OR/14/014 Data characteristics
|Beamish D and White J. 2014. TellusSW: airborne geophysical data and processing report . British Geological Survey Internal Report, OR/14/014.|
All data were supplied in British National Grid (BNG) coordinates. The data coverage of the complete line-based data set is summarised in Figure 2.
It can be seen that a set of offshore E-W tie-lines in the south of the survey area were included in the delivery. Additionally a survey line ‘gap’ exists on the south coast, to the west of Plymouth. This zone of omitted data was approved in the winter of 2013 during a prolonged period of poor weather. The poor weather combined with both onshore and offshore military restriction zones to effectively halt progress across this area. Permissions had been obtained to perform surveying across the controlled airspace above the Devonport naval dockyard however the survey would have been restricted to elevations of 2000 feet and above. This would have entailed long climbs and descents in a N-S direction and provided data of limited resolution.
Due to the poor weather experienced from mid-October onwards, the contractor undertook increasing amounts of partial-line acquisition. During final delivery, repeated requests to remove partial line overlaps in the data set were only partially successful. As a consequence the delivered data set contains a series of localised overlapping line sections.
Although the line-separation quality control was largely adhered to there are a small number (3) of tall communication-mast ‘avoids’ within the data set.
The CAA permit for the survey ruled that the aircraft should survey ‘at a height of less than 800 feet Minimum Safe Distance (MSD) over congested areas and not less than 260 feet MSD over all other areas’. The two heights referred to are 244 and 79 m respectively. The contractor developed a conventional drape plan, based on topography and aircraft performance. We requested ‘over-sea’ modifications to this drape plan to avoid climbs and descents over areas with major cliff zones (e.g. along the northern coast of the survey area). The contractor was not able to carry out these modifications due to inflexible software. The CAA requirements then formed visual ‘in-flight’ modifications to the drape plan.
The final delivered survey elevations range from 10 to 651 m and the pattern of variation is formed by a spatial convolution of the drape plan and locations of conurbations/structures across the survey area. Survey elevations above ground are shown in Figure 4 using a linear colour scale from 80 to 200 m.
The mean flying height for the TellusSW survey was a somewhat disappointing 91.6 m. Banded zones, presumably due to differing piloting abilities, can be observed in Figure 4.
The contractor delivered data are fully described in the Appendix. The delivered data contain 17 276 495 data points (MAGNETIC) and 855 362 data points (RADIOMETRIC). Some of the data channels contain DUMMY values (as a result of processing) and the available valid data sets effectively contain 17 140 169 data points (MAGNETIC) and 853 322 data points (RADIOMETRIC).
The line-km values within the database are 61 564 801 (MAGNETIC) and 61 896 591 line-km of RADIOMETRIC data. Data assessments shown in Table 1 contain both survey and tie-lines.
|BGS determined data characteristics|
|speed||45.98||92.84||71.21 (m/s)||256.35 (km/hr)|
|Horizontal Gradient MAG||-18.04||39.98||-0.00003||nT/m|
|IGRFCorr_MAG (4th DIFFERENCE, normalised)*||-3.7966||4.4311||0.0000|
|Basemag (4th DIFFERENCE, normalised)||-0.0003||0.0003||0.0000|
|Basemag (5nT / 2min)||0.0||2.62||0.20||nT (over 2 min)|
*There are 25 instances of the IGRFCorr_MAG_4th DIFFERENCE (normalised) having values greater than 0.5. There are 72 instances of the IGRFCorr_MAG_4th DIFFERENCE (normalised) having values greater than 0.1.
**Negative values in ground concentrations arise due to statistical uncertainty in the measurement when very low signal levels are encountered.