Revising gravity terrain corrections in Tasmania

Abstract

SUMMARYTerrain corrections for determination of the complete Bouguer anomaly are empirically evaluated with respect to a number of different techniques, parameters and digital terrain model data sets, for areas in western and northern Tasmania.For the most part, while terrain corrections calculated from very high resolution terrain models (1.2 metres or better) are presumed to deliver the most accurate results, those computed for the same area using only a Statewide 25 metre-cell digital terrain model to within two metres of gravity stations correspond remarkably well. Internally consistent comprehensive terrain correction of acceptable yet maximal accuracy can therefore be calculated for all Tasmanian gravity stations, even if very high resolution DTMs are unavailable.Fully automatic terrain correction computation from two metres to 167 kilometres from gravity stations will result in significantly improved removal of topographic effects over extant manual corrections, which were limited to 22 kilometres.

Terrain correction correction Tasmania - results and implications

“…In rare cases the automatically calculated terrain correction value is less than the original, but generally it is greater, by an average of just under 2 mGal. Much of this can be ascribed to extension of the calculation radius from 22 to 167 km (Duffett 2016). The effect of the greater radius of calculation increases with station height, and this is borne out by comparison of Figures 1 and 4.…”

“…As discussed by Duffett (2016), the method adopted in calculating terrain corrections for all Tasmanian onshore gravity stations is that of Cogbill (1990). Three DEMs of varying resolution were used: The 20 metre DEM described above for distances between 2 metres and 4 km of the station, the ~222 metre DEM combining onshore and offshore topography (Whiteway, 2009) The resulting new complete Bouguer gravity anomaly for Tasmania is shown in Figure 3, with the difference between this and the older version with more restricted TC in Figure 4.…”

“…This was determined primarily by 10 metre contours, originally obtained photogrammetrically. Though generally adequate for TC calculation (Duffett, 2016), remaining shortcomings from the gravity terrain correction perspective are lack of information in lake areas, and a tendency for ridges to present as plateaus. Lakes including hydroelectric impoundments occupy a significant fraction of Tasmania, some in regions of high mineral prospectivity and gravity station density.…”

“…The earlier work (Duffett, 2016) utilised the Statewide digital elevation model produced by the Tasmanian Department of Primary Industries, Parks, Water and Environment (DPIPWE) in 2007. This was determined primarily by 10 metre contours, originally obtained photogrammetrically.…”

“…Availability of high resolution elevation models, coupled with high computing capacity, allows revision of these. An earlier pilot project on portions of western Tasmania (Duffett, 2016) indicated that the older TC values may be substantial underestimates. In some areas, automatic corrections (to the standard 167 km radius) exceed the former by > 5 mGal.…”