Automatic Velocity Calibration
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
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.”
On 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.
Simultaneous Transmission on all channels without cross channel interference is a particular feature of our Groundvue range of GPR systems and not generally available on other systems.
Analysis of AVC Image (left)
The attached image (left) has been produced using a specific “AVC” feature provided by Sandmeier within their ReflexW post processing package. https://www.sandmeier-geo.de/reflexw.html
Start with the trace in the middle. This is one of the AVC output channels and 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.