Methods Used in a Mass-Balance Study of Holocene Sediment Accumulation on the ...
373
tests carried out on map sheet 2716 (Elsfleth) demonstrate that large deviations
occur only in the volumes of depth intervals below -10m NN. This effect can be
explained as follows. Only a small number of boreholes intersected the base of
the Holocene in an unusually deep position, i.e. below about -10 m NN; these
were interpreted by the author of the manually constructed structure contour
maps as belonging to a system of deep, narrow gullies. Any such interpretation
is disregarded by computerized grid calculations that are directly based on borehole data and which in consequence produce a series of isolated, deep, and relatively wide structural depressions ("bull's-eye" artifacts). However, the volume
of sediment in the depth interval -10 to -16 m NN is relatively unimportant as it
comprises less than 2% of the total Holocene volume below NN.
In order to use a standardised procedure and to monitor the results obtained,
it was decided to calculate the Holocene volume below NN on the basis of both
data sources, the manually constructed structural contour maps and directly
from borehole data. ISM proved to be a suitable computer program, because it
works on a VAX under the VMS operating system and allows the user to calculate
the grids and volumes in batch jobs. This is necessary because of the huge data
files. Furthermore, ISM is the only one of the programs used that allows blanking out of certain areas (e.g. Pleistocene hinterland). In the course of gridding,
this function replaces the node values by zero. A grid spacing of 40 m proved to
be appropriate for all computations. Because of the huge number of measurements in the case of the digitized contour lines, only one depth value was considered for each grid node. In contrast, grid calculations based only on borehole
data made use of eight depth values for each grid node.
At present the volume of the Holocene sediment wedge has been determined
for over 50% of the research area (total area 4700 km 2 ) in the coastal lowlands of
Lower Saxony. By the end of 1997, the calculations and interpretation of the results will have been completed
Acknowledgements
The authors would like to thank the Deutsche Forschungsgemeinschaft for their
financial support within the scope of the Priority Program Changes in the
Geo-Biosphere in the Course of the last 15 000 Years. We would also like to thank
the Niedersachsisches Landesverwaltungsamt - Abteilung Landesvermessung -,
the Wasser- und Schiffahrtsdirektion Nordwest in Aurich, the Bundesanstalt fUr
Wasserbau in Karlsruhe, and the Forschungsstelle Kiiste Norderney des Niedersachsischen Landesamtes fUr bkologie for making available the digital elevation
models for parts of the coastal marshland, tidal flats, and subaquatic areas.
References
Barckhausen, J., Preuss, H. & Streif, H. (1977): Ein lithologisches Ordnungsprinzip fUr das
Kiistenholozan und seine Darstellung in Form von Profiltypen.- Geol. Jb.,A 44: 45-77,7
Fig., 3 Tab.; Hannover.
373
tests carried out on map sheet 2716 (Elsfleth) demonstrate that large deviations
occur only in the volumes of depth intervals below -10m NN. This effect can be
explained as follows. Only a small number of boreholes intersected the base of
the Holocene in an unusually deep position, i.e. below about -10 m NN; these
were interpreted by the author of the manually constructed structure contour
maps as belonging to a system of deep, narrow gullies. Any such interpretation
is disregarded by computerized grid calculations that are directly based on borehole data and which in consequence produce a series of isolated, deep, and relatively wide structural depressions ("bull's-eye" artifacts). However, the volume
of sediment in the depth interval -10 to -16 m NN is relatively unimportant as it
comprises less than 2% of the total Holocene volume below NN.
In order to use a standardised procedure and to monitor the results obtained,
it was decided to calculate the Holocene volume below NN on the basis of both
data sources, the manually constructed structural contour maps and directly
from borehole data. ISM proved to be a suitable computer program, because it
works on a VAX under the VMS operating system and allows the user to calculate
the grids and volumes in batch jobs. This is necessary because of the huge data
files. Furthermore, ISM is the only one of the programs used that allows blanking out of certain areas (e.g. Pleistocene hinterland). In the course of gridding,
this function replaces the node values by zero. A grid spacing of 40 m proved to
be appropriate for all computations. Because of the huge number of measurements in the case of the digitized contour lines, only one depth value was considered for each grid node. In contrast, grid calculations based only on borehole
data made use of eight depth values for each grid node.
At present the volume of the Holocene sediment wedge has been determined
for over 50% of the research area (total area 4700 km 2 ) in the coastal lowlands of
Lower Saxony. By the end of 1997, the calculations and interpretation of the results will have been completed
Acknowledgements
The authors would like to thank the Deutsche Forschungsgemeinschaft for their
financial support within the scope of the Priority Program Changes in the
Geo-Biosphere in the Course of the last 15 000 Years. We would also like to thank
the Niedersachsisches Landesverwaltungsamt - Abteilung Landesvermessung -,
the Wasser- und Schiffahrtsdirektion Nordwest in Aurich, the Bundesanstalt fUr
Wasserbau in Karlsruhe, and the Forschungsstelle Kiiste Norderney des Niedersachsischen Landesamtes fUr bkologie for making available the digital elevation
models for parts of the coastal marshland, tidal flats, and subaquatic areas.
References
Barckhausen, J., Preuss, H. & Streif, H. (1977): Ein lithologisches Ordnungsprinzip fUr das
Kiistenholozan und seine Darstellung in Form von Profiltypen.- Geol. Jb.,A 44: 45-77,7
Fig., 3 Tab.; Hannover.