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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
41 - 60 of 116 results
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Comparison of High-resolution P- and SH-wave Reflection Seismic Data in Alluvial and Pyroclastic Deposits in Indonesia
Authors C.M. Krawczyk, W. Wiyono and U. PolomHigh-resolution, P- and SH-wave seismic reflection profiles were acquired at two different locations in Indonesia to test the capability of imaging in pyroclastic sediments. The seismic measurements for both locations were carried out using a 48-channel recording system, and sledge hammer and micro-vibrator ELVIS as sources. The results of seismic data in Semarang show fair to good seismic records for both P- and SH-waves. The raw data contain a high signal-to-noise ratio. Many clear reflectors can be detected. The P-wave data show reflectors down to 250 ms two-way time while the SH-wave records show seismic events up to 600 ms two-way time. This result is in strong contrast to the observations in the Tiris region. Here, the P-wave data show a very low signal-to-noise ratio, and there is no reflection signal visible, only the surface waves and the ambient noise from the surrounding area are visible. The SH-waves give a fair to good result which enables reflector detection down to 300 ms two-way time. Thus, the SH-wave reflection seismic seems to be the suitable method, which could be applied in Indonesia in both alluvial and pyroclastic regions.
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Increasing the Depth of Detectability of ERT Measurements
Authors K. Szokoli, S. Szalai and A. NovákThe issue of depth of investigation has always been a basic problem in geophysical studies. Nowadays, as more and more research should be carried out in built-up areas where space is limited, it became even more important. Since we think that the depth of investigation is basically determined by the detectability of small-size inhomogeneities, the so-called depth of detectability (DD) values, this parameter is investigated in this presentation for the 2D ERT. In the case of the studied prism and dyke models the not-yet investigated g11n arrays gave systematically higher DD values than the pole-dipole and dipole axial arrays which were the best conventional arrays from this point of view. The difference can be significant, up-to 2-2.5-fold in comparison even with these arrays. This makes possible to obtain useful information even where the measuring area is limited, eg. in settlements, in less time and from greater depth. Moreover, due to the imaging properties of arrays having larger DD values are generally better it is easily possible that these arrays have also better imaging properties than those of the traditional arrays.
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First Results from the New Multi-purpose Instrument CapGeo
By T. RadicHigh quality geophysical equipment is expensive. Therefore, people repeatedly ask for instruments, which are adapted to measure more then one physical parameters. An example for this is the idea to combine Transient Electromagnetic Method (TEM) with Magnetic Resonance Sounding Method (MRS). The MRS method delivers the water content and the TEM method delivers the resistivity distribution. Unfortunately, this idea is not really practicable. The loop may be useful for both methods but the electronics must be quite different. Here we present and discuss first results measured with new type of equipment called CapGeo. This instrument was developed for the geophysical institute of TU Braunschweig (Prof. Hördt). The main purposes are capacitive resistivity measurements. But the severe requirements make this instrument eminent suitable for other geophysical exploration methods. Till now we have made some first field test measurements for four different methods: - Capacitive Resistivity (CR) - Spectral Induced Polarization (SIP) - Radiomagnetotellurics (RMT) - Very Low Frequency (VLF), Z/H
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3-D SNMR Inversion of Water Content
Authors W. Warsa, H. Grandis, W. Parnadi and D. SantosoSurface Nuclear Magnetic Resonance (SNMR)is the geophysical method, which allows to determine water content and pore size distribution directly from the surface. A 3-D forward modeling code for SNMR amplitudes and decay times has been developed; after that improved 2-D and 3-D inversion algorithm has been investigated consist of model parametrization regularization schemes. Giving a short review of general inversion schemes used in geophysics, the special properties of SNMR or MRS (magnetic resonance sounding) inversion are evaluated. This inversion will be applied for the SNMR field of water content and decay time. At the end, the SNMR can be applied to dectect subsurface water in suitable geological formation and more depending on the presence of natural and cultural electromagnetic noise.
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Optimizing Distribution of SNMR Pulse Moments and Stacking Strategy
Authors E. Dalgaard, M. Müller-Petke and E. AukenIn this study we are analyzing the balance between number of data points and data quality in SNMR survey design. In SNMR the number of data points increase with the number of pulse moments and data quality increase with the number of records at each pulse moment (stack size). The premise for our analysis is that the total amount of records is fixed to 1500 and the stack size and number of pulse moments are dependent on each other. Thus, for a high number of pulse moments the stack size and thereby data quality are low and for a low number of pulse moments the stack size and data quality are high. From our model uncertainty analysis we show that it is better to discretize the data space densely, by having a high number of pulse moments with a lower data quality compared to fewer pulse moments with a higher data quality. Furthermore, we analyze and compare the logarithmic pulse moment distribution scheme with an optimized pulse distribution scheme. We show that the determination of the model parameters at a certain depth can be improved by adding more pulse moments at corresponding interval.
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Surface-NMR in Urban Area - A No Go?
Authors M. Mueller-Petke and S. CostabelSurface-Nuclear-Magnetic-Resonance (surface-NMR) has shown a continuous development towards a frequently used and established hydro-geophysical tool during the last decades. However, the measured signals are as low as few tens of nanovolts and therefore often contaminated by significant amounts of electromagnetic noise. Consequently, surface-NMR has been used on remote area with great success but is also known to be a technique that needs to be chosen with respect to low noise conditions. Several developments both on instrumentation and processing allow overcoming these strong restrictions on applying surface-NMR. Multi-channel devices use remote references to cancel harmonic noise. High sampling rates of the used analog-to-digital converter allow for advanced de-spiking schemes. Finally, the combination of formerly known figure-of-eight loop layouts with remote references provides access to areas close or even within the borders of settlements. We give several examples demonstrating these developments and highlighting the new capabilities of the technique.
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Geophysical Investigations of a Rural Water Point Installation Program in Nampula Province, Mozambique
Authors F. J. Chirindja, E. Sjöstand, O. Enkel and T. DahlinThere are many projects in Mozambique for poverty reduction. One of these projects is funded by Millennium Challenge Account (MCA) and is aimed to install a total of 600 rural water points in the provinces of Nampula and Cabo Delgado. Each water point consists of a drilled well, a water pump and a communal washing basin. Vertical Electrical Sounding (VES) was used for assessing the suitability of the drill sites but despite this many boreholes have come out with an insufficient yield and the failure rates in certain areas are as high as 50 %. Continuous Vertical Electrical Sounding (CVES), also known as Electrical Resistivity Tomography (ERT), was carried out in an attempt to explain the high failure rate. In total 11 sites were investigated, including seven boreholes with sufficient yield and five boreholes with insufficient yield. A perpendicular cross with two 400m survey lines were made over 7 boreholes and single 400m survey lines were made over 5 boreholes. Due to lateral variation the geology in study area is best described in 3D therefore ERT appears to be a suitable method for groundwater exploration and could probably lower the failure rate.
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Spectral Induced Polarization Measurements on New Zealand Sands - Dependence on Fluid Conductivity
More LessSpectral induced polarization (SIP) measurements have been made for different fluid conductivities on a sample of sand typical of the host material in New Zealand coastal aquifers. Fitting the data with a Cole-Cole model shows that neither the mean relaxation time nor the shape exponent depend upon the fluid conductivity. This is consistent with measurements of the hydraulic conductivity made on the same sample which also show no dependence on fluid conductivity. Simple predictions of permeability based on the Kozeny-Carmen relationship are of the correct order of magnitude but tend to overestimate the permeability. These results lay the groundwork for future measurements aimed more directly at drawing correlations between the SIP response and hydraulic properties of New Zealand aquifers.
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Electric and Electromagnetic Measurements on the North Sea Island of Langeoog
Authors S. Costabel, T. Günther, U. Noell, B. Siemon and W. VoßElectric and electromagnetic methods were conducted on the North Sea island of Langeoog to investigate its freshwater lens that is managed by the local water works to supply the island with drinking water. Helicopter-borne electromagnetic (HEM), as well as surface geophysical methods were applied, i.e., electrical resistivity tomography (ERT), transient electromagnetic (TEM), and magnetic resonance sounding (MRS). We found that, beside the depth of the fresh-saltwater interface, the combined interpretation of TEM and MRS allows a reliable identification of lithological layering, whereas ERT or TEM alone exhibit a high degree of ambiguity regarding subsurface lithology and groundwater salinity. Future research is focused on the joint inversion of the surface methods, as well as on the combined inversion of surface and HEM data. In this way, 3-D multi-parameter models will be generated as additional input for numerical modeling of groundwater dynamics inside the freshwater lens.
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Comparison between the Hydraulic Properties of Two Low-permeability Alluvial Systems Supported by Geophysical Methods
Authors A. Guinea, W. Timms and I. AcworthThe mechanisms for vertical flow through clayey alluvial deposits are poorly understood. This type of deposits are usually studied as sets of separated permeable (aquifers) and less permeable (aquitards) strata assuming water flow is negligible over the short to medium term. Nevertheless, in some clayey deposits this simplification proves to be inadequate as there is evidence of vertical groundwater flow. This study was aimed at improving the understanding of hydrological mechanisms for vertical flow, determined by the degree of homogeneity or heterogeneity of the alluvial deposit. In order to study the vertical hydraulic connection through clay-silt dominated deposits, near surface and borehole geophysics have been combined with other hydrological techniques at two different sites with a contrasting degree of heterogeneity in the Liverpool Plains within the Namoi River catchment (NSW, Australia). At the Breeza DPI farm site, situated in an area of groundwater extraction for irrigation studies indicates active deep drainage through heterogeneous clayey strata and hydrochemical changes occurring in the underlying aquifer systems. By contrast, at the Cattle Lane site, located upstream in a dryland farming area with no groundwater extraction, there appears to be little significant vertical flow through a more homogeneous clayey deposit.
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A Heat Injection and Pumping Experiment in a Gravel Aquifer Monitored with Crosshole Electrical Resistivity Tomography
Authors T. Hermans, S. Wildemeersch, P. Jamin, P. Orban, S. Brouyère, A. Dassargues and F. NguyenThermal tracing experiments are becoming common in hydrogeology to estimate parameters governing heat transport processes and to study geothermal reservoirs. Electrical resistivity tomography (ERT) has proven its ability to monitor salt tracer tests, but few studies have investigated its performances in thermal tracing experiments. In this study, we monitor the injection and pumping of heated water using crosshole ERT in a panel crossing the main flow direction. Difference inversion time-lapse images clearly show the heterogeneous pattern of resistivity changes, and thus temperature changes, highlighting the existence of preferential flow paths in the aquifer. Comparison of temperature estimates from ERT and direct measurements in boreholes show the ability of ERT to quantify the temperatures in the aquifer and to draw the breakthrough curves of the thermal tracer with a relative accuracy. Such resistivity data may provide important information to improve hydrogeological models. Our study proves that ERT, especially crosshole ERT, is a reliable tool to follow thermal tracing experiments. It also confirms that ERT should be included to in situ techniques to characterize heat transfer in the subsurface and to monitor geothermal resources exploitation.
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Reliability of ERT-derived Temperature - Insights from Laboratory Measurements
Authors T. Robert, T. Hermans, G. Dumont, F. Nguyen and D.E. RwabuhunguWe performed laboratory measurements on fully saturated sand samples in the context of deriving reliable temperature from time-lapse electrical resistivity tomography (ERT). The experiment consisted in monitoring an increase of temperature in sand samples with electrical resistivity measurements. We neglected the effect of surface conductivity since experiments showed two orders of magnitude between surface and fluid conductivities. We show that using simple linear relationship between fluid electrical conductivity and temperature alone does not allow reliable temperature estimates. Indeed, chemical analyses highlight the importance of accounting chemical reactions occurring when temperature changes, including dissolution/precipitation processes. We performed two experiments based on typical in-situ conditions. We first simulated the injection of a less conductive tap water and second, the injection of heated formation water. In the second case, minerals solubility decreases and precipitation occurs, leading to an increase of bulk resistivity. This mechanism competes with dissolution of minerals when tap water is injected, since tap water is not in equilibrium with the medium. In any case, further research is needed to fully understand the mechanisms and to develop a fully integrated law to derive better temperature estimates.
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3-D Time-lapse Electrical Resistivity Monitoring of Injected CO2 in a Shallow Aquifer
Authors J. Doetsch, E. Auken, A.V. Christiansen and G. FiandacaContamination of potable groundwater by leaking CO2 is a potential risk of carbon sequestration. With the help of a field experiment, we investigate if surface electrical resistivity tomography (ERT) can detect dissolved CO2 in a shallow aquifer. For this purpose, we injected CO2 at a depth of 5 and 10 m and monitored its migration using 320 surface electrodes on a 126 m × 20 m grid. A fully automated acquisition system continuously collected data and uploaded it into an online database. The large amount of data allows for time-series analysis for data quality and noise estimation. A baseline inversion reveals the geology at the site consisting of aeolian sands near the surface and glacial sands below 5 m depth. Time-lapse inversions clearly image the dissolved CO2 plume with decreased electrical resistivity values. We can follow the CO2 plume as it spreads and moves with the groundwater, and the spatial distribution of the plume agrees well with direct measurements of the groundwater resistivity. Future work will include improved time-series analyses and quantitative comparison with the groundwater resistivity measurements.
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Joint Analysis of Lunar Surface Waves - The Apollo 16 Dataset
By G. Dal MoroUnderstanding the near-surface structure of the Moon is important not only for scientific reasons, but for possible future technological applications related to mining, geotechnical activities, oxygen production etc. The Lunar regolith is represented by a layer of unconsolidated debris constituted by fine soil (average density around 1.5-1.8 gr/cm3) occasionally including breccia, rocks and boulders from the bedrock. Its thickness generally seems to range from about 5 m on younger maria to about 20 m on older highlands and appears highly stratified with many buried soil horizons. Although most of the Lunar literature necessarily dates back to the 70's, the opportunities provided by recent technical developments allow the re-processing of Lunar data capable of providing a deeper understanding of the formation and evolution of our satellite. The current study focuses on the re-processing of some data acquired in the framework of both the active and passive experiments of the Apollo 16 mission (April 16-27, 1972). In particular, a joint analysis of Rayleigh-wave dispersion (ASE data) and Horizontal-to-Vertical Spectral Ratio (HVSR) (LSPE data) is performed also considering the effects of attenuation and with the aim of retrieving the local shear-wave velocity profile.
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Unambiguous Determination of the Vs Profile via Joint Analysis of Multi-component Active and Passive Seismic Data
Authors G. Dal Moro and L. KellerThe determination of the shear-wave velocity profile is a major request in site characterisation studies for geotechnical and seismic-hazard studies and it is often accomplished via surface-wave analysis according to Multichannel Analysis of Surface Waves (MASW). In order to overcome non-uniqueness of the solution and possible interpretative issues, for the present study we consider the joint analysis of different and independent multi-component data: Rayleigh and Love waves (from MASW and frequency-wavenumber analysis), Horizontal-to-Vertical Spectral Ratio (HVSR) and p- and s-wave Seismic Refraction Tomography (pSRT and sSRT). Obtained Vs profile is eventually compared with the data obtained from a Vertical Seismic Profiling (VSP).
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Detailed Digital Terrain Models for Supporting the Analysis and Interpretation of Near-surface Geophysical Data
Authors J. Tronicke and U. BönigerToday's near-surface geophysical acquisition approaches readily allow for combining geophysical measurements with high-resolution positioning solutions based on the global-positioning system (GPS) or self-tracking total stations (TTS). Under favourable site conditions, such surveying strategies allow for efficiently acquiring high-quality geophysical data with a positioning accuracy in the centimetre range. In addition to accurately locating geophysical measurements, such positioning data can also be used to generate a detailed digital terrain model (DTM) of the surveyed area. Here, we present two case studies where a detailed DTM supports and complements the interpretation of near-surface geophysical data. While the first case study comprises magnetic and electromagnetic induction data collected to locate buried utilities, the second case study presents a 3D ground-penetrating radar survey recorded to detect archaeologically relevant structures. In both cases, the topographic data resulting from the employed TTS-based surveying strategy are analyzed using different topographic attributes supporting the geophysical data interpretation. We conclude that, from our experience, inclusion of a detailed DTM into the geophysical data processing flow, can provide additional valuable information for data analysis and interpretation.
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Combined Geophysical Survey at the A2 Tunnel Maastricht
Authors O. Brenner and D. OrlowskyIn advance of the construction of the A2 tunnel in Maastricht a geophysical survey was necessary in order to investigate the position of layers, faults, flint layers, occurrence of rock deposits and potential cavities within the limestone layer. For this purpose three different geophysical methods were combined: CMP refraction seismic, refraction tomography, reflection seismic. The whole survey was performed with three different sources to get the best results for each depth: air sound source, sledgehammer and accelerated weight drop source. After processing and during the interpretation process the results of 18 boreholes were included. Finally all relevant layers could have been detected and corresponded to the results of drilling. Furthermore minor and major faulting in the survey area could be detected. Potential cavities in the limestone were not detected.
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Geophysical Investigation for the Detection of Liquefaction Phenomena in an Archaeological Site, Lechaion, Greece
More LessA geophysical survey was designed in order to detect ground liquefaction phenomena in a wrecked Early Christian Basilica in the archaeological site of the ancient harbour of Lechaion, located in the southeastern coast of the Corinth Gulf, Greece. EM, GPR and ERT measurements led to the detection of stratigraphy and resistive zones of coarse material which were possibly used in ancient times for geotechnical purposes and could be related with the liquefaction phenomenon characteristics. ERT sections have detected the liquefaction zones successfully suggesting that the method can be a useful tool for such applications. EM and GPR methods add valuable information regarding lateral and vertical changes in sedimentation.
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The High Resolution Short Length P&S Wave Velocity Mesurement
Authors T. Kobayashi and H. AzumaThe shorter seismic velocity measurement is currently required to obtain more detailed and precise velocity for the assessment of the surface of rock, such as the rock tests sites. The physical properties that are surveyed with rock test, such as Unconfined Compression Test and Shear Test, are important to calculate earthquake-resistant building designs. To achieve high precision with the short length measurement, we should use the high speed sampling data logger, the wide range sensors, and the high frequency seismic source, compared to the existing seismic measurement systems. In this paper, we demonstrated the importance of three components for accuracy in the short range velocity measurements: estimating the errors before beginning an investigation, selecting the optimum measurement system, and assessing the velocity result of errors. We will present the new measurement system developed for increasing precision and these successful results in the field.
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Detecting Fracture Anisotropy Using Geological Mapping and Vertical Electrical Resistivity Survey
Authors G.O Mosuro, K.O. Omosanya, I.O. Kehinde and V.U. UkaegbuThe two areas were studied and have shown different trend of fracture orientations at surface and in the subsurface. Area 1 showed overlap between fractures oriented at shallow subsurface reveal by electrical resistivity anisotropy polygon plot and that of fracture orientation on the surface rock exposures in the study area; suggest that fractures are penetrative and hard-linked. Thus, the fractures at both levels are produced by similar tectonic. Whereas, lack of overlap between fracture orientations at both surface and subsurface at Area 2 suggests that the surface fractures are not deep seated, and that fractures at both scales are thought to be produced by different tectonic events
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