Intrepid Geophysics

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Intrepid Geophysics

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FitzGerald D.,Intrepid Geophysics | Holstein H.,Aberystwyth University
SEG Technical Program Expanded Abstracts | Year: 2014

In the last 10 years, quite a few full tensor gravity gradiometry (FTG) surveys have been acquired over rift systems (e.g. Carlin Trench, East African Rift and Bonaparte Gulf). A new effort has been made to extend the estimation of strike, dip and throw of faults, by firstly examining prior algorithms. This has resulted in some unexpected innovations that have much wider applicability than first expected. The choice is made to use rift systems to demonstrate the technology, as the signal is unambiguous, strong, and usually some existing structural knowledge is available to help validate and calibrate the new technique. Also, there can be independent observations of the geology, derived from field mapping, drilling and seismic profiles. © 2014 SEG.

FitzGerald D.,Intrepid Geophysics | Milligan P.,Geoscience Australia
SEG Technical Program Expanded Abstracts | Year: 2013

Australia, via the efforts of the Government Geological surveys, has a program of releasing ever bigger, higher resolution, continental-scale datasets. The recently released isostatically corrected gravity data imagesmany deep and large-scale crustal features. This is a key dataset for understanding the primary structure of the deep crust across thousands of kilometres. Direct "inversion" of this dataset to a consistent 3D fault surfaces network explains more than 50% of the primary information. The method of choice relies on multi-scale edge detection or "worming". This continues to enjoy increasing popularity in the regional mapping domain. Large-scale minerals and oil exploration mapping often make use of this technique. With the current shift to 3D geology modelling, issues arise to improve/generalise the worming technology and get 3D contacts that can be interpreted, particularly the sub-set that indicates a primary fault network. Methods to rapidly compute a consistent 3Dfault network for the entire Australian continent, linking the dominant 20 km deep features back to the surface, are described. If measured gravity curvature gradients are available an even better, more detailed use of these methods at the prospect scale is now available. © 2013 SEG.

Gerald D.F.,Intrepid Geophysics | Paterson R.,Intrepid Geophysics
SEG Technical Program Expanded Abstracts | Year: 2013

The critically important steps to get best value from your gravity gradiometry data, assuming your contractor has done his job well in designing and acquiring the data, is the preparation of the representation of the potential field gradients. The ~200m resolving power of existing gradiometer systems approaches what is necessary for minerals applications. In particular, beyond the aircraft, the topographic surface represents the largest and most proximal density contrast encountered in an airborne survey. Hence terrain effects can have significant impact on AGG data. The critical steps are: Terrain correction and determining 'best' terrain density Gridding, using all the measured gradients to constrain the interpolation Smoothing/de-noising by using the 3rd order tensor constraints Anti-alias filtering of the gradient signals so that wave lengths are properly represented in all directions Transformation of the gradients by integration to estimate the gravity or magnetic field Terrain corrections are a necessary step in the processing of observed AGG data in rugged terrain, in order to highlight subsurface density variations with a minimal overprint from the terrain. We propose a simple and rapid AGG tensor-based method to estimate an optimum bulk terrain density for subsequent terrain-correction. Each of the currently deployed systems for acquiring gradiometry is evolving driven by competition and the users' needs. Mining applications of the technology to directly detect ore-bodies that show up as anomalies can now be successful provided the dimensions are of the order of 200m or more. High resolution 3D geology models of operating mines can be used to calibrate gradiometry surveys. © 2013 SEG.

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.

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.

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.

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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