Near Surface Geophysics - Latest issue

As global groundwater levels continue to decline rapidly, there is a growing need for advanced techniques to monitor and manage aquifers effectively. This study focuses on validating a numerical model using seismic data from a small‐scale experimental setup designed to estimate water volume in a porous reservoir. Expanding on previous work with synthetic data, we analyse seismic data acquired from a controlled experimental site in Laukaa, Finland. By employing neural networks, we directly estimate water volume from seismic responses, bypassing the traditional need for separate determinations, for example, of reservoir water‐table level and porosity. The study models wave propagation through a coupled poroviscoelastic–viscoelastic medium using a three‐dimensional discontinuous Galerkin method. The proposed methodology is validated against experimental data, aiming to improve precision in mapping current water volumes and contributing to the development of sustainable groundwater management practices.

Groundwater management in arid and semi‐arid regions, particularly around the Mediterranean Sea, poses significant challenges in terms of exploration, exploitation and sustainability. This study focuses on understanding the structural and sedimentary variations on the Sahel domain in Eastern Tunisia, Western Mediterranean province. Groundwater resources are predominantly tapped through shallow and deep wells within syncline structures filled with Neogene sediments, predominantly siliciclastic deposits but a clear delimitation of aquifers is lacking. The gravity method combined with seismic interpretation, petroleum and water well description is employed to explore the subsurface geology, aiming to enhance comprehension of the basins and sub‐basins structure. The latter is a key component to recognizing water recharge and discharge pathways and to develop effective and long‐term groundwater exploration strategies. First, the residual gravity field is produced. Derivative maps, an Euler deconvolution solution map and a 2D gravity inversion model are subsequently generated to delineate different anomalies and to estimate the depth to basement and subsurface density contrasts. The different maps and the 2D gravity inversion models show that: (1) The Sisseb‐El Alem and Sousse regions are separated by a deep NE–SW positive anomaly, related to a basin structure that can reach up to 3 km depth; (2) the Sisseb‐El Alem region is composed of many elongated sub‐basins with N–S, E–W and NE–SW directions (local negative residual anomalies) separated by NW–SE and E–W structures. Finally, the proposed method provides a valuable basis for better management of hydrogeological exploitation.

Dien Bien basin (DBB), located in the Northwest of Vietnam, is facing a high seismic hazard. To understand the characteristics of the subsurface cover, microtremor measurements were conducted at 452 sites and analysed using the horizontal‐to‐vertical spectral ratio (HVSR) technique. The results indicate that the shape of the HVSR curves reflects local site characteristics, whereas the fundamental frequency (F0) is associated with the thickness of the subsurface cover. In the plain area, HVSR curves typically display clear peaks, while in the hilly‐mountainous zones (hard soil or exposed rock), they exhibit multiple high‐frequency dominant peaks or appear relatively flat. Seismic microzonation results show that F0 ranges from 0.50 to 15.87 Hz. Low‐frequency values (<1 Hz) are only observed in the plains, where high‐rise buildings (>7 floors) are more prone to resonance with local site conditions. In contrast, high‐frequency values (>3.5 Hz) are mainly distributed in hilly‐mountainous zones, where low‐rise buildings (<2 floors) are more likely to be affected. Accordingly, the hilly zones are relatively safe zones for high‐rise construction, while the strip along the Nam Rom River represents a seismically vulnerable area. The shallow cover thickness varies between 6 and 157 m, increasing from North to South and from the mountain to the plain zones. The hard rock surface is a depression‐shaped, with a rough‐stepped bottom in the longitudinal direction, smoother in the latitudinal direction and gradually extending southwards. This study demonstrates that the site characteristics of the DBB are primarily controlled by the thickness of the subsurface cover and were well evaluated by microtremor surveys.

This study presents a methodological framework for estimating electromagnetic wave velocities in ground‐penetrating radar (GPR) data based exclusively on the analysis of diffractions. The approach integrates diffraction separation using the plane wave destruction algorithm and subsequent velocity refinement through the residual diffraction moveout (RDM) technique. The methodology was initially validated on synthetic datasets with controlled geological complexity, including models with lateral and vertical velocity variations. These synthetic models were intentionally designed to simulate environments rich in diffracted events, replicating conditions similar to those observed in real data. Results from synthetic tests demonstrated the capacity of the methodology to isolate diffraction hyperbolas effectively, estimate local velocities and generate coherent velocity models suitable for migration. The workflow was then applied to real GPR data acquired on the Detroit Plateau, Antarctic Peninsula. The resulting velocity model aligned well with previous studies based on common midpoint analysis, highlighting structural variations associated with firn densification and the firn–ice transition zone. Kirchhoff migration using the RDM‐derived velocity model led to more precise reflector imaging and improved subsurface structural interpretation. This study demonstrates that diffraction‐based velocity modelling can provide accurate and high‐resolution subsurface characterization, particularly in environments with low lateral velocity contrast where conventional methods may be limited.

Near‐surface seismic refraction tomography is a powerful tool for imaging shallow subsurface structures, yet conventional approaches often fail to resolve sharp velocity contrasts at geological interfaces due to inherent smoothness constraints. We present a hybrid methodology that combines Hagedoorn's Plus–Minus (T±) method with refraction tomography, constructing geologically plausible 2D starting models with predefined interfaces to overcome these limitations. Validation through synthetic benchmarks and field applications—performed with identical iteration counts and inversion parameters for fair comparison—demonstrates consistent superiority in scenarios involving abrupt velocity transitions (permafrost boundaries, water tables and bedrock interfaces). Synthetic tests show that although conventional tomography fails to recover plausible models for interfaces with significant topography, the hybrid approach accurately resolves both interface geometries and velocity distributions. Field applications confirm these advantages. In a hydrological survey, the method delineated horizontal water tables at 5.2 ± 1.0 m depth versus smooth, non‐physical solutions from conventional tomography. In Alpine permafrost zones, it resolved extreme lateral velocity contrasts (such as 2.1–5.3 km/s at 20 m depth) while maintaining inversion stability. This work establishes a practical and robust workflow for generating geologically constrained starting models directly from refraction data, significantly advancing the resolution of sharp interfaces in near‐surface seismic tomography for both academic and industrial applications.

The southern margin of the Junggar Basin is abundant in oil and gas resources, with substantial exploration potential. However, the region's complex surface conditions, marked by significant elevation variations, abrupt near‐surface structural changes and uneven gravel layer thickness in the foreland, present significant challenges for static correction. These complexities severely impact imaging accuracy and impede further exploration efforts. To address these challenges, we propose a multi‐information‐constrained static‐correction method that integrates first‐arrival traveltime tomography with refraction traveltime migration. The tomography is constrained by jointly utilizing seismic first‐arrival times, micro‐well logs and geological outcrop data, yielding a geologically plausible near‐surface velocity model. This model then serves as the input for refraction traveltime migration, which is employed to delineate the geometry of the low‐velocity weathering layer with higher fidelity, ultimately leading to more accurate static corrections. The application in the Junggar Basin foreland shows that our method produces a sharper, more geologically consistent weathering‐layer interface compared to conventional tomography using a velocity contour datum. This leads to superior static corrections, evidenced by enhanced reflection continuity and sharper focus in the final stacked image.