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13th SAGA Biennial Conference & Exhibition
- Conference date: 06 Oct 2013 - 09 Oct 2013
- Location: Kruger National Park, South Africa
- Published: 06 October 2013
21 - 40 of 108 results
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Evaluation And Interpretation Of Full Tensor Squid Magnetic Gradiometer Data
Authors A. Vorster, L. Polomé and S. Lettsence Device) systems will most likely provide the mineral exploration industry with a new tool that provides significant benefits over current airborne magnetic systems. Full tensor measurement of the magnetic field allows the application of filtering, gridding and visualisation techniques that greatly improve the information and resolution relative to data acquired with standard sensors. Potential benefits to the mining industry include faster execution of exploration work programmes and a reduction in exploration costs. Remote sensing of magnetic remanence as a physical property through the use of SQUID magnetometers has not been used as an exploration tool before and the SQUID system’s ability to measure remanence indicators will add significant benefits to exploration programmes. In 2007 Anglo American and De Beers entered into an agreement with the Institute of Photonic Technology (IPHT) in Germany to develop and operationalise a full tensor airborne magnetic SQUID system. Final field tests with a helicopter tow-bird mounted system were successfully completed in February 2012. In December 2012 a test survey was flown for De Beers over an area near Finch in the Northern Cape. The survey produced excellent results with noise levels generally below 10 pT/m. Visualisation of the full tensor and filtered products showed significant detail and resolution in the final data and allowed for the extraction of a wealth of additional geological information from the data.
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Africaarray Seismological Studies Of The Structure And Evolution Of The African Continent
Authors R.J. Durrheim, A.A. Nyblade, M.C.B. Brandt, F. Tugume, G.D. Mulibo, E.M. Kgaswane, A. Mangongolo, M.S.D. Manzi, N. El Tahir and L. LootsThe AfricaArray programme seeks to build geoscience capacity and conduct research that supports development in Africa. This paper reports on investigations of the structure and evolution of the African continent that have been concluded since the review presented at the 2009 SAGA Conference. The AfricaArray observatory network has been expanded from 33 to 49 stations, and 25 continuous GPS and 22 meteorological stations have been installed. A temporary seismic array has been deployed in Mozambique and Madagascar. Brandt and Mulibo elucidated the relationship between the African Superplume, Superswell and the East African Rift System by studying the seismic velocity structure of the mantle. Kgaswane jointly inverted P-wave receiver functions (PRFs) and surface waves, and found that the Kalahari Craton lower crust is largely mafic, except for a few terrains such as the Kimberley. Kgaswane also produced evidence that supports a link between the eastern and western lobes of the Bushveld Complex. Mangongolo used surface wave tomography to define the south-western boundary of the Congo Craton. El Tahir used PRFs to investigate the crustal structure of the Khartoum Basin. Tugume determined the Moho depths and Poisson’s ratios of the Precambrian crust in East Africa. Manzi reprocessed 3D reflection seismic data covering part of the Witwatersrand goldfields using seismic attribute analysis methods, and has provided new constraints on the evolution of the Basin during the Neoarchean. Loots interpreted a 105 km 2D seismic reflection profile immediately to the north of the Cape Fold Belt, imaging the Karoo and Cape Supergroup rocks and the seismic fabric of the basement. A zone of strong reflectors was found beneath the Beattie Magnetic Anomaly.
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What Rock Magnetism Reveals Of The Karoo Basin
Authors L.P. Maré, M.O. De Kock, B. Cairncross and H. MouriA comparative study of the variation in magnetic properties in eight boreholes situated across the Karoo Basin was performed. Four of these boreholes incepted one or more dolerite sills. The aim of this study is to determine the thermal impact of the magma intrusions on the surrounding sediments by using several magnetic experiments. The methods that will be discussed include low field anisotropy of magnetization (LFAMS), magneto-stratigraphy using the classic baked contact test, as well as the magnetic alteration index (A40). Re-magnetization due to the heating effect of the dolerite intrusions are limited to short distances within the contact aureole. Those boreholes that intersect dolerite sills indicate that the heating effect occurred no wider than half of the sill thicknesses. These results also suggest a widespread elevation in the geomagnetic temperatures of the Karoo stratigraphic sequence to temperatures above 150 °C that gradually increases from southwest to northeast across the Karoo Basin.
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Bushveld Cooling And Palaeomagnetism
Authors S.J. Webb, R.G. Cawthorn and W.A. MorrisEarly palaeomagnetic studies on the Bushveld Complex yielded widely separated poles for the various zones suggesting that the Bushveld Complex was emplaced over an extended period of time, possibly in separate pulses over a period of ~50 million years. However, modern high resolution age data have revealed that the Bushveld Complex was emplaced in a relatively short time span, likely to be less than 1 million years. These new age data prompted new palaeomagnetic studies that resulted in a single pole for all of the zones of the Bushveld Complex from magnetite with high unblocking (HB) components. These new studies also documented 7 reversals within the 8 km thick sequence of Bushveld Complex lithologies. These reversals were initially interpreted as occurring sequentially in stratigraphic order; however careful examination of the cooling process in conjunction with the reversals reveals that they are duplicated as the Bushveld Complex cools from the top and the bottom. Thus the stratigraphic location of the reversal also depends strongly on the thickness of the Bushveld Complex. In order to examine details of these reversals and their relationship to the cooling process, we are using data from the triaxial fluxgate magnetometers used in borehole orientation systems to identify the precise stratigraphic location of reversals in borehole core.
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Magnetotelluric Experiments Across The Southern Barberton Greenstone Belt, South Africa
Authors S. Kuetter and U. WeckmannThe first formation of the crust is a controversially discussed topic amongst geoscientists. Tectonics on the Early Earth might have been similar to the plate movement and their driving forces that we observe today. However, regarding that some fundamental conditions like the thermal setting were considerably different at this time other processes like vertical mass transport might have played the governing role in tectonics. Unfortunately, there are only few remnants, e.g. the Barberton Greenstone Belt (BGB) in South Africa, where Early Earth’s tectonics can be studied. In the framework of the research initiative Inkaba yeAfrica two high resolution magnetotelluric (MT) field experiments were carried out in 2009 and 2010 across the southern part of the BGB. Unlike the surface structure which is well known from a number of geological studies, there is only little information on its deeper architecture. Imaging the complex subsurface is therefore the major aim of this project. The MT method allows resolving the electrical conductivity distribution at least down to lithospheric depths. Against the surrounding more conductive geological units the BGB is outlined as a highly resistive body. Mapping tectonic features within the BGB like faults and sutures which can be electrically conductive due to mineralizations along shear planes could be the first step to better understand the evolution of the BGB. However, various electromagnetic noise sources, e.g. power lines and electric fences, hinder the analysis of the MT data. Applying advanced filtering methods is therefore mandatory to improve the data quality as much as possible. Based on the good areal coverage of the ~ 200 MT sites (Figure 1), 2D interpretations along the six profiles with lengths between 80 and 110 km and a 3D analysis of the whole data set are conducted. In 2D models, the faults appear as zones of high conductivity down to a depth of 5 to 10 km; however, it seems difficult to follow these structures across neighbouring profiles. Therefore 3D inversion is more appropriate to correctly image this part of the Barberton Greenstone Belt.
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Maximising The Benefit Of Historic Airborne Em Through New Modelling; 36 Surveys Over A Decade For Building A Basin-wide Conductivity Model For Uranium Exploration
More LessSince 2001, Cameco has been acquiring airborne electromagnetic data over their tenements in the Arnhem Land region of northern Australia to assist in uranium exploration. Airborne EM has predominantly been used for mapping the depth to the sandstone-basement unconformity, and geological structures that favour uranium mineralisation. Thirty-six surveys have been acquired using the TEMPEST® system over the last 12 years in the region. These surveys have historically been limited to individual tenements that were being explored at the time, and as such have been typically treated individually and in isolation to one another. Originally, conductivity models were generated using EMFlow on Z component data only. In 2012, Cameco re-modelled these datasets using the latest inversion algorithms utilising the non-geometry corrected “raw” X and Z component data. This has increased the spatial resolution of the resultant conductivity model, providing an improved product for geological interpretation. The individual surveys were merged together with no stitching, and the resultant basin wide compilation shows the excellent stability of the TEMPEST® system throughout the 12 years of surveying. Re-modelling has markedly improved the conductivity models from these surveys and allowed a basin wide compilation to be constructed to aid in exploration, particularly by providing a regional structural framework which is not always obvious at the tenement scale.
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Vtem And Ztem Helicopter Em Case-study Over The Nuqrah Cu-pb-zn-au Sedex Massive Sulphide Deposit In Western Arabian Shield, Ksa
Authors J.M. Legault, A. Prikhodko, C. Izarra, S. Zhao and E.M. SaadawiHelicopter VTEM active source, ZTEM AFMAG passive source EM and aeromagnetic survey results are compared over the Nuqrah Sedex massive sulphide deposit in Western Arabian Shield of KSA. Field data and 1D-2D- inversions are used to show that all surveys map major controlling structures that host the Nuqrah deposits. VTEM directly detects more massive sulphide mineralized vent portions of Sedex orebodies; whereas ZTEM likely defining larger, less conductive and weakly mineralized distal portions of Sedex system. ZTEM also maps possible conductive down-dip extension of Nuqrah South Sedex below 750m depths.
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Looking For Graphite To Find Copper
More LessExploration for copper in the Zambian Copperbelt has been guided for most of the past hundred years by the search for a known favourable stratigraphic horizon known as the Ore Shale. Historical efforts have also been informed by a belief that the ores are syngenetic and stratiform. Both of these criteria are now known to be too restrictive. Economic mineralisation beyond the Ore Shale has been amply demonstrated by stratigraphically higher deposits close to the traditional Copperbelt, such as Frontier and Lonshi, as well as deposits such as Kansanshi and Sentinel that extend the Zambian Copper Belt all the way to the northwest of the country, and by the most recent discovery at Kamoa, in the DRC, by Ivanplats Ltd, far outside of, and above what was considered Copperbelt stratigraphy. The hydrothermal nature of many of the ores has also been validated, with the attendant role of structure, both deep-seated and local. These cases demonstrate that copper-bearing fluids precipitate their metals in (or near) the first reducing horizon they reach above the basement. The geophysical contribution becomes focused on mapping basement topography and graphitic (reducing) horizons, as well as crustal and local structure that can focus this fluid flow. Examples from a geographic spread of deposits along the Zambian Copperbelt highlight the role of electromagnetic imaging in the search for copper.
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The Geophysics Of The Jacomynspan Proje
Authors M. Ushendibaba, S.J. Du Plessis, R. Hornsey and N. NonoiseAfrican Nickel Ltd applied a number of geophysical methods from May 2011 to March 2013 with state of the art equipment at the Jacomynspan nickel-copper project in the Northern Cape in order to guide drilling and to detect the better mineralised parts of the deposit. The Ni-sulphide mineralisation is hosted in a 100m thick mafic to ultramafic sill that dips 650-750 south. A high resolution aeromagnetic survey was conducted covering the main target zone and the strike extent of historical drilling. The survey successfully delineated the sub-crop of the east-west trending ultramafic rocks. This information was used to plan ground electromagnetic (EM) surveys. Physical property measurements in the form of magnetic susceptibility and electrical conductivity were collected on historical drill core. The relative conductivity readings show the mineralised harzburgite to be more conductive than the gneissic host rock and the tremolite schist mineralisation. It was anticipated when planning the EM techniques, that the current would gather in massive sulphide stringer zones and the net-textured higher grade mineralised harzburgite. Therefore the EM plate model targets could potentially indicate these units rather than the lower grade tremolite schist mineralisation. A moving loop Time Domain Electromagnetic (TDEM) survey was carried out using a Low Temperature SQUID sensor over the main target zone to assess whether multiple stacked conductors exist, to map the best mineralised zones, and to explore for massive sulphide mineralisation. Downhole Electromagnetic (DHEM) surveys were also carried out in order to confirm whether off-hole mineralisation was present. To image both deep and near surface geological structure and mineralisation, vector Audio-frequency Magnetotellurics (AMT) was carried out on the main target zone. The results confirm that the intrusion is steeply dipping to the south and clearly mapped the higher grade harzburgite mineralisation down to 500m depth. The final 3D model was integrated into Micromine software with modelled geology and EM plates. This work confirms that AMT is applicable at Jacomynspan to rapidly and cost-effectively prospect for higher grade mineralisation.
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Magnetic Tensor Measurements Validated By Means Of A Simulated Magnetic Source
Authors B. Du Plooy and R. StolzAn experimental facility was established in which a helicopter borne magnetic gradient sensor was flown over a known magnetic source. A comparison of the measured gradients and the calculated gradients served as a validation of the proper functioning of the complete system.
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High-temperature Squid (hts) Tem Trials In Africa
Authors T. Le Roux, C. Naudé and B. SteenkampTrial surveys have been carried out successfully with High Temperature SQUID (HTS) sensors for Time Domain EM applications in Africa where previously SQUID technology had been available only for LTS under exclusive rights to Anglo American. The HTS sensor coupled with a 24-bit SMARTem receiver proved a practical and cost effective field solution capable of acquiring much better low-noise TEM data than conventional coils and 16-bit receivers. A trial line of TDEM and MIP data over a Xade Complex target in Botswana is shown here to illustrate that effective penetration of severely conductive (100 Siemens) Kalahari and Karoo cover can be achieved. This means that the HTS technology now available has tremendous potential for exploring for sulphide or gold targets elsewhere in Africa below previously debilitating conductive cover with these geophysical methods.
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The Terrascanner - A Ground Based Magnetic Mapping System
Authors H.-g. Meyer, S. Linzen, M. Schneider, M. Schulz, S. Dunkel, J. Bemmann, M. Meyer and R. StolzThe aim of the Terrascanner instrument developed at IPHT Jena is the fast geomagnetic prospection of large areas. This enables applications like as archaeology, building-ground-analytics, geology and detection of unexploded ordnance. The instrument is based on nine thin-film planar-type SQUID gradiometers of first order, three of which are assembled in each of the three liquid helium cryostats. A fast SQUID electronics and a low-drift and low-noise data acquisition systems enables rapid exploration using an all-terrain vehicle. Hence, extended archaeological or geologic objects are mapped with high efficiency and quality of magnetic data. In order to allow high spatial resolution (better than 10cm) and real time geo-referencing of the sensor signals a high-end differential GPS system with base station was implemented. Therefore, precise topographic models of the scanned area can be calculated which are useful for interpretation of the data. In this work we present data acquired with the Terrascanner during a campaign within the Orkhon valley, Central Mongolia, carried out in autumn 2010. The joint interpretation of the magnetic and topographic maps of surveyed areas of up to 1km x 1km proved to be very helpful for large-area archaeological investigations of e.g. steppe regions like the Mongolian ones. We will show additional examples of magnetic scans for archaeology and geology in Germany.
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New Developments On The Full Tensor Magnetic Gradiometer System
Authors R. Stolz, V. Zakosarenko, M. Schulz, M. Schmelz, A. Chwala, M. Meyer and H.-g. MeyerINFLUINS is a German research program which investigates the movement of fluids (and associated gases) in the Thuringian sedimentary basin. The phrase "integrated fluid dynamics" in the project name highlights the interest in the relationship between near surface and deeper fluid and material flows. One major part of the program is to examine shallow and medium depth structures, for instance to understand fluid-fault interactions, using magnetic field measurements. The main tool is an instrument measuring the full tensor of the Earth’s magnetic field (FTMG) in airborne or ground based operation. The sensors are planar gradiometer of first order with a base line of 3.5 cm and a sensitivity of below 100 fT/(m*sqrt(Hz)) based on low temperature Superconducting Quantum Interference Devices so called SQUIDs. The gradiometers effectively suppress the Earth’s magnetic field by about four orders of magnitude and are well suited to map the gradient of the Earth’s magnetic field. A low sensitive reference magnetometer triplet is used in order to enhance the suppression by another three orders of magnitude. The system can be operated in a nonmagnetic bird towed by a helicopter at low altitude or on a nonmagnetic cart towed by a 4x4 car. Both operations allow the investigation of huge areas in short time and record the full tensor gradient of the magnetic field. Here, we introduce the new features of an airborne FTMG system which provides superior performance in fixed wing and helicopter based applications compared to conventional Caesium type magnetometers. First of all we could half the length of the liquid helium cryostat. This enabled to build a new tow fish with a much more aerodynamic shape. Additionally, we improved the performance of the inertial measurement unit which leads to a better transformation from the body to a georeferenced coordinate system. First results of the new system in a newly developed bird flown in three survey areas in the Thuringian basin are shown. Standard deviation of noise level is lower than 60pT/m throughout a whole survey area. Examples of 2D maps of tensor components with high spatial resolution are presented.
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A 3d Potential Field Model Of The Pilanesberg Complex Shape And Structure
Authors S-a. Lee, S.J. Webb, M.Q.W. Jones, R.J. Durrheim and M. GanerødThe Mesoproterozoic Pilanesberg Complex, South Africa, is the world’s largest alkaline intrusion. While surface field relationships suggest an inward dipping structure, it is unclear how these dips extend to depth. The 3D geometry of the Pilanesberg Complex is also unknown. 2D and 3D forward and inversion modelling of gravity and magnetic data are used to set limits on the 3D shape of the Complex. Based on age and chemical affinity, it is known that the Pilanesberg Complex forms part of a larger system of alkaline intrusions that includes two dyke swarms that radiate to the north-west and south of the Complex, as well as smaller circular clinopyroxene intrusions throughout the Bushveld Complex. The Pilanesberg dyke swarms and the circular clinopyroxenite intrusions are reversely magnetised to that of the normally magnetised Pilanesberg Complex, suggesting that a magnetic reversal occurred during emplacement of the system.
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New Evidence For The Asthenospheric Origin Of The Cameroon Volcanic Line From 1d Shear Wave Velocities
Authors A.-p. Tokam, R. Durrheim, A. Nyblade, C. Tabod and S. NguiyaThe composition of the lithosphere beneath Cameroon and the origin of the Cameroon Volcanic Line (CVL) are still debated by the Science Community. Although many studies based on regional or global observations provide good arguments for the setting of the CVL, none of them are strong enough to be considered as unique. In this work we used the joint inversion of Rayleigh wave group velocities and Rayleigh wave group velocities to provide shear wave velocity profiles of the lithosphere beneath Cameroon. Preliminary results show that lithosphere is, on average, faster beneath the Congo Craton than the Pan-African lithosphere beneath Cameroon. Due to the limited resolution of the dispersion curves, the maximum investigation depth was taken to be 200 km. The calculated velocity-depth profiles do not show any sharp discontinuity that could be interpreted as the lithosphere-asthenosphere transition. In addition there is no clear evidence of the existence of a low velocity zone beneath any geologic province within Cameroon. The smooth velocity contrasts observed on the velocity models are believed to be influenced by lateral mantle heterogeneities rather than vertical ones. The shear wave velocities for the uppermost mantle are in general greater than 4.3 km/s at all stations. This is higher than the values obtained in the Main Ethiopian Rift, and suggest that the lithosphere is not globally perturbed by thermal anomalies. This suggests that the source of volcanism along the CVL is from small scale convection in the asthenosphere and occucontrolled by lithospheric fractures that are probably driven by the cold (and fast) edge of the Congo Craton.
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Uncovering Prospective Palaeo-proterozoic Basins In Western Australia In The Search For Distal Footprints Of Large Ore Systems
Authors T. Munday, S. Johnson, R. Hough and I. TylerThe Bryah Basin is part of the Capricorn Orogen, a collision zone between the Archaean Pilbara and Yilgarn Cratons in Western Australia. The Basin is host to significant mineralisation, including mesothermal orogenic gold, copper–gold volcanogenic massive sulphides. Among the challenges in the exploration for these mineral systems is the paucity of outcrop and the extent and variability of a complex regolith cover. To better understand this regolith, a regional-scale, fixed-wing time domain AEM survey was undertaken over the Bryah Basin in 2012. The resulting data were inverted using a smooth model layered earth inversion, taking account of system geometry to better resolve regolith variability in the near surface, to define regolith thickness and to understand the geometry of the regolith-basement boundary. The inverted data show the most dominant regolith features are associated with deep sediment filled palaeovalleys. The regional regolith framework determined from this study provides a basis for better understanding and interpreting an extensive regolith geochemical data set with respect to metalloid anomalies that may be linked to buried Cu-Au mineral systems. It also indicates where cover is relatively thin and therefore offers the opportunity to prioritize areas for exploration.
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Delineating The Kitumba Iocg Deposit With The Orion 3d Dcip System
Authors D.L. Mcgill, K.J. Killin, T. Woolrych and J. RuddAn ORION 3D DC/IP survey was conducted over Blackthorn Resources’ Kitumba IOCG deposit in Zambia. The survey results provided 3D models that successfully delineated the known deposit, and provided an enhanced understanding of the three-dimensional geometry of the mineralization. With this improved understanding of their deposit, Blackthorn was able to refocus their ongoing drilling program to best target possible extensions of the existing mineralization.
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Evaluation Of Various Electrical And Electromagnetic Methods In The Exploration For Weathered Kimberlites In Wajrakarur Kimberlite Field, Ap, India
By S. VermaIn Wajrakarur Kimberlite Field in Dharwar Craton, south India, there are more than 100 Proterozoic kimberlite pipes and some of the world famous diamonds like Kohinoor, Nizam, Orloff, etc. were won from this field. The top regions of many of these kimberlites are weathered and altered and have the presence of clayey minerals, thus providing good targets for electrical and EM exploration. As kimberlites outcrop in a limited area, the weathered top material acts as a localized 3D conductor with confined spatial distribution. Highly resistive Archaean granitic gneiss host the kimberlites and the geological setting is ideal for the application of various electrical and EM methods. Depending on the availability of equipment, different electrical and EM surveys were carried out over a long period stretching more than a decade to study some of these kimberlites. Here a comparison of the performance of the following methods is presented: electrical resistivity tomography (ERT), frequency or time-domain electromagnetic (FEM or TEM) surveys, VLF or Radio MT surveys, etc. Suitability of a particular method in delineating such 3-D conductors critically depends on factors like scale and cost of the survey, available time-frame, required details on the subsurface, availability of survey space and logistic constraints, topography, habitation, vegetation, ambient electromagnetic noise, interpretational rigor, etc. Choice of the most appropriate method thus becomes important in efficiently procuring the desired information optimally. The results are expected to be useful in optimizing survey strategy not only for kimberlites but for a variety of mining and urban problems requiring mapping of 3-D conductors.
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Application Of Aem To Shallow Geothermal Potential Mapping
Authors A. Viezzoli, A. Menghini, A. Manzella, D. Montanari, S. Maggi and A. SantilanoThe “VIGOR” project aims at assessing geothermal resources in 4 regions of Southern Italy. The SkyTEM survey is a part of the planned geophysical activities and has been chosen since it is able to collect resistivity data on wide areas, with great resolution and in short times. To date, the application of AEM methods to geothermal targets has been limited, due to the depth at which the target is usually found. Kirsh and Siemon (2008) suggested its application in low enthalpy study. In this project AEM is tested in an area of known geothermal activities, and where a wealth of ancillary data is also present, in western Sicily. The results are composed in a 3D model of the electrical resistivity of the subsurface, and then interpreted for geothermal potential, following 2 main paths. The first is to produce a model of heat exchange capacity. The second aims at identifying directly conductive anomalies that might be associated to locally shallow hydrothermal fluids. “Termini” area is covered by extensive geological surveys. Since geological conditions of Sicily, even at shallow depth, are very complex, this area provided a good place for defining the resistivity values of the main geological units outcropping in the region. The “Western Sicily” area covers the main thermal manifestations of Western Sicily. Based on near real time preliminary results from the more regional mapping at 1 km line spacing, three infill areas were selected as being the most promising to test the concept. The obtained resistivity volume has then been the base for a detailed lithological and geothermal interpretation. Lithological and geological maps were used to constrain surface condition and to understand the resistivity ranges of the different lithological units. On the base of AEM derived resistivity values, and of laboratory measurements of thermal and electrical conductivity on samples, it was possible to establish the main links between lithology, electrical resistivity and thermal conductivity. The work in progress aims at extending the correlation also at depth, producing a 3D model of thermal exchange capacity for the areas surveyed. This detailed interpretative modeling provides also the basis for detecting resistivity anomalies within carbonate units, which may possibly represent hydrogeological or hydrothermal bodies.
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The Importance Of The Tdem Waveform And Characterization Of Discrete Conductors
Authors F. Effersø, N.S. Nyboe and P. GisseløIn airborne time-domain EM (ATEM) the signal-to-noise ratio (SNR) is paramount for the detection of small responses from discrete conductors. In this paper it is examined how shortening the linear current turn-off can enhance the target responses significantly for discrete 3D conductors. For the objective of the paper two methods were applied (1) synthetic 2D thin sheet modelling as approximations to 3D discrete conductors and (2) comparison of real datasets collected over discrete conductors using two different ATEM systems having respectively a 200 s and a 1,200 s turn-off ramp. The findings of both the synthetic modelling and the real datasets show that the target response can be amplified by a factor 2 or even more if the length of the current turn-off ramp is shortened from 1,200 s to 200 s. The enhancement of the target response, and thereby the SNR, occurs for a large group of discrete conductors for which the time constant is comparable to or smaller than the duration of the current turn-off ramp. Shortening the current turn-off ramp will improve the capability to detect such conductors.
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