| 2D seismic | Seismic surveys wherein the geometry of the source-receivers spans a two-dimensional space. |
| 3D seismic | Seismic surveys wherein the geometry of the source-receivers spans a three-dimensional space. |
| 4D seismic | Seismic surveys wherein the geometry of the source-receivers spans a three-dimensional space, but where acquisitions are repeated over time. |
| Accuracy | Quantifies how close a measurement is to a real value. For DTS systems, the real values are absolute temperatures. Accuracy must not be confused with resolution or precision. |
| Active distributed acoustic sensing | The use of distributed acoustic sensing in cases where the signal is comprised both of engineered acoustic sounds as well as naturally occurring signals. |
| Active DTS | A method of DTS data collection based on using a heated fibre optics cable(s) and monitoring the thermal response during the heating and/or the cooling phase. Also commonly known as the heat pulse method. Active DTS can be used for measurement of thermal properties and directly dependent quantities (e.g. soil moisture, heat fluxes), and of advective fluxes of water and air. |
| Advanced acoustic processing | Acoustic processing to determine the speed of sound in the production fluid at different depths. This information indicates in-situ fluid type and can be used to resolve hold-up in two-phase flow. |
| Anti-Stokes Backscatter | The component of Raman backscattering that is shifted to a lower wavelength from the incident light. This backscattered signal is strongly temperature dependent. |
| Attenuation | A measure of the loss of intensity with distance of the light travelling in an optical fibre, usually expressed in dB or dB/km. |
| Backscattered Light | Also referred to as backscatter, is the portion of light that, during the forward propagation of a pulse through the optical fibre, is scattered back towards the light-emitting source because of collisions of the photons with the constitutive elements of the optical fibre. |
| Brillouin Signal | The backscattered signal shifted in wavelength from the incident light pulse through energization of the silica lattice structure. This component of backscatter is most commonly used for distributed strain sensing. Brillouin wavelengths are shifted less from the incident wavelength that the Raman backscatter. |
| Calibration | When used in conjunction with DTS this term refers to means of calibrating the physical quantities that allow inferring temperature from DTS raw signal. A good calibration is crucial to avoid the DTS temperature readings to be affected mainly by offset (constant shift between measured and real absolute temperatures) and slope issues (variation with distance of DTS readings of a constant temperature). Multiple calibration methods are available including both single-ended and double-ended techniques. |
| Cladding | One or more layers of material in direct outer contact with the fibre core. The cladding has a lower refractive index than the core causing light to be contained within the fibre through total internal reflection. |
| Core | The central component of an optical fibre in which the majority of light travels. This material has a higher refractive index than the cladding in order to maintain total internal reflection. |
| Differential Attenuation | Delta attenuation between the different loss of light intensity with distance for the Stokes and anti-Stokes Raman backscatter. This physical quantity requires careful calibration in order to set the correct slope of a DTS trace (DTS readings of a constant temperature do not change with distance). |
| Distributed Acoustic Sensor (DAS) | A sensor that allows for the collection of acoustic signal fully distributed along an optical fibre. The most advanced DAS systems allow for the collection of full-waveform acoustic data. |
| Distributed Temperature Sensor (DTS) | A sensor that allows for the collection of temperature data fully distributed along an optical fibre. |
| Doppler shift | Change in frequency of a wave (or other periodic event) for an observer moving relative to its source. |
| Double-ended | A method of DTS measurement and calibration that requires an Optical Fibre in U shaped configuration. Each temperature trace is provided by combining two raw data traces measured sequentially from the two ends. Particularly useful for DTS calibration of multiple spliced and/or tight bended Fibre Optic Cables as it provides integrated differential attenuations in space. However, an extremely accurate measurement of the optical fibre length is necessary. It must not be confused with the duplex method. |
| Duplex | A method of DTS measurement and calibration that requires an Optical Fibre in U shaped configuration. Single-ended measurements are performed sequentially from the two ends. Differently from double-ended method, the raw data traces are not combined. |
| Fibre Optic Cable | A complete assembled product including optical fibre(s), strength members, and protective jacket(s) that provides protection to the optical fibre(s) from environmental factors. |
| Fusion Splicing | A method of joining two optical fibres end to end using intense heat. This method can provide connections with very low attenuation. |
| iDAS enabled flow meter | Real-time monitoring of multi-phase flow with Silixa’s iDAS |
| Incident Light | The light that is coupled into an optical fibre at the DTS and travels down the fibre from the near end toward the far end of the fibre. |
| Multimode Fibre | Optical fibre that has a relatively large core diameter (when compared with singlemode fibre) of usually 50 or 62.5 µm. This type of fibre supports more than one light propagation mode. |
| Optical Fibre | Consisting of the core surrounded by the cladding, the optical fibre provides a means of transmitting signal at very high bandwidths over long distances through a process known as total internal reflection. Optical fibres are usually made of silica. |
| Passive distributed acoustic sensing | The use of distributed acoustic sensing in cases where the signal is comprised solely of naturally occurring acoustic signals. |
| Passive DTS | The standard method of collecting temperature data along an optical fibre using DTS instrumentation, without artificially heating the fibre optic cable as in the active DTS methods. |
| Precision | Quantifies the smallest variation of a quantity that an instrument is able to detect and properly measure above the noise. For a DTS system, the smallest fluctuations in temperature that can be resolved. Precision can be used as synonymous of resolution, and is an index of the repeatability (in both space and time) of a measurement. It must not be confused with the accuracy. An instrument can be very precise, but still poorly accurate, and vice-versa. |
| Raman Scattering (Raman Effect) | Light that is shifted in wavelength from an incident light pulse through vibrations of the silica molecular structure. The Raman backscatter caused by an incident light pulse propagating through an optical fibre is most commonly used for DTS. Raman wavelengths are shifted further from the incident wavelength that the Brillouin backscatter. |
| Rayleigh Scattering (Rayleigh Effect) | Light scattered at the same wavelength of the incident light. In an optical fibre Rayleigh scattering is caused by microscopic density and refractive index variations. The Rayleigh backscatter caused by an incident light pulse propagating through an optical fibre is most commonly used for DAS. |
| Repeatability | Refers to the precision (or resolution) at which a measurement can be repeated in space and time. It must not be related to the accuracy. For a DTS system, the index of repeatability is provided by the temporal resolution. |
| Sampling Resolution | The sampling interval spacing along an optical fibre that a DAS or DTS system collects. Though related, the sampling resolution should not be confused with the spatial resolution of the instrument. |
| Single-ended | A method of measurement and calibration in which temperature traces are collected from one end of an optical fibre. |
| Singlemode Fibre | Optical fibre that has a relatively small core diameter (when compared with multimode fibre) of usually 8 or 9 µm. This type of fibre supports only a single light propagation mode. |
| Singlemode optical fibre cable | A fibre optic cable wherein the fibre is designed to transmit light only in the transverse mode. |
| Spatial Resolution | The minimum distance that a distributed sensing system needs in order to measure entirely, or almost entirely, a step change in signal along an optical fibre. For DTS systems, this is usually calculated as the distance needed to differentiate between 10% and 90% of an applied temperature step. |
| Step loss | Sharp loss in intensity of light propagation along an optical fibre, very localized in space. It is usually due to connectors and fusion splices, and appears as a step in the Stokes and Anti-Stokes traces. If not considered, DTS readings at larger distances than the step loss may be biased. |
| Stokes Backscatter | The component of Raman backscattering that is shifted to a higher wavelength from the incident light. This backscatter is not strongly temperature dependent. |
| Temperature Resolution | Indicates the precision of the DTS system to measure temperature (i.e. how noisy is the instrument). It is usually calculated in space as the standard deviation of the DTS readings along short optical fibre sections (in order to do not consider differential attenuation effects) held at constant temperature. It can be also calculated in time for a fixed point along the optical fibre. |
| Temporal Resolution | This is the time required to collect an individual trace along an optical fibre. |
| Termination | The completion at either end of an optical fibre or cable. Examples include connectors and U‑bends. |
| True acoustic data | iDAS records the true amplitude and phase of the acoustic signal allowing advanced signal processing techniques to be applied to the data. |
| U Shaped Configuration | Deployment of a Fibre Optic Cable with both ends connected to a DTS instrument. This configuration allows performing measurements in two directions. It is necessary for double-ended and duplex methods. |
