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Near Surface Geoscience 2013 - 19th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 09 Sep 2013 - 11 Sep 2013
- Location: Bochum, Germany
- ISBN: 978-90-73834-38-5
- Published: 09 September 2013
21 - 40 of 116 results
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Very Low Frequency Method Combined with an Unmanned Aerial System
Authors R. Eröss, J.B. Stoll, B. Tezkan and R. BergersSo called Unmanned Airborne Systems (UAS) can be used beneficial for geophysical measuring campaigns. Their advantages reach from faster data acquisition compared to ground measurements to higher versatility (e.g. ground vicinity) to conventional aerial systems. The geophysical Very Low Frequency (VLF) method exploits this advantages especially good. This is because it is well suitable for mapping and therefore extremely powerful in combination with aerial systems. The system presented here is a combination of the unmanned helicopter Scout B1-100 from Aeroscout and the Super High Frequency induction coil Triple (SHFT) and Analog Digital Unit (ADU)-07 data logger from Metronix. The sensor and logger went trough a weight reduction process and had to be deployed on a special suspension developed by Aeroscout. With a payload of 18 kg and an operation time of one hour the system is ideal for mid sized measuring campaigns, i.e. several hundred meters. A proof of concept study was carried out in March 2012 on a test area close to Neuchâtel. Two man-made anomalies, a pipeline and a power cable were investigated to test the practicability of the system. The feasibility of airborne VLF measurements with an UAS to detect buried anomalies is demonstrated by first results.
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A Practical Field Experiment of Multiple Transmission Resistivity Profiling Using Code Division Multiple Access
Authors Y. Yamashita, F. Lebert, J.C. Gourry and B. BourgeoisWe have conducted field experiments for evaluating practicality of a multiple transmission DC resistivity profiling using Code Division Multiple Access (CDMA) technique. In a multi-transmission resistivity profiling, it is possible to inject current at multiple points simultaneously by using coded current waveforms among which covariance between mutual waveforms equal to zero. Multi-transmission resistivity profiling technique has the potential to enable very rapid measurement compared to mono-transmission conventional resistivity profiling. In this paper, we have conducted experiments for evaluating workability and noise-robustness. The results of field experiments show that the efficiency of a multi-transmission resistivity measurement is quite high regarding to current duration time. We have found that erratic low-frequency noise badly affects the measurement result but it was possible to improve this result by post-processing the measured potential waveform.
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Influence of Electrode Heights on 4-point and 2-point Measurements Based on Capacitively Coupled Electrodes
Authors A. Przyklenk and A. HördtTo survey resistive grounds and estimate electrical parameters, the capacitive resistivity imaging technique (CRI) can be used. It overcomes coupling problems that are known from DC techniques. Instead of metal stakes, thin plates are used to inject AC current into the subsurface. In field measurements capacitively coupled electrodes often do not have direct contact with the ground, because of vegetation or a rough surface. We are focusing on electrode height variations and its effect on 4-point and 2-point impedances theoretically and experimentally. To keep controlled conditions, we measured in the laboratory in a box (1.2m x 1.2m x 0.8m) filled with dry sand and gravel. We increased electrode heights from 1µm to 0.8m and repeated the measurements to verify the reproducibility. Results show, that the range of electrode heights can be separated into sections that are characterised by different properties with respect to sensitivity to height variations and electrical parameters of the subsurface. It is also beneficial to measure 2-point impedances. They can be used to assess 4-point impedance measurements. For investigations on unknown soils, it is recommended to use a wide range of electrode heights. This allows a reduction of non-uniqueness with respect to electrical parameters and electrode heights.
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Data Quality Quantification for Time Domain IP Data Acquired along a Planned Tunnel near Oslo, Norway
Authors T. Dahlin, E. Dalsegg and T. SandströmTests were done measuring resistivity and time domain induced polarisation using standard multi-core cable spreads and a special layout with separate cable spreads for transmitting current and measuring potentials. For both types of cables spreads both normal and reciprocal measurements were done in order to estimate the measurement errors. The tests were done along a planned tunnel stretch outside Olso in Norway. The electrode contact was variable with resistances in the range 0.6 - 25 kΩ. The results gave low median error levels for both types of cable spreads, but the single cable spread showed a significantly larger variation with more scatter in the IP data. Data for both types of spreads gave models that are consistent and appear to delineate the complex geology in a useful way. It is concluded that the single cable spread gives surprisingly good IP data considering the large layouts at this site, which is adequate for inversion of the integrated full decay. If on the other hand the data were to be used for spectral IP inversion of the decay curves for recovering the Cole-Cole parameters the extra effort of measuring with separated cable spreads would probably be well motivated.
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Management of Sinkhole Risks Using Long Term ERT Monitoring - A Laboratory Experiment
Authors J. Deceuster, J.E. Chambers, P. Goderniaux, O. Kuras, P.B. Wilkinson and O. KaufmannAccurate methodologies are required to manage risks linked to land-use planning in covered kart terrains, especially in densely urbanized areas. The main risk lies in the occurrence of sinkholes at the base of buildings or infrastructure. We conducted a laboratory experiment to evaluate the contribution of ERT monitoring in the long term management of such karstic risks. After presenting the design of the laboratory experiment, we detail the selected scenarios and the acquisition protocols tested. The methodology proposed to process the data and manage the inversion results relies on two steps: (1) we estimate the resistivity variations due to measurement and inversion errors based on Monte-Carlo simulations and (2) we define a resistivity changes index for every cell of the 3D model. The methodology is tested on a 3D surface survey including inline dipole-dipole, equatorial dipole and Wenner-Schlumberger arrays. A 10 cm in diameter plastic ball is used to model a 3.5 m sinkhole at depths ranging from the surface to 20 cm, using a water resistivity of 12 Ohm.m. Based on the proposed methodology, we show that this target can be suitably detected when its top reaches 10 cm or less in depth.
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Electromagnetic Monitoring in the Zone of Seismic Activisation
Authors A. Sanchaa and N. NevedrovaElectromagnetic monitoring is carried out in the epicentral zone of catastrophic Chuya earthquake (Mountain Altay, Russia) with the magnitude of 7.3. On the basis of transient electromagnetic method (TEM)soundings with central loops field data analysis the evolution of geological massive tension is investigated. Regular TEM measurements are carried out in several regime points that placed in the west part of the Mountain Altay Chuyskaya basin (fig.1). Geoelectrical parameters of rocks in that points carried out in low seismic activity period were estimated according to the 80th years archive data. Comparison of quantitative interpretation of up-to-date (2004-2011) and archive measurements results allow to trace the process of restoration of geoelectrical parameters of rock massive in aftershock period.
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Slidequake Generation at Softrock-landslides
Authors M. Walter, J. Gomberg, W. Schulz, P. Bodin and M. JoswigOur study describes the conditions for the generation of initial fracture processes, i.e. slidequakes, at three different creeping softrock-landslides: the Slumgullion landslide in the San Juan Mountains, Colorado, USA, the Heumoes slope in the Austrian Alps, and the mudslide in Super-Sauze, French Alps. Our investigations identified seismic and aseismic slip in each slide, with slidequakes focusing at significant bedrock structures or at lateral boundaries. At the Slumgullion landslide, the majority of slidequakes occurred at the lateral boundaries of the landslide, while no events were detected along the planar basal surface. At Heumoes slope, the slidequakes cluster in the slope area with the lowest surface displacement rates. A significant bedrock rise, oriented perpendicular to the direction of slope movement, divides the landslide geometry in two basins, and probably impedes motion, slows the slide, and leads to slidequakes. At the Super-Sauze mudslide, the slidequakes are preferentially generated in its centre where the deformation rates are highest. There, the slidequake generation is directly linked to in-situ bedrock crests that border several gullies oriented in line to direction of the entire slope movement. Synoptic comparison of these three scenarios underlines the importance of landslide-bedrock and landslide-lateral boundary interactions under gravitational loading.
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Near-surface Fracture Discovery from Landslides by Sonogram Screening
More LessFracture signals from near-surface processes like creeping landslides and pending rockfalls carry important information on formation stability and promise to enhance early warning approaches significantly. In principal, these fracture signals behave like any local earthquake signal. However, two aspects differ so much that for long time even the existence of these signals was unknown: (I) low energy of local magnitudes Ml = –2 and below, unknown source region and thus sub-optimal network layout with source-receiver distances of several hundred to thousand meter demand signal detection with SNR near one, (II) short signal duration, and a priori unknown signatures result in frequent confusion of fracture signals with noise bursts. In any field installation, noise forensics are crucial to differentiate between signals-of-interest and local noise. Classical detector approaches like STA/LTA are not suited to perform these tasks with sufficient rate of success due to very high false positive rates. Instead, we developed a scheme of visual inspection by seismologists, based on an optimum spectral display of detectable energy spots by combined sonograms of seismic mini-arrays. Applications from landslide monitoring in Austria and France and rockslide precursory detection in Austria demonstrate the appropriateness of our approach.
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Quick-clay Landslides in Sweden - Insights from Shear-wave Reflection Seismics and Geotechnical Integration
Authors C.M. Krawczyk, U. Polom, A. Malehmir and M. BastaniAs part of a joint project studying clay-related landslides in Nordic countries, we successfully tested the use of high-resolution shear-wave reflection seismics to survey shallow structures that are known to be related to quick-clay landslide processes. Three 2-D reflection seismic profiles were recorded in Frastad, southern Sweden, above the main slide area, using a 120 m long streamer of 120 SH-geophones at 1 m spacing, and the ELVIS micro-vibrator as source (10 s long sweep ranging from 20 to 160 Hz). The longest profile along a gravel road shows clear internal structuring of the up to 50 m thick marine sediments as well as strong undulations of top basement underneath. Different sedimentary sequences can be distinguished, and the quick-clay sequence is interpreted in 15-20 m depth, which correlates well with the height of the most recent scarp. The shear-wave velocities suggest extremely low values of 100-120 m/s, which geotechnically prohibits building areas. In addition, test measurements on a stubble field parallel to the gravel road showed the first time that the suppression of Love waves is not only restricted to paved surfaces. In addition, the gap between structural data from ground-penetrating radar and P-wave seismics can be closed.
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Estimation of Electrode Movements from Sparsely Distributed Reference Points for Time-lapse ERT
Authors S.S. Uhlemann, P.B. Wilkinson, J.E. Chambers, P.I. Meldrum, O. Kuras and L. OxbyThe improving capabilities of ground imaging techniques results in an increased application of geophysical techniques to the characterization of landslides. Time-lapse electrical resistivity tomography (ERT) can provide valuable data (i.e. changes in moisture content) to understand landslide processes in more detail. However, it relies on the exact knowledge of the electrode positions, which on a landslide will change over time. We present three different techniques to estimate electrode positions over time from a given set of reference points. The velocity approach is derived from a linear interpolation of directional movement of the reference points to the initial electrode positions. Another method, the general approach, assumes that the relative position of a point to two vectors remains constant over time. A third approach uses kriging to interpolate a ground movement to the initial electrode positions. The three different methods are applied to a synthetic example and real data, and their performance compared. The general approach is shown to be the most accurate one.
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Ground Truthing and Integrated Geophysical Surveying for the Safety Assessment of Dike Systems
By T. InazakiAn integrated geophysical technique we proposed has begun to be utilized as the vulnerability assessment of dike systems in Japan. Characteristic features of the technique are in combination of seismic and electrical methods, in multi-stage application of geophysical surveying, and in joint inversion of geophysical survey results with reference to geotechnical data. The proposed technique mainly consists of high-resolution surface wave method using Land Streamer, capacitively-coupled resistivity method using OhmMapper, multi-frequency Slingram electromagnetic survey method. The geophysical properties evaluated by these methods, S-wave velocity and resistivity, are used to evaluate permeability, stiffness and seismic resistance. We also demonstrate the usefulness of crossplot analysis as a simplified joint inversion based on the empirical relationship between geophysical and geotechnical data. Ground truthing is one of the most efficient ways to obtain the relationship mentioned above. Then we conducted the integrated geophysical surveying and several field measurements at a site where dike was to be partly excavated, and correlated the survey and measurement results obtained before and after the excavation. As a result, we could successfully identify anomaly structures in and beneath the dike systems at the excavation site. Furthermore, the surface geophysical survey results were concordant with the ground truthing data.
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Shear Wave Velocity Structure for Seismic Microzonation of Lorca town (SE Spain) from MASW Analysis
The average shear-wave velocity to a depth of 30 m (Vs30) has been obtained in the Lorca town (SE Spain). Measurements of Vs30 were made using the Multichannel Analysis Surface Waves (MASW) method. Those Vs30 values let us classify the urban area of Lorca town according to both the EC8 and the NEHRP site class. Then, these zones were related to the shallow geology of the town. Regarding the damage distribution in Lorca town due May 11th, 2011 Lorca earthquake, the most affected buildings were located on zones classified by the EC8 as B2 (from 360 m/s to 500 m/s) or C (from 180 m/s to 360 m/s). The geology in those zones is predominantly composed of shallow layers of Holocene unconsolidated sedimentary filling (including colluvial, aluvials and anthropogenic fillings) and layers of Pleistocene consolidated sedimentary fillings of glacis. Areas of the town composed of Tertiary (Neogene) medium hard rock were found Vs30 values above 500 m/s (zone B1) and where the building damages were not relevant.
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Wind Farm Site Characterisation using MASW Survey
By A. RichardThis study presents a non-intrusive geophysical investigation using Seismic Refraction and the Multi-channel Analysis of surface waves (MASW) techniques at the consented 10- wind turbine generator (WTG) locations at Tullo Wind Farm, in Aberdeenshire, Scotland. The aim was to provide geotechnical parameters including the ground stiffness profile at each turbine location. The results are intended to inform ongoing Site Investigation (SI) activity and inform detail design of turbine foundation across the site. At some turbine locations, the attenuation of relatively high frequency components of the transmitted seismic energy was recorded. This is most likely as a result of boggy nature of the near surface conditions. It is highlighted that there is a reduced confidence of the estimated P-wave velocity values for these cases. The S-wave velocity estimates including Vs30 were derived from MASW analyses. Data from most locations exhibited good dispersive characteristics that provided clear, laterally consistent phase-velocity frequency plots enabling 2D visualisation. However, a lack of dispersive behaviour at STI1, ST3 & TET3 locations limited the lateral extent of S-wave velocity determination. The deformation properties such as Elastic moduli were calculated based upon assumed densities of 1.8Mg/m3 for each geological units across the site.
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How to Explore Deep Geothermal Reservoirs in Populated Areas
Authors S. Bissmann, D.J. Orlowsky and B. LoskeWithin the framework of the high financial investment for deep geothermal projects it is especially important to include geoscientific work as possible. Consequently, comprehensive geological and geophysical investigations are needed to predict the reservoir development and its overburden. The application of 3D seismic data to predict reservoir conditions has become the standard approach having replaced initial surveying by way of 2D seismic line data in conjunction with reprocessed older survey data. Progress has been made in enhancing the quality of seismic imaging by use of state-of-the-art processing such as pre-stack depth migration and in deriving relevant factors (faulting/fracturing, facies, diagenesis, karstification) for predicting reservoir properties from this data. The ensuing results are of great help to significantly reduce exploration risks. Furthermore, the results allow planning a target orientated well path.
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Geothermal System Interpretation Based on 1D Magnetoteluric Model at Akesahu, North Maluku
Authors S Darmawan and G.M.L. JunursyahA region of east Indonesia with a big geothermal energy potential is Akesahu in the province of North Maluku. Akesahu is located on the island of Tidore. An integration investigation included geological, geochemical, and geophysical survey has been completed to determine of subsurface mapping in this area. There are seven hot springs are found in this area as a surface manifestation of geothermal system. Furthermore, Hg and CO2 mapping has been supporting an investigation due to of ability to indicate a heat fluid leak and structure zone mapping. The presences of these hot spring and also the other supporting data from geological and previously geophysical data could be help on magnetotelluric data interpretation. Based on these previous survey data, data compilation are consistent shown a reservoir prospect in Akesahu area is in the northeastern of Tidore Island. At least, there are three layers shown in LINE-01 and LINE-02 MT section. In addition, the presences of other structures or weak zones that appear on MT seem consistent with the interpretation based on the other data sets and it’s proved by 1-D magnetotelluric data which show the top reservoir is 900 m.
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GPR Monitoring on an Organic Target Decomposition
Authors E. R. Almeida and J.L. PorsaniIn this work the authors present GPR results of the monitoring of an organic target decomposition. The experiment consists in use a buried pig (Sus scrofa) as an organic target, which simulates a human body. The pig was buried at the University of São Paulo campus located in Pirassununga city (São Paulo State) in December, 2012 and has been surveyed periodically with the GPR method. The objective is to evaluate the effects of its decomposition on the GPR reflection data. The results show that the anomaly caused by the body as better viewed in the 270MHz radargrams, while depth slices made with 900MHz data were better to identify the trench borders. The research is still in progess, and the results are important for understand the use of GPR method in searching for human remains.
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ERT for Localizing 17th Century Tunnels at a Jesuit Mission in Buenos Aires, Argentina
Authors A. Osella, V. Grunhut, H.P. Martinelli, M. de la Vega and N. BonomoWe performed a geophysical study in a historical site of Buenos Aires, corresponding to a Jesuit Mission established during the 17th and 18th centuries. It consisted of a Church, the Cloisters, a School and a Procurator Office; also several tunnels were built, connecting the Mission with different public buildings of the town. In 19th century the Faculty of Sciences was constructed over one of the site, functioning until its demolition in 1973. At present, this space is a cobbled square. With the aim of preserving and restoring the buried structures, we worked in this square, looking for tunnels and also for remains of the basement of the old building. Taking into account both the conductive features of the soil, mainly formed by clays and silt, and the urban characteristics of the environment, we performed pre-feasibility studies to determine the penetration-depth of different methods. We obtained reliable data surveying with dipole-dipole profiles with apertures of 1.5 and 3 m, provided that the electrodes were buried at least 15 cm among the cobble stones. By correlating the resulting nine ERTs with the original plans, old structures and evidences of remains of tunnels were identified.
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Study of Industrial Disturbances Affecting Magnetic Prospecting Data in Archaeological Sites
By S.E. ItkisThe study of industrial disturbances was focused on “static” sources made of iron and reinforced concrete (e.g. water pipes, stakes), and “active” or electromagnetic-field producing elements (e.g. electrical power lines, generators and geophysical equipment). The measurements and forward modeling provided the patterns and main parameters (amplitudes, gradient and blind-zone sizes) for magnetic anomalies of static disturbances. Forward modeling was also applied to an analysis of the demagnetization effect of highly magnetic objects and the threshold, the latter being the rate at which the anomaly amplitude increases when that of the values begins to slow down. The disturbig effect introduced by a non-stable electromagnetic field cannot be eliminated by single-sensor magnetometer measurements but can be suppressed by the use of a gradiometer survey.
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Application of Areal Transient Electromagnetic Method Measurements Directed on Polymetal Prospecting
Authors N. Nevedrova, A. Sanchaa and S. BabushkinThe results of transient electromagnetic method soundings areal measurements directed on polymetal prospecting are presented in this paper. Measurements are carried out by transient electromagnetic method (TEM)soundings within Ergozhu ore field in the Irkutsk region, Russia. The way of excitement of electromagnetic field in a large size generating contour was chosen and measurements were executed by small receiver loops which were moved along on system of profiles with a detailed step. As a result of processing of TEM field data geoelectric parameters of rocks and ore objects which are allocated with abnormal values of the specific electric resistance are determined. Areal and spatial distribution of geoelectric characteristics of an ore field is received. Geoelectric sections and maps of the main structural horizons with allocation of ore objects are constructed. Areal survey and chosen sizes of loops provides necessary resolution of the TEM in the conditions of considerable lateral and vertical inhomogeneity of rocks. Thus, the purpose of work consists in identification of spatial placement of perspective ore zones according to the TEM on the basis of three-dimensional visualization of results of observation.
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Shallow Shear Wave Reflection Surveying on Alpine Glaciers – Comparison of Results from Gepatschferner and Colle Gnifetti
Authors U. Polom, A. Diez, C. Hofstede, M. Stocker-Waldhuber and O. EisenIn the summer season 2010, a shallow reflection seismic experiment using the small vibrator source ELVIS was carried out on the overburden firn and ice cover of the Colle Gnifetti, Monte Rosa group, Swiss/Italian Alps. This site is widely used for method testing, since the physical properties of ice are similar to those of polar regions. The unique experiment approved for the first time the shallow high-resolution vibroseis method using P- and S-waves for seismic targets on firn and ice masses at least to nearly 60 m depth. As a consequence of this successful experiment, the method was subsequently applied in April 2012 on the Gepatschferner, a glacier in the Austrian Alps, using S-waves only. In contrast to the commonly planted receivers at Colle Gnifetti, a land streamer modified for snow application was used as receiver system. The source was slightly modified by a ski to support operation on soft snow. The recorded data at Gepatschferner achieved clear reflections from the ice base boundary in nearly 150 m depth and from the glacier bed below.
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