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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
61 - 80 of 108 results
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Coseismic And Aseismic Deformations Associated With Mining-induced Seismic Events Located In Deep Level Mines In South Africa
Authors A. Milev, P. Share, R. Durrheim, M. Naoi, M. Nakatani, Y. Yabe and H. OgasawaraTwo underground sites in a deep level gold mine in South Africa were instrumented by the Council for Scientific and Industrial Research (CSIR) with tilt meters and seismic monitors. One of the sites was also instrumented by JApanese-German Underground Acoustic emission Research in South Africa (JAGUARS) with a small network, approx. 40 m span, of eight Acoustic Emission (AE) sensors. The rate of tilt and the seismic ground motion were analysed in order to understand the coseismic and aseismic deformation of the rocks. A good correspondence between the coseismic and the aseismic deformations was found. The rate of coseismic and aseismic tilt, as well as seismicity recorded by the mine seismic network, are approximately constant until the daily blasting time, which takes place from about 19:30 until shortly before 21:00. During the blasting time and the subsequent seismic events, the coseismic and aseismic tilt shows a rapid increase. Much of the aseismic deformation, however, occurs independently of the seismic events and blasting and was described as ‘slow’ or aseismic events. During the monitoring period a seismic event with MW 2.1 occurred in the vicinity of the instrumented site. This event was recorded by both the CSIR integrated monitoring system and JAGUARS acoustic emission network. More than 21,000 AE aftershocks were located in the first 150 hours after the main event. Using the distribution of the AE events the position of the rupture area was successfully delineated. The tilt changes associated with this event showed a well pronounced after-tilt. The distribution of the AE events following the main shock was related to after-tilt in order to quantify post-slip behaviour of the source. No evidence was found for coseismic expansion of the source after the main slip. Therefore the hypothesis of the post-seismic creep type behaviour of the source was proposed to explain the large amount of tilt following the main shock. Previous studies using numerical modelling and analytical tools show that for this specific event and rupture area, the amount of measured tilt is highly sensitive to seismic activity in specific regions, in particular, to the bottom corner of the source area. Thus, it is concluded that the post-seismic creep behaviour, if present, most probably occurred in the bottom corner of the rupture area, a region also characterised by large amounts of aftershock activity.
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Azimuthally Anisotropic Avo And Sand-shale Content
Authors L.J. Gavin and D. LumleyAnomalous horizontal stress conditions within reservoir rock can result in azimuthally anisotropic elastic properties, the effects of which can be observed in 3D seismic data. The amount of stress-induced anisotropy can vary depending on the sedimentary rock type, and on the subsurface stress conditions. In this study, we model azimuthal AVO (AzAVO) and compare it to observed AzAVO in seismic data from the Stybarrow Field, North-West Shelf, Australia. This forward modelling approach allows us to better understand the spatial variation in observed anisotropy over the extent of the survey area. Finally, we investigate the relationship between azimuthal anisotropy (AA) and sand-shale content, and use the relationship to explain variations in AA observed in the seismic data.
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Integrated Earth Model Building With Seismic And Em Methods: Improved Subsurface Imaging For Exploration And Development Decisions
Authors M. Mantovani, M. Virgilio, E. Tartaras, A. Lovatini, T. Dean and G. KramerAs exploration for hydrocarbon resources moves to increasingly complex geological environments, the integration of multiple physical measurements is needed to build better Earth models. Seismic has enjoyed a long history as the geophysical workhorse in the oil & gas industry for imaging the subsurface and guiding exploration and development decisions. While this still holds true, it has now become clear that multiple physical measurements are needed to build improved earth models, particularly as exploration for hydrocarbon resources moves to increasingly complex geological environments. Examples of challenges are ultra-deep water exploration where drilling risks are high include sub-salt and sub-thrust exploration where seismic imaging faces significant problems.
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Integrated Multivariate Lateral Predictions For Reservoir Properties Via Geostatistical Modeling
Authors O.J. Rotimi, B.D. Ako and Zhenli WangHydrocarbon production is a function of various elements such as reservoir quality, petrophysical properties, geophysical properties and also volume. Lateral extent of reservoir body is important information for estimation of hydrocarbon initially in place and the recoverable quantity. A method of integrated lateral prediction towards understanding reservoir geometry and rock properties towards enhanced hydrocarbon production is explored with dataset from Bohai bay, Northern China. The methods of conventional well log analysis, seismic interpretation and geostatistical modeling has been employed for this study. Computation of useful logs like Volume of shale, porosity, permeability, Acoustic impedance and Elastic impedance was primarily done on well logs followed by the identification of potential reservoir formation. Lateral extent of reservoir horizons was traced on seismic reflection data with precision through continuity and dominance. Seismic attributes were also computed to assist in better understanding of the characteristics of the rock units in structure and stratigraphy. Computation of variograms and data transformation was done prior to geostatistical simulations for all rock properties considered. The result shows an all encompassing realization on a static model of lateral rock properties within the zone of interest thereby making it easy for making decisions locating parts of the field with high values in rock and fluid properties as potential portions towards hydrocarbon production.
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Integrated Interpretation Of Potential Field And Seismic Data In The Karoo Basin, South Africa
Authors S.E. Scheiber-enslin, S.J. Webb, J. Ebbing and D.G. EberleRecent interest in the Karoo basin of South Africa has been sparked by the possibility of extensive shale gas reserves. Here we present an integrated study of historical reflection and refraction seismic (Soekor and academic), borehole, MT and regional gravity and magnetic data, with the goal of evaluating the shale gas potential. The reflection seismic data, constrained by 5 deep boreholes and MT data are used to map the contact between the base of the Karoo (Dwyka Group) and the underlying basement. This provides an immediate constraint on the thickness of the Karoo (~5.5 km). By constraining the gravity data with the structure known from the reflection seismic and density data from boreholes, this contact can be extended to regions where the seismic data are sparse and suggests that the basin in the region near 24° longitude is thinner than on either side. The flexure of the basin also suggests that at the time of deposition, the Kaapvaal Craton was stiff, preventing a deep basin from developing further northwards on craton. Additionally the magnetic and borehole data highlight pervasive dolerite sills and dykes. These sills and dykes can be highly fractured at surface, but at depth are possibly impermeable, providing seals and traps. They are also an important source of heat and need to be accounted for when evaluating the gas potential. The borehole data indicate sills can occur at any depth throughout the basin. The near surface sills are easily mapped with the coarse national magnetic data and single flight line modelling suggests that the edges of deeper sills can be delineated with higher resolution magnetic data. This approach indicates the importance of integrated studies to evaluating the shale gas potential in the Karoo.
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Seismic Monitoring In Namaqualand/bushmanland Region
Authors H. Malephane, R.J. Durrheim and M.A.G. AndreoliThe Namaqualand-Bushmanland region has numerous features that make it attractive for the storage of radioactive waste. In the late 1970s a programme to find a suitable site for low- and intermediate-level waste was launched and Vaalputs was identified. Such site has been in operation at Vaalputs, ca. 100 km S of Springbok since 1986, and seismicity is one of several key factors that are monitored as part of the on-going disposal operations. We evaluate the region’s seismic history with the aim of assessing its long-term stability and geodynamic setting. The two-station network of short-period seismometers that was installed in 1989 was replaced with a three-station network in 2012, which comprises one broadband and two short period seismometers. Data from these networks, the South African National Seismological Network, and the International Seismological Centre has been used to compile a catalogue of the general seismicity of the region. A previously known cluster of earthquake foci, with MMax = 5.8, termed the “Grootvloer cluster”, is found to consist of three distinct seismic source zones, namely: (1) the Springbok area source, which is attributed to mining activities that ended around 2000; (2) the Great Escarpment area source, which could be attributed to continental margin instability, and (3) the Bushmanland Plateau area source, which is not easily understood. However, we believe that such sub-cluster reflects the strain in the upper and middle crust in response to the transpressional force of a robust, horizontally–oriented 1, a vector known in the literature as the Wegener Stress Anomaly. The overall cumulative trend of the number of events displays a significant increase in the rate, from 13 events/year for the period 1989 to 1995 to 25 events/year for the period 2005 to 2009. These changes seem to coincide with periods of increased global seismic moment release.
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Large Scale Mapping Of Groundwater Resources In India With Results From Test Sites In Different Geological Terrain
Authors E. Auken, S. Chandra, G. Vignoli, A. Shakeel, M.K. Sen and S. GuptaAs the most part of the world, India is also getting more and more dependency on groundwater resources and hence mapping and cautious management become imperative. Ministry of Water Resources of the Government of India has launched a flagship project to tackle this problem. AQUIM stands for pilot project on aquifer mapping and it is a great challenge, not merely because includes the acquisition, in less than one year, of 13,800 line-km of helicopter-borne transient electromagnetic (HTEM) data over six different areas across India (Figure 1), but also because its ultimate aim is to develop a cost-effective tool to locate, characterize, and, eventually, manage water resources. An accurate picture of groundwater through aquifer mapping allows water management plans at the appropriate scale to be effective for the protection of this invaluable resource, while it is becoming physically and economically scarcer and scarcer. This is a necessary prerequisite to achieve drinking water security, improve irrigation strategies and develop sustainability, especially in a country, like India, characterized by an impetuous economic and population growth. Depending on the outcomes, AQUIM is planned to be scaled across the entire country within the next decade. The original scientific plan of AQUIM starts with compiling of the existing database in the area and conceptualize the hydrogeological set up followed by SkyTEM survey, ground truthing and validation of the derived model by ground based data, joint inversion and data integration, deriving 2/3D hydrogeological model at 1:50,000 scale and demarcating the aquifer disposition precisely usable to village level up to 200 m in hard rock and 300 m depth in alluvial. CGWB will use the above information to carry out aquifer modeling for effective management and development of groundwater resources. CSIR - National Geophysical Research Institute and the HydroGeophysics Group (HGG) at Aarhus University signed an agreement concerning the acquisition, processing and inversion of the HTEM data from the six areas of the AQUIM project.
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Seismic Imaging To Help Understand And Manage Water Quality In Benin, West Africa
Authors K. Lindsay, J. Bradford, S. Silliman, N. Yalo and M. BoukariThe coastal city of Cotonou in Benin, West Africa relies on the Godomey aquifer for fresh drinking water. Increased pumping of the aquifer to accommodate rapid population growth has led to saltwater intrusion into the aquifer. Previous work shows a groundwater divide between the aquifer and the Atlantic coast suggesting the source of salinity is nearby Lake Nokouѐ, a large shallow lake with high salinity levels during the dry season. We use shallow seismic reflection imaging to characterize the lithology of the Godomey aquifer with the goal of using this information to improve regional water management strategies. Seismic reflection profiles from the Godomey well field show the subsurface lithology to be a complex system of discontinuous and topographically variable strata. Initial results from the first of two field seasons provide critical information about lithology that will improve the current hydrologic model, influence water management strategies, and guide future geophysical studies.
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Modelling Of Ground Electromagnetic And Electrical Resistivity Data For Characterisation Of Shallow Aquifers In The Limpopo Province, South Africa
Authors P.K. Nyabeze, F. Monteiro Santos and O. GwavavaGround electromagnetic and resistivity datasets were modelled using two different inversion methods. The main objective was to deduce accurate depth models that show variations in the subsurface geology. Groundwater bearing structures were investigated using geophysical methods consisting of electromagnetic induction (EI) and electrical resistivity tomography (ERT). An inversion program EM4Soil that models multi frequency electromagnetic data was used to generate 2-Dimensional depth models for shallow thermal spring aquifers. ERT data was modelled using Res2DINV software. The positive correlation of the inversion models showed that the two methods of inversion are complementary. The integrated geophysical survey approach proved to be an important tool for the rapid and cost effective determination of the geometry of shallow aquifers.
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Very Early Times In Aem - Resolution Of The Near-surface For Hydrogeological Applications
Authors C. Schamper, E. Auken, F. Jørgensen, F. Effersø and K. SørensenSince the 2000’s helicopter transient electromagnetic (HTEM) systems have widespread in hydrogeological/environmental investigations. They are usually more limited in terms of near-surface resolution compared to helicopter frequency EM systems (HFEM). A new version of the SkyTEM system, called Mini-SkyTEM or SkyTEM 101, is now capable of measuring at very early times, only few μs after the turn-off of the current in the transmitter loop. The system payload is very light (350 kg) with a transmitter loop of only 130 m² (Figure 1a). The particularity of all SkyTEM systems is the measurement of both a low (LM) and a high (HM) moments (Figure 1b). This dual-moment configuration allows getting information both from near surface and deep layers. The HM gives a depth of investigation of ~100 m for an average resistivity of 50 Ωm, but the more interesting feature of the system is its earliest time in the LM which is only 2-3 μs from end of ramp (~ 3μs). The very early times or the high frequency content can be interpreted only if two key points are handled. The first one is the calibration of the EM system. The AEM system has been calibrated following the procedure described by Foged et al. (2013) where a time and amplitude factor shifts are estimated above a well-known reference site to match the measurements and the modeled reference response. The accuracy of this calibration is critical in the present case, since a bad time shift estimation of less than 1 μs can cause non-negligible differences in the top 30 m. The second point to consider is the primary field residual, or coil response (CR), which affects the first gates right after the turn-off of the current. This CR is induced by small residual current in the transmitter wire and needs to be modeled especially when ground resistivity and/or flight altitude increase, reducing the strength of the ground EM response. This CR is measured at high altitude. Since measurements have confirmed its stable shape, only its amplitude, which varies due to small bending of the frame during the acquisition, has to be determined during the inversion as an amplitude correction factor. With the consideration of the CR, it is possible to interpret the very early times right after the turn-off of current (Schamper et al., 2012).
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Vaal Gamagara Water Scheme: Using Airborne Tdem, Magnetic And Ground Gravity To Augment The Supply
Authors E.H. Stettler, F.E. Wiegmans and R.C. WhiteheadThe Vaal Gamagara water supply scheme is a R300 Million intervention by the South African Department of Water Affairs and Forestry (DWAF) to interconnect the Vaal river water supply with the towns of Postmasburg, Kuruman, Hotazel , the surrounding mines as well as the cattle and game farms. Considerable savings in the project costs were achieved by prospecting and developing groundwater wells that deliver some 4 million litres per day into the scheme. Locating these zones permitted DWAF to extract a safe, secure water supply, and was accomplished from a combination of airborne magnetic and TDEM and ground gravity surveys combined with a fundamental understanding of the geology.
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The Optimisation Of Non-linear Geophysical Inverse Problems
More LessOver-determined inverse problems possess a substantial amount of information which is averaged together by the least-squares inverse method to produce an average solution. If the best solution is used rather than the average solution then not only does the method converge more rapidly than the standard method, but it has a larger basin of attraction and is computationally far more efficient. The method is demonstrated using synthetic gravity models.
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3d Magnetic Modelling And Inversion Incorporating Self-demagnetisation And Interactions
Authors P.K. Fullagar and G.A. PearsSelf-demagnetisation can significantly reduce the amplitude and alter the orientation of magnetisation in highly magnetic bodies, thereby modifying the observed magnetic field, sometimes substantially. In tabular bodies, for example, the transverse magnetisation is reduced, with the result that the magnetisation vector rotates towards the plane of the body. Furthermore, when highly magnetic bodies are in close proximity, the assumption of uniform inducing field is violated. Highly magnetic bodies can modify the local magnetic field appreciably, with the result that the magnetisation induced in one susceptible body is affected by the magnetisations of all its neighbours. It is important to take such interactions between highly magnetic bodies into account. Potential field modelling and inversion software “VPmg” has been upgraded to account for self demagnetisation and interaction between magnetic bodies. The algorithm computes H-field perturbations at the model cell centres in two stages: initialisation and optimisation. During initialisation, a demagnetisation tensor is estimated for each cell, from which a first estimate for the H-field perturbation is derived. During optimisation, the H-field field estimate is refined iteratively via an inversion procedure. Remanence is taken into account as well as induced magnetisation. The algorithm has been validated for homogeneous spheres, spheroids, slabs, and cylinders. It has also reproduced magnetic interactions between two horizontal cylinders, previously published by Hjelt. Explicit verification for complex heterogeneous bodies requires a suitable independent algorithm for benchmarking. The application to inversion in a highly magnetic environment is illustrated in a field data example.
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Improving The Subsurface Image With Simultaneous Joint Inversion Of Land Seismic And Airborne Gravity Gradiometry Data
Authors G. Walker, M. Mantovani, e. Medina, L. De Luca and A. LovatiniThe quest for subsurface resources (hydrocarbons, minerals or geothermal) requires explorers to clarify as much as possible the geological model of the area under investigation and its potential targets and to test these hypotheses through drilling. An image of the subsurface is critical in order to achieve both tasks, to reduce uncertainty the use of multiple geophysical methods is a well-proven strategy. We show how the quantitative integration of land seismic and airborne gravity gradiometry (AGG) data, using the 3D simultaneous joint inversion (SJI) algorithm described by De Stefano et al. (2011), within an imaging workflow, can provide not only a good-quality image but also multiple physical properties to reduce the uncertainty and guide the interpretation.
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Inverting For Reservoir Change: 4d Image-domain Tomography
By J. ShraggeAdjoint-state methods (ASMs) have proven successful for calculating the gradients of the functionals commonly found in geophysical inverse problems. The 3D ASM image-domain tomography (IDT) formulation of the seismic velocity estimation problem highlights imperfections in migrated image volumes and, using appropriate penalty functions (e.g., differential semblance), forms an objective function that can be minimized using standard optimization approaches. For time-lapse (4D) seismic scenarios, the 3D ASM-IDT approach can be extended to multiple datasets and offers high quality estimates of subsurface velocity change. I discuss two different 4D inversion strategies: absolute and relative. The absolute approach uses the difference of independent 3D inversions to estimate a 4D model perturbation. The relative approach inverts for the model perturbation that optimally matches the monitor image to the baseline image - even where migrated energy is imperfectly focused. Both approaches yield useful 4D slowness estimates; however, we assert that the relative approach is more robust given the ubiquitous presence of non-repeatable 4D seismic acquisition noise and imperfect model estimates.
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Experimental Evaluation Of 3d Geoelectrical Resistivity Imaging Using Orthogonal 2d Profiles
Authors A.P. Aizebeokhai, A.I. Olayinka, V.S. Singh and O.A. OyebanjoNumerical evaluation of 3D geoelectrical resistivity imaging was conducted using orthogonal set of 2D pseudo-sections generated over two synthetic models, horst and trough models. The models represent geological environment that simulates a typical weathered profile and refuse dump site in a crystalline basement complex, respectively. Different arrays including Wenner-alpha (WA), Wenner-beta (WB), Wenner-Schlumberger (WSC), dipole-dipole (DDP), pole-dipole (PDP), and pole-pole (PP) arrays were used for the data generation. The 2D apparent resistivity data were collated to 3D data set and then inverted using a full 3D inversion code. The effectiveness of the technique for 3D resistivity imaging as well as the imaging capabilities of the selected arrays is evaluated. The observed anomaly effect and normalized model sensitivities of the arrays indicate that DDP and PDP arrays are more sensitive to the 3D features, while WSC show moderate sensitivity to 3D features. Field example in which the technique was applied in a 3D geoelectrical resistivity imaging for engineering site investigation in the crystalline basement complex of southwestern Nigeria is also presented.
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Integrated Geophysical Tomography Around Buildings Having Foundation Defects At Some Residential Sites In Zaria Area, Northwestern Nigeria
More LessIntegrated geophysical imaging technique has been applied in the detailed investigation of foundation of buildings in order to identify the geologic formations and mechanisms of rocks and soils causative to the identified defects. The study was also aimed at the estimation of the dependent elastic constants of the subsurface formations which is useful to engineers. The study area is part of the Zaria Batholith which comprises series of granites that intruded the country rock gneisses. It falls within the Nigeria Basement Complex which is underlain by Precambrian rocks at the elevation of about 670 m above the mean sea level. It has a Tropical-continental climate with distinct wet and dry seasons. Structurally, the study area is characterized by deformations which are mainly due to NW-SE and NE-SW fracture patterns in the Nigerian basement complex caused by a trans-current movement. The study area precisely is located within 70 35’ 17’’E and 70 41’ 17’’E longitude, and 110 7’ 50’’N and 110 11’ 22’’N latitude on the Nigeria National grid. Three buildings affected by such defects were selected for investigation. Two of them were located at the Staff quarters of Ahmadu Bello University, Zaria, Nigeria and the other, at the students’ hostel of the same University. While the former were solely characterized by severe foundation-based cracks with unknown cause at sight of superstructure, the latter is a case of severe crack due to leaning having a relative rotational angle of about 5.50. Modern geophysical field equipment, namely; 24-Chanel Seismograph (Terraloc Model MK6) and Lund Imaging system (Terrameter Model SAS 4000/1000) with its electrode Selector (Model ES464) having 42 take-outs, were used for the data collection of the seismic refraction tomography and the electrical resistivity imaging respectively. Eight profile lines were laid parallel and close to each of the buildings during the investigation. While the geophone spacing of 5.0 m was used in the seismic refraction method, the take-out spacing for the electrodes ranged between 1.5 m and 5.0 m in the electrical resistivity method. The travel-time data for the p-wave velocity and Electrical Apparent Resistivity data were collected and processed with computer software namely; REFLEX-W version 3.0 and RES2DIV version 3.4 respectively. Both software were based on a high resolution inversion modelling techniques for data processing. Spearman’s formula for rank correlation analysis was applied on the pair of interpreted tomograms. The rank correlation results obtained from the two methods showed that there is positive correlation in the range of (0.3170 – 0.9883 0.0001) between the structural displays of their lithological stratification. The interpreted results from the seismic refraction tomography technique show that the compressibility of the soils within the foundation depths (0-5 0.1 m) ranges from 2.21797 x 10-09 to 1.45565 x 10-10 ( 0.08 x 10-09) Pa-1), while the estimated allowable total foundation bearing pressure ranges from about <4.5 x 108 Pa to < 1.31 x 1011 Pa ( 0.08 x 10-09) Pa). The depth range to the bedrock is 7 – 18 ( 0.1) m, while the overburden and the weathered basement thicknesses have been estimated to be in the range 3 – 26 0.1 m and 7 – 30 0.1 m respectively. The aquiferous zones based on the interpreted results, mostly occur both at the lower depths of the overburden and within the weathered basement. The integrated interpretations show that expansive soils such as clays, predominantly characterize the near-surface of the three sites. The clays usually undergo seasonal swells and shrinkages which have posed danger to the structures. Also based on the results, fractures, synclinal structures of shallow bedrocks, weakness of intact rocks, structural arrangement of the subsurface lithology, soil creep/crawl and to some extent, the growth of tree roots, have partly contributed to ground movements and differential settlements of most of the buildings in the area. These have resulted in the structural defects on the buildings. Conclusively it was observed that while two of the sites (sites 1 and 2) show low tendency of progressive structural failure, the other (site 3), poses a high tendency of a progressive structural failure.
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Geoelectrics Of The Volcano Mutnovsky (kamchatka)
Authors I. Eltsov, S. Bortnikova, G. Panin and S. BortnikovaThe work is based on materials obtained in the course of field work on a series of fundamental research in Kamchatka (Russia). Today, after a disastrous great Japanese earthquake and tsunami (11.03.2011) is very crucial to develop models of formation and evolution of the fluid-magmatic systems in adjacent areas of modern volcanism. The main purpose of this study was to determine the structure and migration routes of high-mineralized solutions in the near-surface area of thermal fields of Mutnovsky volcano. Mutnovsky volcano construction, one of the largest and most active volcanoes in Kamchatka, has a maximum height of 2323 m above sea level. Volcano consists of four contiguous, sequentially formed stratovolcanoes with vertex calderas and subsidiaries intercaldera facilities and all of its complicated array is complicated by numerous cones of adventive eruptions [Selyangin, 1993]. In periods of “quiet” activity of the volcano is implemented through an extensive network of hydrotherm in the active craters and on their slopes, as well as on the numerous hydrothermal sites and individual sources to the north of the volcano. The biggest manifestation of the activities in the area is the North-Mutnovskoe thermal field. The next object was studied in this work is Donnoe fumarole field. Almost all the manifestations of hydrothermal activities at these two sites: the sources, reservoirs and mud pots have been investigated by geochemical methods [Bortnikova et al., 2009]. A large geochemical variety was found and, as a consequence, the favorable conditions of electromagnetic geophysical methods were ascertained. The complex of geoelectric methods was focused primarily on the definition of channels structure of fluid migration. The main features of fluids composition that significantly affect the responses using the methods of resistivity: • variety of hydrochemical parameters in closely spaced areas of thermal unloadings; • abnormally high levels of certain elements (Cr, Ni, Co, Ti, V), indicated the depth origin of hydrotherms; • sudden changes of chemical elements concentrations in solutions on the near-surface barrier. The genesis of thermal solutions, especially in the near-surface of the volcanic structures is a key for understanding of the functioning of the fluid-magmatic system and, consequently, allows approaching to the correct description of the active volcanoes functioning. According to the frequency induction sounding subsurface geoelectric zoning area is detected to a depth of 7 m (Fig. 1). In the above example features of fluid migration are clearly visible.
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Using Geophysics To Find Suitable Rock Aggregates As Replacement For Fluvial Sand In Botswana
Authors L. Persson, O. Ntibinyane, M. Göransson and V. LekulaThis work presents the first results from an ongoing cooperation project between the Department of Geological Survey (DGS) in Botswana and the Geological Survey of Sweden (SGU). The purpose of the project is to find suitable rock to produce aggregate for concrete and roads in Botswana. We show in this paper how different geophysical methods can be used in this process. Airborne data together with the available bedrock information are used to find suitable areas for bedrock sampling and later strength testing. Measurements of the gamma radiation are crucial since rock material with very high natural gamma radiation should not be used as building material in houses. Where airborne gamma radiation data are available it should be used initially to identify certain areas with high gamma radiation. The airborne magnetic data together with ground magnetic measurements were used to define the lateral extent of certain rock types. Measurements of the magnetic susceptibility on outcrops are essential here in order to determine the cause of a magnetic anomaly. Our results from gamma radiation measurements on outcrops have revealed a syenitic rock with very high radiation that should not be used for building purposes. Also the Gaborone granite complex show high gamma radiation while mafic and sedimentary rock show low gamma radiation.
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Different Type Of Tem Signals Along Only One Profile: Benefit Study For Beginners
More LessThe superparamagnetic (SPM) effect caused by laterites, “red soil” in Africa distorts TEM signal. Usually the SPM effect occurred if coaxial loops are used. The instrument TEMFAST48 has been used. Different types of signals distorted my SPM and induced polarization (IP) effects have been recorded along only one profile. Interpretation of TEM data has been carried out using the software TEM-RESEARCHER. This software allows mathematical modelling of the TEM signals distorted by induced polarization (IP) and SPM effects. Eight blocks with different geology have been delineated along one profile with a length of 7.4 km, with each block separated by a fault. The result of interpretation is a geoelectrical profile in very good agreed with geological data collecting from outcrops.
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