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) data 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. Some of the Applications of ground penetrating radar (GPR) are given below.
Applications of Ground Penetrating Radar (GPR)
Buried Utilities Detection
Cemetery and Grave Surveys
Underground Storage Tanks and Drums
Void and Sinkhole Surveys
Bedrock Depth and Geologic Hazards
GeoModel, Inc. provides ground penetrating radar survey services nationwide.
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.
Using GPR to Locate Utilities
How GPR Locates Unmarked Graves at Cemeteries
GeoModel, Inc. conducts ground penetrating radar surveys for locating unmarked gravesites and confirming marked gravesites at cemeteries.
The GPR profile and cross-sectional diagram (above) shows several gravesites (caskets) detected using ground penetrating radar (GPR). The soils at this site were very sandy and the caskets produced distinct anomalies. The GPR image shows the condition of the graves, from distinct (more recent internments) to collasped or deteriorated graves (older internments). Click here for more information on GeoModel, Inc.’s grave locating services.
GPR for Locating Bedrock and Soil Horizons
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 300 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.
Ground Penetrating Radar (GPR) Frequently Asked Questions (FAQ)
What can GPR find?
GPR can find buried objects and other underground features that are reflective for radar waves. Examples of what GPR can find are underground pipes, grave sites, underground voids, and other buried objects and features.
How deep can GPR detect?
Generally, for most areas of the United States, the depth of investigation is about 10 feet. In areas with clay soils, this depth is only up to a few feet. In areas of sandy soils, this depth can be as much as 30 feet.
Can GPR detect through concrete?
GPR can detect through concrete even if it is reinforced. It can even detect the rebar in the concrete for engineering purposes.
Is GPR portable?
GPR is very portable and is easily moved over each site with just one operator. GPR can even be packed in suitcases and shipped as airline baggage.
Is GPR easily interpreted?
Depending on the objects or features being investigated, with some instruction, GPR data profiles may easily be interpreted, or may require years of experience to interpret.
GeoModel, Inc. Background
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.