Active Layer Temperature Monitoring in Blocky Material
Susanne
Hanson, Glaciology and Geomorphodynamic Group, Department of Geography,
University of Zürich,
Switzerland, e-mail: shanson@geo.unizh.ch
Introduction
The processes in the active layer and its
connection to the atmosphere is of great importance for the state of the frozen
ground below. These processes depend to a great extend on the actual material of
the active layer (e.g. boulders, solid bedrock, gravel or soil) and the
fieldwork should be designed with regards to what sub-surface is being measured.
The here discussed procedures focus especially on active layer monitoring in
areas of coarse boulders which usually covers considerable parts of the high
alpine environment. Two main procedures are considered. One simple and quit
cheap; the other more sophisticated and definitely more expensive and difficult
to complete.
In bore hole palaeoclimatology, it is commonly assumed that a direct coupling
exists between air and ground temperatures. This assumption is valid only if
variables affecting ground-surface temperature exchange have remained constant
through time. It should therefore be taken into awareness that ground
temperatures can contain non-climatic noise due to changes in ground-surface
variables, including vegetation cover, duration and thickness of snow cover,
precipitation and melt water, wind speed and thereby wind pumping within the
boulders, net radiation, phase change and boundary conditions. All this, as well
as if one measure site or a network of sites is necessary should therefore be
considered when designing the fieldwork in proportions to what is expected to be
evaluated from the data.
Processes which influence the temperature regime within the active layer has been discussed in the literature (e.g. Williams and Smith, 1989; Humlum, 1997; Humlum, 1998; Harris and Pedersen, 1998; Kane et al., 2001; Hinkel et al., 2001; Schmidt et al., 2001)
Miniature temperature
data loggers (mini loggers)
Mini loggers can be used as a simple and cheap
approach when establishing a temperature profile above and within the active
layer. The mini loggers are described in detail elsewhere in this publication.
The mini loggers can normally be programmed to measure and store data at a
variety of time spans and are simply placed with the desired spacing within the
boulders. Mini loggers, though do not have endless memories and the more often
temperatures have to be registered the shorter the time the mini loggers will be
operating. Further disadvantages are that the batteries have to be replaced to
not loose data and that the resolution of the data often is quite coarse.
Bore hole and thermistor
chain
The system recording the data using this approach consists of the thermistor sensor chain, lowered in a bore hole, and connected to a data logger and a storage module normally driven by a battery or a solar panel. The data logger is the brain of a data acquisition system. They do the measurements at a specified scan rate and process the data. The data logger needs to be programmed to measure and download data at a certain time and form. The storage module is the device used to transport the data and programs between the field site and the office or transferring the data directly to a laptop. Furthermore, it also increases the data logger's storage capacity by storing data and programs in a solid state module or a memory card. The thermistor chain itself consists of the thermistor sensors which are placed with the desired space between them and wrapped up in a plastic coating.
The drilling
This part of the fieldwork is not to be
underestimated, especially not if the drilling takes place in loose material or
boulders. It is heavy work and do involve quit a lot of practice. If one has to
do the drilling one-self several things have to be taken into consideration.
The type of drill needed is dependent on how deep
the bore holes will be, the diameter of the hole and the material within the
drilling takes place. The drilling requires electricity and water supply which
can turn out to be a problem in remote areas. It is possible to drill dry but it
is much to prefer drilling with water. This will cool the drilling head during
the drilling, smoothen the drilling and is a good deal faster than dry-drilling.
For the design of the bore hole system, the first
step is to decide the diameter of the bore hole. To reduce air circulation
within the bore hole choose the diameter of the drilling tubes as small as
possible. Otherwise the air circulation within the bore hole, once established,
can disturb the natural temperature profile. The chain should be fastened to a
weight at the end to be sure that there is weight enough to keep the chain
straight and get it all the way to the bottom. This weight can have a larger
diameter than the actually chain which is to keep in mind when deciding the
diameter. Consider as well the option of wrapping neoprene tightly to one side
of the chain. When the chain is lowered in the afterwards the sensors will be
pressed against the wall of the bore hole. This will result in a more precise
measurement of the material and also prevent air circulation in the hole.
Next step is to decide the length of the bore tubes.
It is a good idea to divide the full length of the bore hole, and thereby the
tube length, into a certain number of meters. Or said in another way: if the
hole has to be 6m deep use 6 * 1m. tubes plus one 1/2m. tube. This is due to
practical reasons during the drilling.
Third step is to decide for the drilling crowns. It
is necessary to know the geology of the material where the drilling will take
place. In loose or coarse material several different crowns that can be changed
according to varying material during the drill are needed. This often implies at
least one crown for loose material and one that fit the main hardness of the
boulders. Let the company guide you according to the material, how deep the bore
holes will be and how many holes needs to be drilled.
A final very important consideration is wherever the bore hole should be installed with an inner tube. An inner tube is a thin plastic tube which is installed during the drilling. The advantage of installing an inner tube is that a lot of problems will be avoided during the actual drilling as it prevent the hole from collapsing every time the tubes are pulled out of the hole to be emptied. The disadvantage is that it will also act as a thin barrier between the thermistor sensors and the material which temperature is to measure.
The
drill with drilling tubes, Corvatsch, 2002. Photo: P. Blétry.
The sensor chain
The chain itself consists of the sensors and the
bundle of their wires coated tightly with a plastic mantle. A temperature sensor
consisting of a semiconductor that provides rapid and large changes in
resistance for relatively small changes in temperature. Several configurations
of the wires can be devised to minimize the risk of loosing data from all the
sensors if some part of the chain is damaged. Again it has to be considered that
the very mantle protecting the sensors also reduce the precision of the
measurements just as it is the case with the coating of the bore hole.
It is possible to fabricate the chain oneself but it
is definitely easier to let the company from where the sensors are bought
construct it. The extra money can quickly turn out to be good value in the long
run. Whatever the choice is the following issues has to be considered.
First the decision has to be made on how fine the
spacing between the sensors on the chain should be. The temperature sensor
spacing in the bore hole is usually dependent on the vertical resolution
required and the stratification of the sub-surface. The top 50cm will experience
the most rapid near-surface short-term temperature variation due to the diurnal
changes of the atmosphere above. There is thus a certain logic in less
spacing between the sensors in the upper 50cm to 1 m of the chain. The
registration of the exact surface temperature is quite tricky as the actual
boundary is infinitely thin. A mini logger or another thermistor sensor placed
nearby as a reference is one approach.
The thermistor chain should be calibrated depending
on the accuracy and temperature range of interest as well as the absolute
accuracy of the whole system should be taken into account. If measuring over a
wide range of temperatures calibration of each thermistor in several temperature
ranges is needed. This will give a calibration curve for each sensor, for each
temperature range. These are then applied individually for the conversion of the
measured resistance in the corresponding range of temperature. In such a case it
makes sense to log the data as measured resistance. If the actual temperature
range is narrow the data logger can be programmed to calculate and store the
data as temperatures.
The measured temperatures in the bore hole will for
some time reflect the fact that drilling generates a lot friction heat which is
transmitted into the surrounding material. It is hard to say how long time it
takes before the system is back to its original state as it depends on the
actual situation. Three months to half a year has been reported. The temperature
profile has to be examined closely till it is evident that it is no longer
cooling and first from then on the data can be used.
The durability of the sensors has to be judged as well. Over time the sensors should be calibrated again if possible. This is possible only if the chain can be pulled out e.g. the chain is lowered in a bore hole which is coated or drilled in firm bedrock. If the chain is lowered in a hole drilled in loose material the chances that the hole is collapsed at some point is high and it can not be recommended to pull the chain out as it will probably not be possible to lower it back in again.
Setting up the system
Always inspect if the measuring site is also prone
to avalanches or other natural hazards. When the system is established cover all
exposed wires on the ground with rocks. That will to some extend protect the
wires from e.g. animals which chews in the cables and from weathering of wind
and stone fall. Do also remember to do a GPS point measurement to be able to
locate the site under a thick snow cover. When the system is first established
there is not much maintaining other than checking that the system is running and
repair ongoing weathering.
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Bockheim, 2001: Patterns of soil temperature and moisture in the active layer
and upper permafrost at Barrow, Alaska: 1993-1999. Global and Planetary Change,
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Humlum, O., 1997: Active layer thermal regime at
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S.I. Outcalt, 2001: Non-conductive heat transfer associated with frozen soils.
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p.
Latest update: 29. January 2003.