Near Surface Geophysics - Volume 21, Issue 1, 2023

Detecting the top and base subsea permafrost from 2D seismic reflection data in shallow marine settings is a non‐trivial task due to the occurrence of strong free surface multiples. The potential to accurately detect permafrost layers on conventional 2D seismic reflection data is assessed through viscoelastic modelling. Reflection imaging of permafrost layers is examined through the evaluation of specific characteristics of the subsurface, acquisition parameters and their impact. Results show that limitations are related to the principles of the method, the intrinsic nature of the permafrost layers, and the acquisition geometry. The biggest challenge is the occurrence of free surface multiples that overprint the base permafrost reflection, with the worst‐case scenario the case of a thin layer of ice‐bonded sand. Wedge models suggest that if the base permafrost is dipping, it would intersect internal and free surface multiples of the seafloor and the top permafrost and be detected. Also, the amplitude ratio of the base permafrost reflection and the multiples decreases with the increasing thickness of permafrost. Therefore, the crosscutting relationship between the reflection at base permafrost reflection and the multiples might not be enough to detect the base permafrost for thicker permafrost layers. Finally, the experiment results show that, for partially ice‐bonded layers, the attenuation combined with the low reflectivity of the basal interface limits the likelihood to resolve the base permafrost, especially for thick permafrost layers.

We present a global–local full‐waveform inversion (FWI) of surface waves to estimate a high‐resolution shear wave velocity model of the near‐surface at the site of Grenoble (France). The seismic data we use have been acquired in the framework of the InterPACIFIC project. A first attempt is made by employing the multichannel analysis of surface waves (MASW) to invert the fundamental modes of Rayleigh waves. This inversion is solved through a global optimization driven by the neighbourhood algorithm. However, the limiting 1D assumption, together with the severely ill‐posedness of the MASW inversion motivate us to exploit all the information content of the recorded Rayleigh waves (i.e., travel‐times and amplitudes) by implementing a global–local FWI approach. We first perform a global FWI in which a genetic algorithm (GA) is used to minimize the L2 norm difference between observed and simulated seismic waveforms up to 30 Hz. This inversion employs a two‐grid scheme in which the subsurface is described by a fine grid input to the forward modelling, whereas the unknowns are defined on a coarse grid. A bilinear interpolation brings the coarse‐grid model into the fine grid. To accelerate the convergence, two strategies are considered; offset marching and frequency marching. Then, the long‐wavelength velocity profile provided by the GA inversion is used as the starting point for a local FWI for further refinement of the predicted model. In this case, we employ a frequency‐marching strategy up to a maximum frequency of 60 Hz. Both the global and local approaches use a finite difference code for the forward computation. Despite the limited a priori information infused into the inversion framework, the presented global–local FWI scheme provides reliable results as demonstrated by the good match between recorded and predicted seismograms and by the agreement of the estimated velocity with both nearby borehole data and stratigraphic information on the study site. Our global–local strategy also achieves better data fitting and improved matching with the available borehole data with respect to the velocity profile estimated when the local FWI is started from the MASW predictions.

Shear‐wave velocity constitutes an important characterization parameter in terms of engineering properties. Several techniques of surface wave analysis have been widely adopted for building near‐surface 1D, or even 2D, S‐wave velocity models, but comparable 3D approaches, such as the surface wave tomography (SWT) method, are usually limited to global or regional seismological studies. Moreover, a reliable 3D subsurface imaging in a complex and noisy near‐surface environment becomes difficult when using the fully automated processing techniques that are commonly suggested for the SWT method. Here, a new strategy that is well adapted to geotechnical investigations is proposed to accomplish automatically the frequency‐dependent mapping of Rayleigh waves, both for group and phase velocity, throughout an active seismic network. The specific methodology can be implemented in both 2D and 3D active seismic data acquisition schemes involving both uniform and non‐uniform layouts of receivers. It combines the advantages of a newly created common‐midpoint (CMP) cross‐correlation (CC) analysis technique, with the precision of the travel‐time tomography method that is usually used in regional seismological studies. The main analysis is based on partitioning the different wave propagation directions, weighting the CC frequency–velocity analysis around one‐wavelength and stacking the CMP dispersion images. The tomographic inversion is applied to frequency‐dependent virtual travel‐times, produced from the azimuth‐dependent results of the specific analysis, in order to generate, as output, the local dispersion curves. The output of the proposed processing method can be interpreted with any preferred inversion algorithm for group and phase velocity, either individually or jointly. Simple synthetic tests together with a 3D seismic experiment in well‐known conditions, using standard refraction and multichannel analysis of surface waves (MASW) equipment, confirmed the effectiveness of the proposed methodology in detecting both lateral and vertical S‐wave velocity variations. A ‘construction site’ case study finally highlighted the potential of the new tool in a very difficult and noisy environment.

In this decade, electro‐geophysical methods are widely used in different environmental subjects. Studies on soil remediation when polluted by dense non‐aqueous phase liquids (DNAPLs) has become a certain need for all countries. Geoelectrical methods have shown their potential to facilitate evaluating decontamination processes. Our challenge in this study was to understand how coupled temperature and saturation changes affect electro‐geophysical parameters in a contaminated 2D sample. The primary objective was to evaluate the efficiency and potential of spectral‐induced polarization (SIP) for monitoring the recovery of DNAPLs in contaminated porous media. A set of 2D tank experiments investigated the impacts of temperature and saturation changes on the electrical complex resistivity of a saturated porous medium under non‐isothermal conditions. The measurements were made with a coal‐tar and water fluid pair in a porous medium that has been simulated by 1 mm glass beads. Hot water was circulated around the tank and an immersion heater used to heat the porous medium in the tank at different stages. The SIP technique (also called complex resistivity) was used to measure the complex electrical resistivity of a medium in the frequency domain. The experimental results for a simple drainage case were validated using numerical modelling. The complex electrical resistivity was used to obtain the saturation field before and after imbibition. For this purpose, the generalized Archie's law obtained for the same fluid pair with 1D cells (with a vertical flow) was used. Our results from electrical resistivity measurements for saturation fields are in accordance with 2D tank images and can illustrate the saturation change with pointwise resistivity measurements. The results show that saturation change has the primary role in electrical resistivity variation compared to temperature (5%–7%). We also studied the effects of temperature change on the Cole–Cole parameters, and the results confirm our previous findings with the same variation trend in these parameters. The results from varying electrical complex resistivity from the 2D tank (with vertical and horizontal flow) in the laboratory conditions will help us to understand the coupled temperature and saturation effects on complex resistivity in a real polluted site case.

Principal calcium sulphate rocks, such as gypsum and anhydrite, originate from evaporitic processes. Gypsum is considered an industrial raw material with high economic value, but anhydrite has no economic interest. Therefore, delineating the interface between sedimentary cover and gypsum, and differentiating gypsum from anhydrite has significant importance in the production planning of open pits. An electrical resistivity tomography study was carried out in the Bala gypsum formation (Central Anatolia, Turkey) to map the gypsum–anhydrite interface. Because open‐pit mining is economical up to a certain depth due to the cost of excavation, delineation of the upper boundary of gypsum is also necessary. As the survey's primary purpose is to delineate the interfaces of the sedimentary cover–gypsum and gypsum–anhydrite layers in shallow depths (50 m), a structure‐based model search scheme that includes a sequential use of global and local search methods has been applied for the derivation of the subsurface resistivity model. The conventional cell‐based conceptual model using the damped least‐squares inversion method is also used for comparison purposes. The strong resistivity contrast leads to an easy identification of cover and gypsum for both conceptual models. However, only the structure‐based conceptual model delineates the interface between gypsum and anhydrite. The estimated interfaces derived from the structure‐based model are consistent with the borehole data. This case history for gypsum investigation indicates the value of selecting an appropriate conceptual model for a specific exploration problem.

Electrical resistivity tomography (ERT) has seen increased use in the monitoring the condition of river embankments, due to its spatial subsurface coverage, sensitivity to changes in internal states, such as moisture content, and ability to identify seepage and other erosional process with time‐lapse ERT. Two‐dimensional ERT surveys are commonly used due to time and site constraints, but they are often sensitive to features of anomalous resistivity proximal to the survey line, which can distort the resultant inversion as a three‐dimensional (3D) effect. In a tidal embankment, these 3D effects may result from changing water levels and river water salinities. ERT monitoring data at Hadleigh Marsh, UK, showed potential evidence of 3D effects from local water bodies. Synthetic modelling was used to quantify potential 3D effects on tidal embankments. The modelling shows that a 3D effect in a tidal environment occurs (for the geometries studied) when surveys are undertaken at high water levels and at distances less than 4.5 m from the electrode array with 1 m spacing. The 3D effect in the modelling is enhanced in brackish waters, which are common in tidal environments, and with larger electrode spacing. Different geologies, river water compositions, and proximities to the model parameters are expected to induce a varied 3D effect on the ERT data in terms of magnitude, and these should be considered when surveying to minimize artefacts in the data. This research highlights the importance of appropriate geoelectrical measurement design for tidal embankment characterization, particularly with proximal and saline water bodies.