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11th SAGA Biennial Technical Meeting and Exhibition
- Conference date: 16 Sep 2009 - 18 Sep 2009
- Location: Swaziland, South Africa
- Published: 16 September 2009
121 - 140 of 157 results
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A Lithological and Structural Study of the Witteberg Group (Cape Supergroup) with Focus on the Witpoort Formation Near Kirkwood, Eastern Cape
Authors W. Olivier, P. W. K. Booth and G. BrunsdonThis study of Late Palaeozoic Witteberg Group rocks (Cape Supergroup) near Kirkwood, Eastern Cape, focused mainly on quartzitic rocks because these rock types crop out in mountainous terrain where lithological and structural data collection was possible. A detailed structural analysis has been carried out and the results compared to the regional structural pattern of the Cape Fold Belt in the Eastern Cape. Field methods include air photo interpretation of lithological and structural features, and the measurement of structural elements with the aid of a Brunton compass. Thin sections of the main rock types analysed by microscopic and SEM techniques show that arenaceous rocks are composed predominantly of quartz, whereas argillaceous rocks contain mainly micaceous minerals with lesser quartz, feldspar and iron oxides. Strata are folded into open anticlines and synclines that plunge at shallow angles to the east-southeast as well as in the opposite direction and show northward vergence. The general east–west orientation of thrust faults, and their southward dips indicate that they formed during a northward-directed stress field that was present during the Late Palaeozoic. Predominantly south-dipping normal faults formed during the Mesozoic, as part of the process of the break-up of Gondwana.
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Reversed Multi-offset VSP Traveltime Tomography Employing Direct-push Technology for Near-surface Seismic Velocity Imaging
Authors H. Paasche, U. Werban and P. DietrichFor a wide variety of environmental and engineering geophysical applications reliable knowledge of the seismic velocity distribution in near-surface sediments is essential. We evaluate the suitability of the minimally invasive direct-push technology for rapid 2D traveltime tomography imaging P-wave velocity variations in near-surface unconsolidated sediments. We acquire reversed multi-offset vertical seismic profiles (VSP) employing geophones placed on the earth’s surface and a seismic source temporarily installed in the subsurface using direct-push technology. The 2D seismic velocity distribution is tomographically reconstructed from first arrival traveltimes obtained from a number of closely spaced reversed multi-offset VSP measurements. First, we test this approach in a synthetic study that utilises realistic geometries. This is followed by a field data example measured over alluvial deposits in a former river flood plain. The P-wave velocity model finally obtained contains information about high and low velocity anomalies and structurally matches the results of cone penetration tests (CPT) and gamma radiation data acquired along the profile. Additionally, the method enabled a significantly larger penetration depth compared to a conventional refraction tomography survey.
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Zonal Cooperative Inversion of Partially Co-located Crosshole Tomographic P- and S-wave Traveltime Data Sets under Automated Incorporation of a Priori Information
Authors H. Paasche, J. Tronicke and P. DietrichIn many near-surface geophysical studies it is now common practice to collect co-located disparate geophysical data sets to explore subsurface structures. Reconstruction of physical parameter distributions underlying the available geophysical data sets usually requires the use of tomographic reconstruction techniques. To improve the reliability of the obtained model parameters, the information content of all data sets should be considered during the tomographic model generation process, e.g., by employing joint or cooperative inversion approaches. Here, we extend the recently developed zonal cooperative inversion methodology based on fuzzy c-means cluster analysis and conventional single-input data set inversion algorithms for the cooperative inversion of two crosshole tomographic traveltime data sets with partly co-located model areas. This is done by considering recently published modifications made to the fuzzy c-means cluster analysis. Additionally, we show how supplementary a priori information can be incorporated in an automated fashion into the zonal cooperative inversion approach. The only requirement is that the additional information considered can be expressed numerically, e.g., by physical parameters or indicator variables. The approach results in a single zoned multi-parameter model, which is consistent with all available geoscientific information and outlines the major subsurface units. Additionally, physical parameter models underlying the zoned multi-parameter model are obtained for each of the input data sets.
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Automated Integration of Large Geophysical Data Sets using Three Partitioning Cluster Algorithms: a Comparison
Authors H. Paasche and D. EberleSince the advent of modern desktop computers, attempts have been made in various geoscientific fields towards rapid, automated and objective information extraction from suites of co-located data sets. Multivariate unsupervised classification techniques, such as cluster algorithms, have been proven valuable tools for largely automated information extraction and are for example routinely used for structural exploration and integration of multi-spectral remote sensing data sets. However, so far very few attempts have been made towards using unsupervised classification techniques for rapid, automated and objective information extraction from large geophysical data sets. In this study, we employ the crisp k-means, fuzzy c-means (FCM) and Gustafson-Kessel (GK) cluster algorithms and compare their suitability for rapid and largely automated integration of complementary geophysical data sets comprising airborne radiometric and magnetic as well as ground-based gravity data. All three data sets cover a survey area of 5000 km2 located south-east of Johannesburg, South Africa. Integrated geophysical maps outlining dominant subsurface structures are obtained from each of the used cluster algorithm. Fuzzy cluster algorithms, such as the FCM and GK algorithm provide additional quantitative information about the trustworthiness of the detected subsurface units, which is considered very valuable when interpreting the finally obtained zonal maps. We will also show that the GK algorithm is most robust when it comes to the integration of data sets containing a few extreme anomalous values, e.g. as typically present in magnetic data sets, resulting in strongly skewed histograms of the data.
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MODIKWA – a Successful Example of a Shallow Ore-body Imaging by Surface 3D Seismic
Authors C. Pretorius, M. Gibson, Eric Gillot and M. DenisSurface 3D is now a widely imaging tool used in the Western Bushveld to derive continuous structural image for ore bodies with a depth ranging from 600m to 1700m below surface. The cost of such surveys is directly linked to the source and receiver surface sampling. Such structural model was requested for the Modikwa Platinum mine, in the Eastern Bushveld, with an ore body whose depth is ranging from 200m till 450m. In order to balance the costs versus the expectations, a trial was conducted in 2004 whose main focus was to determine the most optimum surface sampling scenario and also benchmark the results of surface seismic versus initial model derived from boreholes. This test has delivered as expected an acquisition model for shallow UG2 surveys in Eastern Bushveld and also a structural model with reliable small features such as fault throws down to 12m due to the achieved vertical resolution. In 2006 the main survey took place and the expected structural image was achieved after processing. The fault network when compared to the initial model had its accuracy and reliability push down till fault throws around 12m for a cost balancing current density of boreholes.
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Exploration and Mining Geophysics and Remote Sensing in 2009 – Where Have We Come From and Where Are We Going To?
By C. PretoriusGeophysical and remote sensing (RS) techniques, integrated with the other geosciences and engineering disciplines, have made a significant contribution across the full mining lifecycle (MLC), extending from exploration to mine closure. Although much of the published data deals with the high profile contributions that geophysics and RS have made to exploration and mining feasibility studies, many of the exciting new contributions are expected in the mining production part of the MLC. In the future there will probably be an increasing focus on geophysical and RS contributions to mine safety, mineral resource management, geotechnical systems, hyperspectral core logging and ore sorting, as well as improved ground and airborne exploration multi-systems. A balanced portfolio across the MLC in the multi-disciplinary Geoscience / Engineering / Metallurgical environment should ensure sustainable success.
Our stock of in-house corporate geophysicists and remote sensors with an owner-manager / shareholder culture needs to be replenished. These individuals are key drivers of the macroeconomic demand for specialist geosciences. The in-house specialists should capitalise on past successes and continue to capture the methodologies in quality assurance systems, best practice manuals, procedures and guidelines in cooperation with our geological and engineering colleagues. These formalised best practices should be appropriately communicated and rolled out and will help our companies to maintain a high standard of compliance with important industry codes, for example in safety risk management and mineral resource management. Organisations such as SAGA and our tertiary education institutions can make a significant contribution to the formal compilation of best practice guidelines for the geophysical and RS geosciences.
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Recent Seismicity in the Mozambique Region and its Impact / Effects on South Africa
Authors T. Pule and I. SaundersThe seismic pattern of the southern extension of the East African Rift in Mozambique is presented in this study with special reference to the M=7.0 earthquake that occurred on 23 February 2006. Seismicity in Mozambique is associated with tectonic activity along the East African Rift, which forms the boundary between the African (Nubian) plate in the west and the Somalian plate in the east. The seismic event that occurred in the Manica province of Mozambique on 23 February 2006 was the largest event prior to 2006 was recorded in 1951 as an M=6.6 earthquake. However, it should be noted that a maximum credible earthquake of M=7.3 was assigned during the Global Seismic Hazard Assessment Programme study for a broad zone encompassing the area. Damage caused by the earthquake was limited though it was felt across a large part of southern Africa. Reports of two fatalities were received from Beira indirectly linked to the earthquake while most damage was centered on the town of Machaze where an unconfirmed number of people were either killed or injured (unconfirmed reports indicate 5 fatalities and 30 injuries). Reports also indicated that numerous houses and businesses were damaged during the earthquake. The estimated intensity levels in Durban and Johannesburg from reports indicate II-III on the Modified Mercalli scale. The shaking levels of the event reveal a surprising trend that is counter-intuitive to normally observed damage from similar events, given that Durban is more than 1000km from the epicenter. This work looks at the Gutenberg-Richter frequency magnitude distribution of earthquakes in this region.
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Electrical Conductivity of the Cape Fold Belt, South Africa.
Authors O. Ritter, U. Weckmann and Kristina TietzeWithin the framework of the German - South African geo-scientific research initiative Inkaba yeAfrica several geophysical field experiments were conducted along the Agulhas-Karoo Transect in South Africa. This 600km long transect crosses the Cape Fold Belt (CFB) with the Kango and Oudtshoorn Basins, the Namaqua-Natal Mobile Belt, the Karoo Basin and the transition into the Kaapvaal Craton. Magnetotelluric imaging reveals structural details of the CFB and its collision zone with the Namaqua Natal Mobile Belt (NNMB) at a scale of the entire crust. The CFB is generally characterized by similarly high electrical conductivities as observed in the NNMB. Exceptionally high electrical conductivities are found in the Mesozoic/Cenozoic inliers, the Kango and Outshoorn Basins, whereas the Swartberg and the Outeniekwa Mountain ranges seem to have mid crustal resistive roots.
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Geological Applications for FTMG
More LessThe geological applications of the Full Tensor Magnetic Gradiometer, or short “Mag SQUID”, are manifold. At a similar resolution to the commercially available high-resolution aeromagnetic surveys, the Mag SQUID provides vector data and information on remanence. The vector data is of high importance as it may allow a better 3D modelling of magnetic bodies such as kimberlites, dykes and iron-rich ultramafic replacement pegmatitoids (IRUPs). In the data one may detect mineralisation directly, but the information can also be used to sense negative structural interferences with the mining process such as dykes and IRUPs (hazard zones). Further we expect to detect weaker magnetic features, e.g. the so-called non-magnetic dykes in the Mpumalanga coalfields. The anticipated information on remanence is required for enhanced anomaly discrimination and determination of age of intrusive or alteration events.
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Chemostratigraphy of Wells A-U1, A-F1, K-M1 and A-G1 in Block 3A Used to Facilitate Reservoir-scale Interwell Correlations in the Orange Basin Offshore the Southwest Coast of South Africa
By X. SchalkwykChemostratigraphy is the study of the inorganic geochemistry of sedimentary sequences where it is used to study variations in the geochemistry of sediments as a correlation tool. This is done by analyzing cutting samples, conventional core samples, sidewall cores or outcrop samples which allows regional to reservoir-scale interwell correlations to be made (F. Schwab, 2003). This project will entail the correlation of four wells for oil and gas provinces, with petrophysical logs and in the end determine potential reservoir compartmentalization. The four wells will serve as a good basis for correlation of sedimentary successions based on subtle changes in concentrations of key major, trace, and rare earth elements. With the aid of logs (Gamma ray, Resistivity log, Neutron – Density log, Spontaneous Potential [SP] and Sonic log the relevant reservoir zones can be determined. The main aim of this project is to use chemostratigraphy analysis as the bases of reservoir-scale interwell correlation. And to also compare geochemistry of the core or side wall samples (if available) or ditch cuttings with the stratigraphic column to establish possible correlations. From these aims the end result will be to estimate the recoverable hydrocarbon reserves and the possible prediction of future production for the reservoirs. Chemostratigraphy then enables the main reservoir units to be differentiated and correlated on the reservoir-scale and subregionally. Core data will provide a direct measurement of well properties but for this project ditch cuttings will enable a more detail description of major, minor and trace elements. The proposed area for this study is located in the Orange basin which is a quintessential passive margin that formed during the break up of Gondwana and the subsequent rifting of the South Atlantic Ocean. The syn-rift portion of the basin is characterized by a series of grabens and half-grabens that trend approximately parallel to the present day margin. The syn-rift fill, which is Upper Jurassic and Lower Cretaceous in age, predominantly comprises siliciclastic and lacustrine strata with a widespread occurrence of volcanic sequences (Gerard and Smith et al, 1983).
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Folding and Faulting in the Weltevrede Formation on Varsfontein, Steytlerville Area
Authors T. E. Scharf, P. W. K. Booth and G. BrunsdonA field study of the lowermost stratigraphic unit of the Witteberg Group (Cape Supergroup, Steytlerville area, Eastern Cape), has shown that rocks of the Weltevrede Formation are composed predominantly of folded argillaceous rocks intercalated with lesser thinly-bedded arenaceous beds. The purpose of this study is to document and provide detailed lithological and structural data, and interpret these in relation to regional geological and structural patterns in the area. Rocks of the Weltevrede Formation show colour variations in both shales and quartzites as a result of variable quantities of iron oxide minerals in the rock. Sedimentary structures are plentiful, especially in quartzites and are useful in the interpretation of facing direction of strata. Trace fossils characteristic of the Witteberg Group, especially Zoophycos and Monocraterion are frequently present in all rock types. Strata are folded into open anticlines and synclines that plunge at shallow angles towards the east-southeast. Folds all verge northwards, indicating deformation forces originated from the south. One set of faults have an east-west strike orientation, whereas a second set strike approximately north-south. The former are interpreted as products mainly of flexure-slip folding, whereas the latter are probably related to a prominent strike-slip fault in close proximity to the study area. From the orientation of structures in the study area we conclude that all structures in the study area conform with the regional structural pattern, and therefore have an integral relationship with deforming forces that were present during the Cape Orogeny, during the Late Palaeozoic.
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Results from the Initial Field Trials of a Borehole Gravity Meter for Mining and Geotechnical Applications
Authors H.O. Seigel, C.J.M. Nind, A. Milanovic and J. MacQueenScintrex is in the final stages of the development of a borehole gravity meter, for mining and geotechnical applications. It is designed to log inside NQ (57 mm I.D.) drill rods to a depth of 2,000 m, using standard four (4) conductor wireline cable. The achieved sensitivity is better than 5 μgal, and is operable in boreholes inclined from 300 to vertical. École Polytechnique of Montreal has developed forward modelling software, as part of this project. Partial financial support was provided by the Ontario government (IRAP) and through a CAMIRO project sponsored by BHP Billiton, Vale Inco, AREVA Resources Canada and Schlumberger. The first field test of the prototype probe was successfully conducted in December 2008 for Vale Inco in a borehole located in Norman Township near Sudbury, Ontario. The results of this test show a large amplitude bipolar residual gravity anomaly, with the crossover at 1,400 m down the hole where the borehole intersected sulphides. A repeat log of the hole indicates that the Gravilog system achieved operational specifications close to its targets. The second field test was conducted in March 2009 for AREVA in a newly drilled, vertical borehole at Shea Creek in northern Saskatchewan. The results clearly show the unconformity, and the bulk density calculations show an indication of a low density zone at the base of the Athabasca sandstone. The data is undergoing further analysis. The third and fourth field tests are planned in July and August 2009 for Schlumberger and BHPB respectively. Gravity measurements inside boreholes provide evidence of density variations both in the immediate vicinity and at a distance from the hole. Scintrex’s development of a new borehole gravimeter will, for the first time, allow the application of gravity logging in typical mining and geotechnical boreholes. Primary applications of the Gravilog system in mining include the sensing and mass-estimates of massive sulphide bodies, either intersected by or in close proximity to the borehole; or accurate bulk density measurements of formations intersected by the hole. In some cases (e.g. iron deposits) there is a semi-quantitative relationship between bulk density and grade of the deposit.
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Interpretation and Modelling of VTEM Data in Difficult Circumstances, Bon Accord Nickel-Sulphide Deposit
By G. R. SelfeThe Bon Accord nickel-sulphide deposit, near Barberton, is surrounded by cultural noise in the form of an operating talc mine, a railway line, a metal pipeline and high voltage power lines which combine to render ground TDEM follow-up and modelling ineffective. A heliborne VTEM survey was therefore flown over the deposit in order to try and delineate the orebody and generate additional targets for follow-up. Although compromised by noise, the VTEM data could be modelled directly using EMIT’s Maxwell software. The modelling matches drilling results closely and has engendered confidence in the new VTEM targets awaiting drilling.
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New Discovery of Ni-mineralisation in Northern Namibia Geophysical Case Study
By G. R. SelfeIndications of potential Ni-mineralisation at Ombuku, on the western edge of the Kunene Anorthosite Complex in northern Namibia, were obtained from a regional stream sampling program undertaken by Anglo American plc from 1998 to 2003. Ground IP, AMT and subsequent TDEM geophysical surveys and a VTEM survey flown by ANL, have delineated several drill-targets around the new ultramafic satellite intrusions discovered at Ombuku. Drilling to date has intersected 0.4% Ni and 0.15% Cu over 24.66m (0.3% Ni cut-off) and 0.53% Ni and 0.24% Cu over 4.3m (0.5% Ni cut-off), in disseminated sulphides. This paper discusses the geophysical surveys undertaken and the potential for identification of a large Ni-sulphide resource through further exploration.
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Prediction of DC Current Flow Between the Otjiwarongo and Katima Mulilo Regions, Using 3D DC Resistivity Forward Modelling and Magnetotelluric and Audio-magnetotelluric Data Recorded During SAMTEX
SAMTEX (Southern African Magnetotelluric Experiment) is a multinational project that was initiated in 2003 to study the regional-scale electrical conductivity substructure of southern Africa and to infer from it the tectonic processes involved in the formation of the southern African subcontinent. Audio-magnetotelluric (AMT) data recorded during the most recent phase of the experiment are evaluated to investigate the local-scale conductivity substructure in the Otjiwarongo and Katima Mulilo regions, where in future the installation of high-voltage direct current (HVDC) earth electrodes will commence. Both of the AMT surveys are situated close to the edge of the orogenic Damara Mobile Belt in northern and north-eastern Namibia. Previous studies using magnetotellurics (MT), magnetometer arrays and geomagnetic observatory data all provide evidence of the relatively conductive nature of the Damara Mobile Belt. The Damara Mobile Belt represents in part the collision between the Congo and Kalahari cratons during the amalgamation of South Gondwana and its high conductivity is explained by the presence of interconnected fluids and conductive materials (graphites, sulphides). In contrast, the lithospheric structure of the ancient Archaean cratons, the Congo and Kalahari, are generally found to be electrically resistive and therefore it is hypothesized that the return path of DC current, flowing along the path of least resistance between the two electrodes, is most likely to lie somewhere within or in the vicinity of the Damara Mobile Belt. To obtain a better understanding of the current flow we propose using geological information, previous results of studies of the conductivity of the Damara Mobile Belt and surrounding regions and 2D inversion results from the AMT and MT data recorded during SAMTEX in northern Botswana and Namibia, as input to a 3D DC resistivity forward modelling code, and try to predict the return path that the DC current will follow.
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Advances in Drill Rig Deployed Radars
Authors Tim Sindle and Carina KempThe deployment of geophysical instruments in underground mining is a rarity. The primary reason for this is that the implementation is often seen as cumbersome and time consuming by geologists and mine managers alike.
This paper explores the recent improvement in deploying borehole radar in hard rock mines. Firstly the paper examines the minimum gear needed to complete the task to simplify logistics. It then looks at the processing of data collected by borehole radar deployed on a core drill rig. The non-ideal stop-start motion of the deployment presents issues in the data quality. With the aid of an onboard accelerometer and a time logging based procedure, these quality issues are addressed by examining three different algorithms to process the data. The strengths and weaknesses of each are discussed, before concluding that an intelligent combination of velocity and statistical methods will reliably produce smooth “winch like’ data.
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TDEM Survey at Shea Creek Uranium Deposit Utilising a Low-Temperature Superconductor SQUID
Authors J.P. Smit and T. Le RouxThe Shea Creek uranium deposit is situated in the Athabasca basin, northern Saskatchewan, Canada. A number of TDEM (time-domain electromagnetic) profiles were surveyed in a variety of commonly used survey configurations at mainly two locations over the conductor associated with the Shea Creek uranium deposit using the IPHT - Supracon - Anglo American low-temperature superconductor (LTS) SQUID (superconducting quantum interference device). The LTS SQUID is a B-field sensor that effectively measures small secondary transient EM decays in a conductive earth due to a sudden change in the primary field of a transmitter loop located beneath, on or above the surface of the earth.
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An Overview of Geophysical Experiments Across the Continental Margins of Southern Africa
More LessIn the framework of the multi-disciplinary Inkaba yeAfrica initiative a number of geophysical experiments have been carried out across the continental margins of Southern Africa. Across the southern margin the so-called Agulhas-Karoo Geoscience Transect extends from the offshore Agulhas Plateau, across Southern Africa’s sheared margin, across the Cape Fold Belt and into the Karoo Basin. Across the western margin, a classic volcanic rifted margin, the Springbok profile was conducted across the offshore Orange Basin, extending to the onshore exposure of the Gariep Belt. Goals of the experiments included broadening our understanding of the features crossed by the transects, especially in the framework of formation of Southern Africa’s margins and the break-up of Gondwana.
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Airborne TDEM by He-filled Balloon
More LessPerforming Time Domain Electromagnetic surveys in rugged terrain is a challenging task and as an alternative to mundanely laying out loops or using a helicopter system, a central loop configuration Time Domain EM system has been fitted to a 5m diameter He–filled balloon with a capacity to lift a ~48kg payload. The transmitter and receiver loops have a diameter of 10 and 5m, respectively. The balloon is handled by an operator and 3 assistants and measures while drifting above the topography. The TDEM electronics was custom built by Elta-Geo in Novosibirsk, Russia and records the soundings to a HP IPAQ handheld PC via BlueTooth communication. A maximum transmitter current input of up to 20 Amp is possible using a conventional half sine waveform.
One of the areas where the balloon was employed was the N’teisha gold occurrence in Yemen where the gold is situated in narrow shear zones, between 0.5 and 2m wide. Although the shear zones are possibly too narrow to be detected by the TDEM system an interesting conductivity anomaly was detected in the vicinity of the shear zones that still has to be further investigated.
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Magnetic Full Tensor Gradiometry
Authors R. Stolz, M. Schulz, V. Zakosarenko, N. Bondarenko, A. Chwala and L. Fritzsch and H.-G. MeyerAirborne geophysics is a commonly used tool for the geophysical prospecting of large areas with high spatial resolution. Especially the mapping of anomalies of the Earth’s magnetic field is well suited for airborne operation. Up to now the measurement of the total field and it’s gradients with caesium vapour magnetometers is the most widely used method in airborne magnetic surveys. But there are advantages of using the tensor gradient instead of the total field gradients, which will be shortly discussed.
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