Annual Report for 2010/11 and Forward Programme - Sellafield Ltd

Figures 4 to 6 show the same data within the Fledermaus package and show the extent ofthe bathymetry survey, the Sellafield pipeline and the two Navigation Markers buoys (redcubes). Note a vertical exaggeration of only 6 times has been applied to the data. This 3Drepresentation of bathymetry data can be viewed in the free iView4D viewer available fromwww.IVS3d.com.Investigation of the bathymetry at a scale of 0 to 10m, which is required to observe,characterise and map seabed bedforms, showed that a number of artefacts were visible inthe data (Figures 4 – 6). These of themselves would not prevent larger-scale features beinganalysed but were of similar amplitude to the smaller-scale features indicative of sedimenttransport. The likely causes of these artefacts include:1) Incomplete coverage2) Incorrect speed of sound correction applied3) Loss of bottom track4) Either poor weather conditions (large waves) and/or an incorrect calibration of theMotion Reference Unit (MRU) creating an artefact of typically 30 cm in the elevationof the seabed.5) Excessive lateral or vertical movement of the vessel, enhanced if the vessel deviatesfrom a regular sampling gridThe possibility of reprocessing the swath bathymetry to remove these artefacts wasinvestigated, but the original raw datasets produced by the Environment Agency were nolonger in existence and thus reprocessing was not possible.In summary, due to the presence of artefacts larger than the bedforms this project set out tostudy, the available swath bathymetry did not allow us to meet the original aims of objective1. The quality is insufficient to observe, delineate or characterise sediment transportfeatures, such as sand waves and ripples, on horizontal scales of 1-10m. No further workwas undertaken using the swath bathymetry data.This conclusion was relayed to WSCL and SL when discovered. Further communication withthe EA Geomatics Unit revealed that they were using ‘a winter survey vessel in far from idealconditions with the MRU working to its limit’. Best practice was applied to the dataprocessing following the advice from a manufacturer's consultant (SEA). The data aretherefore believed to be as good as it was possible to achieve and no further processing ispossible to improve the data quality. The EA also point out that the data only have a 2mhorizontal and +/- 25cm vertical accuracy. The EA therefore suggested ‘it was alwaysunlikely that [we] were going to be able to pick out more subtle seabed features’.