This project concerns the propagation of electromagnetic (EM) waves through sea water (clear, murky and polluted) for frequencies in the range lOMHz to 30GHz. The propagation characteristics depend upon a combination of the conductivity (permutation) and dielectric (epsilon) properties of the various conditions of sea water.
Theoretically the spatial and temporal variations of the electric (E) and magnetic (H) fields are obtained from Maxwell's equations with the dielectric properties being given by Debye's equations. Experimentally these solutions can be examined in the laboratory using metal resonance cavities filled with simulated sea water with various quantities of included sand. Initially measurements of the attenuation coefficient (alpha) and the velocity (v) of the E (or H) field waves may be obtained from which permutation and epsilon(=epsilonI+jepsilonII) will be obtained as a function of frequency. If the cavity is unlined then usually the conductivity dominates in this frequency range, however, if the cavity is lined, with an insulator, then the dielectric term dominates. Precision measurements may be obtained by using state of the art' spectrum analysers and data logging systems. Measurements obtained using these techniques will allow accurate measurements of both permutation and epsilon to be obtained for comparison with a relaxation theory. The cavity technique can also be readily used in either a dock or coastal water environment. Hence the variations of permutation and epsilon with frequency may be compared for conditions of clear, murky and polluted sea water. The results of this investigation will be aimed at two important offshore applications. The first application is as a pollution monitor. The cavity technique will allow permutation, epsilonI and epsilonII to be measured. Increased sea water conductivity should effect permutation, epsilonI and epsilon II whilst increased turbidity should mainly effect epsilonII. The cavity can be miniaturised and used with custom electronics to enable it to be carried by an ROV or placed on a buoy. The second application is to allow through water propagation between a transmitting aerial and a receiving aerial for a distance of the order of SOm and a propagation frequency of about lOOMHz. This will confirm previous but scarce results, obtained about two decades ago, which showed that such transmission was possible. This new investigation will benefit from the modern equipment now available from satellite and mobile radio research which allow signal attenuation up to -200dB to be successfully recover and interpreted.
Such communications systems will have a major impact on subsea sensors and will have particular applications for positioning and navigation of ROV's within structures and also providing communications between the base station (buoy or benthic station) and the ROV. The EM wave bandwidth will be sufficient to transmit real time video images from the on-board ROV cameras to the base station. The project will be undertaken by four partners namely two Universities (Liverpool and Gent) and two SME's (Comex Technologies and Stenmar). This partnership has the necessary blend of basic experimental and theoretical skills as well as the required commercial evaluation and exploitation.
Funding SchemeCSC - Cost-sharing contracts
AB23 8GD Aberdeen