The use of Ditributed Temperature Sensing in environmental applications
The measurement and use of heat as a tracer
is commonplace in hydrology and hydrogeology (e.g.
Anderson,
2005). In streams, the mapping of streambed temperatures can be used to
detect and
quantify discharge of groundwater (e.g. Bense
and Kooi, 2004; Conant Jr,
2004). In boreholes, temperature logs can be used to assess and
quantify
regional groundwater flow conditions (e.g. Taniguchi,
1993), vertical flow within boreholes (Klepikova,
2011), and to reconstruct paleo surface temperatures (e.g. Huang
et al., 2000). The advent of
Distributed Temperature Sensing (DTS) allows such systems to be studied
at both
high temporal and spatial resolution which may otherwise be unviable
when using
traditional ‘point’ methods of temperature measurement. With DTS a
laser pulse
is coupled into a fibre optic cable which then acts as the
‘thermometer’ along
which temperature is measured (Tyler et
al., 2009). Data
have already been collected at a site
in Uden, the Netherlands,
where DTS was used to investigate the thermal patterning on the land
surface
resulting from upwelling groundwater in an area of heterogeneous
sediments.
Here, a fibre optic cable was laid out on the ground surface to form a
grid. It
was found that the temperatures showed a good correlation to the type
of vegetation
cover which was used as a proxy for groundwater depth. The combined
forcing of
the shallow groundwater and vegetation cover caused distinct thermal
patterns of
several degrees, which inverted relative to the mean ground surface
temperature
on a diurnal basis.
Research
is now ongoing at three sites. At
Holme-next-the-Sea, Norfolk,
a cable is to be deployed to trace marine water as it propagates along
a
channel within a salt marsh in response to tidal forcing. It is thought
that
the channel also receives groundwater from the underlying chalk, and
further
measurements are planned to be taken from a cable which is now buried
in the
sediments of the channel. A fibre optic cable is also to be installed
in
selected water courses within the River Wensum Demonstration Test
Catchment
(DTC). Here the DTS installation is used to detect water entering the
drainage
channels as surface run off, shallow through flow, or from subsurface
field
drains. The temperature data are then to be used to help understand the
complexity of nutrient and sediment transport from the surrounding
heavily
farmed fields. At a smaller spatial scale we are planning to test the
method
and application of DTS in thermal tracer tests between closely spaced
boreholes
in fractured crystalline aquifers.
References
Anderson,
M.P. 2005. Heat as a ground water
tracer. Ground Water 43, no. 6:
951–968
Bense,
V.F. and H. Kooi, 2004, Temporal and
spatial variations of shallow subsurface temperature as a record of
lateral
variations in groundwater flow, Journal
of Geophysical Research, VOL. 109
Conant Jr.,
B.,
2004. Delineating and quantifying ground-water discharge zones
using streambed temperatures, Ground Water, v 42, no. 2, p. 243-257.
Huang, S.,
Pollack, H. N., and Shen, P.Y.,
2000. Temperature trends over the past five centuries reconstructed
from
borehole temperatures. Nature,
403: 756-758.
Klepikova, M.V., Le
Borgne, T., Bour, O., Davy, P., 2011. Methodology
for using
borehole temperature-depth profiles under ambient, single and
cross-borehole
pumping conditions to estimate fracture hydraulic properties, Journal
of
Hydrology, 407: 145–152
Taniguchi,
M., 1993, Evaluation
of vertical groundwater fluxes and thermal properties of aquifers based
on
transient temperature-depth profiles, Water Resour.
Res., 29(7),
2021–2026
Tyler,
S.W., J.S. Selker, M.B. Hausner, C.E. Hatch, T. Torgersen and S.
Schladow. 2009.
Environmental temperature sensing using Raman spectra DTS fiber optic
methods. Water
Resources Res. doi:10.1029/2008WR007052 4(187):673-679.