Design and Feasibility Assessment of a Passive Underwater Acoustic Tag with a Two-Dimensional Code

Conventional underwater optical tags have limited operating range and reliability, whereas active acoustic tags require onboard power and periodic maintenance, restricting long-term deployment. This study proposes a passive underwater acoustic tag that encodes information in a two-dimensional array of strongly scattering corner reflectors and weakly scattering, sound-absorbing coating elements. A coupled acoustic–structure frequency-response model was established to quantify the scattering and absorption characteristics of the two element types. A multifrequency imaging and decoding method was then developed by combining near-field delay-and-sum (DAS) beamforming with an adaptive threshold derived from the prescribed number of black elements. Coupled acoustic–structure finite-element simulations were performed for five representative 4 × 4 patterns over 65–70 kHz. Under ideal free-field conditions, the two element types remained acoustically distinguishable throughout the simulated band, and the proposed method recovered all five prescribed patterns. These results demonstrate the feasibility of the concept under the modeled conditions. Because the tag contains no onboard power supply or integrated circuitry, it may provide a simple, low-maintenance platform for long-term underwater identification. Potential applications include equipment localization and traceability, subsea pipeline tagging, and robotic inspection and navigation.