The research team, led by Jinrong Cao of the Geological Carbon Dioxide Storage Technology Research Association and the Research Institute of Innovative Technology for the Earth in Japan, combined three observational streams in a single experiment: distributed fiber-optic strain sensing, time-lapse X-ray computed tomography, and coupled numerical modeling of two-phase flow and poroelastic deformation. Their specimen was a cylindrical tuffaceous sandstone core, 34.83 millimeters in diameter and 179.50 millimeters long, drilled from the Sarukawa oil field in Oga, Akita Prefecture, an area selected for a Japanese advanced carbon storage program targeted to begin operations by 2030. The rock had a porosity of 25.06 percent and an intrinsic permeability of 5.41 millidarcys, with a mineral framework dominated by the zeolite clinoptilolite at roughly 63 percent, alongside quartz, albite, anorthite, and heulandite.

The sensing element was a single-mode silica fiber bonded to the core surface with epoxy at an optimized wrap angle of 80 degrees relative to the core axis. This helical geometry converts one continuous fiber into a dense array of quasi-independent strain sensors, resolving circumferential, or hoop, strain at one-centimeter spatial resolution along the full specimen length. The interrogator, a Neubrex 7020 unit, works by measuring Rayleigh frequency shifts in backscattered laser light using a tunable wavelength coherent optical time-domain reflectometer. Because strain and temperature both shift the Rayleigh spectrum, the team held the experiment at a stable 40 degrees Celsius and applied a calibrated strain coefficient of minus 0.153 gigahertz per microstrain, yielding a strain resolution of about half a microstrain for comparable measurements, far finer than the tens to roughly one hundred microstrains observed during injection.

Before introducing carbon dioxide, the researchers ran a single-phase water flow test to pin down permeability and validate their poroelastic model. When upstream pressure reached 10.5 megapascals, circumferential strain near the inlet jumped to approximately 75 microstrains within a single sampling interval of 36 seconds or less, while the downstream end of the core registered only a few microstrains. Numerical simulations showed that the sub-second initial response could not be resolved experimentally, but by roughly half a second to one second the simulated strain gradient matched the first measured profile, and a steady inlet-to-outlet strain gradient established itself within about one second. This close agreement between measurement and model confirmed that, in fully water-saturated rock, deformation is governed by pore water pressure gradients acting through Biot-style poroelastic coupling.

The main event was the supercritical carbon dioxide flood. Time-lapse CT scans, taken at 60 seconds per volume using an iodide-rich contrast solution to sharpen fluid-rock contrast, tracked the drainage process in detail. During the first 1.6 minutes, corresponding to 0.036 injected pore volumes, carbon dioxide accumulated mainly near the inlet. By 2.67 minutes the saturation field had become spatially nonuniform, reflecting variability in pore accessibility and capillary entry conditions across the zeolite-rich framework. At 4.27 minutes the front reached the specimen midpoint, and breakthrough occurred at 8.0 minutes, when average carbon dioxide saturation hit 19.4 percent. Differential pressure during drainage held steady between 0.45 and 0.48 megapascals at a flow rate near one milliliter per minute.

The striking result was how differently the strain field evolved from the saturation field. At the onset of injection, a pronounced strain gradient appeared immediately: strain near the inlet climbed to roughly 80 microstrains, while even the outlet showed a small but detectable response. The strain front was far more spatially diffuse than the comparatively sharp saturation front imaged by CT. By breakthrough, strain had reached about 100 microstrains near the inlet and 20 microstrains near the outlet. Most tellingly, at a position 0.09 meters from the inlet, a distinct strain increase appeared near the very start of injection, while local carbon dioxide saturation remained at zero for the first four minutes. The delay between strain onset and saturation onset grew with distance from the inlet, exactly the pattern expected if pressure disturbances propagate through the connected aqueous phase faster than the capillary-controlled gas front advances.

The coupled numerical model, a two-dimensional axisymmetric finite-element representation of the full specimen, provided the physics-based bridge between the observable strain and the unmeasured internal pressure. The team calibrated two-phase flow parameters exclusively against the CT-derived saturation profiles, then compared the predicted strain against the fiber measurements as an independent test. Simulated pore gas and pore water pressures evolved concurrently but with different spatial patterns, related through capillary pressure, and the modeled strain reproduced both the magnitude and the timing of the measured response. Because the sandstone’s apparent drained bulk modulus of roughly one to two gigapascals makes it relatively compliant, a given pressure perturbation produces a large, measurable strain. The authors are careful to stress that the fiber does not measure pressure directly; the strain signal is a model-constrained poroelastic proxy for the evolving pressure footprint, not a standalone pressure gauge.

The engineering implications reach well beyond the laboratory. Operational surveillance of storage sites typically relies on wellhead and bottomhole pressure, flow rates, sparse downhole gauges, and episodic plume imaging, none of which uniquely constrains the internal pressure field. Distributed strain sensing adds a spatially continuous constraint that can be folded into coupled flow and geomechanical models, helping to characterize uncertain formation properties and to evaluate deformation relevant to reservoir, caprock, and wellbore integrity. Field deployments already hint at feasibility: distributed fiber sensing has been demonstrated under field-scale injection conditions at the CO2CRC Otway site in Australia, a pumping test at Mobara in Japan captured pressure-related reservoir deformation, and behind-casing water-injection measurements show that injection-induced strain is detectable through a casing-cement-formation system.

The authors are equally candid about the limitations that separate their benchtop result from routine field practice. The adhesive layer bonding the fiber to the core was not independently calibrated, so perfect strain transfer was assumed. Field fibers are usually installed axially behind casing, where recorded amplitude depends on cable orientation, completion design, cement properties, gauge length, and strain-transfer efficiency, and axial strain is not generally equivalent to the hoop strain measured here. Stiffer formations or rocks with a lower Biot-Willis coefficient would produce smaller strains that may fall below detection thresholds, meaning an absent strain signal cannot be read as absent pressure communication. The homogeneous axisymmetric model also cannot capture localized effects of bedding, fractures, or anisotropic permeability, and longer-term chemical interactions between carbon dioxide, brine, and reactive minerals may gradually alter rock stiffness and fracture behavior.

Even with those caveats, the study delivers a compelling proof of concept: the mechanical consequences of injection arrive before the plume does, and a helically wound strand of glass can watch them happen. Future priorities include experiments on rocks with contrasting stiffness and capillary behavior, direct internal pressure measurements, three-dimensional models that explicitly represent the helical fiber geometry, and field studies that integrate distributed strain with pressure gauges, time-lapse geophysics, distributed acoustic and temperature sensing, and surface deformation observations. As global net-zero pathways call for roughly ten gigatons of carbon dioxide storage per year by 2050, tools that convert invisible pressure transients into measurable, interpretable strain signals could become an essential part of the monitoring toolkit, giving operators and regulators earlier and more defensible warnings about where the subsurface is being mechanically stressed.

Subject of Research: Distributed fiber-optic sensing of poroelastic strain precursors during supercritical CO2 core flooding in sandstone

Article Title: Poroelastic strain precursors during laboratory carbon dioxide core flooding revealed by helically bonded fiber sensing

Article References: Cao, J., Park, H., Xue, Z., Nakajima, T., Amer, R., & Ouchi, W. (2026). Poroelastic strain precursors during laboratory carbon dioxide core flooding revealed by helically bonded fiber sensing. Communications Engineering, 5(1), Article 164. https://doi.org/10.1038/s44172-026-00783-5

Image Credits: AI Generated

DOI: 10.1038/s44172-026-00783-5

Keywords: carbon capture and storage, distributed fiber-optic sensing, poroelasticity, CO2 core flooding, X-ray computed tomography, pore pressure, geomechanics, sandstone, capillary pressure, geological storage, strain sensing, two-phase flow

The research, published in Results in Engineering by Johannes Rathgen and Vincent Oettel of the iBMB Division of Concrete Construction, tackles a problem that has haunted bridge engineers for decades. Shear failure is among the most treacherous failure modes in structural concrete: unlike ductile flexural failure, which announces itself through large deflections and visible sagging, shear failure typically arrives suddenly and without significant warning, governed by the unstable propagation of diagonal cracks inclined toward the load. Design codes handle this uncertainty conservatively, and analyses of extensive experimental databases show that current approaches can substantially overestimate the danger for beams with little or no shear reinforcement. The German team’s calculations, for instance, underestimated the measured failure loads of three low-reinforced beams by factors approaching 1.85, confirming that the true reserve of these structures is often much larger than the codes admit. Monitoring, the authors argue, can unlock that reserve safely by revealing how a structure actually behaves under load.

To test that idea, the researchers cast six reinforced concrete beams spanning a deliberate range of shear reinforcement ratios, from one beam with no stirrups at all to others provided with between 1.3 and 8.1 times the minimum ratio required by the German National Annex to Eurocode 2. All beams used normal-strength concrete with a maximum aggregate size of 16 millimeters, B500B steel bars of 20 millimeters as longitudinal reinforcement, and B500A stirrups of 6 and 8 millimeters. Each beam was loaded to failure in three-point bending, with the load applied at midspan and a shear span of 90 centimeters, deliberately chosen to prevent direct load transfer to the supports. The material properties were carefully characterized on the day of testing: mean cylinder compressive strengths ranged from 36.3 to 56.5 newtons per square millimeter, and mean yield strengths of the reinforcement were determined by tensile testing according to EN ISO 15630-1.

The instrumentation was the heart of the experiment. The team used a LUNA ODiSI 6104 interrogator working on the principle of optical frequency domain reflectometry, a technique that turns a single polyimide-coated optical fiber into thousands of distributed strain measurement points with a gage pitch of just 0.65 millimeters, acquired at 1 hertz. Two contrasting sensor configurations were compared in the shear-critical regions near the supports. In one region, the fiber was bonded in an inclined loop at 45 degrees, aligned with the principal tensile stresses of the uncracked beam so that emerging shear cracks would cross it nearly perpendicularly. In the other region, the fiber ran in three horizontal sections parallel to the beam’s bottom surface, spaced 6 centimeters vertically. The horizontal layout is far simpler to install and requires no prediction of the expected crack pattern, but it sacrifices some sensitivity because inclined cracks intersect it at oblique angles. To keep the fragile fibers alive through large crack openings, the researchers bonded them with a slightly elastic UV-curing resin, allowing controlled debonding instead of catastrophic fiber breakage.

The results were striking. In the beam without shear reinforcement, which failed at 145 kilonewtons, the inclined sensor detected the critical shear crack at approximately 76 percent of the failure load and pinpointed its position to within a centimeter at sensor coordinate 8.2 centimeters. Continued loading produced a pronounced and largely continuous rise in measured strain, an unmistakable early indication of impending failure. In the beams with low shear reinforcement, the picture was even more encouraging: the crack that ultimately governed failure was first flagged at 58 percent of the ultimate load in one beam, 52 percent in another, and 77 percent in a third with an asymmetric stirrup arrangement. The team condensed these observations into a shear-failure warning index, defined as the ratio of the detection load to the failure load, offering a quantitative measure of how much warning a monitoring system can realistically provide before brittle collapse.

Perhaps the most consequential finding concerns what happens after the load is removed. Because bridge inspections are performed under service loads rather than at the extreme levels that may have formed cracks, many shear cracks close up and become invisible to the naked eye. The fiber optic measurements, however, reliably detected and localized cracks even after they had largely closed upon unloading. This means the sensors can supply a structural record that visual inspection cannot, capturing evidence of past cracking events that would otherwise vanish. For the vast population of older bridges that show no visible distress despite calculated shear deficits, this capability could fundamentally change how condition assessments are made, replacing worst-case assumptions with measured structural response.

The study also revealed that detecting cracks is the easy part; classifying them is the real challenge. Flexural cracks form roughly perpendicular to the beam axis, while shear cracks incline at around 45 degrees, but a crack that begins as a benign flexural crack can gradually rotate and develop into the critical flexural-shear crack that destroys the beam. In one test, the top horizontal sensor section registered a crack at 63 percent of the failure load that looked entirely flexural; only when a rapidly propagating shear crack merged with it did the fatal crack emerge, becoming detectable in the lower sections at 77 to 79 percent of the failure load. The researchers therefore propose a conceptual two-step classification procedure: first, estimate crack inclination from the pattern of strain peaks across adjacent, closely spaced sensor sections of a loop-shaped fiber; second, track potentially critical cracks near the supports over successive load levels, watching for continuous strain growth that signals dangerous propagation.

That classification framework, the authors emphasize, remains a concept rather than a validated method, but it points toward something genuinely transformative: an automated, condition-based early warning system for shear-deficient bridges. Combined with predefined warning and action levels, the approach could drive a traffic-light scheme in which green indicates all measured strains remain within expected ranges, yellow triggers additional assessment, and red demands prompt investigation, weight restrictions, or closure. A fixed strain threshold of 1 per mille proved suitable under laboratory conditions for separating crack-induced strain peaks from measurement noise, and its simplicity makes it attractive for automated data evaluation, though the researchers caution that it is a configuration-specific criterion whose robustness under field conditions, varying materials, and environmental exposure still requires validation.

The broader implications reach well beyond the laboratory. Strengthening a deficient bridge with ultra-high performance concrete overlays or carbon fiber reinforced polymer strips demands long planning lead times, temporary closures, and considerable expense, while wholesale replacement is simply infeasible given limited personnel, machinery, and materials. Distributed fiber optic sensing, by contrast, can be installed on existing structures with minimal intervention and no major structural modification, enabling both long-term and large-scale monitoring of the regions where shear failure would begin. The study also found that sensor placement matters more than orientation: multiple horizontal sensor sections stacked near the support, particularly the lowest section in the tension zone, caught cracks earlier than expected, and the horizontal configuration avoids the complex principal-stress trajectory calculations that the inclined layout demands. Notably, no clear relationship emerged between the crack-to-fiber intersection angle and early detection performance, weakening the case for the more elaborate inclined installations.

Laboratory beams are not bridges, and the authors are careful about the limits of their work. Only four of the six tests ended in shear failure, some measurement data were lost to a defective remote module and sensor damage, and the findings apply strictly to the tested configurations and loading conditions. Long-term and variable loading, environmental effects, and full-scale field trials remain necessary before the method can be trusted on live structures. The Braunschweig team is already extending the work to haunched beams, whose tapered support regions alter the stress distribution and crack patterns in ways that complicate sensor placement. Still, the core message stands: light traveling through a fiber thinner than a human hair can reveal the silent, invisible cracks that precede one of structural engineering’s most sudden failure modes, and in doing so may help keep thousands of aging bridges open safely while the slow work of renewal catches up.

Subject of Research: Monitoring shear crack formation in reinforced concrete beams using distributed fiber optic sensors

Article Title: Experimental investigation of fiber optic sensors in shear-critical regions of reinforced concrete beams for monitoring shear behavior

Article References: Experimental investigation of fiber optic sensors in shear-critical regions of reinforced concrete beams for monitoring shear behavior. (n.d.). https://doi.org/10.1016/j.rineng.2026.113052

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113052

Keywords: fiber optic sensors, structural health monitoring, reinforced concrete, shear failure, bridge assessment, distributed fiber optic sensing, crack detection, Eurocode 2, shear reinforcement, concrete beams, early warning system, TU Braunschweig

The challenge that motivated the study is deceptively simple to state but remarkably hard to solve. Groundwater-induced land subsidence is one of the most widespread geohazards associated with excessive groundwater exploitation, and it produces broad, uneven patterns of sinking that carry serious economic and social consequences. Traditional monitoring tools such as leveling benchmarks and GPS receivers provide accurate measurements, but only at discrete points scattered across the landscape. That sparse coverage makes it nearly impossible to characterize the full regional footprint of subsidence, let alone to work out which buried layers are compressing and driving the deformation. Remote sensing techniques such as Interferometric Synthetic Aperture Radar, or InSAR, can map surface deformation over huge areas in all weather, but by themselves they reveal little about what is happening hundreds of meters underground, where the real mechanical action unfolds within layered aquifer and aquitard systems.

The new framework, developed by Hongwei Sang, Ke Fang, Qimeng Liu, Liang Yuan, and Bin Shi, closes that gap in three sequential steps. First, the researchers applied Persistent Scatterer InSAR, known as PS-InSAR, to derive detailed regional maps of surface deformation. Second, they quantified the temporal relationship between the satellite-derived deformation and regional terrestrial water storage, or TWS, using Pearson correlation analysis, allowing them to flag areas where sinking was strongly tied to hydrological variation. Third, they turned to distributed fiber optic sensing, DFOS, installed in a borehole, to identify the specific compressible strata responsible for the deformation at depth. Each step covers a different scale, and together they build a continuous chain of evidence from regional surface patterns all the way down to individual geological layers.

For the satellite component, the team processed 171 images acquired by the Sentinel-1A satellite between January 2017 and December 2022. The C-band radar instrument revisits the area every twelve days, and the researchers used the SNAP, ISCE, and StaMPS processing chain to extract line-of-sight deformation velocities at roughly half a million monitoring points across the study area. Careful corrections were applied throughout, including precise orbit correction, removal of the topographic phase using the 30-meter Shuttle Radar Topography Mission digital elevation model, three-dimensional phase unwrapping, and atmospheric correction based on the Generic Atmospheric Correction Online Service, GACOS, which uses high-resolution weather-model products to strip out delays introduced by the atmosphere. Because the study area sits on an exceptionally flat alluvial plain at an average elevation of less than six meters, the line-of-sight measurements serve as a reasonable first-order proxy for vertical settlement.

The satellite data painted a telling picture. Line-of-sight velocities ranged from minus 22 millimeters per year, indicating movement away from the sensor, to plus 10 millimeters per year. The most pronounced deformation anomalies appeared in the central sector of the study area, coinciding with zones of land subsidence documented in earlier investigations. But surface maps alone cannot answer the crucial question of cause. To address that, the researchers turned to terrestrial water storage data from the GLDAS V2.2 CLSM product, obtained through the Google Earth Engine platform at a spatial resolution of about 25 kilometers and a daily temporal resolution. TWS aggregates groundwater, soil moisture, surface water, snow and ice, canopy interception, and wet biomass, and thus serves as a regional-scale indicator of overall water storage conditions, including groundwater variability.

The correlation analysis revealed a striking story of changing hydro-mechanical behavior. During 2017, areas with Pearson correlation coefficients between 0.4 and 1.0, the threshold the team adopted to flag moderate to strong correspondence, clustered in the central part of the study area, precisely where deformation rates were highest. Two representative InSAR points, P1 and P2, showed correlation coefficients of 0.958 and 0.811 respectively in 2017, indicating an almost synchronized dance between falling water storage and accumulating subsidence during a period when groundwater exploitation still dominated the system. Similar positive correlations persisted in 2018 and 2019, though generally weaker.

Then, after 2020, something changed. The spatial distribution of correlation coefficients shifted toward lower and even negative values, and the percentage of InSAR pixels exceeding the 0.4 threshold fluctuated downward, from 16.9 percent in 2017 to 4.3, 9.1, and 7.1 percent in 2020 through 2022. Regional terrestrial water storage began to rise, reflecting the gradual recovery of water storage following the enforcement of groundwater extraction restrictions, yet surface deformation continued to accumulate regardless. The explanation lies in the physics of consolidation. According to Terzaghi’s consolidation theory, when groundwater recovers, pore-water pressure increases within the permeable confined aquifer, but the surrounding low-permeability aquitards dissipate their excess pore pressure far more slowly. Residual consolidation of these compressible strata therefore continues long after the hydrological turnaround, producing a time lag between hydrological recovery and surface deformation. The subsurface, in effect, keeps replaying the memory of past over-extraction.

To pin down exactly where that lingering deformation resides, the team exploited the borehole fiber optic monitoring system at borehole SZ1, established in 2013 in Shengze Town, a silk-textile hub in southeastern Suzhou whose township industries once drove severe aquifer over-extraction. The system uses a Brillouin Optical Time Domain Reflectometer, BOTDR, which measures the Brillouin frequency shift generated by the interaction between incident light and acoustic phonons within the optical fiber, converting that shift into axial strain. Configured with a 20-kilometer measurement range, one-meter spatial resolution, and a strain accuracy of plus or minus 50 microstrain, the fiber laid continuously along the borehole records strain profiles across the full vertical extent of the layered aquifer system, something no array of point sensors could achieve.

The fiber optic observations delivered a clear verdict on depth. Negative strain, representing compressive deformation, concentrated within the Af2 confined aquifer and the adjacent Ad2 and Ad3 aquitards. Intriguingly, although groundwater extraction occurs primarily within the Af2 aquifer itself, the largest cumulative deformation occurred in the neighboring aquitards. This counterintuitive pattern is exactly what classical hydro-mechanics predicts. Groundwater withdrawal first reduces pore-water pressure in the high-conductivity aquifer, transmitting pressure changes rapidly and producing only limited elastic deformation. The low-permeability aquitards, by contrast, drain slowly, sustain delayed consolidation, and accumulate disproportionately large inelastic compression. The Ad2 and Ad3 layers therefore contribute far more to the observed subsidence than the aquifer that is actually being pumped, a conclusion that matches Terzaghi’s theory and previous investigations of multilayer aquifer systems.

The broader significance of the study extends well beyond one town in the Yangtze River Delta. Suzhou itself has a long history with subsidence; by the end of 2005, deep groundwater extraction had been completely prohibited in the Suzhou-Wuxi-Changzhou region, and subsidence rates slowed as water levels rose, yet localized zones of concentrated sinking persist. The authors acknowledge limitations, including the absence of descending-orbit radar acquisitions, the fact that TWS is not a direct substitute for groundwater-level measurements, and the reliance on a single representative borehole. Even so, the framework is designed to be general rather than site-specific. Similar combinations of regional surface deformation and subsurface compaction afflict the North China Plain, California’s Central Valley, Jakarta, Mexico City, and the Konya basin in Türkiye, and the researchers argue that pairing regional InSAR with representative borehole fiber optic monitoring offers a practical, scalable strategy for identifying groundwater-induced subsidence and understanding its stratigraphic mechanisms. For water managers confronting the slow-motion disaster of sinking cities, the message is clear: the ground remembers what we pump, and now we can read that memory from orbit and from deep within the earth itself.

Subject of Research: Identification of groundwater-induced land subsidence through integration of satellite InSAR, terrestrial water storage, and borehole distributed fiber optic sensing

Article Title: Regional identification of groundwater-induced land subsidence using integrated surface and subsurface observations

Article References: Sang, H., Fang, K., Liu, Q., Yuan, L., & Shi, B. (2026). Regional identification of groundwater-induced land subsidence using integrated surface and subsurface observations. Environmental Earth Sciences, 85(15), Article 390. https://doi.org/10.1007/s12665-026-13113-x

Image Credits: AI Generated

DOI: 10.1007/s12665-026-13113-x

Keywords: land subsidence, groundwater extraction, InSAR, PS-InSAR, terrestrial water storage, distributed fiber optic sensing, aquitard consolidation, Sentinel-1, BOTDR, Yangtze River Delta, Suzhou, geohazard monitoring