The research, conducted by Abdulhakim Ahmad and Aniefiok Francis Akpaneno of Federal University Dutsin-Ma, set out to bridge two worlds that rarely talk to each other in corrosion science. On one side sits the traditional approach: teams of geophysicists hauling resistivity meters across the landscape, driving current into the ground through electrode arrays and measuring how easily electricity flows through the soil. Low resistivity generally signals moist, salt-rich, electrically conductive soil, which is precisely the kind of environment in which buried metal corrodes fastest. On the other side sits the promise of remote sensing: satellites that sweep over the entire planet every few days, capturing spectral fingerprints of vegetation, moisture, exposed soil and salinity at no cost to the user. If those spectral fingerprints could be statistically tied to soil resistivity, corrosion risk maps covering vast territories could be drawn from a laptop.

To find out whether that promise holds, the team worked at forty-four Vertical Electrical Sounding stations scattered across Sabon Gida, a semi-arid site in the Sudan Savanna zone where annual rainfall of roughly 600 to 800 millimeters is concentrated between May and September, and temperatures routinely climb above 40 degrees Celsius. Using a Schlumberger electrode configuration with current electrode spacings reaching 30 meters, they measured the electrical resistivity of the uppermost two meters of soil, the depth interval where most buried infrastructure lives. The inversion of those measurements produced root mean square errors between 0.316 and 2.19 percent, indicating the resistivity models were trustworthy. At five representative locations spanning the full range of corrosivity classes, the researchers also collected twenty soil samples from four depth intervals down to two meters and measured their pH, which ranged from 6.3 to 7.1, indicating neutral to slightly acidic conditions.

The satellite side of the study relied on a single cloud-free Sentinel-2 Level-2A image acquired on 4 March 2026, during the dry season and just weeks after the field survey. Because Level-2A products are atmospherically corrected to bottom-of-atmosphere surface reflectance, they are suitable for quantitative work rather than mere visual interpretation. From this image the team computed four spectral indices, each built from ratios of reflectance in specific bands. The Normalized Difference Vegetation Index, or NDVI, exploits the fact that healthy vegetation strongly reflects near-infrared light while absorbing red light during photosynthesis. The Normalized Difference Water Index, in the McFeeters formulation, contrasts green and near-infrared reflectance to gauge surface moisture. The Bare Soil Index combines blue, red, near-infrared and shortwave-infrared bands to flag exposed ground. Finally, the Salinity Index, built from the shortwave-infrared and red bands, serves as a relative proxy for salt-affected surfaces. Spectral values were extracted at each of the 44 sounding stations using the point sampling tool in QGIS, creating a single integrated dataset of satellite and ground observations.

The descriptive statistics alone told an important story. Shallow-layer resistivity ranged from a low of 23.11 ohm-meters to a striking 1779 ohm-meters, with a mean of 387.40 and a coefficient of variation of 91.74 percent, the highest of any variable measured. Soil pH, by contrast, was remarkably uniform, varying only between 6.65 and 6.85 with a coefficient of variation of just 0.42 percent. The satellite indices were also comparatively stable: NDVI values were negative throughout, between minus 0.0975 and minus 0.0641, confirming the sparse, dry-season vegetation cover one expects in the Sudan Savanna, while the Salinity Index varied only from 0.4068 to 0.4511. In other words, the surface environment as seen from orbit was relatively homogeneous, while the subsurface electrical properties fluctuated wildly over short distances. That mismatch, the authors note, is itself a scientific finding: local geology matters more than what the camera in the sky can see.

Pearson correlation analysis made the mismatch quantitative. The strongest link between resistivity and any spectral index was with the Salinity Index, at a modest r of 0.287, followed by NDVI at 0.138 and the Bare Soil Index at 0.126, while the water index showed essentially no relationship at all, with r of minus 0.005. Meanwhile, the spectral indices correlated strongly with each other: NDVI and NDWI were negatively correlated at r of minus 0.645, NDWI and the Bare Soil Index positively at 0.711, and NDVI and the Bare Soil Index negatively at minus 0.517, all statistically significant. This pattern shows the satellite indices are internally coherent, reliably tracking real surface conditions, but those surface conditions simply do not map cleanly onto the electrical behavior of the soil two meters down, which is governed by lithology, clay content, pore-water chemistry, weathering and groundwater, none of which a multispectral imager can observe directly.

The multiple linear regression delivered the study’s headline numbers. With resistivity as the dependent variable and soil pH plus the four indices as predictors, the model explained only 19.7 percent of the variance in shallow resistivity, and, crucially, was not statistically significant overall, with an F statistic of 1.867 and a p-value of 0.123. The Salinity Index stood out as the only individual predictor to reach significance, with a coefficient of 19,500 and a p-value of 0.025, suggesting that salt-related surface conditions do exert a measurable influence on the electrical behavior of the shallow subsurface. Multicollinearity diagnostics using the Variance Inflation Factor showed acceptable values for most predictors, with only NDWI slightly elevated at 5.811, and residual checks found no substantial violations of the regression assumptions. The authors are careful to flag a puzzle, however: the positive relationship between the salinity index and resistivity runs opposite to the textbook expectation that more salt should mean more conductivity and lower resistivity, a contradiction they attribute to the confounding effects of soil texture, mineralogy, moisture and subsurface heterogeneity, and to the fact that the index is a spectral proxy rather than a direct salinity measurement.

Why does this matter beyond one Nigerian town? Because the result is a caution against a shortcut that is becoming increasingly tempting as free, high-resolution satellite data floods the environmental sciences. Corrosion of buried metallic infrastructure is one of the leading causes of failure in underground engineering systems worldwide, and in data-scarce regions the allure of mapping risk from orbit is obvious. Yet this study demonstrates with clear statistics that surface spectral conditions explain less than a fifth of the variation in the property that actually controls corrosion electrochemistry. The resistivity of a soil is the product of an intricate subsurface interplay between water, ions, clay minerals and pore geometry, and no optical satellite currently sees any of it. What the satellites do provide, the authors argue, is a valuable complementary layer: the Salinity Index maps, the vegetation patterns and the moisture indicators help characterize the environmental context in which corrosion proceeds, and can guide where expensive ground surveys should be concentrated.

The study is also refreshingly honest about its own limits. It rests on a single dry-season image, while vegetation, moisture and salinity in semi-arid environments swing dramatically between seasons; the 10-to-20-meter pixel size may smooth over local variability; conventional Pearson correlation and ordinary least squares regression ignore spatial autocorrelation; and no direct measurements of electrical conductivity, chloride, sulphate or corrosion rates were made. The authors propose a roadmap for successors: multi-temporal Sentinel-2 stacks, radar-derived soil moisture, land surface temperature, topographic indices, machine-learning models, geostatistical interpolation, Moran’s I statistics and geographically weighted regression, all anchored by laboratory measurements of the chemical parameters that drive corrosion directly.

For the engineers planning the next pipeline or water main through the Sahel’s expanding towns, the practical message is clear. Sentinel-2 is a superb scout, cheap and persistent, capable of flagging salt-affected zones and tracking the seasonal drying that concentrates soluble salts near the surface through evaporation. But when the question is how fast a steel pipe will corrode at a specific spot two meters underground, the electrodes still have to go into the ground. The most robust framework, this study concludes, is not satellite versus survey but satellite plus survey, an integration that turns limited field campaigns into smarter, better-targeted defenses for the hidden metallic arteries on which growing cities depend.

Subject of Research: Statistical evaluation of Sentinel-2 spectral indices as indicators of shallow soil resistivity for soil corrosion assessment in semi-arid Nigeria

Article Title: Statistical evaluation of sentinel-2 spectral indices as environmental indicators of shallow soil resistivity for soil corrosion assessment in a semi-arid region of northwestern Nigeria

Article References: Ahmad, A., & Akpaneno, A. F. (2026). Statistical evaluation of sentinel-2 spectral indices as environmental indicators of shallow soil resistivity for soil corrosion assessment in a semi-arid region of northwestern Nigeria. Discover Geoscience, 4(1), Article 359. https://doi.org/10.1007/s44288-026-00732-x

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00732-x

Keywords: Sentinel-2, soil corrosion, soil resistivity, remote sensing, spectral indices, salinity index, NDVI, electrical resistivity survey, semi-arid environment, Nigeria, infrastructure, geophysics

The motivation comes from one of the deepest puzzles in modern physics. The standard model of particle physics, built on the symmetry group SU(3) x SU(2) x U(1), describes the strong, weak and electromagnetic interactions with spectacular success, while general relativity handles gravity. Yet many theorists suspect that this edifice is incomplete. One popular possibility is an extra U(1) symmetry, often called a dark U(1), whose mediator would be an extremely light spin-1 boson resembling a generalized dark photon. Such a particle would generate an extraordinarily feeble new force whose couplings to ordinary matter would involve combinations of baryon number B, lepton number L, or their difference B minus L. A spin-0 mediator could produce a similar effect. Because these forces would couple differently to different materials, they could produce apparent violations of the equivalence principle, the exact identity between inertial and gravitational mass that underlies general relativity and was famously tested by Baron Eötvös more than a century ago.

The sharpest current probe of such effects is the MICROSCOPE satellite mission, which monitored the relative acceleration of two freely falling test masses, one of titanium and one of platinum, aboard a drag-free spacecraft orbiting at an average altitude of roughly 710 kilometers. The mission found no deviation, constraining the Eötvös parameter, which measures the relative difference in accelerations, to a level of a few parts in ten to the fifteen. Translating this null result into limits on the strength of a hypothetical new force, however, requires knowing exactly how the Earth’s extended mass distribution sources that force, and this is where the new mathematics enters.

For a new interaction mediated by a particle of mass m, the force has a finite range lambda equal to the inverse mass in natural units. If the range greatly exceeds the Earth’s radius, the potential outside the planet is essentially the same as that of a point charge at the center. But as the range shrinks toward the satellite altitude, the potential at a given point is increasingly generated by the parts of the Earth closest to it, down to a minimum distance equal to the altitude itself. Fayet shows that the outside potential can always be written as the Yukawa potential of a pointlike source multiplied by a correction factor Phi(x), where x is the ratio of the Earth’s radius to the force range. This factor, always greater than or equal to one, is what he calls the hyperbolic form factor.

The definition is elegantly simple: the hyperbolic form factor is the bilateral Laplace transform of the density distribution, expressible as the average of the hyperbolic cosine of the dot product of the wave vector and position vector, or equivalently as the average of the hyperbolic sine of kr divided by kr over the body. Remarkably, this quantity is related by a duality transformation, or by analytic continuation through the substitution of imaginary for real wave numbers, to the ordinary form factor familiar from scattering theory, which involves the ordinary sine and cosine. The two are two faces of the same underlying function, and the connection extends to a compact formula in which the product of the wave number and the form factor appears as the bilateral Laplace transform of the quantity two-pi times r times the density. An inversion formula even allows one to reconstruct the density profile itself from an analytic continuation of the form factor, closing the loop between structure and potential.

Fayet also introduces a companion concept, the effective density. For each force range, one can ask what uniform sphere would generate the same outside potential as the real, layered Earth. The answer is a density that decreases as the range shortens, sliding from the global average density for very long-range forces down toward the density near the surface for short ranges. This makes physical sense: a short-ranged force is sourced preferentially by the outer shells of the planet, which are less dense than the iron core. The effective density thus encodes, in a single curve, how the sensitivity of an experiment migrates inward or outward as the hypothesized mediator mass changes.

The practical payoff comes from a surprising discovery about the Earth itself. Rather than needing a detailed numerical model with many layers, Fayet finds that remarkably simple analytic density profiles reproduce the form factor of a realistic five-shell Earth model, which distinguishes inner and outer cores, inner and outer mantles, and crust, with shell boundaries at radii of 1121, 3480, 5701, 6341 and 6371 kilometers. A density falling off as one over the radius yields a form factor equal to the square of the hyperbolic sine of x over 2 divided by x over 2, accurate to within about one percent up to x equal to 4. Even better, a hybrid profile combining the one-over-r behavior with a linear decrease gives a closed-form expression that matches the five-shell calculation to within 0.7 percent all the way up to x equal to 64, corresponding to force ranges above 100 kilometers or mediator masses below roughly two times ten to the minus twelve electron-volts. The power series expansion of this analytic expression, beginning with one plus 0.0833 x squared, sits extraordinarily close to the five-shell result, and the tiny differences that do exist are in fact mathematically mandatory, since identical form factors would imply identical densities through the inversion formula.

These approximations matter because the experimental limits on new-force couplings scale with the form factor. For a mediator mass of ten to the minus twelve electron-volts, the coupling limits derived from MICROSCOPE are weakened by a factor of about 34 compared with the massless case. Concretely, the analysis yields upper limits of roughly 3.6 times ten to the minus twenty-four on the magnitude of a coupling to B minus L for a spin-1 mediator, and about 2.6 times ten to the minus twenty-three for a coupling to baryon number alone, with slightly different numbers, differing by a factor of about 1.2, in the spin-0 case. The limits grow rapidly with mediator mass, and the analytic and numerical approaches agree to better than 0.4 percent across the relevant mass range, meaning future analyses can replace layered numerical models with a single compact formula without loss of accuracy. An even simpler approximation, in which the square root of the form factor is just the hyperbolic sine of x over 2 divided by x over 2, suffices to within about 2.3 percent for ranges down to a tenth of the Earth’s radius.

Beyond the immediate application to equivalence-principle tests, the framework has broad reach. The same hyperbolic form factors can be computed for Gaussian density distributions, where they reduce to simple exponentials, and for the electron cloud of a hydrogen atom in its ground state, where the ordinary form factor reproduces the well-known result governing electron scattering. The formalism also illuminates curious resonant behavior: for certain density profiles, the ordinary form factor vanishes at specific values of the wave number, corresponding to internal solutions of the field equations with vanishing outside potential, the static analogue of a stationary wave confined within the sphere. And because the coupling limits depend only weakly on the fine details of the Earth’s deep interior, further refinements of the internal structure are unlikely to change the conclusions. What began as an exercise in redefining a form factor may thus sharpen one of the most sensitive searches for physics beyond the standard model, turning the whole planet into a precisely calibrated instrument for detecting forces too faint to imagine.

Subject of Research: Hyperbolic form factors describing Yukawa potentials of extended bodies and their application to constraining new finite-range forces using Earth's density distribution

Article Title: Hyperbolic form factors for Yukawa interactions, and applications to the Earth

Article References: Fayet, P. (2026). Hyperbolic form factors for Yukawa interactions, and applications to the Earth. The European Physical Journal C, 86(9), Article 1125. https://doi.org/10.1140/epjc/s10052-026-15933-4

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-15933-4

Keywords: Yukawa potential, hyperbolic form factor, dark photon, equivalence principle, MICROSCOPE mission, new forces, Earth density, dark sector, fifth force, Laplace transform, particle physics, geophysics

The research, carried out by Nasser Laouami of the National Applied Research Centre in Earthquake Engineering in Algeria, tackles one of the most persistent gaps in seismic hazard assessment. Many strong-motion stations worldwide were installed decades ago, often on the roofs or basements of buildings, with little or no accompanying subsurface investigation. Drilling new boreholes at each site is costly and time-consuming, so the average shear-wave velocity of the top 30 metres, known as Vs30, remains unknown or poorly constrained for a large fraction of the global seismic network. Yet Vs30 is the single parameter that most building codes, including the American NEHRP provisions and Eurocode 8, use to assign a site class and therefore to determine the design forces a structure must withstand. Without reliable Vs30 values, the ground-motion predictions that feed into hazard maps carry an extra layer of uncertainty.

Laouami’s approach exploits a phenomenon that seismologists have long used as a diagnostic tool: the horizontal-to-vertical spectral ratio, or H/V. When earthquake waves arrive at a station, the horizontal components of motion are typically amplified relative to the vertical component at frequencies where the shallow soil column resonates. That resonance frequency depends on the thickness and stiffness of the sedimentary layers, while the shape and amplitude of the H/V curve encode information about the impedance contrasts between layers. In principle, the entire H/V spectrum is a fingerprint of the subsurface velocity structure. The challenge has always been to invert that fingerprint, meaning to find a layered velocity model whose predicted H/V curve matches the observed one, without falling into the trap of non-uniqueness, where many different models fit the data equally well.

What makes the new method notable is that it requires no prior information about the subsurface. Most inversion schemes in geophysics lean heavily on initial guesses: an approximate depth to bedrock, a rough velocity for the top layer, or a fixed number of layers. When such constraints are wrong, the inversion can converge to a misleading solution. The new technique instead assumes that the strong-motion wavefield at the station is dominated by vertically incident body waves, a reasonable approximation for stations recording earthquakes at regional distances, and then searches for the velocity profile that best reproduces the observed H/V spectrum. The inversion is statistical in character, exploring a wide space of possible layered models and identifying the family of solutions consistent with the data rather than committing to a single profile that may be an artefact of the starting conditions.

To test the method rigorously, Laouami turned to one of the best-characterized seismic datasets on Earth: Japan’s KIK-NET network, operated by the National Research Institute for Earth Science and Disaster Resilience. KIK-NET stations are paired with boreholes that reach stiff engineering bedrock, and the shear-wave velocity profile at each site has been measured directly. That makes the network an ideal proving ground, because the true answer is known. The study used strong-motion records and measured velocity profiles from 66 KIK-NET stations, spanning a wide range of soil conditions from soft alluvial basins to relatively stiff sites. The inverted profiles were compared layer by layer against the borehole measurements, and the key classification parameter, Vs30, was compared station by station.

The results were evaluated in two configurations. In the first, the inversion ran completely unconstrained, with no prior information whatsoever about the subsurface. In the second, a small piece of shallow information was supplied: the average shear-wave velocity of the top five metres, or Vs5, which can be obtained cheaply from non-invasive surface measurements. The comparison revealed a clear and useful pattern. Without any prior information, the method produced reliable velocity profiles for sites whose Vs30 values reach approximately 600 metres per second, which covers the softer end of the soil spectrum, including many of the most seismically hazardous sedimentary settings. For stiffer profiles beyond that threshold, the unconstrained inversion began to lose accuracy, but adding the modest Vs5 constraint restored performance, extending reliable estimates to stiffer sites as well.

The quantitative agreement is impressive for a technique that requires no drilling. For most of the 66 stations, the relative error between the observed and predicted Vs30 was less than 20 percent, and the error in the predominant frequency of the H/V curve, the frequency at which the site resonates most strongly, was around 15 percent. In geotechnical and geophysical practice, both levels of accuracy are generally considered acceptable for preliminary site characterization. Perhaps more importantly, the method captured the overall shape of the velocity profiles, including the depth of major impedance contrasts, rather than merely matching a single averaged number. That matters because site response is not determined by Vs30 alone; the full layering controls how ground motion is amplified across the frequency range that affects buildings of different heights.

The implications extend well beyond Japan. Dense seismic networks exist in many earthquake-prone countries, but borehole measurements are far rarer, particularly in developing regions where instrumentation has expanded faster than site characterization budgets. A technique that extracts velocity profiles from recordings that stations are already producing, at no additional field cost, could rapidly fill the characterization gap. It could also serve as a first-pass screening tool: stations flagged by the H/V inversion as sitting on unusually soft or strongly resonant soils could be prioritized for follow-up borehole investigation. In regions governed by seismic codes that rely on Vs30-based site classes, such as the Algerian RPA2024 regulations or the NEHRP provisions, even a preliminary estimate with 20 percent accuracy can meaningfully improve hazard maps and design decisions.

The method also fits into a broader scientific conversation about what the H/V spectral ratio actually measures. Earlier theoretical work, including diffuse-field formulations of the H/V ratio developed by researchers such as Kawase, Sánchez-Sesma and Matsushima, showed that the ratio can be interpreted through the properties of the wavefield and the layered medium beneath the receiver. Laouami’s contribution is to demonstrate, against ground truth, that a body-wave assumption applied to strong-motion records is sufficient to recover the velocity structure without prior constraints, at least for a substantial range of site stiffness. The technique complements rather than replaces established non-invasive methods such as surface-wave analysis, which suffers from its own well-documented non-uniqueness problems, and it inherits the practical advantage of requiring only a single three-component station rather than an array of sensors.

Limitations remain, and the author is careful about them. The assumption of vertically incident body waves may weaken at sites with complex three-dimensional structure or strong lateral heterogeneity, and the accuracy degrades for very stiff sites unless shallow information is supplied. The validation set, while diverse, comes from a single national network with high-quality borehole data, so performance in other tectonic and sedimentary environments will need independent confirmation. Nevertheless, the core result stands: for a wide range of soil profiles, the ground beneath a seismic station can be mapped from its own earthquake recordings, with errors small enough to support site classification. In a field where every unknown parameter translates directly into uncertainty in shaking estimates and building safety margins, a reliable, non-invasive, zero-drilling method for recovering shear-wave velocity profiles is a genuinely valuable addition to the earthquake engineering toolkit, and one that could be applied to thousands of existing stations worldwide almost immediately.

Subject of Research: Non-invasive estimation of S-wave velocity profiles from single-station H/V spectral ratio inversion of strong-motion records

Article Title: Estimating S-wave velocity profiles from single-station horizontal-to-vertical spectral ratio inversion without prior information: application for KIK-NET accelerometric stations

Article References: Laouami, N. (2026). Estimating S-wave velocity profiles from single-station horizontal-to-vertical spectral ratio inversion without prior information: application for KIK-NET accelerometric stations. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02660-0

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02660-0

Keywords: shear-wave velocity, H/V spectral ratio, inversion, Vs30, site characterization, KIK-NET, strong-motion records, seismic station classification, earthquake engineering, geophysics, non-invasive methods, soil amplification

The research, led by Yan Lévesque of the Institut National de La Recherche Scientifique and Université du Québec à Chicoutimi, together with Romain Chesnaux, Julien Walter and Lamine Boumaiza, tackles one of the most persistent problems in hydrogeology: calibrating a numerical model when direct measurements of groundwater levels are sparse. Groundwater models are the workhorses of water resource management, used to predict how aquifers will respond to pumping, drought and climate change. Yet a model is only as good as the data used to constrain it, and in many regions of the world, observation wells are few and far between.

To build their model, the team assembled an unusually rich geological picture of the moraine system. They drew on 94 boreholes drilled through the glacial deposits, five stratigraphic cross-sections that traced the layering of sediments across the landscape, and three complementary geophysical techniques: 20 transient electromagnetic surveys, 6 electrical resistivity tomography surveys, and 6 ground-penetrating radar surveys. Each method probes the subsurface in a different way. Transient electromagnetic surveys induce currents in the ground and measure how they decay, revealing contrasts in electrical conductivity that distinguish saturated from unsaturated materials. Electrical resistivity tomography injects current through electrodes and maps resistivity variations in two-dimensional slices, while ground-penetrating radar uses high-frequency radio waves to image shallow stratigraphy with centimeter-scale resolution.

From this combined dataset, the researchers constructed an integrated three-dimensional geological framework capturing the complex, heterogeneous and anisotropic character of the aquifer system. Glacial deposits such as moraines are notoriously difficult to model because they were deposited by ice in chaotic, laterally discontinuous layers. A single borehole may reveal coarse sand at one location while a neighboring hole, only a few hundred meters away, encounters dense till. Geophysics helps bridge those gaps by interpolating between the hard constraints of the boreholes with continuous geophysical profiles.

With the geological architecture in place, the team turned to numerical simulation. They used FEFLOW, a widely applied finite element software package for modeling groundwater flow, mass and heat transport in porous media, to run flow simulations and define initial estimates of hydraulic parameters such as hydraulic conductivity, which describes how easily water moves through the subsurface. These initial values were then refined through calibration, the process of adjusting model parameters until simulated groundwater levels match observed ones as closely as possible.

The crucial innovation lay in what the researchers used as calibration targets. Instead of relying solely on water levels measured in wells, they employed groundwater levels derived from the geophysical data themselves. Because the electrical resistivity of sediments changes sharply at the water table, where dry unsaturated material gives way to saturated material, geophysical surveys can be inverted to estimate the depth of the saturated zone. The team used the FePEST module, an interface between FEFLOW and the PEST parameter estimation software, to automatically adjust hydraulic parameters until the model reproduced these geophysics-derived water levels.

The real test came afterward. The researchers evaluated the calibrated model against an entirely independent dataset: piezometric measurements from 26 observation wells distributed across the study area. These wells had not been used in the calibration, so they provided an unbiased measure of the model’s predictive skill. The comparison between calibrated and observed groundwater levels yielded a root mean square error of 3.69 meters, a relatively low value for a regional-scale model of heterogeneous glacial terrain, indicating good agreement with field observations.

That result carries weight beyond one moraine in Quebec. In many parts of the world, including remote northern communities, agricultural regions with sparse monitoring networks, and developing countries investing in groundwater resources for the first time, the cost of drilling observation wells limits how well aquifers can be understood. Geophysical surveys are faster and often cheaper per unit of spatial coverage than drilling, and this study demonstrates that their output can serve not merely as a qualitative check on a model but as quantitative calibration data that actively refines hydraulic parameters.

The approach also reframes the traditional division of labor between geophysics and hydrogeology. Conventionally, geophysical data have been used to build geological models, delineating aquifer boundaries and layer thicknesses, while calibration has depended on hydraulic measurements. The new work shows that geophysically derived groundwater levels can play both roles, supporting the geological framework and simultaneously constraining the flow model. This dual use of a single dataset maximizes the value of every field campaign and reduces the uncertainty that accumulates when models are calibrated against too few points.

The findings arrive at a moment when pressure on groundwater is intensifying worldwide. Aquifers feed rivers, sustain ecosystems and supply roughly half of the world’s drinking water, yet many are being depleted faster than they recharge. Reliable regional models are essential for managing this invisible resource, and the Quebec study offers a practical template: combine boreholes, multiple geophysical methods and automated calibration to extract the maximum information from limited field data. As the authors note, integrating geophysical datasets with conventional hydrogeological observations improves parameter estimation and enhances the reliability of regional groundwater flow models, a step toward water management decisions grounded in a clearer picture of what lies beneath.

Subject of Research: Calibration of a 3D regional groundwater flow model using groundwater levels derived from geophysical surveys in Quebec

Article Title: Calibration of a 3D regional groundwater model using geophysics-derived groundwater levels

Article References: Lévesque, Y., Chesnaux, R., Walter, J., & Boumaiza, L. (2026). Calibration of a 3D regional groundwater model using geophysics-derived groundwater levels. Hydrogeology Journal. https://doi.org/10.1007/s10040-026-03144-w

Image Credits: AI Generated

DOI: 10.1007/s10040-026-03144-w

Keywords: groundwater modeling, geophysics, hydrogeology, aquifer calibration, transient electromagnetics, electrical resistivity tomography, ground-penetrating radar, FEFLOW, Saint-Narcisse Moraine, Quebec, hydraulic conductivity, water table

The TFFSM is an unusual instrument in the geophysical toolbox. Developed in China, it borrows the observational philosophy of magnetotellurics, the grand old method that reads natural electromagnetic fields to peer into the Earth, but with a crucial twist. Rather than recording raw time-domain signals and extracting frequencies later through spectral analysis, TFFSM devices use hardware-based frequency selection: they lock onto predefined frequencies in advance, targeting specific subsurface depths while suppressing noise at the circuit level. The approach requires minimal post-processing, and its instruments are light enough for a single operator to carry. One commercially produced frequency selector has been deployed in more than 180 countries, yet many academic geophysicists have remained skeptical, largely because rigorous theoretical work on the method has been scarce.

The heart of the controversy lies in what actually causes the anomalies that TFFSM detects. The new study, led by Tianchun Yang of Hunan University of Science and Technology together with colleagues in China and Italy, set out to answer that question with an unusually well-constrained experiment. Because the water well’s geology is known in exquisite detail from drilling logs—25 meters of Quaternary clay overlying siltstone, glutenite, and conglomerate, with a static water level at 27 meters and two major water-bearing karst cavities at depths of roughly 40 and 56 to 77 meters—any anomaly the instruments recorded could be checked against ground truth. The survey line itself was constrained to less than 30 meters by hardened pavement, forcing the team to work in tight quarters beside the well, which intersects a small southwest-trending secondary fault.

The physics behind the method’s signal is subtle and elegant. In a layered Earth, natural electromagnetic fields induce currents that flow preferentially in horizontal planes. When a near-surface body with different electrical conductivity is present—say, a water-saturated cavity—it accumulates electric charge at its boundaries, distorting the horizontal electric field measured at the surface. Using Gauss’s law and the quasi-static approximation, the researchers show that this secondary electric field is proportional to, and in phase with, the primary field. The result is a frequency-independent multiplicative shift in the measured electric field, a phenomenon known in magnetotellurics as the static effect. Conventionally, static shift is treated as a nuisance that corrupts deep-crustal soundings and must be corrected away. TFFSM, in a stroke of methodological audacity, deliberately embraces it: the static offset becomes the signal, and its spatial pattern maps shallow electrical heterogeneities such as groundwater.

The experimental design was methodical. First, a rapid triple-frequency survey at 25, 67, and 170 hertz—frequencies chosen because they coincide with Schumann resonances and lightning-driven magnetotelluric fluctuations while avoiding odd multiples of the 50-hertz power-line frequency—scanned the line in about three seconds per station. The team then deployed three commercial frequency selectors, the PQWT-TC150, TC300, and TC1200, with maximum detection depths of roughly 150, 300, and 1200 meters. These were run in two configurations: each instrument using its own electrode pair, and all three sharing a single pair. Finally, electrode spacing was varied from 10 meters down to 6 and then 2 meters, and AMT measurements were taken at two stations using a Chinese Academy of Sciences SEP system.

The results were strikingly consistent. All three instruments, in both electrode configurations, recorded a pronounced low-potential-difference anomaly at the 8-to-9-meter mark of the survey line—directly above the known water-bearing structure. The pseudo-sections built from the multi-frequency data displayed the characteristic noodle-like vertical streak that is the fingerprint of static shift, confirming that groundwater produces exactly this signature in TFFSM data. Notably, whether the instruments shared electrodes or used separate ones made essentially no difference to the results, underscoring the method’s famously relaxed grounding requirements. The static effect was strongest in the TC150, whose higher minimum frequency of about 100 hertz keeps its signal clean, while the deeper-sounding TC1200, sampling down to 8 hertz, showed a weaker expression because low-frequency signals are inherently feeble and more vulnerable to cultural noise.

The electrode-spacing experiments revealed a practical rule that commercial manufacturers may need to heed. As spacing shrank from 10 to 6 to 2 meters, peak potential differences collapsed from about 30 millivolts to under 10 millivolts, the anomaly narrowed, and at 2 meters the static effect vanished almost entirely. The water-bearing cavities lie deeper than 40 meters, and with electrodes only 2 meters apart, the induced potential difference simply fell below the detectable threshold. Smaller spacing also destabilized the low-frequency data, with curves below roughly 20 hertz becoming erratic. The authors conclude that fixed cable lengths of 10 or 20 meters are a design limitation, and that users should adaptively choose electrode spacing according to the depth of their target—larger spacing for deeper objectives, potentially approaching 1000 meters.

The comparison with AMT was where the study delivered its most dramatic verdict. In the campus environment, the AMT power spectral density curves for both electric and magnetic field components fluctuated wildly and broke down across the frequency band, contaminated by traffic, high-voltage transmission lines, and electrical appliances. The derived apparent resistivity curves were severely corrupted, especially below 10 hertz, and failed to represent the true subsurface. In practice, the authors note, such conditions render AMT unusable for routine exploration. The TFFSM curves at the same stations, by contrast, varied smoothly and gradually—a resilience the team attributes to hardware-level frequency selection and noise suppression, which sidesteps the time-domain acquisition and spectral processing that make conventional AMT so noise-sensitive.

The implications reach beyond groundwater. The authors propose that future AMT instruments could borrow the frequency-selective acquisition strategy, sampling discrete frequencies sequentially so that filters can be pre-configured to suppress ambient noise. More immediately, the study positions TFFSM as a viable tool for urban geological prospecting, where electromagnetic interference, confined spaces, and hardened surfaces defeat most conventional methods. Prior work has already shown the technique can deliver high-quality imaging near high-voltage lines when the survey line runs parallel to them, and its lightweight, portable hardware suits the cramped logistics of city surveys.

For a method once dismissed as theoretically undercooked, the water well experiment offers something rare: a clean, physically grounded explanation of what its anomalies mean. Groundwater does not reflect electromagnetic waves back like radar in any dominant way; instead, it stamps a static, frequency-independent distortion onto the telluric electric field, and that distortion can be read as a map of hidden water. By turning magnetotellurics’ most notorious artifact into a diagnostic tool, and by demonstrating robustness where AMT fails outright, the Xiangtan team has given a five-decade-old technique the theoretical legitimacy it long lacked—and perhaps pointed the way toward a new generation of noise-hardened electromagnetic instruments for the crowded, electrified cities of the future.

Subject of Research: Comparative field testing of the telluric field frequency selection method and audio-magnetotelluric method for groundwater detection near a known water well

Article Title: A comparative experiment of telluric field frequency selection method and audio-magnetotelluric method next a water well

Article References: Yang, T., Yang, Z., Qin, Q., Hussain, Y., Yu, Q., & Zhu, M. (2026). A comparative experiment of telluric field frequency selection method and audio-magnetotelluric method next a water well. Discover Geoscience, 4(1), Article 367. https://doi.org/10.1007/s44288-026-00704-1

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00704-1

Keywords: telluric field frequency selection method, audio-magnetotelluric method, groundwater exploration, static effect, geophysics, electromagnetic methods, water well, electrode spacing, hydrogeology, karst aquifer, urban geophysical prospecting, natural electromagnetic fields

The study rests on a simple but technically demanding question: what happens to molten rock under the conditions that prevail at Mars’ core–mantle boundary, roughly 2,000 kilometers beneath the surface, where pressures reach tens of gigapascals and temperatures climb above 2,000 kelvin? Earlier in the planet’s history, Mars was almost certainly covered by a global magma ocean. As that ocean cooled, minerals crystallized and settled, and the question of what the last dregs of melt looked like—and where they went—has hung over Martian science for decades. If the final melts were dense enough, they could have drained downward and pooled at the base of the mantle, forming a long-lived silicate layer sandwiched above the liquid iron core.

To test that possibility, the researchers performed high-pressure experiments on synthetic compositions modeled after the silicate melts expected to be produced during the crystallization of a Martian magma ocean. Using multi-anvil press apparatus capable of reaching the gigapascal-scale pressures of the deep Martian interior, they equilibrated samples at controlled temperatures, quenched them rapidly to preserve their textures and chemistry, and then analyzed them with electron microscopy and other microanalytical techniques. From these recovered samples, the team determined the density of the melts as a function of pressure, temperature and composition, along with the melting behavior of the deep mantle assemblage.

The central result concerns buoyancy. For a magma layer to persist at the core–mantle boundary, it must be denser than the overlying solid mantle so that it does not rise and disperse, yet it must remain molten rather than freezing solid. The experiments show that late-stage melts enriched in iron and incompatible elements—components that preferentially remain in the liquid as crystals grow—become sufficiently dense under Martian deep-interior conditions to be gravitationally stable at the base of the mantle. In other words, the last liquids of a crystallizing magma ocean would naturally sink, collect and stagnate at the very bottom of the mantle, exactly where a basal magma layer has been hypothesized.

Just as important, the measurements constrain how much heat such a layer would have to exchange with its surroundings to remain molten. A magma layer at the core–mantle boundary sits atop a hot liquid iron core and beneath crystalline mantle rock, so its survival depends on a delicate thermal balance. The experimental data on melting temperatures and melt densities allowed the team to build thermal evolution models of the Martian interior, tracking the layer’s fate over the planet’s 4.5-billion-year history. Those models indicate that, for plausible core temperatures and mantle heat flows, the layer does not necessarily freeze; instead, it can be maintained as a thin, stable reservoir of molten silicate for geologically long timescales, sustained by heat flowing out of the core.

The implications extend directly to seismology. NASA’s InSight lander, which recorded Martian seismic activity until late 2022, produced the first direct glimpses of the planet’s interior, and analyses of its data have hinted at structure near the base of the mantle—some studies even proposed an entirely molten or partially molten layer above the core to explain the observed seismic velocities and attenuation. A basal silicate magma layer offers a physical mechanism for such signals: molten rock attenuates seismic waves strongly and slows shear waves dramatically compared with solid rock. The new experiments provide the compositional and thermodynamic anchor that seismic modelers need, linking a specific melt composition and thickness to specific seismic signatures rather than treating the layer as a free parameter.

The results also speak to the long-term thermal and chemical evolution of Mars. A persistent magma layer at the core–mantle boundary acts as an insulating blanket between the mantle and the core, regulating how efficiently heat escapes the core. That, in turn, influences whether the core can generate a dynamo-driven magnetic field and how the mantle convects over time. Mars lost its global magnetic field billions of years ago, and understanding the thermal insulation provided by a basal melt layer helps explain the timing and efficiency of core cooling. The layer would also sequester heat-producing and incompatible elements—uranium, thorium, potassium and others—concentrating them at the base of the mantle and altering the planet’s internal heat budget in ways that standard models, which assume a chemically uniform mantle, do not capture.

There are caveats, and the authors are careful about them. The experiments constrain the behavior of candidate melt compositions under simplified conditions; the real Martian core–mantle boundary may host heterogeneous materials, partial melting of mantle rock in contact with the core, or mixtures of silicate melt with light elements transferred from the core. The thickness of any surviving layer depends sensitively on the initial sulfur and iron content of the core, the efficiency of mantle convection and the exact crystallization sequence of the ancient magma ocean. Still, the experimental demonstration that deep Martian melts are gravitationally stable at the base of the mantle removes one of the biggest objections to the basal-magma-layer hypothesis: that such melt would have been buoyant and would have risen away long ago.

The work also carries lessons for other planets. Magma ocean crystallization is a universal stage in terrestrial planet formation, and analogous basal melt layers have been proposed for early Earth, where dense iron-rich melts may have accumulated above the core–mantle boundary and influenced the chemistry of plumes that rise from that region today. If a small planet like Mars can retain a molten basal layer for billions of years, the same physics may operate on Venus, Mercury and rocky exoplanets, where the presence or absence of such layers could alter volcanic activity, magnetic-field generation and long-term habitability. Each new laboratory dataset on melt density and melting curves thus becomes a tool for reading the interiors of worlds no spacecraft will ever drill into.

Future observations could settle the question. Additional seismic data, whether from a future Mars geophysical network or from advances in analyzing the existing InSight archive, could be compared directly with the experimentally derived elastic and attenuation properties of the candidate melt compositions. Improved constraints on Mars’ core size and density, its tidal response and its moment of inertia will further narrow the permissible layer thickness. For now, the experiments turn an intriguing speculation into a physically grounded scenario: beneath the cold, dusty surface of Mars, at the deepest reach of its rocky mantle, the last traces of a primordial magma ocean may still be glowing—molten, dense and stubbornly enduring at the edge of the planet’s iron heart.

Subject of Research: Experimental constraints on the existence of a long-lived basal magma layer at the Martian core–mantle boundary

Article Title: Experimental constraints on a long-lived magma layer at the Martian core–mantle boundary

Article References: Pierru, R., Gréaux, S., Dominijanni, S., Man, L., Kono, Y., Kakizawa, S., Higo, Y., Badro, J., Frost, D. J., & Antonangeli, D. (2026). Experimental constraints on a long-lived magma layer at the Martian core–mantle boundary. Nature Geoscience. https://doi.org/10.1038/s41561-026-02104-z

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02104-z

Keywords: Mars, core–mantle boundary, magma ocean, high-pressure experiments, silicate melt density, InSight seismology, planetary interiors, thermal evolution, Martian mantle, basal magma layer, geophysics, Experimental

The urgency of the work is grounded in real events. At Xocén, the roof of an underground cavern gave way suddenly in the middle of a Maya community, toppling a two-century-old Ceiba tree and opening a sinkhole roughly 57 meters in diameter and up to 36 meters deep. The researchers believe intense rainfall and the passage of heavy construction machinery in the days before the collapse triggered the failure of a cavern ceiling that had been weakening for a long time below the surface. At Saamal, an open cenote popular with visitors, partial collapse of the steep cliff walls suggests that even mature, apparently stable karst features can remain structurally active for years.

The scientific setting is extraordinary. The Yucatán’s cenote density traces back to the Chicxulub impact crater, the roughly 200-kilometer-wide scar left by the asteroid that ended the Cretaceous period. The crater’s fractured rim created a zone of enhanced permeability, the so-called Ring of Cenotes, along which carbonate dissolution concentrated over millions of years. When acidified rainwater percolates through limestone, it enlarges fractures and voids until the remaining roof can no longer support its own weight. Because karst terrains show few visible warning signs at the surface until failure occurs, they account for a disproportionate share of sudden ground-collapse hazards worldwide.

Conventional geotechnical investigation relies on drilling and excavation, methods that are expensive, slow, and impractical to deploy across many sites immediately after a collapse. ERT offers an alternative. By injecting electrical current into the ground through arrays of electrodes and measuring the resulting voltage differences, the technique produces cross-sectional images of subsurface resistivity. Competent dry limestone appears highly resistive, while water-saturated, clay-rich, or heavily weathered rock conducts electricity well. Air-filled voids also register as strong resistive anomalies. These contrasts map directly onto the mechanical properties that govern collapse susceptibility.

At Xocén, the team deployed a Syscal Pro system with Wenner and Schlumberger electrode arrays along three profiles totaling up to 110 meters each, reaching effective investigation depths of roughly 16 to 25 meters. After filtering unreliable readings and inverting the data with a smoothness-constrained least-squares algorithm, the final models achieved root-mean-square misfits between about 2 and 2.5 percent. The inverted sections revealed a consistent vertical stratification: a resistive cap of dry limestone more than 1000 ohm-meters, an intermediate transition zone of partially weathered rock, and a laterally continuous conductive interval below 250 ohm-meters interpreted as water-saturated, highly altered limestone, whose top approximates the local water table.

Most striking was a vertically persistent conductive anomaly in one profile, where resistivity dropped below 150 ohm-meters, linking the surface to the deep conductive layer. The researchers interpret this feature as a fracture-controlled infiltration corridor, a preferential pathway through which water percolates, accelerating dissolution and progressively weakening the rock mass. This geometry suggests that the Xocén collapse was not an isolated surface event but the geomorphological expression of a pre-existing weakened subsurface domain, with future instability most likely concentrated at the interfaces between the resistive cap, the transition zone, and the deeper conductive material.

At Saamal, five closely spaced profiles were acquired along the vulnerable cliff rim using a SuperSting R8 system, with electrodes just one meter apart to resolve the approximately 3-meter-thick limestone ledge involved in the recent failures. A robust, blocky inversion captured the sharp contrasts near the wall. The sections showed a thin weathered conductive veneer half a meter to a meter thick overlying competent carbonate rock, plus a very high-resistivity domain exceeding 1600 ohm-meters adjacent to the cliff, interpreted conservatively as a dry, possibly fractured carbonate block. Because air-filled cavities and dry rock yield similarly high resistivities, the authors caution that resistivity magnitude alone cannot confirm open voids. To visualize domain continuity between profiles, the team trained a neural network to interpolate the independently inverted two-dimensional sections into a three-dimensional resistivity volume, a supporting tool rather than a true 3D inversion.

A depth-of-investigation analysis following the established Oldenburg and Li method confirmed that the models are constrained by real data to mean depths of about 19.7 meters at Xocén and 2.9 meters at Saamal, guarding against over-interpretation of poorly resolved regions. The authors are candid about limitations: no borehole, geotechnical, or piezometric control was available, so the inferred domains remain hydrogeophysical rather than directly verified, and the resistivity thresholds are site-specific rather than universal. They recommend corroboration through fracture mapping, ground-penetrating radar, shallow seismic surveys, and repeat ERT monitoring after rainfall events.

The broader significance lies in translating geophysical images into operational disaster-risk decisions. The study proposes that collapse-prone sectors be identified not by a single universal signature but by site-specific combinations of shallow weathered conductive veneers, laterally connected saturated zones, fracture-controlled infiltration pathways, and sharp resistivity interfaces. Classifying ground into high, intermediate, and low susceptibility zones gives authorities a defensible basis for restricting access, prioritizing monitoring, and planning land use around cenotes in populated or heavily visited areas. As reported collapses of cenote roofs increase across the Yucatán, this rapid, affordable screening framework offers communities and tourism operators a practical first line of defense against one of nature’s most sudden geological traps.

Subject of Research: Electrical resistivity tomography screening of collapse susceptibility in karst cenote environments of the Yucatán Peninsula, Mexico

Article Title: Electrical Resistivity Tomography for collapse susceptibility screening in karst cenote environments: A case study in the Yucatán Peninsula, Mexico

Article References: Juárez, S. L., Coyoacán, Aleman, J. C. O., Castañeda, C. C., Hernandez, J. F. E., Perez, D. A. P., & Martinez, J. M. (2026). Electrical Resistivity Tomography for collapse susceptibility screening in karst cenote environments: A case study in the Yucatán Peninsula, Mexico. Environmental Challenges, Article 101642. https://doi.org/10.1016/j.envc.2026.101642

Image Credits: AI Generated

DOI: Not provided

Keywords: cenotes, karst, electrical resistivity tomography, Yucatán Peninsula, sinkhole collapse, Chicxulub crater, geophysics, Xocén, Saamal, disaster risk, limestone dissolution, Ring of Cenotes