"Towards the surface, backscatter tends to be plentiful, providing sufficient reflections for the ADCP to operate at maximum efficiency; however the deeper you go in the ocean, the less backscattering occurs. This substantially limits the range of any current profiler, and effectively prevents it from collecting a 2000m profile. To overcome this, recent validation trials have proven that by mounting the ADCP on the sea floor facing upwards, it points into the area of the stronger backscatter, thus dramatically improving the range that would otherwise be obtained by a downward-facing instrument deployed at the same depth. Incorporating acoustic modems to transmit bi-directional data in real time has a further advantage in that it allows real time manipulation – and hence enhancement – of the ADCP’s performance. Armed with this information, Shell Global Solutions (US) and RD Instruments, working in co-operation with Floatation Technologies, Linkquest, and Evans-Hamilton, set about designing a downward/upward-looking system for offshore operations. A validation test for this configuration was performed in the Gulf of Mexico, where Shell Oil was drilling from the Transocean Sedco Forex Deepwater Nautilus semi-submersible in 1574m water depth. The rig was already fitted with an RDI 75kHz Vessel Mounted (VM) downward-looking ADCP mounted in the rig’s port pontoon some 9m below the surface. This current profiler provided real time data for the top 600 – 800m of water. An RDI Workhorse Long Ranger 75kHz ADCP was chosen as the bottom-mounted, upward-looking instrument. The main reason for the choice was that this instrument has the longest profiling range, up to 600m, of any self-contained current profiler available."
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