Simon Cox

Ontology paper: The SSN ontology of the W3C semantic sensor network incubator group

The W3C Semantic Sensor Network Incubator group (the SSN-XG) produced an OWL 2 ontology to describe sensors and observations - the SSN ontology, available at http://purl.oclc.org/NET/ssnx/ssn. The SSN ontology can describe sensors in terms of capabilities, measurement processes, observations and deployments. This article describes the SSN ontology. It further gives an example and describes the use of the ontology in recent research projects.

On the formation and growth of faults: an experimental study

Abstract By using an experimental geometry, which includes a slot loaded to produce a mode 3 shear stress concentration, shear fractures have been grown in two fine-grained rock types. The deformation is accomplished by initially generating an array of oblique, mainly tensile, cracks at the tip of the slot, which are subsequently linked to form a rupture zone. This suggests a more general model for the formation of brittle shear rupture. A through-going shear surface will form if a damage zone is concentrated enough in the appropriate orientation. This may occur at the edges of an existing shear strain discontinuity, such as a fault, particularly in a mode 3 displacement field. Thus under some conditions faults may grow or link up as true macroscopic shear ruptures. This may be part of a solution to the apparent paradox of the existence of long faults. We suggest features that would indicate this sequence of formation in the field and review some geological examples.

MICROCRACK FORMATION AND MATERIAL SOFTENING IN ROCK MEASURED BY MONITORING ACOUSTIC EMISSIONS

Abstract Brittle deformation in rocks is accompanied by the formation of microcracks which emit elastic energy partly as acoustic emissions. Acoustic emission parameters such as amplitude may be related to geometric parameters such as crack size. We have analyzed catalogues of acoustic emission events recorded during compression tests in rock in terms of the information they give about the accumulated state of damage in a material. We combine this measured damage state with a model for the softening behaviour of cracked solids, and show that reasonable predictions of the mechanical behaviour are possible. Several strategies have to be used to allow for incomplete recording of the acoustic emissions. An independent calibration of the scaling relation between the acoustic emission parameters and the microcrack geometry remains outstanding, although the results here suggest constraints on the scaling relation. We show, however, that this quantitative approach is markedly superior to the more traditional methods of acoustic emission analysis in correlating the acoustic activity with the weakening of the material.

A modern regression approach to determining fault displacement-length scaling relationships

A number of studies have supported the hypothesis that fault displacement is systematically related to fault size by a power law. The relationship is important for estimating bulk strain in faulted terrains. However, the exponent of this relation has been the subject of some dispute: in particular as to whether the exponent is unity (i.e. a linear relationship exists between displacement and length) or larger. The techniques used to determine the exponent have been inconsistent and far from rigorous in their application of statistical tests and terminology. We have re-analysed several of the data sets using more careful techniques. We find that the power law with an exponent of unity explains most sets of data when analysed separately. We also applied a weighted joint regression analysis to combined data covering nearly 6 orders of magnitude. The best estimate of the common slope (slope in log—log space = exponent of a power law in linear space) is 0.946 with a standard error of 0.0426. Statistical tests confirm that this exponent is consistent with a value of unity, implying a linear relationship between fault displacement and length within each data set. However, the different data sets have varying intercepts in the log—log space, indicating differing slopes for the linear relation.

Clay mineral transformations and weakening mechanisms along the Alpine Fault, New Zealand

Abstract The formation of clay minerals within active fault zones, which results from the infiltration of aqueous fluids, often leads to important changes in mechanical behaviour. These hydrous phyllosilicates can (1) enhance anisotropy and reduce shear strength, (2) modify porosity and permeability, (3) store or release significant volumes of water, and (4) increase fluid pressures during shearing. The varying interplay between faulting, fluid migration, and hydrous clay mineral transformations along the central Alpine Fault of New Zealand is suggested to constitute an important weakening mechanism within the upper section of this crustal discontinuity. Well-developed zones of cataclasite and compacted clay gouge show successive stages of hydrothermal alteration, driven by the cyclic, coseismic influx of meteoric fluids into exhumed amphibolite-facies rocks that are relatively Mg rich. Three modes of deformation and alteration are recognized within the mylonite-derived clay gouge, which occurred during various stages of the fault’s exhumation history. Following initial strain-hardening and frictional melting during anhydrous cataclastic breakdown of the mylonite fabric, reaction weakening began with formation of Mg-chlorite at sub-greenschist conditions (<320 °C) and continued at lower temperatures (<120 °C) by growth of swelling clays in the matrix. The low permeability and low strength of clay-rich shears are suitable for generating high pore-fluid pressures during faulting. Despite the apparent weakening of the c. 6 km upper segment of the Alpine Fault, the upper crust beneath the Southern Alps is known to be actively releasing elastic strain, with small (

Scaling of rock friction constitutive parameters: The effects of surface roughness and cumulative offset on friction of gabbro

We describe experiments in which large (14×40 cm nominal contact area) blocks of gabbro were sheared in a direct shear apparatus at room temperature, 5 MPa normal stress, and slip velocities from 0.1 to 10 μm/s. The apparatus was servocontrolled using a displacement feedback measurement made directly between the gabbro blocks. Two surface roughnesses were studied (rough, produced by sandblasting, and smooth, produced by lapping with #60 grit) and accumulated displacements reached 60 mm. Measurements of surface topography were used to characterize roughness and asperity dimensions. Step changes in loading velocity were used to interrogate friction constitutive properties. Both rough and smooth surfaces showed appreciable displacement hardening. The coefficient of friction μ for rough surfaces was about 0.45 for initial slip and 0.7 after sliding 50 mm. Smooth surfaces exhibited higher μ and a greater tendency for unstable slip. The velocity dependence of frictiona−b and the characteristic friction distanceDc show systematic variations with accumulated displacement. For rough surfacesa−b started out positive and became negative after about 50 mm displacement andDc increased from 1 to 4 μm over the same interval. For smooth surfaces,a−b began negative and decreased slightly with displacement andDc was about 2 μm, independent of displacement. For displacements <30 mm, rough surfaces exhibit a second state variable with characteristic distance about 20 μm. The decrease ina−b with displacement is associated with disappearance of the second state variable. Our data indicate thatDc is controlled by surface roughness in a complex way, including but not limited to the effect of roughness on contact junction dimensions for bare rock surfaces. The data show that simple descriptions of roughness, such as rms and peak-to-trough, are not sufficient to inferDc. Our observations are consistent with a model in whichDc scales with gouge thickness.

Optimising the application of the hough transform for automatic feature extraction from geoscientific images

Abstract We have adapted the Hough transform to extract linear features successfully from geoscientific datasets. The Hough transform is used in an automatic technique, which makes use of a parameter space to describe features of interest in images. This method has been widely applied in machine vision for recognition of features in highly structured images. Geoscientific data is more demanding. Features of interest within scenes of natural environments exist on all scales, are often partially obscured and the images are usually noisy. Pre-processing of images before the HT is essential. Adaptations of the HT to cope with particular properties of geoscientific data include: optimising the dimensions of the discrete transform domain; using feature-modelling to cancel lines found; transforming multi-scale tiles of the original image and correcting amplitudes in the transformed domain to account for the position of features. These specific adaptations produce a method for automatic feature detection which requires the user to select only two parameters. Output of the procedure is rich in feature content and accurate, leaving a clean result for statistical analysis. This optimised HT is robust for natural scenes, coping in particular with short line-segments.

Analogue experiments and numerical modelling on the relation between microgeometry and flow properties of polyphase materials

Abstract Constant stress experiments, with shear strains of up to 185, were carried out with two-phase composites of power-law creep organic crystalline rock analogues: camphor and octachloropropane (OCP). Isotropic mixtures of camphor inclusions (hard) in an OCP matrix (soft) show initial softening coinciding with the development of foliation, followed by hardening coinciding with buckling and folding of the camphor foliation. A mixture of OCP inclusions in a camphor matrix softens rapidly while shear is localized by linking of the soft OCP inclusions at a bulk shear strain of less than unity. Numerical modelling shows that the fraction at which the phases change their roles of connected phase and the inclusion phase seems to be the most important parameter for relating the microgeometry to the composite flow properties. The connectivity changes as the microgeometry evolves during deformation. Therefore the properties of a polyphase material change during deformation, as was observed with the analogue experiments. These changes may lead to localization of deformation in shear zones.

A formal model for the geologic time scale and global stratotype section and point, compatible with geospatial information transfer standards

The geologic time scale is a complex data structure composed of abstract elements that represent time intervals and instants and their relationships with specific concrete representations in the geologic record as well as the observations made of those concrete representations. The International Union of Geological Sciences’ International Commission on Stratigraphy guidelines recommends a very precise usage of the relationships between these components in order to establish a standard time scale for use in global correlations. However, this has been primarily described in text. Here, we present a formal representation of the model using the Unified Modeling Language (UML). The model builds on existing components from standardization of geospatial information systems. The use of a formal notation enforces precise definition of the relationships between the components. The UML platform also supports a direct mapping to an eXtensible Markup Language (XML)‐based file format, which may be used for the exchange of stratigraphic information using Web-service interfaces.

The anisotropy of surface roughness measured using a digital photogrammetric technique

Abstract The characterization of fracture surface morphology is important for the understanding of shear and normal deformation behaviour of rock masses, and for their transport properties. It has been suggested that natural fracture surfaces might be fractal. If true, this would have dramatic implications for these rock properties and would provide a sound basis for future modelling. Evaluating this hypothesis requires measurement of fracture surface morphology over a large range of scales. A software package has been developed that can automatically reconstruct surface morphology information from stereo pairs at any scale, using edge correlation and point matching. The data derived using this technique compare favourably with results generated from a traditional surface profilometer survey, and results are presented from the analysis of the anisotropy of blasting-induced tensile fractures in granite from Mt Alexander Quarry, Harcourt, Victoria, Australia.