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

Nataline Simon,Gilles Porel,Benoît Nauleau,Behzad Pouladi,Olivier Bour,Laurent Longuevergne,Nicolas Lavenant

doi:10.3390/s20020570

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.

Highlights

Initiated in the 1980s, the use of fiber-optic distributed temperature sensing (FO-DTS) has been considerably improved, and this technology is nowadays widely applied in a large range of applications in various scientific and industrial disciplines, such as the oil and gas industry, leakage and fire detection, structure health monitoring, civil engineering, etc. [1,2]
Our results suggest that the correlation length provides an estimate of spatial resolution close to the manufacturer’s values, while the derivative method and the “90% step change” method provide an estimate that takes into account both the capabilities of the units and the effect of fiber-optic cable, such as heat conduction that occurs during experiments, especially close to abrupt temperature step changes
We showed that the estimation of the effective spatial resolution of DTS data during an experiment depends on the technical specifications of the DTS unit and on the methods used that may be more or less sensitive to fiber-optic cable properties

Summary

Introduction

Initiated in the 1980s, the use of fiber-optic distributed temperature sensing (FO-DTS) has been considerably improved, and this technology is nowadays widely applied in a large range of applications in various scientific and industrial disciplines, such as the oil and gas industry, leakage and fire detection, structure health monitoring, civil engineering, etc. [1,2]. Works have been dedicated to metrological characterization and specification of DTS devices (principle and physic of the measurement, technology limitations) [6,13,14,15,16,17], improvements in calibration methods [18,19], and in instrumentation deployment (DTS unit and cable selection, field installation) [8,13,15] In this context, Tyler et al [13], Selker et al [15], and Failleau et al [17] demonstrated the importance of considering the spatial resolution of DTS systems. This distinction is required to define the ability of the device to detect punctual change in temperature and to fully understand the meaning of collected data [13,15,17]

Objectives

Methods

Results

Discussion

Conclusion