Double-Ended Calibration of Fiber-Optic Raman Spectra Distributed Temperature Sensing Data

Abstract

Over the past five years, Distributed Temperature Sensing (DTS) along fiber optic cables using Raman backscattering has become an important tool in the environmental sciences. Many environmental applications of DTS demand very accurate temperature measurements, with typical RMSE < 0.1 K. The aim of this paper is to describe and clarify the advantages and disadvantages of double-ended calibration to achieve such accuracy under field conditions. By measuring backscatter from both ends of the fiber optic cable, one can redress the effects of differential attenuation, as caused by bends, splices, and connectors. The methodological principles behind the double-ended calibration are presented, together with a set of practical considerations for field deployment. The results from a field experiment are presented, which show that with double-ended calibration good accuracies can be attained in the field.

Upscaling lacustrine groundwater discharge rates by fiber-optic distributed temperature sensing

“…Based on the differences in groundwater and surface water temperatures, spatial patterns of groundwater discharge can be identified by tracing temperature anomalies at the sediment-water interface. Temperatures can be traced along fiber-optic cables of several kilometers length with currently 0.3-4 m spatial resolution and measurement precision of 0.05-0.1 C for sampling intervals of 30 s [Selker et al, 2006a;Hausner et al, 2011;Van de Giesen et al, 2012]. In contrast to the aforementioned methodologies, FO-DTS is useful for spatially detailed measurements at larger scales, and therefore has the potential to provide temperature information for tracing LGD with high spatial resolution at scales exceeding previous detailed investigations of local flow.…”

“…The FO-DTS method applied in this project analyses the offset in the backscatter of Raman Stokes (temperature independent) and anti-Stokes (temperature dependent) signals from a 10 ns light pulse to undertake and locate temperature measurements along the fiberoptic cable [Selker et al, 2006a, b]. The applied DTS system (Sensornet Halo) is capable of measuring temperature at high precision (0.05 C) with a sampling resolution of 2 m [Sensornet, 2009] and a spatial resolution of 4 m [ Van de Giesen et al, 2012]. For the temperature survey, a gelcoated, plastic covered two channel fiber-optic cable (BruPro, Brugg/CH) was deployed at the sediment surface (ensured by carefully inspecting cable position during installation) in a setup of four parallel loops with 0.5, 1, 2, and 3 m distance to the shoreline (Figure 2).…”

Practical considerations for enhanced-resolution coil-wrapped distributed temperature sensing

“…In other words, the Stokes pulse loses more of its extreme modes as it passes through a bend, despite the same angular change of the guiding fibre for both Stokes and anti-Stokes modes. The altered differential attenuation of the Stokes and anti-Stokes signals was observed by Arnon et al (2014), and is one of the reasons why Hausner et al (2011) and Van de Giesen et al (2012) promoted manual calibration of the differential attenuation for separate sections of the cable. Unfortunately, since the loss in this case is asymmetrical with the direction of light travel (as may be understood by thinking about the first 100 m of cable, where the light entering the coil will experience significant differential attenuation, but the light leaving the coil will have settled down to a constant rate of attenuation), the double-ended procedures presented in the literature are not applicable to these unique defects.…”

Reply to comment by J. S. Selker et al. on “Capabilities and limitations of tracing spatial temperature patterns by fiber‐optic distributed temperature sensing”

“…Such shortfalls obviously impede a thorough assessment of the application specific capabilities and uncertainties of FO-DTS. With this in mind, the experiments of Rose et al [2013] were designed to complement recent benchmark studies on calibration strategies and sampling designs in single-ended [Hausner et al, 2011] and double-ended [ Van de Giesen et al, 2012] monitoring modes as well as investigations of the impact of single-ended and double-ended mode monitoring and combinations thereof, such as two-way single-ended averaging [Krause and Blume, 2013].…”