Acquisition of Satellite Derived Bathymetry Data for Offshore Engineering Projects - Island Case Study

Abstract

Abstract Recent advances in satellite technology such as improved image resolution and use of multi-spectral bands have increased its potential as an alternative source of hydrographic data. Recent applications in the surveying industry have proved the usefulness of Satellite Derived Bathymetry (SDB) as a low cost source of good quality bathymetry data. In order to support ADNOC's efforts to fast track new field development projects and timely delivery of high value data, ZADCO as part of its survey campaign selected SDB as a tool to source quality near shore hydrographic data. Offshore bathymetry derivation was realized using WorldView-3 satellite scenes. These scenes were processed by following a rigorous processing methodology for the correction of atmospheric adjacency. The processed and corrected satellite image provided a spatial resolution of nearly four metres and covered depths from the 0m depth contours up to the 15m depth contours. The accuracy and reliability of the satellite images are dependent on the optical conditions of the satellite scene, the sensitivity of the satellite's sensor and the accuracy of the calibration. After detailed multi-level quality checks for various systematic and non-systematic errors, the most reliable bathymetry that was found to meet ZADCO selection criteria was noted to be between 0m and 3m water depths, with an error of ±0.3m. Water depths between 3m and 5m presented themselves with an error of ±0.4m, whereas for depths between 5m and 10m, the errors varied between 0.5m to 1.0m. It was found that in the extreme nearshore areas with depths between 0m and 3m water depths, SDB could be accepted for use as it was falling within the tolerance limits of ±0.3m, with most areas having a tolerance of ±0.2m. While satellite derived bathymetry between the 0m to 5m water depths was found to be of acceptable quality, those over drying heights were found to be completely unreliable. The reliability factor that could be established for SDB data between 0m and 3m implied that dangerous and logistically challenging nearshore survey operations using conventional methods, that is, using shallow draught survey motor boats could be done away with. This also presented an interesting corollary in that it can also remove a huge HSE risk factor associated with shallow water survey boat operations, which are otherwise a significant risk due to the presence of underwater obstacles such as corals, rocks, reefs and surface dangers such as wave action and tidal influences. In conclusion, high resolution multi-spectral satellite imagery corrected for atmospheric and underwater refraction effects combined with the correct processing techniques and multi-level quality control mechanism provides a reliable source for bathymetry in extreme nearshore areas. Based on the results achieved it was concluded that SDB data can be used for engineering planning and estimation applications, for quick turnaround.