Intrepid Geophysics

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Intrepid Geophysics

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Kohrn S.B.,Lockheed Martin | Bonet C.,Intrepid Geophysics | DiFrancesco D.,Lockheed Martin | Gibson H.,Intrepid Geophysics
Transactions - Geothermal Resources Council | Year: 2011

Gravity methods are sensitive to the subsurface distribution of geologic materials of different densities, and have proven value in geothermal exploration. In some geothermal settings, measurements of the earth's gravity gradient (i.e., gravity gradiometry) may provide advantages over the more traditional measurements of the earth's scalar gravity field. These include higher resolution of targets less than approximately 10 km deep, and better edge detection for interpreting faults, boundaries of geologic bodies, and other structural features. To determine if the gravity gradient signal from a geothermal exploration target is within the detection limits of commercially available sensor technology, the gravity gradient response was modeled for a simplified 3D geological model of the Salton Sea Geothermal Field in Southern California. This is a water-dominated geothermal field in the Salton Trough with a known 20 mGal residual gravity anomaly. The resulting gradient of vertical gravity in the z direction (Gzz) at the Salton Sea Geothermal Field ranged from -53 to -31 Eötvös (1 Eötvös = 0.1μGal/m, which is equivalent to 0.1 ppb of the Earth's gravity field). The local density highs are clearly visible in the calculated gravity gradient response, and are consistent with the known gravity anomaly. Additionally, modeled hypothetical faults associated with the pull apart basin setting are more clearly evident in the gravity gradient data compared with the scalar gravity data. This has significance for exploration of blind geothermal systems, especially where faults do not have surface expression in the cover geology.

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.

Zengerer M.,Intrepid Geophysics
76th European Association of Geoscientists and Engineers Conference and Exhibition 2014: Experience the Energy - Incorporating SPE EUROPEC 2014 | Year: 2014

This presentation will show some best-practice examples of maximising borehole petrophysical and geochemical log data together with geological and seismic data, to produce ideally constrained 3D starting geological models for inversion with magnetics, gravity and gravity gradients. Results have far-reaching implications for targeting of Uranium and Base Metal Mineralisation, as well as producing extremely wellconstrained Basin and Basin Density models, and even predicting mineralisation away from known drilling. The presentation will highlight approaches to problem-solving for 3D inversion in different geological contexts with some recent modelling examples from both industries, using novel geostatistical modelling with domain kriging, wavelength and residual filtering, and gravity, gravity gradient and magnetics data. The presentation will also demonstrate how the results of inversion and modelling can be used in exploration targeting at earlier stages with significant application for the Uranium industry and other types of exploration.

FitzGerald D.,Intrepid Geophysics | Courrioux G.,Bureau de Recherches Géologiques et Minières
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.

Silic J.,Jovan Silic and Inc | Paterson R.,Intrepid Geophysics | Fitzgerald D.,Intrepid Geophysics | Archer T.,Reid 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.

Holstein H.,Aberystwyth University | Hillan D.,CSIRO | Fitzgerald D.J.,Intrepid Geophysics
76th European Association of Geoscientists and Engineers Conference and Exhibition 2014: Experience the Energy - Incorporating SPE EUROPEC 2014 | Year: 2014

Thin planar sheets are useful target models for geological structures such as dykes and veins, which are essentially two-dimensional on survey scales. We derive the gravity potential, field and field gradient for such targets. We concentrate on triangular sheets, because these can be assembled into a drape surface, with continuous densities across the boundaries, so as to approximate general curved surfaces. We verify the correctness of the anomaly formulae via numerical testing, and model a scenario with dyke curvature and density compaction.

Fitzgerald D.,Intrepid Geophysics | Holstein H.,Aberystwyth University | Foss C.,CSIRO
SEG Technical Program Expanded Abstracts | Year: 2011

We have developed an optimization method for automatic dyke delineation from observed magnetic and gravity gradient traverse data. A non-linear least squares algorithm is used to find model dyke parameters that best fit the computed gradient tensor data to the observed data. The eigen-system of the observed magnetic gradient tensor data is used to provide starting model dyke parameters for an iterative non-linear least squares solver. This greatly enhances the ability of the solver to find a plausible dyke model for matching observed and synthetic tensor gradients locally. The method works well on synthetic examples. Multiple surveys using a Full Tensor Magnetic Gradient (FTMG) signal instrument from IPHT, have been made in Southern Africa. A real case study with remanence, taken from the Platreef near Pretoria, shows that the gross observed gradient features can be recovered by our procedure, but the residuals in the gradient fit hint strongly at the need for more complex dyke models. There is more directly inferable structural geology in this tensor signal than can be found in a conventional TMI signal. © 2011 Society of Exploration Geophysicists.

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.

Fitzgerald D.,Intrepid Geophysics | Holstein H.,Aberystwyth University | Letts S.,Spectrem Ltd.
72nd European Association of Geoscientists and Engineers Conference and Exhibition Incorporating SPE EUROPEC 2010, Workshops | Year: 2010

In recent years, Anglo/De Beers have championed the development of a Full Tensor Magnetic gradient (FTGM) signal instrument from IPHT. Multiple surveys of this quantity have been made in Southern Africa. With the advent of this new potential field full tensor gradient instrumentation, new methods have been developed to de-noise and process these curvature gradients. Traditional Fourier domain and minimum least squares residual of the linear differential tensor relationships have been adapted. This leads to levelling, gridding and grid filtering innovations. The result is a full tensor grid representation of the curvature gradients that is coherent and compliant with the physics at all points in the grid. All of the observed data is thus honoured in the Tensor grid. Isolating the signal and then refining it to be sure there are no distortions have dominated efforts to date. Superior anomaly interpretation regarding the full magnetic history and inferences can then be made. A survey from the Groblersdal Platinum mine is shown in the context of the structural geology interpretation. In particular, the dolerite dykes and faults are seen. The Hornfels footwall contact is very strong. The phase map traces the Platreef contact. The Upper Zone magnetites are more pervasive and fine layered structure is revealed there. None of the granites can be seen. A 3D geology model is in preparation. The observed FTG signal will be compared to the predicted thin-body responses from the model. There is more directly inferable structural geology in this tensor signal than can be found in a conventional TMI signal.

Hassen I.,Tunis el Manar University | Gibson H.,Intrepid Geophysics | Hamzaoui-Azaza F.,Tunis el Manar University | Negro F.,University of Neuchatel | And 2 more authors.
Journal of Hydrology | Year: 2016

The challenge of this study was to create a 3D geological and structural model of the Kasserine Aquifer System (KAS) in central Tunisia and its natural extension into north-east Algeria. This was achieved using an implicit 3D method, which honors prior geological data for both formation boundaries and faults. A current model is presented which provides defendable predictions for the spatial distribution of geology and water resources in aquifers throughout the model-domain.This work has allowed validation of regional scale geology and fault networks in the KAS, and has facilitated the first-ever estimations of groundwater resources in this region by a 3D method.The model enables a preliminary assessment of the hydraulic significance of the major faults by evaluating their influence and role on groundwater flow within and between four compartments of the multi-layered, KAS hydrogeological system. Thus a representative hydrogeological model of the study area is constructed. The possible dual nature of faults in the KAS is discussed in the context that some faults appear to be acting both as barriers to horizontal groundwater flow, and simultaneously as conduits for vertical flow. Also discussed is the possibility that two flow directions occur within the KAS, at a small syncline area of near Feriana.In summary, this work evaluates the influence of aquifer connectivity and the role of faults and geology in groundwater flow within the KAS aquifer system. The current KAS geological model can now be used to guide groundwater managers on the best placement for drilling to test and further refine the understanding of the groundwater system, including the faults connectivity. As more geological data become available, the current model can be easily edited and re-computed to provide an updated model ready for the next stage of investigation by numerical flow modeling. © 2016 Elsevier B.V.

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