A Comparison of Different Methods to Estimate the Effective Spatial Resolution of FO-DTS Measurements Achieved during Sandbox Experiments

Abstract

For many environmental applications, the interpretation of fiber-optic Raman distributed temperature sensing (FO-DTS) measurements is strongly dependent on the spatial resolution of measurements, especially when the objective is to detect temperature variations over small scales. Here, we propose to compare three different and complementary methods to estimate, in practice, the “effective” spatial resolution of DTS measurements: The classical “90% step change” method, the correlation length estimated from experimental semivariograms, and the derivative method. The three methods were applied using FO-DTS measurements achieved during sandbox experiments using two DTS units having different spatial resolutions. Results show that the value of the spatial resolution estimated using a step change depends on both the effective spatial resolution of the DTS unit and on heat conduction induced by the high thermal conductivity of the cable. The correlation length method provides an estimate much closer to the value provided by the manufacturers, representative of the effective spatial resolutions along cable sections where temperature gradients are small or negligible. Thirdly, the application of the derivative method allows for verifying the representativeness of DTS measurements all along the cable, by localizing sections where measurements are representative of the effective temperature. We finally show that DTS measurements could be validated in sandbox experiments, when using devices with finer spatial resolution.

Numerical and Experimental Validation of the Applicability of Active‐DTS Experiments to Estimate Thermal Conductivity and Groundwater Flux in Porous Media

“…The accuracy of groundwater flux estimates is even better since our experimental results suggest it can reach a few percent which is much better than the one obtained with hydraulic methods (de Marsily, 1976). Moreover, while hydraulic methods provide in general an integrated value of hydraulic conductivity, active‐DTS experiments can provide continuous groundwater flow measurements with an excellent spatial resolution close to 1 m or less (Simon et al., 2020).…”

“…Finally, this method could address the challenge of characterizing the distribution of fluxes in the subsurface. Depending on the spatial resolution of the DTS units and on experimental conditions (Simon et al., 2020), the method can provide flux estimates all along a FO cable at a very high sampling resolution that is not reachable with other conventional methods. This results in the possibility of precisely mapping heterogeneities and understanding flow dynamics, such as for instance in sandy aquifers or in the hyporheic zone, where the spatial distribution of flow can be driven by very local heterogeneities (Cardenas, 2015).…”

“…Considering the spatial resolution of the DTS device and the limited size of the sandbox, the feasibility of conducting active‐DTS experiments in such conditions was discussed in a previous publication (Simon et al., 2020). We validated experimental results for most measurement points, except section GH that was highly affected by heated adjacent sections of cable.…”

“…Considering the flow direction and the transport of heat generated along the cable from upstream to downstream (from DE to IJ), section DE is the only one whose temperature evolution is not affected by upstream heated sections. The representativeness of DTS measurements along this section was validated by Simon et al (2020) who studied the effective spatial resolution during active-DTS experiment using the same data set as the one used in this study. The value ΔT corresponds to the difference between the temperature measured over time and the initial temperature.…”

Estimation of Temperature and Associated Uncertainty from Fiber-Optic Raman-Spectrum Distributed Temperature Sensing

“…The spreading of the pulse is caused by the finite width of the emitted laser pulse, the measurement response of the detector, and intermodal dispersion of the pulse while propagating through the fiber. The spreading of the pulse may be experimentally estimated following Simon et al [19]. Most of the scattered energy has the same wavelength as the emitted laser pulse (Rayleigh scattering), but a small part has different wavelengths.…”