Fiber Optic Distributed Temperature Sensing

Basic Concepts

Analogous to how thermal infrared is used to identify and map bank and water-surface temperature anomalies, fiber-optic distributed temperature sensing (FO-DTS) can trace the thermal signatures of natural processes such as groundwater-surface water exchange (Hare et al., 2015). Because the FO-DTS method uses direct-contact rather than remote sensing, temperature sensitive cables are typically deployed within the water column, along the sediment/water interface, or buried in bed sediments. Measurements can also be performed in boreholes (Bense et al., 2016) and the unsaturated zone, depending on the study objectives.

FO-DTS measurements involve sending pulses of laser light through an armored fiber-optic cable from a control unit that is typically housed in a protective case. Following a laser pulse, photons interact with the molecular structure of the glass fibers and some of the light back-scatters to the instrument. Analysis of temperature-dependent Raman backscatter allows for temperature measurements to be made over time along the cable at sub-meter spatial resolution over several kilometers of linear distance.

Theory

FO-DTS systems function by initiating a laser pulse though an optical fiber and determining temperature along the fiber by measuring the ratio of temperature-independent Raman backscatter (Stokes) to temperature-dependent backscatter (anti-Stokes) of the incident laser pulse (Briggs et al., 2012; Tyler et al., 2009). Timing of this backscatter yields a measure of location along the fiber, which can be resolved to approximately 0.25-m resolution (depending on FO-DTS control unit type) at the scale of several km.

All FO-DTS measurements are integrated over time, and that period is user-defined, as is the linear integration distance (spatial resolution). Temperature measurement precision increases with the square root of time, as more Raman backscatter is collected by the control unit. Temperature precision also increases with integration distance, such that a 2-m measurement is more precise than a 0.25-m resolution measurement when collected during the same length of time. Therefore, FO-DTS is unique in temperature sensing instrumentation as the measurement resolution and precision are in part user defined and can be tailored to the study objectives.

FO-DTS measurements can be made in ‘single-ended’ or ‘double-ended’ mode by most instruments, where for the latter bidirectional laser pulses along the fiber simplifies the calibration processes. This longitudinal calibration is necessary because of the inherent attenuation of the laser pulse as it travels down the optical fiber and passes through spliced connections. Additionally, FO-DTS data calibration is needed to account for variable measurement drift over time as the control unit heats and cools.

This dynamic drift calibration is performed by placing a coiled length of fiber within a bath of known temperature, such as a cooler with ice water mixed by a bubbler, or a small, turbulent stream pool. Temperature of the bath is independently measured over time using a high precision thermistor built into the control unit or a discrete temperature logger supplied by the user.  

The FO-DTS control unit records data in ‘optical’ distance from the photon collector in the instrument.  For special referencing, the actual linear cable locations need to be measured.  These actual linear cable locations are then cross referenced with temperature signals identified in the optical record.  This special calibration then allows spatial referencing of the temperature signal.

Applications

The FO-DTS systems function as a spatially continuous temperature sensor that can be installed along the streambed to identify groundwater seepage, hyporheic return flows, bed sedimentation, and provide data for stream heat budget models. Typically, FO-DTS is only sensitive to gaining surface-water inflows, such as groundwater discharge or tributary confluences, not surface-water losses (Figure 1).

Figure 1. These plots show streambed interface FO-DTS data collected along a trout stream with numerous, discrete groundwater discharge zones. Cold groundwater seeps appear as vertical bands when temperature data are plotted vs time as ‘heat maps (a, b, c).  The USGS ‘DTS-GUI’ software can be used to efficiently trim non-target data such as that collected on the cable spool or temperature calibration bath shown in panels a) and b) to yield a clean analysis data set (c ). If cable georeference data are collected with external GPS the DTS-GUI can also be used to plot data in map view so groundwater discharge zones can more readily be located and sampled (d). (Modified from Domanski et al., 2020).

However, in some instances the FO-DTS cable can be buried within bed sediments at a common depth and losing zones can then be identified. FO-DTS is also deployed in boreholes to map bedrock fracture flow, though as natural borehole temperature signals are often muted, active heating of special cables may be necessary (e.g., Figure 2).

Actively heated cables can also be deployed in the vadose zone to estimate soil moisture content. Other diverse applications include fire detection in buildings and tracking of dam failure. If the fiber-optics are wrapped around a rod or pipe, apparent temperature measurement resolution along this core can be increased to the cm- to mm-scale (e.g., Briggs et al., 2016). When installed vertically with bed sediments or the water column, fine detail regarding heat exchanges and mixing can be tracked and attributed to processes of interest (Figure 2). In addition to onboard instrument software, publicly available software packages have been developed for efficient spatial plotting and analysis (Figure 1, Domanksi et al., 2020).

Figure 2. Vertical wrapped FO-DTS optic fibers can be actively heated to create temperature signals in settings with little natural thermal contrast, such as strong groundwater discharge zones or boreholes (modified from Briggs et al., 2016).

Examples/Case Studies

Bense, V. F., T. Read, O. Bour, T. Le Borgne, T. Coleman, S. Krause, A. Chalari, M. Mondanos, F. Ciocca, and J. S. Selker (2016), Distributed Temperature Sensing as a downhole tool in hydrogeology, Water Resour. Res., 52, 9259–9273, doi:10.1002/ 2016WR018869.

Abstract: Distributed Temperature Sensing (DTS) technology enables downhole temperature monitoring to study hydrogeological processes at unprecedentedly high frequency and spatial resolution. DTS has been widely applied in passive mode in site investigations of groundwater flow, in-well flow, and subsurface thermal property estimation. However, recent years have seen the further development of the use of DTS in an active mode (A-DTS) for which heat sources are deployed. A suite of recent studies using A-DTS downhole in hydrogeological investigations illustrate the wide range of different approaches and creativity in designing methodologies. The purpose of this review is to outline and discuss the various applications and limitations of DTS in downhole investigations for hydrogeological conditions and aquifer geological properties. To this end, we first review examples where passive DTS has been used to study hydrogeology via downhole applications. Secondly, we discuss and categorize current A-DTS borehole methods into three types. These are thermal advection tests, hybrid cable flow logging, and heat pulse tests. We explore the various options with regards to cable installation, heating approach, duration, and spatial extent in order to improve their applicability in a range of settings. These determine the extent to which each method is sensitive to thermal properties, vertical in-well flow, or natural gradient flow. Our review confirms that the application of DTS has significant advantages over discrete point temperature measurements, particularly in deep wells, and highlights the potential for further method developments in conjunction with other emerging hydrogeophysical tools. Copyright © 2016. American Geophysical Union. All Rights Reserved.

Briggs, M.A., L.K. Lautz, and J.M. McKenzie. 2012. A comparison of fibre-optic distributed temperature sensing to traditional methods of evaluating groundwater inflow to streams. Hydrological Processes 26, no. 9: 1277–1290. https://doi.org/10.1002/hyp.8200.

Abstract: There are several methods for determining the spatial distribution and magnitude of groundwater inputs to streams. We compared the results of conventional methods [dye dilution gauging, acoustic Doppler velocimeter (ADV) differential gauging, and geochemical end‐member mixing] to distributed temperature sensing (DTS) using a fibre‐optic cable installed along 900 m of Ninemile Creek in Syracuse, New York, USA, during low‐flow conditions (discharge of 1·4 m³ s⁻¹). With the exception of differential gauging, all methods identified a focused, contaminated groundwater inflow and produced similar groundwater discharge estimates for that point, with a mean of 66·8 l s⁻¹ between all methods although the precision of these estimates varied. ADV discharge measurement accuracy was reduced by non‐ideal conditions and failed to identify, much less quantify, the modest groundwater input, which was only 5% of total stream flow. These results indicate ambient tracers, such as heat and geochemical mixing, can yield spatially and quantitatively refined estimates of relatively modest groundwater inflow even in large rivers. DTS heat tracing, in particular, provided the finest spatial characterization of groundwater inflow, and may be more universally applicable than geochemical methods, for which a distinct and consistent groundwater end member may be more difficult to identify. Copyright © 2011 John Wiley & Sons, Ltd.

Briggs, M.A., E.B. Voytek,  F.D. Day-Lewis, D.O. Rosenberry, and J.W. Lane (2013), Understanding Water Column and Streambed Thermal Refugia for Endangered Mussels in the Delaware River, Environmental Sciences and Technology, 47, doi:10.1021/es4018893

Abstract: Groundwater discharge locations along the upper Delaware River, both discrete bank seeps and diffuse streambed upwelling, may create thermal niche environments that benefit the endangered dwarf wedgemussel (Alasmidonta heterodon). We seek to identify whether discrete or diffuse groundwater inflow is the dominant control on refugia. Numerous springs and seeps were identified at all locations where dwarf wedgemussels still can be found. Infrared imagery and custom high spatial resolution fiber-optic distributed temperature sensors reveal complex thermal dynamics at one of the seeps with a relatively stable, cold groundwater plume extending along the streambed/water-column interface during midsummer. This plume, primarily fed by a discrete bank seep, was shown through analytical and numerical heat-transport modeling to dominate temperature dynamics in the region of potential habitation by the adult dwarf wedgemussel.

Briggs, M. A., S. F. Buckley, A. C. Bagtzoglou, D. D. Werkema, and J. W. Lane (2016), Actively heated high- resolution fiber-optic-distributed temperature sensing to quantify streambed flow dynamics in zones of strong groundwater upwelling, Water Resour. Res., 52, 5179–5194, doi:10.1002/2015WR018219.

Abstract: Zones of strong groundwater upwelling to streams enhance thermal stability and moderate thermal extremes, which is particularly important to aquatic ecosystems in a warming climate. Passive thermal tracer methods used to quantify vertical upwelling rates rely on downward conduction of surface temperature signals. However, moderate to high groundwater flux rates (>21.5 m d21) restrict downward propagation of diurnal temperature signals, and therefore the applicability of several passive thermal methods. Active streambed heating from within high-resolution fiber-optic temperature sensors (A-HRTS) has the potential to define multidimensional fluid-flux patterns below the extinction depth of surface thermal signals, allowing better quantification and separation of local and regional groundwater discharge. To demonstrate this concept, nine A-HRTS were emplaced vertically into the streambed in a grid with 0.40 m lateral spacing at a stream with strong upward vertical flux in Mashpee, Massachusetts, USA. Long-term (8–9 h) heating events were performed to confirm the dominance of vertical flow to the 0.6 m depth, well below the extinction of ambient diurnal signals. To quantify vertical flux, short-term heating events (28 min) were performed at each A-HRTS, and heat-pulse decay over vertical profiles was numerically modeled in radial two dimension (2-D) using SUTRA. Modeled flux values are similar to those obtained with seepage meters, Darcy methods, and analytical modeling of shallow diurnal signals. We also observed repeatable differential heating patterns along the length of vertically oriented sensors that may indicate sediment layering and hyporheic exchange superimposed on regional groundwater discharge.

Hare, D.K., M.A. Briggs, D.O. Rosenberry, D.F. Boutt, and J.W. Lane (2015), A comparison of thermal infrared to fiber-optic distributed temperature sensing for evaluation of groundwater discharge to surface water, Journal of Hydrology, 530, doi: 10.1016/j.jhydrol.2015.09.059

Abstract: Groundwater has a predictable thermal signature that can be used to locate discrete zones of discharge to surface water. As climate warms, surface water with strong groundwater influence will provide habitat stability and refuge for thermally stressed aquatic species, and is therefore critical to locate and protect. Alternatively, these discrete seepage locations may serve as potential point sources of contaminants from polluted aquifers. This study compares two increasingly common heat tracing methods to locate discrete groundwater discharge: direct-contact measurements made with fiber-optic distributed temperature sensing (FO-DTS) and remote sensing measurements collected with thermal infrared (TIR) cameras. FO-DTS is used to make high spatial resolution (typically m) thermal measurements through time within the water column using temperature-sensitive cables. The spatial–temporal data can be analyzed with statistical measures to reveal zones of groundwater influence, however, the personnel requirements, time to install, and time to georeference the cables can be burdensome, and the control units need constant calibration. In contrast, TIR data collection, either from handheld, airborne, or satellite platforms, can quickly capture point-in-time evaluations of groundwater seepage zones across large scales. However the remote nature of TIR measurements means they can be adversely influenced by a number of environ- mental and physical factors, and the measurements are limited to the surface ‘‘skin” temperature of water features. We present case studies from a range of lentic to lotic aquatic systems to identify capabilities and limitations of both technologies and highlight situations in which one or the other might be a better instrument choice for locating groundwater discharge. FO-DTS performs well in all systems across seasons, but data collection was limited spatially by practical considerations of cable installation. TIR is found to consistently locate groundwater seepage zones above and along the streambank, but submerged seepage zones are only well identified in shallow systems (e.g. <0.5 m depth) with moderate flow. Winter data collection, when groundwater is relatively warm and buoyant, increases the water surface expression of discharge zones in shallow systems.

Lowry, C. S., J. F. Walker, R. J. Hunt, and M. P. Anderson (2007), Identifying spatial variability of groundwater discharge in a wetland stream using a distributed temperature sensor, Water Resour. Res., 43, W10408, doi:10.1029/2007WR006145.

Abstract: Discrete zones of groundwater discharge in a stream within a peat-dominated wetland were identified on the basis of variations in streambed temperature using a distributed temperature sensor (DTS). The DTS gives measurements of the spatial (±1 m) and temporal (15 min) variation of streambed temperature over a much larger reach of stream (>800 m) than previous methods. Isolated temperature anomalies observed along the stream correspond to focused groundwater discharge zones likely caused by soil pipes within the peat. The DTS also recorded variations in the number of temperature anomalies, where higher numbers correlated well with a gaining reach identified by stream gauging. Focused zones of groundwater discharge showed essentially no change in position over successive measurement periods. Results suggest DTS measurements will complement other techniques (e.g., seepage meters and stream gauging) and help further improve our understanding of groundwater–surface water dynamics in wetland streams.

Henderson, R.D., Day-Lewis, F.D., and Harvey, C.F., 2009, Investigation of aquifer-estuary interaction using wavelet analysis of fiber-optic temperature data: Geophysical Research Letters, v. 36, L06403, https://doi.org/10.1029/2008GL036926.

Abstract: Fiber-optic distributed temperature sensing (FODTS) provides sub-minute temporal and meter-scale spatial resolution over kilometer-long cables. Compared to conventional thermistor or thermocouple-based technologies, which measure temperature at discrete (and commonly sparse) locations, FODTS offers nearly continuous spatial coverage, thus providing hydrologic information at spatiotemporal scales previously impossible. Large and information-rich FODTS datasets, however, pose challenges for data exploration and analysis. To date, FODTS analyses have focused on time-series variance as the means to discriminate between hydrologic phenomena. Here, we demonstrate the continuous wavelet transform (CWT) and cross-wavelet transform (XWT) to analyze FODTS in the context of related hydrologic time series. We apply the CWT and XWT to data from Waquoit Bay, Massachusetts to identify the location and timing of tidal pumping of submarine groundwater.

Mwakanyamale, K., Slater, L., Day-Lewis, F.D., Elwaseif, M., Ntarlagiannis, D., and Johnson, C.D., 2012, Spatially variable stage-driven groundwater-surface water interaction inferred from time-frequency analysis of distributed temperature sensing data: Geophysical Research Letters, https://doi.org/10.1029/2011GL050824.

Abstract: Characterization of groundwater-surface water exchange is essential for improving understanding of contaminant transport between aquifers and rivers. Fiber-optic distributed temperature sensing (FODTS) provides rich spatiotemporal datasets for quantitative and qualitative analysis of groundwater-surface water exchange. We demonstrate how time-frequency analysis of FODTS and synchronous river stage time series from the Columbia River adjacent to the Hanford 300-Area, Richland, Washington, provides spatial information on the strength of stage-driven exchange of uranium contaminated groundwater in response to subsurface heterogeneity. Although used in previous studies, the stage-temperature correlation coefficient proved an unreliable indicator of the stage-driven forcing on groundwater discharge in the presence of other factors influencing river water temperature. In contrast, S-transform analysis of the stage and FODTS data definitively identifies the spatial distribution of discharge zones and provided information on the dominant forcing periods (≥2 d) of the complex dam operations driving stage fluctuations and hence groundwater-surface water exchange at the 300-Area.

Selker, J. S., L. The´venaz, H. Huwald, A. Mallet, W. Luxemburg, N. van de Giesen, M. Stejskal, J. Zeman, M. Westhoff, and M. B. Parlange (2006), Distributed fiber-optic temperature sensing for hydrologic systems, Water Resour. Res., 42, W12202, doi:10.1029/2006WR005326.

Abstract: Instruments for distributed fiber-optic measurement of temperature are now available with temperature resolution of 0.01°C and spatial resolution of 1 m with temporal resolution of fractions of a minute along standard fiber-optic cables used for communication with lengths of up to 30,000 m. We discuss the spectrum of fiber-optic tools that may be employed to make these measurements, illuminating the potential and limitations of these methods in hydrologic science. There are trade-offs between precision in temperature, temporal resolution, and spatial resolution, following the square root of the number of measurements made; thus brief, short measurements are less precise than measurements taken over longer spans in time and space. Five illustrative applications demonstrate configurations where the distributed temperature sensing (DTS) approach could be used: (1) lake bottom temperatures using existing communication cables, (2) temperature profile with depth in a 1400 m deep decommissioned mine shaft, (3) air-snow interface temperature profile above a snow-covered glacier, (4) air-water interfacial temperature in a lake, and (5) temperature distribution along a first-order stream. In examples 3 and 4 it is shown that by winding the fiber around a cylinder, vertical spatial resolution of millimeters can be achieved. These tools may be of exceptional utility in observing a broad range of hydrologic processes, including evaporation, infiltration, limnology, and the local and overall energy budget spanning scales from 0.003 to 30,000 m. This range of scales corresponds well with many of the areas of greatest opportunity for discovery in hydrologic science.

Tyler, S. W., J. S. Selker, M. B. Hausner, C. E. Hatch, T. Torgersen, C. E. Thodal, and S. G. Schladow (2009), Environmental temperature sensing using Raman spectra DTS fiber-optic methods, Water Resour. Res., 45, W00D23, doi:10.1029/2008WR007052.

Abstract: Raman spectra distributed temperature sensing (DTS) by fiber-optic cables has recently shown considerable promise for the measuring and monitoring of surface and near-surface hydrologic processes such as groundwater–surface water interaction, borehole circulation, snow hydrology, soil moisture studies, and land surface energy exchanges. DTS systems uniquely provide the opportunity to monitor water, air, and media temperatures in a variety of systems at much higher spatial and temporal frequencies than any previous measurement method. As these instruments were originally designed for fire and pipeline monitoring, their extension to the typical conditions encountered by hydrologists requires a working knowledge of the theory of operation, limitations, and system accuracies, as well as the practical aspects of designing either short- or long-term experiments in remote or challenging terrain. This work focuses on providing the hydrologic user with sufficient knowledge and specifications to allow sound decisions on the application and deployment of DTS systems.

References

Bense, V. F., T. Read, O. Bour, T. Le Borgne, T. Coleman, S. Krause, A. Chalari, M. Mondanos, F. Ciocca, and J. S. Selker, 2016, Distributed Temperature Sensing as a downhole tool in hydrogeology, Water Resour. Res., 52, 9259–9273, doi:10.1002/ 2016WR018869.

Briggs, M.A., L.K. Lautz, and J.M. McKenzie, 2012, A comparison of fibre-optic distributed temperature sensing to traditional methods of evaluating groundwater inflow to streams. Hydrological Processes 26, no. 9: 1277–1290. https://doi.org/10.1002/hyp.8200.

Briggs, M.A., E.B. Voytek,  F.D. Day-Lewis, D.O. Rosenberry, and J.W. Lane, 2013, Understanding Water Column and Streambed Thermal Refugia for Endangered Mussels in the Delaware River, Environmental Sciences and Technology, 47, doi:10.1021/es4018893

Briggs, M. A., S. F. Buckley, A. C. Bagtzoglou, D. D. Werkema, and J. W. Lane, 2016, Actively heated high- resolution fiber-optic-distributed temperature sensing to quantify streambed flow dynamics in zones of strong groundwater upwelling, Water Resour. Res., 52, 5179–5194, doi:10.1002/2015WR018219.

Domanski, M., Quinn, D., Day-Lewis, F., Briggs, M., Werkema, D., and Lane, J.W.,2020, DTSGUI; A Python program to process and visualize fiber-optic distributed temperature sensing data, Groundwater, doi: 10.1111/gwat.12974.

Hare, D.K., M.A. Briggs, D.O. Rosenberry, D.F. Boutt, and J.W. Lane, 2015, A comparison of thermal infrared to fiber-optic distributed temperature sensing for evaluation of groundwater discharge to surface water, Journal of Hydrology, 530, doi: 10.1016/j.jhydrol.2015.09.059

Henderson, R.D., Day-Lewis, F.D., and Harvey, C.F., 2009, Investigation of aquifer-estuary interaction using wavelet analysis of fiber-optic temperature data: Geophysical Research Letters, v. 36, L06403, https://doi.org/10.1029/2008GL036926.

Lowry, C. S., J. F. Walker, R. J. Hunt, and M. P. Anderson, 2007, Identifying spatial variability of groundwater discharge in a wetland stream using a distributed temperature sensor, Water Resour. Res., 43, W10408, doi:10.1029/2007WR006145.

Mwakanyamale, K., Slater, L., Day-Lewis, F.D., Elwaseif, M., Ntarlagiannis, D., and Johnson, C.D., 2012, Spatially variable stage-driven groundwater-surface water interaction inferred from time-frequency analysis of distributed temperature sensing data: Geophysical Research Letters, https://doi.org/10.1029/2011GL050824.

Selker, J. S., L. The´venaz, H. Huwald, A. Mallet, W. Luxemburg, N. van de Giesen, M. Stejskal, J. Zeman, M. Westhoff, and M. B. Parlange, 2006, Distributed fiber-optic temperature sensing for hydrologic systems, Water Resour. Res., 42, W12202, doi:10.1029/2006WR005326.

Tyler, S. W., J. S. Selker, M. B. Hausner, C. E. Hatch, T. Torgersen, C. E. Thodal, and S. G. Schladow, 2009, Environmental temperature sensing using Raman spectra DTS fiber-optic methods, Water Resour. Res., 45, W00D23, doi:10.1029/2008WR007052.