Unique GPR based tools

Unique GPR based tools

*NEW FEATURE*  Focussing on Automatic Velocity Calibration & how it can help detect problems in roads, voids and subsurface water.  "AVC is by far the most promising method of reducing the coring requirement for calibration of GPR derived interface depth data on pavements".  Read our client's comments in full.

Also read about dedicated tools for runways (detection of subsurface cracks) and highways (crack depth determination)  - unique to us.  

Automatic Velocity Calibration (AVC)

Radars measure accurately in nanoseconds time.  To turn this into traditional measurements in metres (or feet), it is necessary to calibrate the transmission velocity.  Traditionally this is done by taking cores at certain points along the path of the radar.  It can be done much more QUICKLY, ACCURATELY & CONTINUOUSLY.

Instead of the more usual 1 Transmitter to 1 Receiver pairing, 1 Transmitter is connected to 2, 3 or 4 Receivers, placed at known distances from the transmitter.  It is important for all of the antennas to be triggered simultaneously.  There will be a slight delay between the same signal reaching each of the receivers.  Because the transmission was identical on all channels and the positions of the receivers are known, the GPR data can be used to resolve the transmission speed of the electromagnetic pulses..........continuously.  So, instead of having spot calibration, continuous calibration data is generated.  

This gives continuous and accurate depth measurements.  It also highlights the anomalous areas - typically where water or voids  are present in the subsurface since these have a major impact on the transmission velocity.  AVC is a very useful technique not only for monitoring flexible pavement but also for detecting problems developing in the subsurface as a result of water flow such as sinkholes. 

Client comments: 1.  On Pavement profiling (road inspection) "AVC is by far the most promising method of reducing the coring requirement for calibration of GPR derived depth data on pavements.  GPR itself is a traffic speed technique but whilst it continues to rely so heavily on cores for calibration it is often not regarded as such because coring requires lanes to be closed which is very expensive and disruptive.  Although AVC is unlikely to replace coring completely, it should mean less cores per km of collected data are required as it allows core based velocity calibration to be thought about from a different perspective.  Traditionally velocity calculation is performed at core positions and linearly interpolated between them.  This low resolution velocity profile forms the basis of the interface depth calculation meaning that accuracy and measurement confidence is reduced with increasing distance from a core position.  With AVC the velocity profile for the entire pavement can be calculated before a core is drilled, all that is then required is a core to confirm the velocity at any given point.  This means greater confidence can be attributed to interface depths between core calibration points and the space between cores can be increased."

2.  Anomalous Material Detection. "AVC also allows material moisture content to be assessed more effectively as increasing the moisture content of an otherwise homogeneous pavement material will lower its velocity, this allows greater confidence in the detection and identifying the cause of defects associated with the presence of unwanted water".

Note that Simultaneous Transmission on all channels without cross channel interference is a feature of the GroundVue GPRs, not available anywhere else.

The AVC Image Above - Analysis

Start with the trace in the middle.  This is one of the AVC output channels.  It shows that the radar has travelled along an asphalt surfaced road then on to a concrete one (just before the 500m mark).  The GPR data shows what appears to be a change in depth.  However we already know that the transmission velocity through asphalt >the velocity through concrete.  The data processor picks the main interface from the bottom of the asphalt/concrete for each Receiver's data and, courtesy of the ReflexW software package, an automatic plot of velocity is produced (top trace in blue).  Already you can see some of the anomalous areas.

This velocity is then applied to the original data to give a calibrated depth (lower plot in green).  It is now obvious that the road depth does not vary significantly from the asphalt section to the concrete section.  All of the anomalous areas, potentially requiring investigation, are very clearly indicated. 

Latest News on the Processing is that ReflexW no longer requires the data processor to pick (or trace) the output from all receivers.  Only 1 set of data requires to be analysed and the package will automatically process the remaining data in the same way.  This will be available on the next upgrade of ReflexW (but is available now through Utsi Electronics Ltd).

If you have questions on AVC or you would like to obtain an early release copy of the ReflexW update or you would like a demonstration, please contact us or our agents (see Contacts page).    

And here are some more clever features of the GroundVue systems that you may not know about....................

Crack Depth Determination

GPR is commonly used to determine areas of cracking e.g. in historical buildings, particularly where these are located in earthquake zones.  Traditional antennas are used for this application.  At the request of the UK Transport Research Laboratory, UtEl has developed specialised crack detection measurement tools.  Rather than simply detecting the existence of cracking, these are optimised to measure the depth of cracking.  This was originally used for top down cracking in flexible pavement (Highways) where the depth of the cracks rather than their extent on the surface determines the need for repair.

The image below illustrates the difference between using the crack detection antenna (cdh) and a traditional antenna to determine crack depth.  The cdh reliably locates the crack and indicates both the top and the bottom.  Comparison of the depths will give the depth to which top down cracking has permeated.

The image below is from a paper presented to GPR 2004 showing the comparison of the results of coring and using the crack depth detection head.

 

 There are a number of publications showing the performance of the crack detector head.  Please contact us if you would like more information. 

Airports


For the repair and maintenance of airport runways and taxi areas, structural engineers need to know where subsurface cracking has developed below the tarmac surface.  Top down cracking is visible on inspection but traditionally coring has been used to determine how extensive is the bottom up cracking i.e. the cracks which start at joints in the concrete and permeate up into the asphalt above.  The cdh is a much more reliable tool for locating the joints in the concrete than a traditional antenna and it can also detect cracks which have developed at the joint but not yet reached the surface of the asphalt.  "Traffic Light" reporting can then be used to indicate joints where cracking has not developed (green) or where there are subsurface cracks (red) or other anomalous indications (amber).  An example of a good joint and one which has developed a crack into the asphalt above is shown below (with thanks to Atlas Geophyiscal for permission to use this data).