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Fiber-optic distributed temperature sensing (DTS) systems record measurements along fiber optic cables and provide continuous temperature or strain profiles. OASIS calculates the measurements along the cable into 3D positions within the experiment. Accuracy of DTS temperatures is greatly improved by OASIS deriving dynamic calibration coefficients for each measurement and then calculating on-the-fly calibrated DTS temperatures from the raw Raman backscatter measurements (Stokes and anti-Stokes) .

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OASIS reports the DTS measurements from multiple perspectives:

  • Temperature profile along the cable at a specified time (measurement vs. cable meter)

  • Temperature profile orientated to the experiment at a specified time (measurement vs. position)

  • Temperature changes at a point along the cable over time (measurements vs. time)

  • Temperature changes at a position within the experiment over time (measurements vs. time)

  • Both 2D and 3D thermal maps spatially oriented to the experiment (when sufficient measurements are available).

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The ability to view DTS measurements from multiple perspectives provides details and insights into the temporal and spatial variations of the temperature field in and around the experiment.
 

DTS systems can measure multiple cables every few minutes with values recorded as frequently as every 10 cm along the cable. Data sets can become large quickly. OASIS works and feels  like a simple desktop program while behind the scenes special data structures and optimized SQL are being used to make retrieving data fast so that plots and reports are created quickly and respond dynamically

Distributed Temperature Sensing (DTS)

Measurement locations

Determining DTS measurements positions within the project requires relating measurements along the fiber optic cable to physical positions (coordinates) in the project. OASIS uses fingerprinting (heating the cable locally at known locations and noting the measured cable lengths where the responses are observed).  Spatial calculations are performed on-the-fly at run time so changes in DTS sampling interval (the spacing between measurements along the cable) or changes in the project are accounted for automatically. The geometric objects and custom linear algebra operators can quickly determine the spatial relationships between objects. Comparing and evaluating DTS measurements to other sensor measurements or objects in the project (fixed point sensors, tanks, pipe joints, etc.) is quick and easy. The OASIS cable selector (shown below) allows users to select cable runs and nearby sensors in both tabular and 3D views.

Spatial 3D GIS sensor management database has specialized distributed temperature sensing (DTS) routines to relate DTS measurements along fiber optic cable to actual locations within project. DTS measurement locations in spatial 3D (X, Y, Z) plots. Custom DTS database uses DTS fingerprints to calculate the location of DTS measurements along fiber optic cable.

  • The upper window section allows users to easily select cable runs and DTS parameters of interest. 

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  • In the lower window users can query sensors within a specified distance from the cable. The table’s sortable columns and filter make it easy to select just the sensors of interest.

 

  • Reports (plots and tables) are output in the selected coordinate system.

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  • Similar selector windows are available for reporting full cable runs, looping cable sections, 2D and 3D thermal maps, dynamic calibration coefficients and OTDR step loss measurements.

Cables, sensors and other objects are shown in the example Cable 3D view (below). The view can be zoomed, panned, tilted, etc. making it easy to relate objects spatially. The fiber optic cable can be colored to represent temperatures so changes in temperature around tanks or along pipelines (indicating possible leaks) can be easily located. Hovering the cursor over a sensor icon displays the newest sensor measurement and the distance from the sensor to the cable (orange line). The 3D view makes it easy to select cable measurements and fixed-point sensors for comparison or evaluation. 

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  • The 3D view can be customized using options to display only objects of interest.

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  • Sensor icon color can show measured parameters (pressure, humidity, etc.) or measurement values (low values are dark blue and high values are red). This allows easy identification of sensors by both 3D position and measurement (sometimes referred to as 4D)​

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  • Moving the mouse over a sensor icon displays the sensor name, latest measurement and distance from the cable (shown in orange).

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  • The spatial design allows investigators to discover relationships and connections in data that could be easily overlooked with only a 2D perspective.

DTS calibration

DTS systems generally use fixed coefficients to convert raw Stokes and anti-Stokes measurements into temperature values. The coefficients are often based on a 50/125 fiber and do not thoroughly account for real-world variations or damages in the fiber. The resulting temperatures can have inaccuracies of several degrees or more.

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OASIS can provide comprehensive DTS calibration using two water baths covering the expected experiment temperature range.

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  • The fiber optic cables are routed through the baths.

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  • Design shown is for duplexed single-ended configuration.  Single-ended and double ended system calibration are also possible.

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  • Bath temperatures are measured using conventional high precision temperature sensors. The OASIS data-pipeline automatically collects and appends the data in the database.​​

Signal step losses (abrupt decreases in light signal strength along the cable due to cable damage, splices or sharp cable bends) can significantly affect DTS temperature accuracy. Corrections assuming constant signal loss along the cable are often insufficient for accurate real-world temperature measurements. Step loss magnitude and location are determined using optical time domain reflectometry (OTDR) with the data also being stored in OASIS.

DTS dynamic coefficients

OASIS compensates for signal losses and calculates “dynamic” calibration coefficients for each cable measurement using the raw DTS measurements (Stokes and anti-Stokes) and bath temperatures as the data are collected from the field. The process of selecting representative bath temperatures, raw DTS measurements from the bath and evaluating the equations (summarized by Hausner M.B., Suárez F., Glander, K.E., van de Giesen N., Selker J.S. and Tyler S.W., 2011) uses optimized SQL procedures so it is extremely fast.

On-the-fly temperatures

OASIS calculates calibrated temperatures on-the-fly using the raw DTS measurements and the dynamic coefficients. The system is dynamic because calibrated DTS temperatures are calculated on demand – if equations or parameters are changed then the calibrated temperature are simply recalculated the next time the data are requested.

Improved DTS temperatures

Calibrated temperatures are significantly improved with an average measurement accuracy of 0.1 - 0.3°C depending on fiber optic cable and DTS interrogator.

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  • Accuracy of DTS measurements changes as temperatures change.

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  • Simply using an offset to match the DTS measurements to a specific temperature will not provide accurate values over a temperature range.

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  • Signal step losses can significantly affect DTS accuracy. Corrections assuming constant loss along the fiber are often insufficient for accurate temperatures in the real-world.

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  • Near real-time compensation for signal step losses and calculation of dynamic coefficients for each measurement significantly improve DTS accuracy and precision.

Calibrated temperatures are available just seconds after collection from the field. Calculated temperatures are traceable because all parameters used to derive the values are stored with the measurements in the database.

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