How Long Does It Take for a Ground Penetrating Radar (Gpr) To Scan Ground?

Scan Plus Tech

Ground Penetrating Radar (GPR) is a non-destructive geophysical technique based on the propagation and reflection of electromagnetic waves from 20 MHz to 3 GHz. It is sensitive to changes in the electromagnetic properties of the medium (permittivity, conductivity and magnetic susceptibility). Investigations are usually carried out by moving the antenna on the surface, with or without contact with the ground. Measurements are taken at regular intervals, which allows a quick image of the basement structure.

The GPR records the arrival times of the different waves and adds them as it progresses along the object studied. This sum of measures forms a radargram which is a graphical representation of the structure and its components.

In a final step, the recorded arrival times are converted into distances using the different wave propagation speeds. These propagation speeds are calibrated locally using boreholes.

The ground penetrating radar technique, originally created in the 1960s, is now used in geology (detection of bedrock, specific geological formations, fractures, karstic phenomena, etc.), archaeology (mapping of buried sites), hydrogeology (depth of groundwater, detection of contaminated areas) and civil engineering (inspection of concrete structures, roads, railways and other buried structures). As far as road testing is concerned, GPR has a proven track record for assessing the thickness of the different layers or the quality of the coating (presence of voids, etc.). This technique also makes it possible to visualize the variations in the road structure (determination of homogeneous zones) and thus guides the location of core boreholes or reconnaissance trenches.

In combination with deflection and core measurements, it improves the evaluation of the modules of the different layers by inverse calculations thanks to a better estimation of their thickness and spatial distribution.

A good command of radar technology is important for successful measurement campaigns. Thus, a judicious choice of the type of radar and antenna (including the frequency range or central frequency) can maximize the quality of the data obtained. Similarly, the choice of measurement parameters, then data processing and interpretation, are all factors that require some user experience.

Functioning

Most ground penetrating radar operate in the time domain, emitting very short electromagnetic pulses (in the order of a few nanoseconds) and recording the reflected signal as a function of time (Figure 2). The corresponding signal has a wide frequency band in the frequency range. The center frequency is the one for which the amplitude is highest. Other types of radar (step frequency) emit continuous sine waves for different frequencies and then possibly reconstruct the signal in the time domain using a reverse Fourier transformation.

The Radars

Materials based on the reflection of electromagnetic waves at the interface of materials of different natures allow a bi-directional location with direct graphical representation in an X-Z plane for a given Y slice. They do not determine the diameter of the armatures. Depending on the type and the center frequency of the antenna used, the water state and the diameter of the internal reinforcements sought in the concrete, the detection depth can reach about 600 mm or even 900 mm if «low frequency» antennas are used and the absence of a bed of reinforcements between the facing and the reinforcements sought.

As the center frequency of the antenna decreases, the depth of investigation increases, but the vertical resolution (minimum distance to detect two parallel “interfaces” without intertwining radar signals) also increases (and vice versa). For example, 12 mm diameter armatures can be detected by a 900 MHz antenna, of central frequency, but in the case of a bed close to the facing, the characteristic signal of the armatures will be “scrambled” in the surface echo, while antennas of “high frequencies” will provide a much better Z separation.

The estimation of depths requires knowing the electromagnetic properties (mainly electrical conductivity and dielectric permeability) of the concrete material concerned by the expertise. To do this, a calibration of the dielectric constant (and therefore the speed of the radar waves) is necessary. For this, it is not recommended to use a predefined value of the speed of the radar waves in the concrete to interpret the radargrams. Two calibration methods are generally used:

  • For a structural element of known thickness, the propagation time of the radar waves allows it to increase at an average speed. It is strongly recommended to find an element whose thickness can be measured directly. Failing this, the drawings provided by the project manager should be used. In case of doubt, calibration on cores taken from the structure allows to improve the precision on the depth.
  • Post-treatment calibration of the radar signal allows an estimate of the average speed of the radar waves between the surface and the detected reinforcements, provided that these reinforcements can be considered as “point objects” (in the case of isolated frames when the movement of the radar antenna is perpendicular to these frames). It should be noted that internal water gradients significantly alter the speed of radar waves, and this speed can vary between skin concrete and deeper concrete. It is therefore necessary to settle on “point objects” located at approximately the same depth as the objects sought (in the case of reinforcement detection, we generally settle on the objects sought, so in practice this does not pose a problem).

Radar imaging provides much more information than the pachometric method, and over a greater depth (50 to 60 cm or more depending on the antennas used).

Indeed, it is possible to measure thicknesses, locate sheaths, cables, voids, detachments, heterogeneities, etc.

So if direct results are desired through the real-time display of a diagram of the structures without having to interpret the data, a GPR is needed to scan concrete! This allows real-time localization of targets and X-ray imaging, with scans that display high-resolution images in 2D and 3D for better visualization of structures. The antenna allows inspection of complex areas with vertical and horizontal resolution. The device is equipped with a laser to be able to locate and mark targets with a backup cursor.