Synthetic Radio Frequency Identification Tracer Stones with Weighted Inner Ball for Burial Depth Estimation

Radio frequency identification (RFID) tracer stones are an increasingly popular tool in sediment transport research. A key limitation of RFID tracers is the sensitivity of their detection zone shape to the orientation of the transponder. The variability in detection zone shapes reduces detection rates by obscuring tracer positions and creating complex interference effects when multiple tracer stones are within the antenna detection field. This paper presents two designs and prototypes of the "Wobblestone," a synthetic tracer stone that ensures the transponder remains vertical regardless of the stone orientation. The designs feature a weighted inner ball containing the transponder that is free to rotate under gravity and correct to a vertical orientation. The prototypes were fabricated using a high-density polyurethane resin and corundum powder mixture. The prototypes achieved densities within the range of natural stone and confirmed the durability of the material. Performance tests verified the vertical orientation of the transponder regardless of the stone orientation and demonstrated an opportunity for the determination of tracer burial depth. The new tracers will reduce signal interference from closely spaced tracers by simplifying detection zone shapes and will allow for information on the vertical position of buried particles to be obtained.