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Complex electrical conductivity is increasingly used to monitor subsurface processes associated with microbial activities as microbial cells mostly have surface charges and thus electrical double layers. While highly saline environments are frequently encountered in coastal and marine sediments, there are limited data available on the complex conductivity associated with microbial activities under a high salinity condition. Therefore, we present the spectral responses of complex conductivity of sand associated with bacterial growth and biofilm formation under a highly saline condition of ∼1% salinity and ∼2 S/m pore water conductivity with an emphasis on relaxation behavior. A column test was performed, in which the model bacteria Shewanella oneidensis MR-1 were stimulated for cell growth and biofilm formation in a sand pack, while the complex conductivity was monitored from 0.01 Hz to 10 kHz. The test results show that the real conductivity increases in the early stage due to the microbial metabolites and the increased surface conduction with cell growth, but soon begin to decrease because of the reduction of charge passages due to bioclogging. On the other hand, the imaginary conductivity significantly increases with time, and clear bell-shaped relaxation behaviors are observed with the peak frequency of 0.1- 1 Hz, associated with double layer polarization of cells and electrically conductive pili and biofilms. The Cole-Cole relaxation model appears to well capture such relaxation behaviors, and the modeling results show the gradual increases in normalized chargeability and decreases in relaxation time during bacterial growth and biofilm formation in the highly saline condition. Comparison with previous literature confirms that the high salinity condition further increases the normalized chargeability while it suppresses the phase shift and thus the imaginary conductivity. Our results suggest that the complex conductivity can effectively capture microbial biomass formation in sands under a highly saline condition.

The shape and flexibility of embedded structures, such as tree roots, piles, and anchors, have important impacts on the pullout behavior. However, the rate and manner of mobilization of soil resistances along such structures has not been rigorously explored across a wide range of shapes and structural properties. A spring model for computing compatible displacements of the structure and soil for curved, flexible structures is defined, validated against commonly used methods for computing pile pullout behavior, and then parametrically explored to demonstrate how resistances are mobilized along the length of such structures. The present model allows description of combined axial and transverse loading of these nonlinear structures. The simulation results for the case of normally consolidated clay show that the curvature of a structure causes the distribution of bearing resistance to extend further along the structure than for linear cases. The requirement of equilibrium of the structure produces a coupling between the mobilized bearing and tensile resistance in terms of rate of development and magnitude. Thus, the choices of structure shape impact the magnitude and distribution of mobilized resistance of embedded flexible structures. Implications for anchorage of tree root structures and principles of bioinspired design of anchorage systems are discussed. [ABSTRACT FROM AUTHOR]

Microbial induced calcium carbonate precipitation (MICP) is widely investigated as a sustainable method of soil improvement. The pore-scale precipitation habit of calcium carbonate (CaCO3) and the grain-scale mechanical strength of the cementing bonds play a significant role in determining the mechanical response of MICP-treated soils. This study presents the pore-scale precipitation patterns in MICP-treated sands and the grain-scale cementation strength of two beads cemented by MICP. X-ray computed microtomography imaging of MICP-treated sands with different grain sizes reveal that the surface area, number of contacts, and flow-induced shear detachment as well as bacterial cell loci have a profound effect on the pore-scale precipitation. With different grain size, the precipitation can show either a contact-cementing pattern or a surface-coating habit. Particularly, evolution of the precipitation pattern is observed for large-sized grains from the surface-coating mode to the contact-cementing mode with an increase in CaCO3 content. The grain-scale mechanical strength tests on MICP-bonded bead pairs demonstrate that the failure with MICP bonding occurs in three modes: debonding, internal, and mixed failure modes. Amongst, the debonding failure mode has the greatest strength and the internal failure mode shows the lowest strength. The tensile strength is greater than the shear strength in all modes; particularly in debonding failure mode, the tensile strength of ~35 kPa and shear strength of ~13 kPa. The presented results advance our insight into pore-scale and grain-scale behavior of MICP-treated sands, which can be further extended to develop DEM models and transport models.

Depressurization of hydrate-bearing sediments (HBS) can cause the movement of fine particles, and in turn, such fines migration affects fluid flow and mechanical behavior of sediments, ultimately affecting long-term hydrocarbon production and wellbore stability. This study investigated how and to what extent depressurization of HBS causes fines migration using X-ray computed tomography (CT) imaging. Methane hydrate was synthesized in sediments with 10% fines content (FC), composed of sands with silt and/or clay, and the hydrate-bearing samples were stepwisely depressurized while acquiring CT images. The CT images were analyzed to quantify the spatial changes in FC in the host sediment and thus to capture the fines migration during depressurization. It was found that the FC changes began occurring from the hydrate dissociation regions. This confirms that the multiphase flow caused by depressurization accompanies fines migration. Depressurization of HBS with a hydrate saturation of ~20–40% caused FC reduction from ~10% to ~6–9%, and the extent of fines migration differed with the particle sizes of the host sands and the types of fines. It was found that fines migration was more pronounced with coarse sands and with silty fines. Such observed level of FC reduction is estimated to increase sediment permeability by several factors based on the Kozeny-type permeability model. Our results support the notion that the extent of fines migration and its effect on fluid flow behavior need to be assessed in consideration of physical properties of host sediment and fine particles to identify optimum depressurization strategies.

Continuous improvements at X-ray imaging beamlines at synchrotron light sources have made dynamic synchrotron X-ray micro-computed tomography (SXR-µCT) experiments more routinely available to users, with a rapid increase in demand given its tremendous potential in very diverse areas. In this work a survey of five different four-dimensional SXR-µCT experiments is presented, examining five different parameters linked to the evolution of the investigated system, and tackling problems in different areas in earth sciences. SXR-µCT is used to monitor the microstructural evolution of the investigated sample with the following variables: (i) high temperature, observing in situ oil shale pyrolysis; (ii) low temperature, replicating the generation of permafrost; (iii) high pressure, to study the invasion of supercritical CO2 in deep aquifers; (iv) uniaxial stress, to monitor the closure of a fracture filled with proppant, in shale; (v) reactive flow, to observe the evolution of the hydraulic properties in a porous rock subject to dissolution. For each of these examples, it is shown how dynamic SXR-µCT was able to provide new answers to questions related to climate and energy studies, highlighting the significant opportunities opened recently by the technique.

Pore-scale distribution of supercritical CO2 (scCO2) exerts significant control on a variety of key hydrologic as well as geochemical processes, including residual trapping and dissolution. Despite such importance, only a small number of experiments have directly characterized the three-dimensional distribution of scCO2 in geologic materials during the invasion (drainage) process. We present a study which couples dynamic high-resolution synchrotron X-ray micro-computed tomography imaging of a scCO2/brine system at in situ pressure/temperature conditions with quantitative pore-scale modeling to allow direct validation of a pore-scale description of scCO2 distribution. The experiment combines high-speed synchrotron radiography with tomography to characterize the brine saturated sample, the scCO2 breakthrough process, and the partially saturated state of a sandstone sample from the Domengine Formation, a regionally extensive unit within the Sacramento Basin (California, USA). The availability of a 3D dataset allowed us to examine correlations between grains and pores morphometric parameters and the actual distribution of scCO2 in the sample, including the examination of the role of small-scale sedimentary structure on CO2 distribution. The segmented scCO2/brine volume was also used to validate a simple computational model based on the local thickness concept, able to accurately simulate the distribution of scCO2 after drainage. The same method was also used to simulate Hg capillary pressure curves with satisfactory results when compared to the measured ones. This predictive approach, requiring only a tomographic scan of the dry sample, proved to be an effective route for studying processes related to CO2 invasion structure in geological samples at the pore scale.

This study investigated the P and S wave responses and permeability reduction during bacterial biopolymer formation in unconsolidated porous media. Column experiments with fine sands, where the model bacteria Leuconostoc mesenteroides were stimulated to produce insoluble biopolymer, were conducted while monitoring changes in permeability and P and S wave responses. The bacterial biopolymer reduced the permeability by more than 1 order of magnitude, occupying ~10% pore volume after 38 days of growth. This substantial reduction was attributed to the bacterial biopolymer with complex internal structures accumulated at pore throats. S wave velocity (VS) increased by more than ~50% during biopolymer accumulation; this indicated that the bacterial biopolymer caused a certain level of stiffening effect on shear modulus of the unconsolidated sediment matrix at low confining stress conditions. Whereas replacing pore water by insoluble biopolymer was observed to cause minimal changes in P wave velocity (VP) due to the low elastic moduli of insoluble biopolymer. The spectral ratio analyses revealed that the biopolymer formation caused a ~50-80% increase in P wave attenuation (1/QP) at the both ultrasonic and subultrasonic frequency ranges, at hundreds of kHz and tens of kHz, respectively, and a ~50-60% increase in S wave attenuation (1/QS) in the frequency band of several kHz. Our results reveal that in situ biopolymer formation and the resulting permeability reduction can be effectively monitored by using P and S wave attenuation in the ultrasonic and subultrasonic frequency ranges. This suggests that field monitoring using seismic logging techniques, including time-lapse dipole sonic logging, may be possible.

Understanding the causal factors and mechanisms behind catastrophic landslides and debris flows is crucial for accurate forecasting and disaster risk reduction. Synthetic aperture radar (SAR) data and interferometric SAR (InSAR) technologies provide valuable information for early warning systems’ landslide and debris flow detection and monitoring strategies. This paper applied the Sentinel-1 Persistent Scatterer InSAR (PSInSAR) technique to detect and monitor precursory slope movements over the open-pit mining site in Toledo City, Cebu Island, in the Philippines. The results show that the slope showed instabilities between December 30, 2019, to December 12, 2020, before the actual failure on December 21, 2020. The landslide initiation zone moved with velocities exceeding –10 mm/yr and reaching –90 mm/yr. The topography and morphology of the mining site contributed to the cut slope instabilities. The mining operations and hydrometeorological conditions during the analysis period aggravated the situation, leading to the cut slope failure. Overall, with favorable slope geometry, surface characteristics, and SAR data availability, the Sentinel-1 PSInSAR technique can serve as a landslide early warning system tool and aid decision-making in an actively operating open-pit mine and other landscapes.

In this study, to investigate the effect of multiple countermeasure on the flow characteristics of debris flows, a real-scale experiment was conducted in a natural gully by reproducing a debris flow with a installation of multiple countermeasures. In addition, the topographic changes before and after experiment by debris flow were investigated using UAV-LiDAR. Based on the experiment results, the effect of multiple countermeasures and the topographic changes against the gully erosion and deposition caused by debris flow were also analyzed. The installation of multiple countermeasures significantly decreased the frontal velocity of debris flow. Furthermore, the countermeasure induced the deposition of debris material on the back of the countermeasure.