Small Radio Frequency Identification (RFID) tags are embedded within native pebbles collected from a site of interest and secured with a waterproof resin.
Schematic picture of a tag inside a tracer pebble
The pebbles are then deployed and re-surveyed at set intervals using a specialised scanner and a Global Positioning System (GPS). The RFID tags are passive, meaning that they have no internal power source and can remain in situ for long periods. The tag only broadcasts a unique identification number when detected with the scanner, which can be logged together with the GPS to establish an accurate position.
One of our surveyors on the beach with the mobile RFID reader
Once the retrieval surveys have been carried out we then bring the results into Geographic Information Systems which allows us to analyse the movement of the tracer pebbles in more detail. Images such as the one shown below are produced to help us to visualise the discrete transport pathways after each survey.
Example image of pebbles found during a tracer survey carried out in Portsmouth in 2018.
The coloured dots indicate where pebbles have been found during different survey days