Underwater electromagnetic communications using conduction: channel characterization

This paper explores the properties of short-range broadband wireless communications for underwater operations using electric conduction. Electric field in the water is generated by a pair of electrodes with opposite current and detected by two receiving electrodes. Ranges of operation can be shorter than 1 m, suitable for contactless data collection by remotely operated vehicles (ROVs), and even as short as 1-10 cm, suitable for contactless riser health monitoring for deep sea drilling sensors. Experiments were conducted at frequencies between 100 kHz and 6.5 MHz, using orthogonal frequency division multiplexing (OFDM). Our lab tests were performed in a plastic tank filled with salt water, and our sea test at the ocean surface and 5 m depth (boundary free). Magnitude and phase-delay of the channel transfer function were modeled based on inference from dipole radiation theory in a conducting medium. An exponential attenuation model fitted to the lab measurements indicated inverse cubic range dependence (near-field compliant). A rational-polynomial model provided the best match for the recorded magnitude, especially at low frequencies. The phase characteristic obtained from the ocean measurements exhibited a minimum around 2 MHz, which agrees with theory.