Dakota Aquifer Program--Subsurface Hydrology
Optical-fiber temperature logging, part 4 of 4
Discussion
Preliminary results show that the logs obtained by the DTS method are
similar in response to those obtained in conventional logging and provide
data adequate for a geothermal survey of subsurface conditions. Technically,
the temperatures obtained with the DTS technique have a space resolution
of 1 m, but the borehole resolution appears to be less as recognized by
the differences of interval gradients of the two methods. To achieve
gradient differences at a maximum of about 10 degrees C/km, the DTS data
have to be processed in 10-m intervals or the same order of gradient
difference can be achieved by applying a smoothing function (5-m running
average) on the 1 m DTS data. Because of this resolution the DTS
technique is more limited in determining foniiation gradients in a
thin-bedded stratigraphic sequence. The main limitation on borehole
resolution is linked with a relatively large signal/noise ratio of about
0.1 K whereas the precision of the temperatures (and gradients) recorded
is affected mainly by the sinusoidal temperature noise on the DTS log.
Improvement of the technical characteristics of the DTS tool or filtering
of the measured temperatures can reduce this noise; this is the approach
we are taking.
Conclusions
In its present form the DTS is suitable for use in conventional
terrestrial heat-flow studies although its resolution is about a
factor of 5 to 10 lower than conventional techniques in an actual
borehole situation where resolution is limited by hole conditions
as well as instrument response. Technical problems that may occur
with conventional logging such as improper insulation, variations
in cable resistance, or disturbing electrical currents are eliminated.
Effects on the quality of logcing data caused bv the logging rate
are not relevant.
The most appropriate applications for the DTS logging device may be
the installation in a borehole for extended periods of time to monitor
transient subsurface temperature changes, for example in connection
with natural and artificially stimulated fluid-flow conditions. The
DTS logging device also could be used to investigate the short-time
response of borehole temperature to the drilling or other transient
thermal effects. The cable can be left in the well and temperatures
measured simultaneously and frequently as a function of time and depth.
The hole would not be stirred by the logging and the data would be
synchronous. The determination of a'continuous' thermal-conductivity
profile based on the short-time response part of the transient recovery
also should be possible.
Another advantage in the DTS technique lies in the situation where
there is a large air or gas column in the well. For example, in a
natural gas field, wells could be logged readliy whereas the slow response
of a conventional tool moving in the borehole precludes accurate
temperature measurements in such environments. Because thermal gradients
in air in typical boreholes are more stable than in water (Sammel, 1968),
and intraborehole flow is precluded, this part of the well may give
better, or more useful, data than the part of the curve below the watertable.
Acknowledgements
We would like to thank the GeoForschungsZentrum Potsdam for supporting
the research project of developing the DTS technique for application in
the geosciences and the Kansas Geological Survey for logistic and
financial support of the field work. Part of the comparison study was
supported by NSF Grant #9018278 to SMU.
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Kansas Geological Survey, Dakota Aquifer Program
Updated Sept. 30, 1996
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