Correcting artifacts in transition to a wound optic fiber: Example from high‐resolution temperature profiling in the Dead Sea

Abstract

Spatial resolution fiber-optic cables allow for detailed observation of thermally complex heterogeneous hydrologic systems. A commercially produced high spatial resolution helically wound optic fiber sensing cable is employed in the Dead Sea, in order to study the dynamics of thermal stratification of the hypersaline lake. Structured spatial artifacts were found in the data from the first 10 m of cable (110 m of fiber length) following the transition from straight fiber optic. The Stokes and Anti-Stokes signals indicate that this is the result of differential attenuation, thought to be due to cladding losses. Though the overall spatial form of the loss was consistent, the fine structure of the loss changed significantly in time, and was strongly asymmetrical, and thus was not amenable to standard calibration methods. Employing the fact that the cable was built with a duplex construction, and using high-precision sensors mounted along the cable, it was possible to correct the artifact in space and time, while retaining the high-quality of data obtained in the early part of the cable (prior to significant optical attenuation). The defect could easily be overlooked; however, reanalyzing earlier experiments, we have observed the same issue with installations employing similar cables in Oregon and France, so with this note we both alert the community to this persistent concern and provide an approach to correct the data in case of similar problems. BackgroundHigh-resolution temperature profiling of the thermally stratified Dead Sea was conducted using highresolution (HR) helically wound fiber-optic profiler, aiming to explore the dynamics of thermal stratification of the hypersaline lake [Arnon et al., 2014]. An artifact in the temperature profiling was found, it is an inherent artifact in the used HR profilers. The aim of this paper is to help identifying this artifact and to propose a correction procedure, to enable accurate temperature sensing along the entire profiler. The Observation System SetupThe observation system setup is presented in Figure 1. A profiler, 55 m vertically oriented, duplex HR optic fiber (BRUsens 70 C high-resolution, Brugg Cables, Brugg Switzerland) was suspended from a buoy; the HR cable was connected to a duplex 0.01 m diameter marine cable (BRUsens Submarine, Brugg Cables, Brugg, Switzerland) running to the shore along the bottom of the Sea (Figure 1). Both cables were produced using Corning (Corning, New York) ClearCurve bend-optimized fiber, and terminated with E2000 APC connectors, joined by barrel connectors. The cable employs a duplex version of the ''wrapped pole'' method described by Selker et al.[2006] to achieve the enhanced vertical resolution: two fibers were helically wound around a 0.020 m core, with each fiber contained in a 0.0023 m OD plastic tube. With a 0.002 m polyurethane jacket, this yielded a cable with a 0.026 m OD, wherein each 0.09 m of cable length represented 1 m of optical path, which is the spatial sampling of the instrument (DTS). The two fibers of ...

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

“…A low number of remaining light modes leaves larger relative errors in the Stokes over anti-Stokes ratio, reducing the signal strength and consequently the accuracy of the temperature measurements. Arnon et al (2014) found that a laser signal entering a coil at first experiences relatively large rates of differential attenuation in the first 100 m of fibre, then asymptotically returning to constant rates of attenuation as the signal passed this distance. In their set-up, the coil was of 20 mm in diameter.…”

“…Various authors (e.g. Selker et al, 2006;Vogt et al, 2010; al., 2011; Vercauteren et al, 2011;Arnon et al, 2014) have increased the vertical spatial resolution of DTS measurements by wrapping fibre optic cable around a solid PVC tube, a technique sometimes called high-resolution DTS (HR-DTS), yielding spatial resolutions between 4 and 11 mm. The downside of this approach is that stress on the fibre or the characteristics of the supporting materials such as PVC (polyvinyl chloride) tubes (in this article: auxiliary constructions) might also influence the temperature measurements.…”

“…The downside of this approach is that stress on the fibre or the characteristics of the supporting materials such as PVC (polyvinyl chloride) tubes (in this article: auxiliary constructions) might also influence the temperature measurements. Such issues were recently pointed out by Arnon et al (2014), but aside from this one observed problem, to date no comprehensive analysis has been published on how the physical design of a coil-wrapped DTS measurement set-up affects temperature measurements. Tyler et al (2009) defined spatial resolution as the cable distance required to measure 90 % of a sharp temperature jump.…”

“…However, recent papers discuss several errors in DTS temperature measurements caused by practical considerations of the measurement set-up. Arnon et al (2014) described a signal loss and consequent error in their temperature measurements along a helically wound cable. This loss was mostly visible in the first 100 m of cable and dissipated later on.…”

“…With this, we aim to better relate the signal loss shown by Arnon et al (2014) to coil diameter and fibre type, specifically, bend tolerance.…”