A distributed heat pulse sensor network for thermo-hydraulic monitoring of the soil subsurface

Abstract

Fibre optic distributed temperature sensing (DTS) is used increasingly for environmental monitoring and subsurface characterization. Combined with heating of metal elements embedded within the fibre optic cable, the temperature response of the soil provides valuable information from which soil parameters such as thermal conductivity and soil moisture can be derived at high spatial and temporal resolution, and over long distances.We present a novel active distributed temperature sensing (A-DTS) system and its application to characterize spatial and temporal dynamics in soil thermal conductivity along a recently forested hillslope in Central England, UK. Compared with conventional techniques (needle probe surveys), A-DTS provided values with a similar spread although lower on average. The larger number of measurement points that A-DTS provides at higher spatial and temporal resolutions and the ability to repeat surveys under different meteorological or hydrological conditions allow for a more detailed examination of the spatial and temporal variability of thermal conductivities at the study site. Although system deployment time and costs are higher than with needle probes, A-DTS can be extremely appealing for applications requiring long-term monitoring, at high temporal repeatability, over long (kilometres) distances and with minimum soil disturbance, compared with one-off spatial surveys.Thematic collection: This article is part of the Measurement and monitoring collection available at: https://www.lyellcollection.org/cc/measurement-and-monitoring

BIFoR FACE: Water–soil–vegetation–atmosphere data from a temperate deciduous forest catchment, including under elevated CO2

“…In addition, one of the juvenile plantations close to the catchment outlet (Figure 2, bottom right) has been instrumented since 2016 with active fibre‐optic distributed temperature sensing (FO‐DTS connected to a XT‐DTS™ unit and a heat pulse system by Silixa Ltd., London, UK). The fibre‐optic system can measure soil temperature every 30 s and soil moisture every 6 h at a submeter spatial resolution, resulting in 1850 soil temperature and soil moisture sampling locations across the site, ranging from 10–40 cm depths (Abesser et al, 2020). The current dataset provides 6‐hourly retrievals of soil temperature and soil moisture.…”

Combining passive and active distributed temperature sensing measurements to locate and quantify groundwater discharge variability into a headwater stream

“…To characterize the temporal dynamics of flow, these experiments must be repeated often, which is clearly more constraining than conducting long-term passive DTS measurements because active DTS experiments require more instrumentation (heat pulse system, electrical cables, etc.). However, the repetition of active DTS measurements offers very promising perspectives for environmental monitoring, as recently shown by Abesser et al (2020), who repeated surveys under different meteorological or hydrological conditions in order to monitor the evolution of thermal and hydraulic properties of the soil subsurface.…”

BIFoR FACE: Water-soil-vegetation-atmosphere research in a temperate deciduous forest catchment, including under elevated CO2

“…In addition, one of the juvenile plantations close to the catchment outlet has been instrumented since 2016 with active fibre-optic distributed temperature sensing (FO-DTS) for measuring soil temperature and soil moisture at a submeter spatial resolution, resulting in 1850 soil temperature and soil moisture sampling locations across the site, ranging from 10-40 cm depths (Ciocca et al, 2020). The retrieval from the FO-DTS has a maximum at 38%v/v, a value empirically determined from a soil-specific field calibration against point soil moisture sensors installed adjacent to the fibre-optic cable.…”