Distributed temperature sensing finds applications in a wide range of industries.
The temporal dynamics and spatial distribution of fluid thermal signature (either as a surrogate for or input into energy conservation formulations) have provided process insights by tracking thermal pulses, estimating fluid fluxes and tracing surface water interactions with surrounding media (including soil, groundwater, atmosphere). Heat can be used as a tracer to detect dams’ internal erosion, to estimate groundwater flow rates and borehole inflow locations from both porous and fractured media, and to monitor stream ecosystems.
Distributed Temperature Sensors (DTS) provide the possibility to upscale the measurements to large spatial extents and also to monitor the temporal dynamics of the hydrological processes. Recent technology enhancements have allowed for determination of energy and fluid mass balances at watershed scale. The ULTIMA™ DTS family offers an enhanced temperature resolution as fine as 0.01 °C and sampling resolution every 12.5cm. DTS guarantees an extreme versatility, being equally powerful in short installations (e.g. hundreds of meters), as well as for cable lengths up to 35 km.
- Long term installations with limited maintenance
- Minimum disturbance of the installation site
- Only one logger is necessary for thousands of probes
- Installation in very harsh environments
- Possible to measure not only temperature, but also heat/water fluxes, thermal/hydraulic properties with the same probe
- Ability for active measurements using heatable cables
Silixa will assist our customers through all the steps leading to a successful installation for measuring hydrological processes. In particular providing:
- Ad-hoc advice on the set-up of the installation, according to the customer’s needs
- All the necessary equipment to perform the measurements, including cables, electrical components, cable terminations.
- Guidance on data analysis and interpretation.
Temperature is an important factor in streams ecosystems. Heat tracers have been used to characterize contaminant transport, infiltration rates and energy exchange with groundwater, the atmosphere and radiation. The dynamics of these complicated water bodies is also affected by seasonal and diurnal patterns making characterization particularly challenging. Utilizing temperature measurements that capture spatial and temporal variations at stream ecosystem interfaces provides a mechanism for detailed characterization.
DTS can provide continuous measurements in space and time of surface water bodies over large spatial scales. Robust optical fibre cables, suitable for streams are a cost effective means to map and determine the flow patterns affecting surface water temperatures and provide a mechanism to quantify groundwater discharge into the stream. Optical cables can be easily deployed either along streams with the degree of spatial characterization only limited by the amount of cable that is utilized. Cable can be deployed along the streambed for many kilometres and can also be locally installed in various formats including grids to provide high spatial coverage for temperature monitoring wherever it is required.
Silixa’s distributed temperature sensing technology can collect invaluable spatial and temporal data to characterize surface water groundwater interactions. Cable can be installed in a grid pattern (above) to maximize local spatial coverage or installed along many kilometres of streambed to maximize spatial extent.
Borehole flow monitoring
Subsurface environments are critical not only as water resources but also for energy use including geothermal, CO2 sequestration and oil & gas operations. These sensitive environments are directly affected by anthropogenic activities, hence monitoring is essential. Temperature offers insight into a variety of physical properties and can be used for flow monitoring and flux quantification in the subsurface. Traditionally, point sensors have been used to collect temperature data; however, significant limitations exist.
Fibre optic distributed temperature sensing (DTS) has the ability to collect high temperature resolution data fully distributed in space and continuously in time. Using active or passive DTS measurements enables groundwater flow measurements in both shallow and deep boreholes. DTS can provide data for models for 3D imaging of temperature changes at the aquifer scale, provide a tool to characterise subsurface heterogeneity into boreholes and localise inflows. Ambient groundwater flow, or flow under natural gradient conditions in fractured rock aquifers is an important component of both contaminant and heat transport. Contaminant transport in fractured media is controlled by the fracture network and resulting flow system.
Gaining an understanding of the natural flow system is needed for site conceptual model development including for predicting contaminant plume migration. In addition, natural gradient flow is particularly challenging to measure due to the very low flow rates involved. Monitoring fluid movement in such a difficult environment requires high resolution techniques that are capable of sensing individual fractures or fractured zones.
A fine temperature resolution is required for measuring very low or rapid flow rates and localised flow. The ULTIMA™ DTS can be combined with active heating techniques to create the thermal differential needed for identifying natural gradient flow distributions and borehole flow heterogeneity. The active approach does not rely on natural temperature signal; instead, a heat pulse is output at a constant rate along the measurement cable and the thermal response monitored using DTS. By creating a thermal disequilibrium, a variety of processes can be characterized using the temperature rise during heating and temperature decay during cooling.
Active DTS methods can be used to measure groundwater flux distributions in fractured rock aquifers by utilizing boreholes sealed with flexible liners.
Soil moisture monitoring
Soil moisture content is the amount of water within a certain volume or mass of soil. Together with soil temperature is a fundamental indicator of the eco-hydrological state of a field. A precise measurement of how water and temperature are spatially distributed in soil, and their evolution in time, is crucial to the forecast and/or quantification of many key processes like plants growth, infiltration, runoff, plant and evaporation.
Commonly adopted solutions to measure soil moisture and temperature are point sensors based on time/frequency domain reflectometry. Unless a large amount of these deployed in the soil (with consequent raise of costs and difficulty of installation and logging), most of the spatial and temporal variability of the water and temperature regimes is not captured. Fibre optic distributed temperature sensor (DTS) technology represents a cost effective solution able to provide a precise distributed monitoring of soil temperature and moisture at high spatial resolution (down to a few centimetres), over distances up to kilometres, and with minimum soil disturbance.
A DTS instrument, coupled to a heatable fibre optic cable allows for inferring distributed soil moisture applying the Active DTS method. A low power heat pulse is sent along the cable buried in the soil at selected depth(s), and the resultant thermal response is analysed to calculate soil moisture content.
The image above shows a cable installation for soil moisture monitoring in Europe.