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Towards Improved Field Application of Using Distributed Temperature Sensing for Soil Moisture Estimation: A
Benjamin Apperl1, Matthias Bernhardt1, Karsten Schulz1
1Institute for Hydrology and Water Management, University of Natural Resources and Life Sciences (BOKU), 1190 Vienna, Austria.
Sensors (Basel, Switzerland)
|December 22, 2019
Summary
The dual probe heat pulse (DPHP) method, using actively heated fiber optic (AHFO) cables and distributed temperature sensing (DTS), can now estimate soil thermal properties and water content. A new evaluation method accounts for cable thickness, improving field-scale accuracy without calibration.
Area of Science:
- Geophysics
- Environmental Science
- Soil Science
Background:
- The dual probe heat pulse (DPHP) method is utilized for field-scale soil thermal property and water content monitoring.
- Actively heated fiber optic (AHFO) cables combined with distributed temperature sensing (DTS) are key components of this method.
- Field applications necessitate robust, thicker fiber optic cables, challenging the assumption of an infinitely thin heat source.
Purpose of the Study:
- To develop and validate a new evaluation procedure for the DPHP method that accounts for the finite thermal properties of heating cables.
- To improve the accuracy of soil thermal property and water content estimations at the field scale without requiring a calibration procedure.
- To assess the performance of the new method against established techniques and in situ measurements.
Main Methods:
- Incorporation of a semi-analytical solution to the heat transport equation to model the finite thermal properties of the heating cable.
- Laboratory experiments with varying heating scenarios to infer soil moisture from volumetric heat capacity.
- Analysis of temperature amplitude shifts and heating curve characteristics at the sensing cable.
- Comparison of results with a calibrated infinite line source solution and in situ water content measurements.
Main Results:
- The new evaluation procedure demonstrated a good approximation of thermal properties for strong and short heat pulses.
- Volumetric water content estimates derived from the new method showed accuracy comparable to the calibrated infinite line source solution.
- The study identified challenges related to cable spacing and the soil resettlement process after cable burial.
Conclusions:
- The developed semi-analytical solution effectively addresses the limitations of the infinite line source assumption for thicker AHFO cables in DPHP.
- The method provides accurate soil thermal properties and water content estimations, suitable for field-scale applications.
- Further research is needed to optimize cable installation techniques and address spacing issues for enhanced reliability.