as the seismic sensor. Unlike seismometers that are thought of as point sensors, a fiber‐optic cable senses strain along the entire fiber, which can be thousands of kilometers long. Because individual sensors do not make the measurement, it is referred to as a “distributed sensor measurement.” For ordinary telecommunication applications, a laser generates encoded light signals that pass information along the fiber to a distant receiver. Small defects or changes in the optical properties of the fiber along its length cause the light to be backscattered toward the laser source. The fiber is designed to minimize, by five to seven orders of magnitude as compared to the illuminating light, the amount of attenuation and backscattered light, so that data can be transmitted over large distances. However, this same unwanted backscattered light in telecommunications is used for DAS applications to detect and characterize local changes in the strain of the optical fiber from acoustic and seismic signals as well as from temperature changes.
When seismic waves or small temperature transients mechanically deform an optical fiber, the optical propagation properties of the fiber change, causing extremely small time delays during the travel path of the backscattered light. When a pulse of laser light is introduced into one end of the fiber, these small changes in the optical properties of the fiber create a continuous “shower” of scattered light emanating from virtually all points along the fiber as the pulse passes through. The timing change of the backscattered light forms the basis by which the strain, or deformation, of the fiber can be measured using an optical interrogation system.
2.2.2. Single vs. Multi‐mode Fiber
One of the first features to determine is the type of fiber to use as the seismic sensor. Practically all older fiber installations use multi‐mode fiber to enable the acquisition of temperature measurements using distributed temperature sensing (DTS). Most newer installations use a cable with two or more single‐mode and two or more multimode fibers inside. For DAS applications, single‐mode fiber currently provides the best SNR properties as compared to multi‐mode fiber. Single‐mode fiber has a small inner glass core diameter of 9 microns, which only allows a single, virtually direct path for the light to propagate; the light is totally internally reflected within the glass. On the contrary, multi‐mode fiber has a larger glass core diameter of 50 or more microns. While the light is still completely internally reflected, the wider glass core thickness allows for multiple paths, or modes, to be transmitted through the fiber. More light energy can be pumped into the multi‐mode fiber; however, the interference of the light pulse from the multiple paths can interfere with the quality of the DAS strain measurement. Thus, it is advisable to use single‐mode fiber for DAS measurements whenever possible. Multi‐mode fiber can be used for DAS measurements, but it usually requires additional optical hardware and does not normally provide an SNR as effective as single mode.
2.2.3. Deploying Fiber
The next determination is how the fiber will be deployed as the seismic sensor. Figure 2.1 shows three different deployment methods for DAS acquisition in a well. The retrievable fiber option uses an optical glass fiber installed inside either a wireline cable or coiled tubing. This option is by far the easiest to deploy, because it can be inserted into and removed from a well at any time. However, it is likely to have the lowest quality of strain measurement because the fiber is not directly coupled to the formation. A new method involves deploying a simple, disposable fiber‐optic glass line into the well (Higginson et al., 2017). In addition to the coupling issue, this method could encounter depthing problems, because there is no control of the tension (thus, placement) of the fiber in the well. Another option is to strap the fiber‐optic cable to production tubing; however, the highest quality comes from attaching the fiber‐optic cable to the outside of the casing, and cementing the casing. For this option, the fiber‐optic cable is directly coupled to the formation and has the best SNR of seismic signals propagating in the surrounding rock.
Figure 2.1 Options for acquiring DAS VSP data in a well.
To monitor teleseismic events, existing fiber‐optic telecommunication cables deployed in shallow‐buried conduits could be used (Martin et al., 2017). Described in the following section, the potential issue is that the broadside response of the fiber to strain is controlled by a cosine‐squared sensitivity to the angle of incidence for P waves, putting a null in the sensitivity for events arriving normal to the fiber. For yet another application, fiber‐optic cables buried in shallow trenches can be used to monitor surface waves, and then interferometric means can characterize the shallow earth properties (Martin et al., 2016).
2.2.4. Handling Fiber‐Optic Cables
Fiber‐optic cables typically contain multiple strands of fiber‐optic glass and can be included during the manufacturing of other cable types, such as wireline cables that have multiple electrical wires. Unlike conventional electrical cables, fiber‐optic cables require a different handling strategy. Bends in fiber‐optic cables must be minimized because a tight radius of curvature will allow the laser light trapped in the fiber to leak out of the glass core, thereby reducing the sensitivity of the strain measurements. Another important aspect is that fiber‐optic connections must be made under clean conditions. Making fiber‐optic connections under unclean conditions—where junction boxes are exposed to the wind, sand, dirt, and even oils from the skin—will generate significant optical losses; therefore, it is important to plan for clean areas and facilities where the optical connections can be made.
Figure 2.2 (Left) Conceptual diagram of an IU (inside the dotted black line); (right) relationship between the measurements of I and Q and the resulting extracted phase value Θ. The dotted red line represents the modulus (or length) of the I/Q vector.
2.3. INTERROGATOR UNIT
2.3.1. Types of Interrogators
The laser that emits light pulses and the hardware measuring system to convert the backscattered light to a strain measurement are housed in the interrogator unit (IU). The dotted black line in Figure 2.2, left, shows a conceptual diagram of an IU. A pulse of light is emitted from the laser into a fiber‐optic cable inside the IU. The light then encounters several optical elements inside the IU and exits using a surface cable connected to the length of fiber being used as a strain detector (e.g., in a well or shallow trench near the surface of the earth). The backscattered laser light returns from the sensing fiber and reenters the IU, where the light is routed through more optical devices, eventually encountering a receiver that converts the light into analog electrical signals that are then converted to a digital data stream using digitizers, typically in a supporting computer system.
IUs might use several different optical designs to emit laser light into the optical fiber that converts the backscattered signals into a measurement of strain (Hartog, 2017). Practically all current hardware on the market use a differential phase method to obtain a high‐fidelity and linear measurement of strain. Note that earlier technology, based only on the amplitude of the backscattered signal, did not provide a reliable measurement of strain, because the amplitude of the backscattered light was not linear with strain.
Essential to the reliable measurement of strain is the concept of a dual‐pulse optical system. This methodology creates two pulses of backscattered light combined in an interferometric process to construct the phase difference between these pulses. These two pulses
