Ground Penetrating Radar

How it works

Ground Penetrating Radar (GPR) uses a high-frequency (e.g. 40 to 1,500 MHz – microwave range) EM pulse transmitted from a radar antenna to probe the earth. The transmitted radar pulses are reflected from various interfaces within the ground, and this return is detected by the radar receiver. Reflecting interfaces may be soil horizons, the groundwater surface, moisture saturation, rock/boulder/soil interfaces, ground defect, manmade objects (embedded steel reinforcement in a concrete structure) or any other interface producing a noticeable contrast in dielectric properties (density, moisture content, etc..). The dielectric properties of materials correlate with many of the mechanical and geologic parameters of materials. The emitted radar signal beam is introduced to the ground by an antenna that is in close proximity to the ground (ground coupled method). The reflected signals can be detected by the transmitting (Tx) antenna or by a second, separate receiving antenna (Rx).

The received signals are processed and displayed in real-time on a graphic recorder. As the antenna (or antenna pair) is moved along the surface, the recorder system displays results in a 2D cross-section record or radargram of the earth. As GPR has short wavelengths in most earth materials (due to its high frequency domain), resolution is in direct proportional with the frequency. Higher frequency to be used provided high resolution but shallower penetration in earth materials or manmade objects. Therefore, the signal attenuation in earth materials is high, and depths of penetration seldom exceed 30ft in vertical. High conductivity materials such as type of Clays with a high cation exchange capacity increase the attenuation and decreasing penetration. In addition to that, the presence of solutes or other substances which increase the electrical conductance of groundwater and have the same attenuation and penetration results.

Figure 1 - Schematic illustration of common offset single-fold profiling.

The objective of GPR surveys is to map near-surface objects. For many as-built or concrete scanning surveys, the location of objects such as steel reinforcement, rebar, post-tension cable, electrical conduits in the concrete structure is the objective. Dielectric properties of materials are not measured directly. The method is most useful for detecting changes in the geometry of subsurface interfaces.

Subsurface problems conducive to solution by Mid Frequency 3D GPR methods are numerous and include the following: under pavement voids, sinkholes, pipes and tanks, location of the high moisture saturation in soil, locate groundwater surface, and others. However, as the results cannot be foreseen from the office, considerable latitude is given to the field geophysicist to incorporate changes in methods and techniques.

The physics of electromagnetic wave propagation are beyond the scope of this manual. However, there are two physical parameters of materials that are important in wave propagation at GPR frequencies. One property is conductivity (σ), the inverse of electrical resistivity (ρ). The relationships of earth material properties to conductivity, measured in mS/m (1/1,000 Ωm), are given in the section on electrical methods.

The other physical property of importance at GPR frequencies is the dielectric constant (ε), which is dimensionless. This property is related to how a material reacts to a steady-state electric field; that is, conditions where a potential difference exists but no charge is flowing. Such a condition exists between the plates of a charged capacitor. A vacuum has the lowest ε, and the performance of other materials is related to that of a vacuum. Materials made up of polar molecules, such as water, have a high ε. Physically, a great deal of the energy in an EM field is consumed in interaction with the molecules of water or other polarizable materials. Thus, waves propagating through such a material both go slower and are subject to more attenuation.

where

 V= velocity in m/s,

ε = dielectric constant (dimensionless),

a= attenuation in decibels/m (db/m),

σ = electrical conductivity in mS/m.

Table 1. Electromagnetic properties of earth materials

Wightman, W. E., Jalinoos, F., Sirles, P., and Hanna, K. (2003). "Application of Geophysical Methods to Highway Related Problems." Federal Highway Administration, Central Federal Lands Highway Division, Lakewood, CO, Publication No. FHWA-IF-04-021, September 2003.