GPR – Subsurface Scanning and Imaging

Ground Penetrating Radar (GPR) was primarily developed and used in geophysics and geology for subsurface scanning and imaging. Since then, the method has been successfully adapted in many other disciplines including civil engineering. The multipurpose nature of GPR and its broad range of applications, make it distinct from existing NDT methods. Got good fortune in civil engineering market, GPR can detect subsurface events and objects using the electromagnetic radar impulse, ranging from 10 MHz to 3,000 MHz. The method has been standardized by ASTM D6432

Application of non-destructive testing (NDT) methods are growing in condition assessment of civil structures. NDT methods are commonly used to evaluate the strength of materials (e.g. strength of concrete, consolidation of soil), to detect/localize embedded objects (e.g. pipe and tube, cables, manhole tunnel), to detect symptoms and side effects of damage mechanisms (e.g. delamination, internal cracking), to map the bed rock or boundaries in multi-layer media (e.g. asphalt/concrete/soil), and to detect the thickness or length of a medium (e.g. pile length, tunnel lining).

How GPR works?

GPR consists of a transmitter antenna and a receiver antenna, and a signal processing unit. GPR emits electromagnetic pulses (radar pulses) with specific central frequency to scan the subsurface medium. The reflected waves from subsurface layers, and objects are captured by the receiver antenna. Depending on impulse frequency, GPR is able to detect the internal events and objects at different depths. When there is a need for high resolution scanning, antenna with high frequency is required; however, the depth of penetration will be limited to the very first few centimeters. Lower frequencies are required when the objective is to detect very deep subsurface events and objects.

Applications of GPR

GPR is used in a wide range of civil engineering projects. The versatility of this NDT method makes it quite unique among other NDT methods. GPR can easily be used in subsurface scanning on soil, bridge decks, or simply locating rebar, live conduits, or other utility ducts.  The following briefly presents the most important applications of GPR in civil engineering area:

Concrete Scanning

• Reinforcement (steel rebar, prestressing tendons) locations
• Concrete component thickness
• Location of delamination, voids, and other hidden defects

GPR for concrete rebar imaging

Bridge Inspection

• Mapping subsurface delamination in bridge deck
• Reinforcement (steel rebar, prestressing tendons) locations

GPR in bridge inspection

Subsurface inspections | Locating utility ducts

• Location of embedded objects such as cables, tanks, pipes and tubes
• Location of support anchors for stone façades

GPR for subsurface scanning

Limitations of GPR

Despite this wide range of applications, there are some limitations associated with GPR. These limitations are related to the nature of electromagnetic impulses that are used for inspection purposes. High conductive environments (such as moisture environments, materials contaminated with sea water, clay, metallic materials) are problematic for electromagnetic probe waves. The ions and bipolar components that exist in high conductive environments absorbs/disrupts the electromagnetic probe waves, and decreases the amplitude of reflection. Weakened reflections lead to lower resolution results.