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NAGRA NTB 92-03
Inhalt
- VI -
- If, with feasible pumping and drawdown rates (or injection), more than approx.
S I/min can be extracted (or injected) from the borehole inteNal being tested
(SE- 3m2/s > T tot > 1 E-7 m2/s) , the packer-flowmeter method is recommended.
- If lower permeability sections (SE-4 m 2 /s > T tot > 1 E-1 0 m2/s) with pumping rates in
the range up to S I/min are to be tested and the individual flow paths characterised,
then the conductivity method is more appropriate.
Transmissivities of 2E-S to 2E-7 m 2 /s were measured in borehole M 1400; taken in
relation to the thickness of the sandstone elements, this gives K values of 4E-6 to
4E-8 m/s. 7 of the 9 inflow paints identified were located in meander belts, and the
other two in crevasse channels/crevasse splays. Two meander belts had no identifiable
inflow points. The architectural features located between the inflow points (clay- and
silt-rich flood plain sediments) function as effective hydraulic barriers which prevent any
balancing out of the relatively large differences in· potential obseNed over short vertical
distances in borehole M 1400.
The petrophysical measurements included a sUNey of four boreholes (caliper log,
gamma log, density log and neutron porosity log). In two of the boreholes which were
already cased and infilled with gravel, no quantitative measurements were possible but
the gamma logs could be used for "layer correlations" with the other boreholes.
Measuring conditions in the two uncased boreholes were good and the sandstone units
could be delimited using mainly gamma and density logs. However, in the area of
borehole breakouts, the caliper corrections required for calculating the density and
porosity values are unsatisfactory. Sandy units can be detected using petrophysical
measurements, without any geological input. Because of their low clay content, the
channel sandstones are characterised by very low intensities of gamma radiation. A
special study carried out using the petrophysical logs from borehole M 1400 showed
that, even without any core analYSis, the USM can still be characterised in some detail
(Le. divided into architectural elements) using the petrophysical dataset available.
The following information was obtained on the subsurface layer geometry in the
Burgdorf investigation area. Based on all the data collected from a total of five
boreholes, the strata presumably dip to the ESE, with an angle of approx. 10°. Since
the gamma logs from four of the boreholes are more or less identical over large depth
ranges, the subsurface geometry would appear to be very simple. The presence of
extensive distu rbed zones can be ruled out. However, some small-scale distu rbed
zones could be detected on the basis of drillcore photos (disturbed zones with a dip of
60°; presumably normal faults, azimuth unknown) and analysis of gamma logs. Spatial
orientation of these zones is not possible, and it is therefore impossible to obtain an
exact picture of the three-dimensional structure of the underground of the Burgdorf
geothermal test field.