Improvements Continue To Drive GPR Applications

By Jeff Griffin, Senior Editor | April 2009 Vol. 64 No. 4
Mala USA's GPR

The main limitation of a GPR locator is that it is highly dependent on soil conditions to work its peak performance. In more conductive soil conditions, the radar waves will not return to the receiver and there will not be a clear data set.

GPR manufacturers are constantly upgrading the performance of their antennas and software. Unfortunately, I do not see any solution to the GPR soil condition problem in the near future. But with adjustments and changes in filters and data collection methods, a skilled GPR technician can extract some information out of even a poor soil condition. (Mala USA manufactures X3M and Easy Locator GPR equipment for locating underground utilities.)

Sensors & Software, Greg Johnston, applications geophysicist: Conceptually, good analogies for GPR are fish finders or ultrasound medical imaging (although they both use sound waves rather than radio waves). A GPR locator uses a transmitter to send radio frequency electromagnetic waves into the subsurface and a receiver to collect the signals that reflect back to the surface from objects or materials with contrasting electrical properties. Data is typically recorded at multiple positions along a survey line on the surface.

The big advantage of GPR for the construction industry is the ability to detect nonmetallic objects like plastic and concrete pipes. GPR is nondestructive, easily deployed and provides fast, real time, interpretable images of the subsurface. Further, since GPR provides a complete image of the subsurface, it can also detect unanticipated objects like old foundations or changes in the soil or rock structures that might impact construction plans.

GPR is most effective in materials with low electrical conductivity, generally but not always coarse grained soils like sands and gravels, hard rock (granite, marble, basalt, limestone, salt), concrete, asphalt, fresh water, snow, and ice.

The biggest limitation, by far, is that GPR penetration into the subsurface is limited in materials with high electrical conductivity. For example, clay soils can reduce penetration down to just a few feet. Another limitation is data acquisition time. Collecting individual survey lines is very fast but detailed grid collection over large areas with a single GPR system can take many hours to days. This means that large grids with tight line spacing, the type of data set that greatly assists in the interpretation and understanding of the subsurface are rarely done, except by academics who have the time.

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