OR/13/003 The merged conductivity database

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Beamish, D. 2013. The construction of a merged EM (conductivity) database using Tellus and Tellus Border airborne geophysical data. (Land Use, Planning and Development Programme). British Geological Survey Internal Report, OR/13/003.

The procedures discussed above resulted in LF and HF merged NN grids obtained using a 50 m cell size. This cell size is larger than the along-line distance of the original data but has resulted in a uniform sampling of all the LF and HF data. The inversion results comprising the L1 misfit error (ERR), and the thickness parameter (THK) are also available at the original sampling interval. A master EM sampling database was constructed using the original contractor supplied EM data files. The X (easting) and Y (northing) coordinates from each survey in the Irish national Grid were used. WGS84 Latitude (LAT) and Longitude (LONG) were then obtained by transformation of the X, Y coordinates.

The data base was constructed to avoid any overlaps between the surveys. Survey lines therefore ‘abut’ at the boundaries between surveys. As noted previously, the master database contains line deviations and line overlaps that were contained within the individual surveys. The master database carried forward the DATE (YYYYMMDD), the radar altimeter (RALT) and the GPS height above geoid (WGS84) of each measurement. An additional data channel (Survey Identifier, SID) was introduced to define the survey (TEL=Tellus, CAV=Cavan and TB=Tellus Border). Line numbers from the original surveys were retained but with a prefix of B (for the Tellus Border survey), D (for the Cavan survey) and L (for the Tellus survey). The control master database constructed here provides 14 765 595 data points from 138.5k line-km of data (including the OST survey).

The master database was then populated with the LF and HF estimates of apparent conductivity from the final merged 50 m, NN grids using a grid sampling procedure. The ERR and THK inversion parameters were sampled, at their original sampling interval, into the master database. As noted previously the ERR term is restricted to a maximum value of 500%. The HF ERR is found to have a higher central moment than the LF ERR when examined across the whole data set. The thickness parameter is limited to the range 0.1< THK <100.0. Both ERR and THK parameters are unlevelled.

The merging procedure produced a small number of spurious negative values into the values of apparent conductivity (-29.9 mS/m at LF and -200 mS/m at HF). Both data sets extend beyond the threshold maximum value of 1000 mS/m, again due to the merging procedure. When use is made of the merged apparent conductivity information it is recommended that that estimates of apparent conductivity when RALT >175 m be either omitted (see Figure 25) or treated with caution due to the procedures employed here. Finally the variable sampling interval contained in the master database (see Table 1) should be noted since it is not explicit in the information supplied.

Figure 25    LF merged conductivity grid imaged using a linear colour scale (2 to 50 mS/m) with condition RALT>175 m defined by white zones and all data cut-to coast. Black area denotes OST survey.