Gravity Integration
Many surveys, particularly the older ones, existed only as contour maps or sometimes
paper listings, which first had to be brought into digital form, before they could be
integrated into a coherent database. Specialist workstation based head-up and digiti sing
tablet programs have been written to allow rapid capture and checking of map based data.
All data arriving in digital or analogue form were first validated for erroneous points
and internal inconsistencies. Both visualisation and statistical software systems have
been developed for this purpose, each linking displays of location and data values with
edit windows. A profile based system (MARV ALID) allowing inspection of adjacent
traces of gravity, bathymetry and implied Eotvos correction, is particularly useful for
marine data. A full 3D visualisation system (DVS) allowing rotation and interaction with
the data values, is found appropriate for the generally more randomly distributed land
data. Interactive visualisation of line crossovers (COE) allows data validation as well as
adjustment of marine line based data.
Surveys also need to be checked and adjusted for external consistency. In many
(especially commercial) surveys little interest was taken in absolute gravity values and
large offsets (some greater than 200mGal) exist between adjacent sUlveys. If sufficient
information is available to re-Iocate the survey base stations, then a new gravity tie can be
made to adjust the data. This was only found to be feasible and necessary in a few cases.
Otherwise visualisation and statistical methods, using comparison with adjacent or
overlapping surveys, can be used to estimate what shift is required to ensure consistency.
This is particularly problematic when considering the relationship between onshore and
offshore surveys, but sufficient coastal surveys exist to fix the necessary bulk adjustment
of the two data sets. These shifts were confirmed by comparison with the satellite derived
gravity for marine areas.
Once all the data have been validated and adjusted they can be gridded to form a regular
grid for each area. Marine data have been replaced in some parts with satellite derived
data depending on the coverage of the shipborne data. The DNAG grid for North
America has been included along with the GETECH project grids to give a virtually
global gravity coverage (figure 2).
TERRAIN MODEL
Terrain data are required in the process of gravity compilation for two main purposes.
1. Estimation of heights where no heights have been recovered from the original data.
This is a common problem where heights were originally acquired and used in the
processing, but not archived (40 per cent of land stations for SAGP). These heights
are required in order to convert to a consistent Bouguer reduction density.
230
Many surveys, particularly the older ones, existed only as contour maps or sometimes
paper listings, which first had to be brought into digital form, before they could be
integrated into a coherent database. Specialist workstation based head-up and digiti sing
tablet programs have been written to allow rapid capture and checking of map based data.
All data arriving in digital or analogue form were first validated for erroneous points
and internal inconsistencies. Both visualisation and statistical software systems have
been developed for this purpose, each linking displays of location and data values with
edit windows. A profile based system (MARV ALID) allowing inspection of adjacent
traces of gravity, bathymetry and implied Eotvos correction, is particularly useful for
marine data. A full 3D visualisation system (DVS) allowing rotation and interaction with
the data values, is found appropriate for the generally more randomly distributed land
data. Interactive visualisation of line crossovers (COE) allows data validation as well as
adjustment of marine line based data.
Surveys also need to be checked and adjusted for external consistency. In many
(especially commercial) surveys little interest was taken in absolute gravity values and
large offsets (some greater than 200mGal) exist between adjacent sUlveys. If sufficient
information is available to re-Iocate the survey base stations, then a new gravity tie can be
made to adjust the data. This was only found to be feasible and necessary in a few cases.
Otherwise visualisation and statistical methods, using comparison with adjacent or
overlapping surveys, can be used to estimate what shift is required to ensure consistency.
This is particularly problematic when considering the relationship between onshore and
offshore surveys, but sufficient coastal surveys exist to fix the necessary bulk adjustment
of the two data sets. These shifts were confirmed by comparison with the satellite derived
gravity for marine areas.
Once all the data have been validated and adjusted they can be gridded to form a regular
grid for each area. Marine data have been replaced in some parts with satellite derived
data depending on the coverage of the shipborne data. The DNAG grid for North
America has been included along with the GETECH project grids to give a virtually
global gravity coverage (figure 2).
TERRAIN MODEL
Terrain data are required in the process of gravity compilation for two main purposes.
1. Estimation of heights where no heights have been recovered from the original data.
This is a common problem where heights were originally acquired and used in the
processing, but not archived (40 per cent of land stations for SAGP). These heights
are required in order to convert to a consistent Bouguer reduction density.
230