Ground Penetrating Radar (GPR) uses high frequency electromagnetic waves (radio signals) that are transmitted into the subsurface. Electromagnetic waves are then reflected back to the surface from the anomalies (features with different physical properties from the surrounding) in the subsurface. A receiver on the ground surface records the amplitude of the reflected pulse as well as the travel-time to and from the target. Knowing the velocity of the EM waves in the subsurface, the depth to the target can be calculated from the travel-time.
There are different types of antennas available in the market, each of which is suitable for detecting objects with different size and depth. The rule of thumb is that the higher the antenna frequency, the higher the resolution, but the shallower depth of investigation. On the other hand, lower frequency antennas are suitable for detecting large targets at greater depths. Usually, higher frequencies (> 1000 MHz) are used to locate rebar, conduits, and tension cables in concrete. However, this will give high resolution detail approximately to the depth of 24 inches. Lower frequencies (12.5 MHz-200 MHz) can penetrate from a few inches to a couple of hundreds of feet. These antennas are good for deep geological and geophysical applications. Middle frequency antennas (200-800 MHz) are usually employed for archaeology, forensic, utility locating, and groundwater studies.
Commonly, GPR investigations are carried out using a single-channel system. Recently, commercial multichannel GPR systems have become available. Multichannel GPR systems can consist of different frequency antennas to cover different range of depths. BESS is equipped with a state-of-the-art multichannel GPR system (Stream EM) with massive array of 40 antennas in two polarizations (Figure below). Dual polarization antennas help detecting linear features in both the direction of the traverse and perpendicular to it in one single data acquisition. for more information, please go to Stream EM.
Ground Penetrating Radar has wide range of applications. Some of GPR applications are as following:
- Void Detection
- Mapping Landfills
- Detecting Sinkholes
- Locating Utilities and Underground Storage Tanks (USTs)
- Mapping rebar, post-tension cables, and conduits in concrete
- Bridge Deck Assessment
- Roadway Condition
- Locating unmarked Graves
- Stratigraphic Analysis