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Instruments / Technical Area	Publication Details	Published
I13-2-Diamond Manchester Imaging	Comparison of methods to segment variable-contrast XCT images of methane-bearing sand using U-nets trained on single dataset sub-volumes Fernando Alvarez-Borges , Oliver N. F. King , Bangalore N. Madhusudhan , Thomas Connolley , Mark Basham , Sharif I. Ahmed Methane 2, 1 - 23 DOI: 10.3390/methane2010001 Diamond Proposal Number(s): [16205] Open Access Show Abstract ▼ Abstract: Methane (CH4) hydrate dissociation and CH4 release are potential geohazards currently investigated using X-ray computed tomography (XCT). Image segmentation is an important data processing step for this type of research. However, it is often time consuming, computing resource-intensive, operator-dependent, and tailored for each XCT dataset due to differences in greyscale contrast. In this paper, an investigation is carried out using U-Nets, a class of Convolutional Neural Network, to segment synchrotron XCT images of CH4-bearing sand during hydrate formation, and extract porosity and CH4 gas saturation. Three U-Net deployments previously untried for this task are assessed: (1) a bespoke 3D hierarchical method, (2) a 2D multi-label, multi-axis method and (3) RootPainter, a 2D U-Net application with interactive corrections. U-Nets are trained using small, targeted hand-annotated datasets to reduce operator time. It was found that the segmentation accuracy of all three methods surpass mainstream watershed and thresholding techniques. Accuracy slightly reduces in low-contrast data, which affects volume fraction measurements, but errors are small compared with gravimetric methods. Moreover, U-Net models trained on low-contrast images can be used to segment higher-contrast datasets, without further training. This demonstrates model portability, which can expedite the segmentation of large datasets over short timespans.	Dec 2022
I13-2-Diamond Manchester Imaging	Gas bubble dynamics during methane hydrate formation and its influence on geophysical properties of sediment using high-resolution synchrotron imaging and rock physics modeling B. N. Madhusudhan , S. K. Sahoo , F. Alvarez-Borges , S. Ahmed , L. J. North , A. I. Best Frontiers In Earth Science 10 DOI: 10.3389/feart.2022.877641 Diamond Proposal Number(s): [16205] Open Access Show Abstract ▼ Abstract: Gas bubble in aquatic sediments has a significant effect on its geophysical and geomechanical properties. Recent studies have shown that methane gas and hydrate can coexist in gas hydrate–bearing sediments. Accurate calibration and understanding of the fundamental processes regarding such coexisting gas bubble dynamics is essential for geophysical characterization and hazard mitigation. We conducted high-resolution synchrotron imaging of methane hydrate formation from methane gas in water-saturated sand. While previous hydrate synchrotron imaging has focused on hydrate evolution, here we focus on the gas bubble dynamics. We used a novel semantic segmentation technique based on convolutional neural networks to observe bubble dynamics before and during hydrate formation. Our results show that bubbles change shape and size even before hydrate formation. Hydrate forms on the outer surface of the bubbles, leading to reduction in bubble size, connectivity of bubbles, and the development of nano-to micro-sized bubbles. Interestingly, methane gas bubble size does not monotonously decrease with hydrate formation; rather, we observe some bubbles being completely used up during hydrate formation, while bubbles originate from hydrates in other parts. This indicates the dynamic nature of gas and hydrate formation. We also used an effective medium model including gas bubble resonance effects to study how these bubble sizes affect the geophysical properties. Gas bubble resonance modeling for field or experimental data generally considers an average or equivalent bubble size. We use synchrotron imaging data to extract individual gas bubble volumes and equivalent spherical radii from the segmented images and implement this into the rock physics model. Our modeling results show that using actual bubble size distribution has a different effect on the geophysical properties compared to the using mean and median bubble size distributions. Our imaging and modeling studies show that the existence of these small gas bubbles of a specific size range, compared to a bigger bubble of equivalent volume, may give rise to significant uncertainties in the geophysical inversion of gas quantification.	Jun 2022
	Investigation of pile penetration in calcareous soft rock using X-ray computed tomography Fernando Alvarez-Borges , Sharif Ahmed , Bangalore Narasimha Madhusudhan , David Richards International Journal Of Physical Modelling In Geotechnics DOI: 10.1680/jphmg.20.00031 Open Access Show Abstract ▼ Abstract: Penetration of open- and closed-ended model piles into intact chalk, a soft calcareous rock, was investigated using microfocus X-ray computed tomography (XCT). 3D images of the specimens showed that the piles crushed and densified the chalk in their path, creating a crushed chalk annulus around the shaft, a region of compressed destructured chalk below the tip, and fractures across cemented regions of the specimen. Laser-diffraction particle size analyses of the crushed chalk annulus after exhumation showed limited difference with laboratory-remoulded chalk, which suggested thorough de-cementation. Installation stresses and XCT-derived densities were paired using a simplified cylindrical cavity expansion solution to estimate effective radial stress-void ratio states at the pile tip during penetration. More complex numerical solutions could not be applied using the available data. This approach posed significant problems, as it could not suitably incorporate hardening and non-linear stiffness behaviours of Chalk during pile penetration, nor account for the creation of discontinuities. However, effective radial stress-void ratio estimates were found to converge with the reconstituted critical state line of the material at high stresses and low void ratios. This partially supported the use of a critical state framework to characterise pile penetration in Chalk, as proposed in recent literature.	Nov 2020
	Mechanical behaviour of low-medium density destructured White Chalk F. Alvarez-Borges , B. N. Madhusudhan , D. Richards Géotechnique Letters 10, 1 - 29 DOI: 10.1680/jgele.20.00009 Open Access Show Abstract ▼ Abstract: Low and medium density White Chalk often destructures during civil engineering works, forming a putty that exhibits problematic low-strengths in the short-term but may build-up strength with time. The underlying mechanisms that control the mechanical performance of the material are not well understood. This paper explores the prospect of developing a framework to characterise the behaviour of chalk putty. To this end, triaxial tests were carried out using low and medium density reconstituted chalks produced by different methods. Using void ratio vs mean effective stress results, a unique critical state line (CSL) is proposed, regardless of parent rock origin or the crushing process used to generate the putty material. The CSL was also found to satisfactorily fit independent test data by various authors, even though each independent sample material may have had different ‘intrinsic’ CSLs. This was suggested to stem from the similar particle characteristics of the chalks used. An assessment of the ageing effect indicates increases in strength as a result of densification due to consolidation and secondary compression, however, the end of test state could still be described using the CSL.	Jun 2020

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