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6th International AEM Conference & Exhibition
- Conference date: 10 Oct 2013 - 11 Oct 2013
- Location: Kruger National Park, South Africa
- Published: 10 October 2013
61 - 69 of 69 results
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Helicopter AFMAG (ZTEM) Survey Results over the Ad Duwayhi intrusion related gold deposit (IRGD) in the Western Arabian Shield, KSA
Authors J.M. Legault, C. Izarra, S. Zhao and E.M. SaadawiAd Duwayhi IRGD gold deposit were designed to characterize the deposit signatures. Field data and 2D EM inversion results indicate the Ad Duwayhi intrusive centre features low magnetic susceptibilities and high resistivities that are consistent with the iron-depleted, quartz-sericite-carbonate alteration. Closer inspection of the ZTEM resistivity image suggests that the mineralized breccia zones represent weak resistivity lows within the larger resistive intrusion. The Ad Duwayhi gold porphyry lacks a visible low resistivity alteration halo that distinguishes IRGD’s from porphyry copper deposits. The combined aeromagnetic low and ZTEM resistivity high signatures might allow discrimination of other buried IRGD gold deposits regionally.
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Airborne electromagnetics in manganese exploration – a case study
Authors A Noetzli, M. Lowe, J Kita, K McKenna and N. KusumaputriAustralia since 2002 when the first Hoistem survey was flown for Pilbara Manganese. A XTEM survey was commissioned by Montezuma Mining to test the effectiveness of the technique in a new manganese province near Kumarina, Western Australia. A series of test lines was flown over areas of known manganese ore deposits and revealed a successful correlation between the electromagnetic conductors and the known manganese zones. In addition the test lines revealed new zones of conductivity where surface traces of manganese had been found but not yet drilled. Based on the preliminary results of these test lines the complete Butcherbird exploration tenement was flown in December 2010. The survey successfully mapped the extent of the known manganese zones and assisted in the identification and mapping of several new targets which were subsequently drilled and found to bear manganese oxide. The shape of the EM anomaly maps the resource outlines and the strength of the EM signal correlates strongly with the grade of the resource.
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ZTEM-VTEM-AeroTEM helicopter EM comparisons over the 501 Project Cu-Zn vms at McFauld’s Lake, northern Ontario, Canada
Authors M. Orta, J.M. Legault, A. Prikhodko, G. Plastow, S. Zhao, M. Moreton and C. UlanskyHelicopter AeroTEM, VTEM and ZTEM surveys were flown over the 501 zone in the McFauld’s Lake area, northern Ontario. The 501 zone is a relatively small VMS deposit that appears to respond well to all three active and passive airborne EM systems that have surveyed the property. Comparisons between these data sets and the geology are showcased using 1D-2D-3D EM inversion modeling.
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The analysis of ZTEM data across the Humble magnetic anomaly, Alaska
Authors Daniel Sattel and Ken WitherlyZTEM data acquired across the Humble magnetic anomaly of almost 30,000 nT were analysed for the presence of a magnetic gradient response and the effects from elevated magnetic susceptibilities. Mag3D inversion of the magnetic data indicates magnetic susceptibility values as high as 2.0 (SI). The response of moving the receiver coil through the magnetic-field gradient peaks at 0.01 Hz and drops off strongly with frequency. Lacking information about the field strength at the base station precludes the comparison of amplitudes between computed gradient responses and the survey data, but the comparison of response shapes suggests that the gradient responses are too small to have a noticeable effect on the survey data. ZTEM responses were forward modeled with a UBC-GIF 3D algorithm that takes into account electric conductivities and magnetic susceptibilities , in order to assess the impact of the elevated values derived from the Mag3D inversion. Computing the ZTEM response for these values combined with resistive half-spaces indicates that the response amplitudes and shapes strongly depend on the half-space resistivities. Ignoring the elevated values during an inversion can result in patterns that resemble crop circles. A comparison of the synthetic data with the Humble survey data suggests that the observed responses are more affected by the subsurface conductivities than by the magnetic susceptibilities. The approximate conductivity structure of the survey area was derived with a UBC-GIF 3D ZTEM inversion, which models . Forward-model results of these conductivities combined with the elevated values derived from the Mag3D inversion indicate that the conductivities are underestimated with the assumption. For an environment such as Humble, with deep-seated zones of elevated values, the shallow inverted conductivity structure appears to be reliable, but the deeper structure should be interpreted with caution.
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Voxel Inversion of AEM Data for Improved Model Integration
More Lessthe model space is usually linked to the actual observation points. For airborne surveys the spatial discretization of the model space reflects the flight lines (Figure 1). Often airborne surveys are carried out in areas where other ground-based geophysical data are available. The model space of ground-based geophysical inversions is likewise usually referred to the positions of the measurements, e.g. the electrode positions in geoelectrical investigations, and hence does not coincide with the airborne model. Consequently, a model space based on the measuring points is not well suited for jointly inverting airborne and ground-based geophysical data. Furthermore, geological and groundwater models most often refer to a regular voxel grid, not correlated to the geophysical model space. This means that incorporating the geophysical data into the geological and/or hydrological modeling grids involves a spatial relocation of the models, which in itself is a very subtle process where valuable information is easily lost. Also the integration of a priori information, e.g. from boreholes, is difficult when the observation points do not coincide with the position of the prior information. We have developed a geophysical inversion algorithm working directly in a voxel grid disconnected from the actual measuring points (Figure 1b), which then allows for straightforward integration of different data types in a joint inversion, for directly informing geological/hydrogeological models, and for easier incorporation of a priori information. The new voxel model space defines the soil properties (like resistivity) on a set of nodes, and the distribution of the soil properties is computed everywhere by means of an interpolation function (e.g. inverse distance or kriging). Given this definition of the voxel model space, the 1D forward responses are computed as follows: 1) a 1D model subdivision, in terms of model thicknesses is defined for each 1D data set, creating “virtual” layers. 2) the "virtual" 1D models at the sounding positions are finalized by interpolating the soil properties (the resistivity) in the center of the "virtual" layers (Figure 2a). For 2D/3D forward responses the algorithm operates similarly, simply filling the 2D/3D meshes of the forward responses by computing the interpolation values in the centers of the mesh cells (Figure 2b). This definition of the voxel model space decouples the geophysical model from the position of acquired data, allowing for straight-forward integration of different data types in joint inversion and for directly informing (hydro)geological models. We believe that this new approach will facilitate the integration of geophysics, geology, and hydrology for improved groundwater and environmental management. The presented algorithm is a further development of the AarhusInv program package, which manages both large scale AEM surveys (e.g. Auken et al., 2008, Siemon et al., 2009) and ground-based data (e.g. Fiandaca et al., 2013, Behroozmand et al., 2012).
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Optimizing Airborne Electromagnetic (AEM) Inversions Through Integrating Hydrogeophysical, Hydrogeological, Hydrochemical and Hydrodynamic Data
In the Broken Hill Managed Aquifer Recharge (BHMAR) project, a large (>50) multi-disciplinary group of geoscientists (geomorphologists, sedimentologists, regolith geoscientists, hydrogeologists, hydrogeophysicists, hydrogeochemists, geospatial analysts, remote sensing specialists, geochronologists, groundwater modelers and structural geologists), worked in a team to understand the hydrogeological system and identify MAR and potential groundwater resource targets. A trans-disciplinary approach was critical to successful completion of MAR pre-commissioning maximal and residual risk assessments. Initially, a number of different (Laterally and Spatially Constrained) inversions of the AEM data were carried out, with refinements made as additional information on vertical and lateral constraints became available. However, hydrostratigraphic mapping using all of these inversions was unable to resolve fundamental aspects of the hydrogeological system, particularly in the near-surface (top 20m), where hydrodynamic data indicated a connection between the major rivers and the underlying aquifers, either through incision and/or through faults (and bypass flow). To resolve these issues, a trans-disciplinary approach was used to investigate all underlying assumptions, including the regularization used in the AEM inversion. In modern laterally-correlated inversion of AEM data, the usefulness of the resulting inversion models depends critically on an optimal choice of the vertical and horizontal regularization of the inversion. Set the constraints too tight, and the resulting models will become overly smooth and potential resolution is lost. Set the constraints too loose, and spurious model details will appear that have no bearing on the hydrogeology. In this study we used a pragmatic approach to optimizing the constraints by an iterative procedure involving all available geological, hydrogeological, geochemical, hydraulic and morphological data and understanding. This was accomplished using a Wave Number Domain Approximate (WANDA) Inversion procedure with a 1D multi-layer model and constraints in 3D. This inversion procedure only takes days to run, enabling the rapid trialing to select the most appropriate vertical and horizontal constraints. In this approach, in a process of both confirming and negating established interpretations and underlying assumptions, the inversion results are judged by their ability to support a coherent conceptual model based on all available information. This approach is dependent on integrating a team of scientists, where all facets of data and interpretation are considered and questioned in a trans-disciplinary analysis of the hydrogeological system. Necessary elements for this approach to succeed are the experience and professional insights of the scientists involved and a willingness and ability of scientists from diverse areas to establish a dialogue that will question and refine the inversion constraints and the quality of the final hydrogeological conceptual model. This approach has been essential to the identification and assessment of MAR and groundwater extraction options in the Broken Hill Managed Aquifer Recharge project. The resultant improved 3D conductivity model revealed details of the hydrostratigraphy and Neogene-to-Present tectonics. Prior to the mapping of near-surface hydrostratigraphy and structural features, it had not been possible to explain apparently contradictory data, nor develop a plausible hydrogeological conceptual model. In summary, a pragmatic approach to optimizing the constraints was achieved using an iterative procedure involving all available geological, hydrogeological, geochemical, hydraulic and morphological data and understanding.
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Evidence for a Paleo-Okavango Delta and overlying mega-lake from airborne TEM data
The near juxtaposition of the Makgadikgadi Basin, the world's largest saltpan complex, with the Okavango Delta, one of the planet's largest inland deltas (technically an alluvial megafan), has intrigued explorers and scientists since the middle 19th century. It was also clear from early observations that the Makgadikgadi Basin once contained a huge lake, popularly referred to as Paleo Lake Makgadikgadi. Through a contract with a commercial company, the Botswana Department of Geological Survey has acquired a helicopter time-domain electromagnetic (HTEM) data set across the entire Okavango Delta. The HTEM data were of extremely high quality, mainly due to very low noise levels (there are no large power lines and only a small number of settlements and tourist lodges in the delta) and significant contrasts between the electrically resistive units (i.e., dry and fresh-water-saturated sand and basement) and electrically conductive ones (i.e., saline-water-saturated sand and clay). Our inversions of these data returned electrical resistivity models containing three principal layers: (i) an upper heterogeneous layer that is largely resistive, (ii) an intermediate conductive layer, and (iii) a lower resistive layer. According to borehole logs and groundwater sampling, the electrically conductive layer is due to a combination of clay and saline-water-saturated sediments. We interpret these sediments to have belonged to Paleo Lake Makgadikgadi, which would have extended into the region presently occupied by the Okavango Delta. The total area of Paleo Lake Makgadikgadi would therefore have exceeded 90,000km2, larger than Earth's most extensive fresh-water body today, Lake Superior. Our HTEM data also provide evidence for a southeast-dipping paleo megadelta or megafan beneath the Okavango Delta’s upper fan and the Paleo Lake Makgadikgadi sediments. Seismic refraction/reflection experiments and boreholes suggest that this feature is composed of fresh-water sediments, probably representing a Paleo-Okavango Delta resting on top of basement rock. If this interpretation is correct, then the clay layers of the Paleo Lake Makgadikgadi sediments act as an aquitard protecting the fresh-water sediments of the paleo-delta from salt water contamination above.
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New abilities of quadrature EM systems
Authors T. Vovenko, E. Moilanen, A. Volkovitsky and E. KarshakovIt can be said that quadrature systems failed the competition with powerful time-domain and towed rigid-boom frequency-domain systems. It seems that there is no place for them but they continued to be in use for a long time and the reasons are: first, in contrast to time-domain systems quadrature systems could be applied in weak conductive environment. But one of them, namely, EM4H is still widely used for survey – about 100 000 line kilometers in a year. Experience has shown that new advantages of EM4H have improved essentially survey results and noticeably risen the potential of frequency-domain systems with large receiver-transmitter separation. In the paper basic features of the EM4H system are described and illustrated by survey results. Particular attention is paid to estimation of effectiveness for searching conductive target depending on distance.
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New Strategies for Faster 3D Inversion of Airborne EM Data
Authors Douglas Oldenburg, Dikun Yang and Eldad HaberTraditional Gauss-Newton approaches to solving inverse problems have been successful for inverting many types of geophysical data. However, the need to finely discretize a large volume of the earth and the requirement to solve Maxwell’s equations with many sources makes 3D inversion of airborne EM data very challenging. The main computational bottlenecks concern the time to carry out forward modelling of the data, computation of sensitivities, and solution of the matrix equation to provide an updated conductivity. In previous work we used a single fine scale global mesh for forward modelling and inversion but that can become untractable for very large problems. Here we separate the inversion and forward modelling mesh. The local mesh can be designed to handle a subset of the initial soundings or it can be tailored to an individual sounding. We demonstrate the large speed-up that can be achieved by having a local forward modelling mesh for each transmitter-receiver pair. This mesh is small, easy to store, and it allows a large EM problem to be decomposed into many small highly independent problems for which massive parallelization can be effectively applied. The second improvement focusses upon how to reduce the number of transmitters at each Gauss-Newton iteration. Rather than down-sampling a-priori, we adaptively select soundings chosen at random from the entire population. The number of soundings needed at each iteration increases with the complexity of the conductivity model but the final number of soundings needed to update a model is usually a small fraction of the total number of soundings. A final forward modelling of all of the soundings in the survey serves as a check on the process. The combination of local meshes and adaptive soundings greatly decreases the computation time. Our field test example involving 14362 soundings, 8 time channels for 114896 data, and a mesh with 433520 cells required about 4.3 hours.
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