers the reflections against two-way travel time in nanoseconds and then amplifies the signals. The output signal voltage peaks are plotted on the ground penetrating radar profile as different color bands by the digital control unit.
How Deep Can GPR Go Into the Ground?
The depth to which ground penetrating radar waves can reach beneath the ground surface is mainly dependent on two conditions: 1) the type of soil or rock in the GPR survey area, and 2) the frequency of the antenna used. Ground penetrating radar can reach depths of up to 100 feet (30 meters) in low conductivity materials such as dry sand or granite. Moist clays, shale, and other high conductivity materials, may attenuate or absorb GPR signals, greatly decreasing the depth of penetration to 3 feet (1 meter) or less.
The depth of penetration is also determined by the GPR antenna used. Antennas with low frequencies of from 100 to 200 MHz obtain subsurface reflections from deeper depths (about 25 to 100 feet or more), but have low resolution. These low frequency antennas are used for investigating the geology of a site, such as for locating sinkholes or fractures, and to locate large, deeply buried objects.
Antennas with higher frequencies of from 400 to 1,500 MHz obtain reflections from shallow depths (0 to about 14 feet), and have high resolution. These high frequency antennas are used to investigate surface soils and to locate small or large, shallow, buried objects, such as utilities, gravesites, and also rebar in concrete.
GeoModel, Inc. conducts ground penetrating radar surveys using a SIR-3000 digital control unit and various antennas with frequencies ranging from 100 to 1,500 MHz. The GeoModel, Inc. principals have over 50 years of combined experience and conduct GPR surveys nationwide.