Very Low Frequency Complex Dielectric Constant Measurement of Seawater for Underwater Electromagnetic Applications

A navigation system using low-frequency electromagnetic waves is being developed for the operation of robots under sea ice in the polar regions. The complex permittivity of seawater and sea ice is necessary to model the propagation characteristics of electromagnetic waves in the three layers: sea, sea ice and air. However, in the low frequency range, an electric double layer forms on the surface of the electrodes used for measurement and most of the capacitance to be measured is an electric double layer capacitor. To remove this capacitance and measure the complex permittivity of seawater itself, a method using the difference of impedances measured at two different electrode spacings is proposed and the measurement results are discussed. At present there are some cases where the difference can give a relative permittivity close to the theoretical value, but in most cases there is a large error in the measured value. In this paper the error factors are discussed and a modified proposed method to reduce the measurement error is reported.