Distributed Strain Sensor Research Articles

Reliability Assessment of Fiber Optic Shape Sensing

Fiber optic shape sensing is a promising technology capable of enriching the field of Structural Health Monitoring (SHM), with many potential applications that expand the uses of distributed optical fiber sensors. The operational principle is based on the simultaneous strain acquisition from several optical fiber cores deployed in a single sensing cable (or multicore fiber). The parallel strain traces of these cores can be combined to extract curvature and torsion distributions along the sensing cable, that can be used for the three-dimensional reconstruction of the underlying shape. This study evaluates the reliability of fiber optic shape sensing for damage detection through the use of a model-assisted probability of detection framework specifically developed to simulate a carbon fiber-reinforced polymers specimen under quasi-static loading monitored by distributed optical fiber sensors based on Rayleigh backscattering. Specifically, we compare the performance of a traditional distributed optical fiber strain sensor with the one of a hypothetical distributed shape sensing cable (or multicore fiber). To perform this comparison, we redefine the damage index definition, which is usually based on the measured strain values, to make it compatible with shape sensing, where it can be defined in terms of curvature and torsion. With these preliminary results, we aim to evaluate whether shape sensing technology can be employed for damage detection to detect damage-related deformation, whether we can obtain better results compared to traditional optical fiber sensor applications, and discuss how damage metrics need to be modified. In future activities, these aspects will be further investigated taking into account other case studies and shape sensing configurations.

Optical frequency domain reflectometry-based high-performance distributed sensing empowered by a data and physics-driven neural network.

Optical frequency domain reflectometry (OFDR) based distributed strain sensors are the preferred choice for achieving accurate strain measurements over extensive sensing ranges while maintaining exceptional spatial resolution. However, the simultaneous realization of high spatial resolution, high strain resolution, large strain range, and an extended sensing range presents an exceedingly challenging endeavor. In this study, we introduce and experimentally demonstrate a data and physics-driven neural network-empowered OFDR system designed to attain high-performance distributed sensing. In our experiments, we successfully maintained an impressive sensing resolution of sub-microstrain (0.91 μ ε) alongside a sharp spatial resolution of sub-millimeter (0.857 mm) across a 140-m sensing range. To the best of our knowledge, this marks the inaugural experimental demonstration of OFDR-based distributed sensing, combining sub-millimeter spatial resolution and sub-μ ε strain resolution across a lengthy sensing range over a hundred meters. This pioneering work unveils new pathways for the development of ultra-high-performance optical fiber sensing systems, paving the way for the next generation of intelligent systems tailored for diverse smart industrial applications.

Pre-Failure Strain Localization in Siliclastic Rocks: A Comparative Study of Laboratory and Numerical Approaches

We combined novel laboratory techniques and numerical modeling to investigate (a)seismic preparatory processes associated with deformation localization during a triaxial failure test on a dry sample of Berea sandstone. Laboratory observations were quantified by measuring strain localization on the sample surface with a distributed strain sensing (DSS) array, utilizing optical fibers, in conjunction with both passive and active acoustic emission (AE) techniques. A physics-based computational model was subsequently employed to understand the underlying physics of these observations and to establish a spatio-temporal correlation between the laboratory and modeling results. These simulations revealed three distinct stages of preparatory processes: (i) highly dissipative fronts propagated towards the middle of the sample correlating with the observed acoustic emission locations; (ii) dissipative regions were individuated in the middle of the sample and could be linked to a discernible decrease of the P-wave velocities; (iii) a system of conjugate bands formed, coalesced into a single band that grew from the center towards the sample surface and was interpreted to be representative for the preparation of a weak plane. Dilatative lobes at the process zones of the weak plane extended outwards and grew to the surface, causing strain localization and an acceleration of the simulated deformation prior to failure. This was also observed during the experiment with the strain rate measurements and spatio-temporally correlated with an increase of the seismicity rate in a similar rock volume. The combined approach of such laboratory and numerical techniques provides an enriched view of (a)seismic preparatory processes preceding the mainshock.

Experimental Investigation of Low-Frequency Distributed Acoustic Sensor Responses to Two Parallel Propagating Fractures.

Low-frequency distributed acoustic sensing (LF-DAS) is a diagnostic tool for hydraulic fracture propagation with far-field monitoring using fiber optic sensors. LF-DAS senses strain rate variation caused by stress field change due to fracture propagation. Fiber optic sensors are installed in the monitoring wells in the vicinity of a fractured well. From the strain responses, fracture propagation can be evaluated. To understand subsurface conditions with multiple propagating fractures, a laboratory-scale hydraulic fracture experiment was performed simulating the LF-DAS response to fracture propagation with embedded distributed optical fiber strain sensors under these conditions. The experiment was performed using a transparent cube of epoxy with two parallel radial initial flaws centered in the cube. Fluid was injected into the sample to generate fractures along the initial flaws. The experiment used distributed high-definition fiber optic strain sensors with tight spatial resolutions. The sensors were embedded at two different locations on opposite sides of the initial flaws, serving as observation/monitoring locations. We also employed finite element modeling to numerically solve the linear elastic equations of equilibrium continuity and stress-strain relationships. The measured strains from the experiment were compared to simulation results from the finite element model. The experimentally derived strain and strain-rate waterfall plots from this study show the responses to both fractures propagating, while the fracture at the lower position took most of the fluid during the experiment. Interestingly, a fracture first began propagating from the upper flaw of the two flaws, but once the lower fracture was initiated, it grew much faster than the upper fracture. Both fibers were intercepted by the lower fracture, further verifying the strain signature as a fracture is approaching and intersecting an offset fiber.

Optimising CO2 storage resource utilisation: insights from the Otway Stage 4 field program

The optimised utilisation of storage resources and accelerated commercial carbon dioxide (CO2) storage development is vital for carbon capture and storage to play its important role in reducing emissions at scale. CO2CRC seeks to address this challenge through an international collaboration – the Otway Stage 4 field program – that demonstrates a suite of cost-effective reservoir management techniques with the potential to improve CO2 storage capabilities, particularly in lower-quality reservoirs, maturing them for commercial readiness. Aiming to enhance injectivity, improve sweep, increase model resolution, and develop performance monitoring capabilities, Otway 4 will substantially improve CO2 storage resource usage, furthering economical viability for decreasing CO2 emissions to the atmosphere. Otway Stage 4 includes acquisition and analysis of CO2 saturation and chemical data during plume migration and trapping, combined with investigation of fine-scale geological heterogeneity’s role in CO2 flow dynamics, to refine modelling workflows and, ultimately, develop strategies for optimising commercial CO2 storage; demonstrating whether CO2 microbubbles (MBs) significantly increase storage efficiency compared to standard CO2 injection, thereby unlocking previously untenable reservoirs for CO2 storage; enhancing seismic monitoring to comprehensively assess storage performance and MB behaviour, including quantitative derivation of CO2 saturation; and demonstrating the capability of Distributed Strain Sensing to quantitatively measure geomechanical changes associated with CO2 injection. In addition to Otway 4’s forward plans, laboratory and modelling work, including core flood analysis, to understand CO2 MB and surfactant behaviour at pore scale, dynamic modelling of CO2 flow through heterogeneous reservoir, and selecting suitable injection intervals at the Otway site are discussed.

Temperature insensitive distributed wide-dynamic-range strain sensing based on polarization-maintaining photonic crystal fiber

In this paper, a temperature insensitive wide-dynamic-range distributed strain sensor with centimeter-level spatial resolution based on optical frequency domain reflection technology is proposed. It adopts a polarization-maintaining photonic crystal fiber as the sensing fiber, and experimental results demonstrate that its temperature sensitivity is much smaller than that of the standard polarization-maintaining fiber. Benefited from the specially designed cross-correlation algorithm, the sensor achieves the maximum measurable strain reaches 7000 μϵ with spatial resolution of 1.66 cm. This provides a new approach to address current issues with special fiber sensors, such as single-point sensing and low spatial resolution, which shows great significance for structural health monitoring of buildings, bridges and tunnels with urgent requirement of large dynamic range.

Modeling of Fiber Optic Strain Responses to Shear Deformation of Fractures

Identifying distributed strain sensing (DSS) patterns (or signatures), particularly those arising from different hydraulic fracture geometries, has gained significant attention and research effort. Recent works have generated a catalogue of signatures for planar hydraulic fractures in an elastic rock formation. Yet, in numerous cases (e.g., fault motion and some geothermal reservoir stimulation), the main mode of deformation is a shear on a fracture or a network of natural fractures (particularly during low pressure injection/circulation). However, the specific fiber signatures that result from such shear deformation have not been studied. In this study, we use a three-dimensional poroelastic hydraulic fracture simulator to capture the strain signatures resulting from the shear deformation of fractures in various orientations with respect to the monitoring well. Five key cases are examined: one where the fracture strike is perpendicular to the fiber, another with the strike running parallel to the fiber, a third case where the fracture strike is at 45 degrees to the fiber, a fourth case with a strike slip fault perpendicular to the fiber, and a fifth case where fiber is intersecting the fracture. Theoretically and physically meaningful results were obtained in all five cases, which completely differ from the heart-shaped signature of tensile fracture propagation. It was discovered that the strain pattern changes with the shear deformation direction with respect to the fiber. The model is then used to simulate the response of a fracture network at Utah FORGE to injection to assess whether a signature might be expected in response to the planned injection and circulation rates, and, if so, what strain pattern might be expected. The simulation confirms that a strain response can indeed be observed. More importantly, the fiber response that would be detected in the monitoring well would be a combination of strain signatures from dilation and shear deformation of differently oriented natural fractures. The results in this study provide useful insights on the application of fiber to other stimulation and/or circulation scenarios where shear deformation of a fracture or fracture network plays a major role.

Self-Healable Multifunctional Fibers via Thermal Drawing.

The development of soft electronics and soft fiber devices has significantly advanced flexible and wearable technology. However, they still face the risk of damage when exposed to sharp objects in real-life applications. Taking inspiration from nature, self-healable materials that can restore their physical properties after external damage offer a solution to this problem. Nevertheless, large-scale production of self-healable fibers is currently constrained. To address this limitation, this study leverages the thermal drawing technique to create elastic and stretchable self-healable thermoplastic polyurethane (STPU) fibers, enabling cost-effective mass production of such functional fibers. Furthermore, despite substantial research into the mechanisms of self-healable materials, quantifying their healing speed and time poses a persistent challenge. Thus, transmission spectra are employed as a monitoring tool to observe the real-time self-healing process, facilitating an in-depth investigation into the healing kinetics and efficiency. The versatility of the fabricated self-healable fiber extends to its ability to be doped with a wide range of functional materials, including dye molecules and magnetic microparticles, which enables modular assembly to develop distributed strain sensors and soft actuators. These achievements highlight the potential applications of self-healable fibers that seamlessly integrate with daily lives and open up new possibilities in various industries.

Comparison of Gas Signature and Void Fraction in Water- and Oil-Based Muds Using Fiber-Optic Distributed Acoustic Sensor, Distributed Temperature Sensor, and Distributed Strain Sensor

Summary Accurate estimation and prediction of gas rise velocity, length of the gas influx region, and void fraction are important for optimal gas kick removal, riser gas management, and well control planning. These parameters are also essential in monitoring and characterization of multiphase flow. However, gas dynamics in non-Newtonian fluids, such as drilling mud, which is essential for gas influx control, are poorly understood due to the inability to create full-scale annular flow conditions that approximate the conditions observed in the field. This results in a lack of understanding and poor prediction of gas kick behavior in the field. To bridge this gap, we use distributed fiber-optic sensors (DFOS) for real-time estimation of gas rise velocity, void fraction, and influx length in water and oil-based mud (OBM) at the well scale. DFOS can overcome a major limitation of downhole gauges and logging tools by enabling the in-situ monitoring of dynamic events simultaneously across the entire wellbore. This study is the first well-scale deployment of distributed acoustic sensor (DAS), distributed temperature sensor (DTS), and distributed strain sensor (DSS) for investigation of gas behavior in water and OBM. Gas void fraction, migration velocities, and gas influx lengths were analyzed across a 5,163-ft-deep wellbore for multiphase experiments conducted with nitrogen in water and nitrogen in synthetic-based mud, at similar operating conditions. An improved transient drift flux–based numerical model was developed to simulate the experimental processes and understand the gas dynamics in different wellbore fluid environments. The gas velocities, void fractions, and gas influx lengths estimated independently using DAS, DTS, and DSS showed good agreement with the simulation results, as well as the downhole gauge analysis.

Distributed Strain Sensing With Phase Unwrapping Error Suppression Based on Φ-OFDR

Distributed Strain Sensing With Phase Unwrapping Error Suppression Based on Φ-OFDR

Multiphysics monitoring of cementation operation: implications for wellbore integrity and hydrogeological characterization

The application of optical fibers for assessing cemented wellbore’s integrity attracted considerable attention recently, because of low cost, decent temporal/spatial resolution and absence of downhole electronics. This study presents an integrated approach to compare measurements from distributed temperature sensing (DTS), distributed strain sensing (DSS) and fiber Bragg grating (FBG), at different stages of the wellbore cementation at Bedretto Underground Laboratory for Geosciences and Geoenergies. Before the cementation, the measurements from DTS provided information about the hydrogeological settings of the wellbore, including the major flow zones, and presence of a highly conductive hydraulic shortcut to a nearby wellbore. During the cement injection, the temperature sensors (DTS and temperature FBG) clearly detected the evolution of the top of the cement. While the mechanical deformation sensors (DSS and strain FBG) did not provide significant insights during this stage, their role became more pronounced in subsequent phases. Results show that the irregularities on the wall have minor influence on the thermo-mechanical response of the wellbore, both during and after cementation. After cementation, the temperature sensors (DTS and temperature FBG) traced different phases of cement-hardening process, while DSS measurements identified areas of major deformation, primarily in fracture/fault zones. It was also observed that localized elevation of temperature and extensional deformation along the wellbore during the cement-hardening are correlated with the presence of permeable structures, most likely due to continuous supply of water. Results of this study show that monitoring of the cemented wellbores using optical fibers, in particular during cement hardening, not only can be used to efficiently assess the wellbore integrity but also can provide us additional important information about the hydrogeological settings of the target reservoir volume.

Behavior Assessment of Exhumed Cast Iron Pipes Using Distributed Strain Sensing

Behavior Assessment of Exhumed Cast Iron Pipes Using Distributed Strain Sensing

Fully Distributed Strain Sensor Based on a Dual-Loop Optoelectronic Oscillator

Fully Distributed Strain Sensor Based on a Dual-Loop Optoelectronic Oscillator

Strain measurement with adaptive local feature extraction method based on special fiber OFDR system.

The optical fiber distributed strain sensor based on the optical frequency domain reflectometer (OFDR) preserves its dominant position in short-distance measurement fields with high spatial resolution, such as biomedical treatment, soft robot, etc. However, owing to the weak intensity of the Rayleigh backscattered signal (RBS) in the single-mode fiber (SMF) and complex computation, the large strain changes cannot be precisely and rapidly demodulated by the traditional cross-correlation method. In this work, the OFDR with backscattering enhanced optical fiber (BEOF) is proposed and demonstrated for fast and large strain measurement. By enhancing the RBS amplitude, the signal-to-noise ratio (SNR) is improved, resulting in a higher similarity between the reference signal and test signal, which is beneficial for the expansion of the strain measurement range. Moreover, the adaptive local feature extraction and matching (ALFEM) algorithm is presented and demonstrated, which replaces the traditional cross-correlation method for strain demodulation and fast measurement. On account of the enhancement ratio of BEOF, the dominant characteristic data segment can be extracted from whole wavelength data. In the experiments, the enhancing ratio of BEOF is designed as 10, resulting in the spatial resolution reaches 400µm and the strain measurement range is greatly increased to 4800µɛ. Further, the effectiveness of the ALFEM algorithm has been verified, in which the strain demodulation time is approximately 25% of that of the traditional method. This scheme fully exploits the enhancement characteristic of the BEOF and is also applicable to the systems based on other types of BEOF, different strain changes and sensing distances.

Distributed fiber-optic temperature monitoring in boreholes of a seasonal geothermal energy storage

Monitoring the in-situ temperature is key for the characterization of a seasonal geothermal energy storage. Distributed fiber-optic temperature sensing (DTS) systems provide temporally and spatially continuous measurement data in near real-time that captures borehole temperature dynamics. In the presented project, three boreholes of a seasonal geothermal energy storage with a vertical depth of down to 500 meters were instrumented with distributed fiber-optic sensors. For this purpose, a standard armored sensor cable was modified to allow for combined DTS, distributed acoustic sensing (DAS) and distributed strain sensing (DSS) at temperatures up to 120 °C. The cable was installed in a loop configuration to allow for temperature calibration using a temperature matching approach. A “Mini-Bend” solution was selected to cope with the limited space between formation and casing and the single cable feedthrough at the wellhead of the pressurized system. Using specially designed centralizers, the fiber-optic cable was installed and hold in place successfully in the 2” annulus outside the casing. Within the present paper, the focus lies on the DTS system and the data acquired during cementation. This continuous temperature data showcases the ability of DTS systems to detect small temperature changes in high detail. During the whole cementation process, the rising level of cement as well as setting temperatures were captured by rising temperature values in the range of a few degrees Celsius. As next project steps, not part of this paper, spatiotemporally temperature data will be recorded during stimulation treatments of the seasonal geothermal energy storage. The authors believe that the experience made with this fiber-optic monitoring system can be a good demonstration of the capabilities of fiber-optic sensing in deep borehole environments.

A Frequency-Scanned Φ-OTDR Implemented by a Frequency Shifting Loop Used for Distributed Strain Sensing

A Frequency-Scanned Φ-OTDR Implemented by a Frequency Shifting Loop Used for Distributed Strain Sensing

The Sensing Performance of a Novel Optical Cable for Tunnel Water Leakage Monitoring Based on Distributed Strain Sensing

The Sensing Performance of a Novel Optical Cable for Tunnel Water Leakage Monitoring Based on Distributed Strain Sensing

High-stretchability and low-hysteresis strain sensors using origami-inspired 3D mesostructures.

Stretchable strain sensors are essential for various applications such as wearable electronics, prosthetics, and soft robotics. Strain sensors with high strain range, minimal hysteresis, and fast response speed are highly desirable for accurate measurements of large and dynamic deformations of soft bodies. Current stretchable strain sensors mostly rely on deformable conducting materials, which often have difficulties in achieving these properties simultaneously. In this study, we introduce capacitive strain sensor concepts based on origami-inspired three-dimensional mesoscale electrodes formed by a mechanically guided assembly process. These sensors exhibit up to 200% stretchability with 1.2% degree of hysteresis, <22 ms response time, small sensing area (~5 mm2), and directional strain responses. To showcase potential applications, we demonstrate the use of distributed strain sensors for measuring multimodal deformations of a soft continuum arm.

Distributed Strain Sensor Based on Self‐Powered, Stretchable Mechanoluminescent Optical Fiber

The ability to locate and quantify large strains will significantly improve the real‐world application scenario of flexible and stretchable strain sensors. However, current methods for implementing stretchable distributed strain sensing still face challenges such as complicated demodulation, multisensor crosstalk, and high power consumption. Herein, a self‐powered and stretchable optical fiber strain sensor is reported with distributed sensing capability based on mechanoluminescent optical fiber, where mechanoluminescent phosphors with different emission color light are discretely integrated onto the outer cladding of the elastomer optical fiber. Based on the wavelength coding technique and time‐domain filtering comparison method, the capability of strain magnitude quantification (10–60%) and strain location identification together in a single stretchable optical fiber is successfully realized, even at multiple positions simultaneously in the strain‐applied situation. Moreover, this stretchable optical fiber strain sensor shows insensitivity to bending, compression, and temperature disturbances and outstanding durability (&gt;8000 cycles). Due to the excellent light confinement of the elastomer optical fiber, demonstrations such as bright‐field measurement, saline water operation, and wearable glove application exhibit its potential as a promising technology for future self‐powered distributed optical sensing systems.

Multi-Disciplinary Monitoring Networks for Mesoscale Underground Experiments: Advances in the Bedretto Reservoir Project.

The Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG) allows the implementation of hectometer (>100 m) scale in situ experiments to study ambitious research questions. The first experiment on hectometer scale is the Bedretto Reservoir Project (BRP), which studies geothermal exploration. Compared with decameter scale experiments, the financial and organizational costs are significantly increased in hectometer scale experiments and the implementation of high-resolution monitoring comes with considerable risks. We discuss in detail risks for monitoring equipment in hectometer scale experiments and introduce the BRP monitoring network, a multi-component monitoring system combining sensors from seismology, applied geophysics, hydrology, and geomechanics. The multi-sensor network is installed inside long boreholes (up to 300 m length), drilled from the Bedretto tunnel. Boreholes are sealed with a purpose-made cementing system to reach (as far as possible) rock integrity within the experiment volume. The approach incorporates different sensor types, namely, piezoelectric accelerometers, in situ acoustic emission (AE) sensors, fiber-optic cables for distributed acoustic sensing (DAS), distributed strain sensing (DSS) and distributed temperature sensing (DTS), fiber Bragg grating (FBG) sensors, geophones, ultrasonic transmitters, and pore pressure sensors. The network was realized after intense technical development, including the development of the following key elements: rotatable centralizer with integrated cable clamp, multi-sensor in situ AE sensor chain, and cementable tube pore pressure sensor.