Ground Penetrating Radar

GPR is an effective and efficient tool that can be used to locate a variety of targets. GPR scans collect anomaly data which can be interpreted in real-time. The geometry of the anomalies located can assist in determining the type of subsurface object producing the anomaly. Linear anomalies can be interpreted to be buried utilities, conduits, trolley tracks, etc., if the return data has the appropriate signature. The GPR unit transmits radar beams into the substrate, and these beams are reflected or refracted back to a receiving antenna. The return data sets exhibit anomalies graphically, as hyprebolas; their crests indicating position and depth below the surface to the target. Again, the geometry and size of the anomalies can be used to identify the target. The GPR is programed at the beginning of each survey, entering such parameters as wave velocity, and depth of penetration. Substrate properties and conditions govern set-up input and the GPR can be re-calibrated during use as these conditions change. An example would be radar wave velocity changes that occur when traversing over concrete sidewalk into grass covered soil. Wave velocities are substrate depenedent, and usually travel more slowly through concrete than through soil. Changes in velocity will make changes in the depth to target algorithems within the GPR software, therefore, instrument calibration is critical.

Initially, a series of single-line traverses are made in both the X and Y directions across the area of interest. When a series of parallel traverses reveal anomalies at similar positions and depth (a linear anomaly), these results can be interpreted as evidence of a conduit, utility or linear structure. Traverses that are run perpendicular to the long axis of the target produce ideal signal returns. Single-line surveys are usually employed when attempting to locate sub-surface conduits or utilites.

The grid survey (a more comprehensive survey), may be used when attempting to locate buried drums or underground storage tanks (UST's). These objects, having discrete geometries, can be missed during single-line surveys and may require an XY grid pattern (having a defined, repeated spacing between traverse lines) to accurately locate them. Grid dimensions (and an origin) are chosen to encompass the largest portion of the area of interest. Multiple grid surveys may be necessary to provide complete coverage of the site. The spacing between each traverse line is of critical importance. Every attempt is made to prepare the grid so that line spacing is smaller than target dimensions. For example, an XY grid having a line spacing of 5ft. or less, would be appropriate for locating a 500 gallon UST with dimensions of 4ft x 5.5ft. The grid pattern is layed out using paint, flags or stakes, and the grid parameters are programed into the GPR. Real-time returns can be interpreted in the field and/or post-processed as profile views; colorimetric plan views; or 3-dimensional renditions. Colorimetric plan views can be examined at any depth interval (slice); and profile views can be examined at any depth slice, on any traverse line in the grid.

Survey results can range from extremely accurate to subjective, depending upon site and substrate conditions. Accessibility is a key factor in obtaining the most comprehensive data set. Ideal substrate conditions would be areas having undisturbed sediment surrounding the intended targets. Two conditions that have a great impact upon the signal return strength are soil moisture and clay-sized sediment particle content. Both water and clay attenuate the signal return strength and high percentages of either in the substrate can impact the quality of the data set.

250 MHz Noggin^plus™

This single channel GPR is made by Sensors And Software Inc. (www.sensoft.ca). The unit has a fixed mount, shielded antenna system that is housed within a wheeled, PVC SmartCart™, facilitating reconnisence surveys over many different types of terrain including short grasses and gravel. The depth of penetration of the radar waves (and associated return signals) is strongest between ten and twenty feet below ground surface. The radar signal is triggered automatically with the cart odometer and real-time data is seen on the LCD screen built into the video data logger (VDL). All data collected is saved internally with the ability to transfer to a flash drive for post-processing or archiving.

1000 MHz Conquest™

The Conquest^®, a high resolution GPR system, made by Sensors And Software Inc., is the perfect tool for on-site concrete mapping or imaging. This intrinsically safe unit combines a full color monitor with state-of-the-art sensor technology, to provide real-time imaging of reinforcement bar; pre-tension or post-tension cables, voids and conduits. Detection of active current through cable or wiring is immediate, and represented both graphically and numerically. The tethered sensor design allows for scans of walls and ceilings in addation to floor slabs, sidewalk and asphalt surfaces. Penetration depth of the radar signal is effective to a minimum of eighteen inches below ground surface. The lightweight and compact design of the system allows for limited access surveys in areas as small as two feet square.