Identification of the Burial Depth of Radio Frequency Identification Transponders in Riverine Applications

Radio frequency identification (RFID) technology enables bidirectional, remote communication between a reader and a transponder (or tag) with a unique ID via an excitation antenna. This study provides a framework for estimating the burial depth of a transponder from a fixed location as a means of assessing scour depth or for other riverine applications. In doing so, a low frequency (LF), passive RFID system is used, which operates at a resonance frequency of 134.2kHz and can penetrate through saturated sediments. The objective of this research is the development and validation of a semitheoretical expression that relates the decay of the received input voltage (VIN) at the reader with the transponder burial depth or detection distance, d, through a series of well-controlled experiments. This study examines the correspondence between VIN and d in air, as well as in water, gravel, and sand, and assesses the role of transponder orientation (perpendicular and parallel with respect to the excitation antenna) on the received VIN. It is found that VIN exhibits a universal behavior as a function of d as it decays inversely with the cube of the distance (approximate to d-3) not only for air but also for water, gravel, and sand. More importantly, it is shown that the orientation angle between the excitation and the transponder miniature antennas as well as the sediment-bed composition/texture (e.g.,sand, clay, gravel) surrounding the transponder can have a significant effect on the magnitude of the VIN and on the decay rate of the VIN versus d curve. A change in the transponder orientation from perpendicular to parallel reduces the VIN by one order of magnitude and the maximum detection d by 26-76% for the different media. The greatest reduction in VIN occurs for sand, likely due to its smaller void ratio that causes excessive defragmentation of the RF waves. These promising findings allow a relatively accurate, remote estimation of d using the inexpensive LF, passive RFID technology and warrant future research and development on this topic.