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Abstract
The primary goal of this dissertation was to examine if soil-gas concentrations are indicators
for shallow rock-salt deposits. The examinations were based on the assumption that rock-salt
is a barrier with respect to ascending gas of geogenic origin. Therefore, the lateral
extensions of rock-salt formations are characterized by increased carbon-dioxid
concentrations within the soil. The study area under consideration comprises an area of 12
km 2 located above the salt-mine Stetten near Haigerloch (Baden Württemberg). The rock-salt
occurs in a depth of 100 to 200m below surface („Mittlerer Muschelkalk). Below the salt layer
the CO2-pressure is 5 to 10 bars. Using existing pneumatic, lithologic and hydraulic data a
model has been developed to explain the overall vertical transport mechanism of geogenic
gases and to generate estimates for the velocities of the ascending carbon dioxide. The
model revealed that the hydrostatic pressure and therefore the thickness of the aquifer within
the Oberen Muschelkalk is the most important mechanism controlling the advective transport
of geogenic carbon dioxid towards the surface. Advective transport of gas only occurs
towards the direction of decreasing pressure. If the hydrostatic pressure on the basis of the
aquifer (located within the Oberer Muschelkalk) is higher than the gas pressure within the
rock formations of the Mittlerer Muschelkalk there is no incline of pressure towards the
surface. Therefore geogenic carbon dioxide within the study area is likely to occur only where
valleys incise the Mittleren Muschelkalk.
To verify the calculations based on this model a location with rocksalt in the sub-surface was
compared to a location where no rocksalt occurs below the surface. Both locations are in
floodplains of streams which incise the Oberen Muschelkalk. The stable isotope
compositions of carbon derived from the CO2 of the soil-gas and the O2 / CO2 ratios of both
locations have been compared. Only the location where no rocksalt occurs in the sub-surface
displays carbon dioxid of geogenic origin. This indicates that rocksalt acts as a barrier with
regard to ascending geogenic CO2 .
The exploration of rock-salt with help of soil-gas meassurements in the study area is severly
limited because only a small fraction of the area is incises the Oberen Muschelkalk.
CO2 concentrations in soils in the Mittleren Keuper were mapped on a larger scale. The
evaluation of this mapping with GIS ArcView revealed two lineaments that parallel the
swabian strike and are characterized by CO2 concentrations which are 50 – 100% higher
than the average carbon dioxide concentrations. One of the lineaments occured where there
is no rock-salt in the Mittleren Keuper. The other lineament follows a geologic fault-line.
Analisis of the stable isotopes of carbon and the concentrations of CO2 and O2 revealed that
the geogenic carbon dioxid induced only a 15% increase in the carbon dioxid concentration
in the soil gas. Therefore, the geogenic carbon dioxide can not explain the marked increase
of the carbon dioxide concentration of this lineament.