Fracture in tuff


  1. Fracture in tuff>
    . Three-dimensional measurement of fractures in heterogeneous materials using high-resolution X-ray computed tomography. Geosphere. .

    Abstract — When present, fractures tend to dominate fluid flow though rock bodies, and characterizing fracture networks is necessary for understanding these flow regimes. X-ray computed tomography (CT) has long been successfully used to image fractures in solid samples, but interpretation of CT data is complicated by the inevitable blurring that occurs when fractures are thin compared to the data resolution. This issue is particularly acute when attempting to quantify fine fractures in scans of larger samples, as typically required for characterizing flow systems on a meaningful scale. A number of methods have been proposed to account for CT blurring, but do not include the ability to account for material inhomogeneity and fracture orientation. We here propose an improved method for fracture measurement that consists of characterizing the blurring as a point-spread function (PSF), and using it, in combination with a calibration for the CT number for void space, in an iterative procedure to reconstruct the fracture and material configuration; we call this the inverse PSF (IPSF) method. Tests on CT scans of homogeneous natural samples show that the IPSF method provides more precise results than others. Further testing demonstrates that it can also recover accurate measurements in heterogeneous materials, although particularly severe inhomogeneities may lead to a locally noisy signal. The accuracy, generality, and adaptability of the IPSF method make it very well suited for characterizing fractures and fractures surfaces in natural materials. The principles behind the IPSF method also apply to the reverse problem of measuring thin features that are denser than their surroundings, such as veins or membranes, when they have one dimension that is small compared to CT data resolution.

  2. Fracture in tuff>
    . Navier-Stokes flow and transport simulations using real fractures shows heavy tailing due to eddies. Geophysical Research Letters. .

    Abstract — Two-dimensional Navier-Stokes flow and transport simulations are conducted for a 15-cm long fracture mapped via X-ray computed tomography. (1) The actual fracture with irregular aperture, (2) a truncated fracture where the largest aperture area is excluded from the domain, (3) the truncated fracture with further thinning of other large aperture areas, and (4) a fracture with uniform vertical aperture equal to the actual fracture’s mean aperture, are subjected to the same pressure gradient. Slight variations in fracture characteristics result in significantly different flow and transport behavior. Flux is much larger for the uniform aperture fracture compared to the actual fracture. A pronounced eddy is present at the largest aperture zone of the actual fracture resulting in a power-law tail absent in other cases. The uniform aperture fracture has the largest effective dispersion coefficient estimated via inversion of a 1D analytical model. The analytical model fit to the other cases is not as robust as in the uniform aperture case.

  3. Fracture in tuff>
    . Effects of inertia and directionality on flow and transport in a rough asymmetric fracture. Journal of Geophysical Research. .

    Abstract — Two-dimensional flow and transport simulations are conducted for a 15-cm-long fracture mapped via X-ray computed tomography. The simulations consider either Navier-Stokes equation (NSE) based flow or Stokes equation (SE) based flow and are run for opposing directions. NSE and SE solutions deviate at larger bulk velocity, and the errors are sensitive to fracture geometry. Transport in all cases is non-Fickian, owing mainly to the presence of a large eddy, and exhibits power law residence time distributions (RTDs). The tailing is more persistent at higher Reynolds numbers (Re), with the exponents of the power law RTDs related to Re via a power function, and becomes asymptotic at higher Re. The late-time portion of the RTD changes with flow direction and is also sensitive to whether the flow is represented by the NSE or the SE. The sensitivity of transport to flow direction and formulation is primarily driven by varying eddy geometry under the different conditions. Our study opens the path to developing robust and mechanistic continuum-scale models of solute transport in fractured geologic media.