Summary 3D X-ray micro-CT (XCT) is a non-destructive 3D imaging method, increasingly used for a wide range of applications in Earth Science. An optimal XCT image-processing workflow is derived here for accurate quantification of porosity and absolute permeability of heterogeneous sandstone samples using an assessment of key image acquisition and processing parameters: Image resolution, segmentation method, representative elementary volume (REV) size and fluid-simulation method. XCT image-based calculations obtained for heterogeneous sandstones are compared to two homogeneous standards (Berea sandstone and a sphere pack), as well as to the results from physical laboratory measurements. An optimal XCT methodology obtains porosity and permeability results within ± 2 per cent and vary by one order of magnitude around the direct physical measurements, respectively, achieved by incorporating the clay fraction and cement matrix (porous, impermeable components) to the pore-phase for porosity calculations and into the solid-phase for permeability calculations. Two Stokes-flow finite element modelling (FEM) simulation methods, using a voxelised grid (Avizo) and tetrahedral mesh (Comsol) produce comparable results, and similarly show that a lower resolution scan (∼5 µm) is unable to resolve the smallest intergranular pores, causing an underestimation of porosity by ∼3.5 per cent. Downsampling the image-resolution post-segmentation (numerical coarsening) and pore network modelling both allow achieving of a representative elementary volume (REV) size, whilst significantly reducing fluid simulation memory requirements. For the heterogeneous sandstones, REV size for permeability (≥ 1 cubic mm) is larger than for porosity (≥ 0.5 cubic mm) due to tortuosity of the fluid paths. This highlights that porosity should not be used as a reference REV for permeability calculations. The findings suggest that distinct image processing workflows for porosity and permeability would significantly enhance the accurate quantification of the two properties from XCT.

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The release of greenhouse gases from both natural and man‐made sites has been identified as a major cause of global climate change. Extensive work has addressed quantifying gas seeps in the terrestrial setting while little has been done to refine accurate methods for determining gas flux emerging through the seabed into the water column. This paper investigates large‐scale methane seepage from the Scanner Pockmark in the North Sea with a new methodology that integrates data from both multibeam and single‐beam acoustics, with single‐beam data covering a bandwidth (3.5 to 200 kHz) far wider than that used in previous studies, to quantify the rate of gas release from the seabed into the water column. The multibeam data imaged a distinct fork‐shaped methane plume in the water column, the upper arm of which was consistently visible in the single‐beam data, while the lower arm was only intermittently visible. Using a novel acoustic inversion method, we determine the depth‐dependent gas bubble size distribution and the gas flux for each plume arm. Our results show that the upper plume arm comprises bubbles with radii ranging from 1 to 15 mm, while the lower arm consists of smaller bubbles with radii ranging from 0.01 to 0.15 mm. We extrapolate from these estimates to calculate the gas flux from the Scanner Pockmark as between 1.6 and 2.7 × 106 kg/year (272 to 456 L/min). This range was calculated by considering uncertainties together with Monte Carlo simulation. Our improved methodology allows more accurate quantification of natural and anthropogenic gas plumes in the water column.

Estimating the range at which an acoustic receiver can detect greenhouse gas (e.g., CO 2 ) leakage from the sub-seabed is essential for determining whether passive acoustic techniques can be an effective environmental monitoring tool above marine carbon storage sites. Here we report results from a shallow water experiment completed offshore the island of Panarea, Sicily, at a natural CO 2 vent site, where the ability of passive acoustics to detect and quantify gas flux was determined at different distances. Cross-correlation methods determined the time of arrival for different travel paths which were confirmed by acoustic modelling. We develop an approach to quantify vent bubble size and gas flux. Inversion of the acoustic data was completed using the modelled impulse response to provide equivalent propagation ranges rather than physical ranges.The results show that our approach is capable of detecting a CO 2 bubble plume with a gas flux rate of 2.3 L/min at ranges of up to 8 m, and determining gas flux and bubble size accurately at ranges of up to 4 m in shallow water, where the bubble sound pressure is 10 dB above that of the ambient noise.