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FitzGerald D.,Intrepid Geophysics | Courrioux G.,Bureau de Recherches Geologiques et Minieres
14th Australasian Tunnelling Conference 2011: Development of Underground Space, Proceedings | Year: 2011

A critical component of any new tunnel design in a complex geological setting is the development of an integrated three-dimensional (3D) geology model with a full set of geotechnical properties of the rocks present. A novel means of predicting the geology based upon sparse observations of structural geology at the surface and from drill holes has been developed and used in several major tunnel studies in Europe. An example of this is the Lyons-Turin twin rail tunnel which is still in a development phase. This tunnel has been in planning and development for over 15 years already. An important aspect of this work is to characterise geological uncertainty along various possible tunnel paths as an input into early design optimisations for time, cost and risk avoidance strategies. Once the tunnel works commence and more detailed geological data is collected by daily mapping, progressive refinement of the geological model is desirable; geological conditions still to be encountered can be more accurately predicted. Faults and their offsets, dykes, geological unconformities, folding, as well as standard sedimentary processes are all correctly modelled in the software. It is important to allow for seamless interchange of geological models at various scales with CAD workflows so that sufficient detail can be readily accessed at the required scale. Also, the proposed tunnel in its correct geometry can become a 'section' in its own right. Progressive detailed local geology models in the vicinity of the advancing head are created from the regional scale work by cutting and pasting a starting model, then adding all the extra detail for the required engineering geology work. For all rocks, most standard geotechnical quantities such as sonic velocities, Young's modulus and Poisson's ratio are handled. These and any other phenomena from borehole observations can have variograms calculated and stratigraphically constrained estimations made in a least biased geostatistical manner, in advance of the tunnel. RQD, or a prediction of rock fractures, is handled by the faulting and folding modelling facilities. This is probably the more interesting issue to pursue, as the fractal nature from small scale fractures and joints, to large-scale faults, can now be characterised more convincingly.

Holstein H.,Aberystwyth University | FitzGerald D.J.,Intrepid Geophysics | Stefanov H.,Aberystwyth University
75th European Association of Geoscientists and Engineers Conference and Exhibition 2013 Incorporating SPE EUROPEC 2013: Changing Frontiers | Year: 2013

Homogeneous prismatic targets, commonly used in geophysical modelling, are special cases of general polyhedral target bodies. In this article, advances made in the polyhedral formulation of the gravimagnetic anomalies are incorporated into the prismatic treatment, which has hitherto maintained reliance on historical pre-polyhedral formulation. By this approach we derive a single compact scheme incorporating all the prismatic anomalies. The benefits are faster algorithms that are easier to maintain and allowance for variants that are numerically more stable. Copyright © (2012) by the European Association of Geoscientists & Engineers All rights reserved.

Holstein H.,Aberystwyth University | Fitzgerald D.,Intrepid Geophysics | Anastasiades C.,Aberystwyth University
72nd European Association of Geoscientists and Engineers Conference and Exhibition 2010: A New Spring for Geoscience. Incorporating SPE EUROPEC 2010 | Year: 2010

Thin planar sheets are useful gravitational and magnetic models of dykes and veins treated as two-dimensional geophysical structures on the scale of the survey. Thus, the anomaly of a polygonal thin sheet of uniform surface density or magnetization in arbitrary orientation has practical interest. The limiting thin-sheet anomaly can be approached from the corresponding polyhedral parallelepiped under decreasing thickness, though the numerical limit cannot be reached this way on account of the floating point finite precision. We derive the analytical zero thickness limit for the gravity potential while maintaining finite total mass. We use the concept of gravi-magnetic similarity to extend the thin-sheet potential formula to include the potential, field and field gradient in both gravity and magnetic cases, thereby generalising other studies that have obtained isolated polygonal thin-sheet anomaly solutions. We compare the anomalies computed by the new formulae to those of corresponding finite thickness targets, and to the finite difference estimates of the field and field gradient obtained from numerically differentiated thin-sheet potentials. In both cases a second order rate of approach to the limit is observed, verifying the correctness of the new formulae. Thin-sheet solutions are attractive for their reduced computational burden compared to full parallelepiped solutions, while the stacking of thin sheets may be used to simulate variable density or magnetization targets. It is anticipated that thin-sheet solutions presented here will find wide application in gravi-magnetic modelling. © 2010, European Association of Geoscientists and Engineers.

Silic J.,Jovan Silic and Inc | Paterson R.,Intrepid Geophysics | Fitzgerald D.,Intrepid Geophysics
1st European Airborne Electromagnetics Conference - Held at Near Surface Geoscience 2015 | Year: 2015

The advantages of 2.5D airborne electromagnetic inversion in 3D geological mapping applications compared to the more commonly used CDI transforms or simple 1D inversions are described using an example from the Bryah Basin in Western Australia. We demonstrate this using a substantially rewritten version of ArjunAir (Wilson et al, 2006), a product of the CSIRO/AMIRA consortium (project P223F). The ArjunAir inversion solver has been replaced with a new GSVD (Paige et al, 1981) solver, with adaptive regularisation which also incorporates a misfit to the reference model and a model smoothness function. The ArjunAir forward modelling code has been revised to fix two errors which manifest at late times around high resistivity discontinuities and in steep topography. We allow the use of a starting or reference geology/resistivity model to influence the inversion. The software has been parallelised using Intel MPI. The software is implemented in a commercial 3D geological modelling package with an intelligent graphical user interface for inversion setup, for introduction of geological reference models and for visualising results. Apparent Resistivity, 2.5D Forward and 1D and 2.5D Inversion methods are integrated in a single 3D geological, electromagnetic and potential field (gravity and magnetics) forward and inverse modelling environment.

Fitzgerald D.J.,Intrepid Geophysics | Paterson R.,Intrepid Geophysics | Holstein H.,Aberystwyth University
74th European Association of Geoscientists and Engineers Conference and Exhibition 2012 Incorporating SPE EUROPEC 2012: Responsibly Securing Natural Resources | Year: 2012

To interpret Airborne Gravity Gradiometry data (AGG), a grid of the observed curvature gradients must be prepared. Methods to interpolate the full tensor and horizontal tensor (FALCON), while simultaneously honouring all the measured components, are now available commercially. Typically, this one step increases the resolution of the measured field components by 50%. We compare two field refinement techniques to further denoise the full tensor component grid estimates. The first method (MITRE), uses 3rd order tensor relations locally, in a manner analogous to Minimum Curvature, but with the correct physics for tensors. The second method uses a locally fitted truncated 3D Fourier series (T3DFS) to derive least squares fitted coefficients for the potential. The results are similar and a further denoising of 40% can be achieved. The aim is to reduce any artifacts from sample aliasing while gridding. Terrain is the single biggest contributor to any measured signal (up to 80%). This is often less than 100m away from the sensor. A detailed high resolution digital terrain model, together with an estimate of the terrain density constitutes a major influence to be removed, to then reveal the buried anomalies is critical. The influence on gridding in this process is reflected upon.

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