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Near Surface Geoscience 2015 - 21st European Meeting of Environmental and Engineering Geophysics
- Conference date: September 6-10, 2015
- Location: Turin, Italy
- Published: 06 September 2015
81 - 100 of 164 results
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Effect of Low Velocity Zones on Seismic Hazard Assessment - The Case of Taygetos Fault
Authors E. Mouzakiotis, V.K. Karastathis and P. KarmisSummaryShallow layers with low shear wave velocities may heavily influence the amplification of strong ground motion in a specific site. Such zones are often not taken into account in calculations of seismic hazard for a region, resulting in underestimated values of calculated ground acceleration. This issue is of great significance in areas that are adjacent to large seismic zones.
We assessed the effect of low velocity layers in the calculation of the amplification of ground motion, as well as in the calculation of the maximum values of acceleration. Additionally we evaluated the ability of several near surface geophysical techniques in detecting velocity reversals. We tested the MASW, MÁLW and refraction tomography techniques in conjunction with crosshole tomography. We found that only the MASW near surface methodology was able to reliably detect velocity reversals. We also proved that by not taking into account such low velocity layers in the calculation of seismic hazard, the results could be underestimated up to 25%.
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The Sensitivity Analysis as a Non-uniqueness Indicator in the Inversion of Rayleigh-wave Dispersion Curve
Authors M. Senkaya and H. KarsliSummaryIn the Rayleigh-wave dispersion curve (RDC) inversion, layer thicknesses are typically fixed during the inversion process and may be estimated using available a priori knowledge. But, the RDC inversion process frequently suffers from non-uniqueness due to the use of erroneous or conjectural layer thicknesses. In this work, sensitivity analysis of S-wave and layer thickness is used to reveal the reliability and accuracy of the RDC inversion results in the non-uniqueness situation. For this purpose, a synthetic data is inverted with three different initial models through the same inversion parameters and their sensitivity patterns are compared. The sensitivity analyses demonstrate that the sensitivity pattern of layer S-wave velocity and thickness may present useful information about the quality of estimated parameters. In this way, the possibility of getting trapped in non-uniqueness may be decreased.
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ERT Characterisation of La Calderilla Volcanic Caldera (Gran Canaria Island, Spain)
Authors M. Himi, A. Rodriguez-Gonzalez, J.C. Tapias, F. Pérez-Torrado and A. CasasSummaryMaar are a singular type of volcanic structure generated by explosive phreatomagmatic eruptions as a result of interaction between rising magma and groundwater. This kind of structures are characterised by circular craters, often filled with water and/or lacustrine sediments and surrounded by a ring of pyroclastic deposits. The whole of the volcano, including the pyroclastic ring surrounding the crater maar is called maar-diatreme.
Recently borehole was drilled at the bottom of the volcanic crater which penetrated about 8.7 m in its sedimentary sequence and the paleobotanical study has supplied the first evidence of paleoenvironmental evolution during the Holocene on the Gran Canaria Island.
The possibility of its sedimentary fill extends beyond of the Holocene is extremely attractive, since, for example, and there are few paleoenvironmental data regarding how much the last glaciation that affected the Canary Islands. In these circumstances, the knowledge of the total thickness of the lacustrine sediments is crucial to design a deeper borehole in the next future. Therefore, the subsurface characterisation provided by geophysics is essential for determining thickness and geometry of the sedimentary filling.
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Outpost Method of Computing Leaking and Normal Modes for Elastic Multilayered Half-space
Authors P. Znak and B. KashtanSummaryDispersion curves analysis of guided waves leaking modes provide P-wave velocity information in addition to the common techniques based on the Rayleigh modes dispersion curves inversion. Accounting for the leaking modes requires a fully automated and precise method for complex roots of dispersion equation computing. We have developed an extension of the so-called outpost method. That allows numerical computation of all the leaking and normal modes presented in the full wavefield in any elastic layered half-space. The method is based on the leaking modes property to come to the vicinity of complex plane real axis with increasing the frequency from zero to infinity. In principle, the approach consists of two steps. Firstly, we search for the moments of leaking modes crossing some line in the complex plane called outpost. We suggest a particular form of the outpost line. It crosses the branch cuts and is closed on the Riemann surface, accounting for all the possible leaking modes arising up in the wavefield. The second step is an individual continuation of every leaking mode as a curve parameterized by frequency starting from the outpost line. We give algorithms for leaking modes curves tracing through their peculiarities, which provides full method automation.
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Analysis of Parameters Sensitivity in Seismic Full Waveform Inversion Applied to Shallow Subsurface
Authors A.S. Spadini, L.D. Diogo, O.B. Bokhonok and R.P. PradoSummaryA Full waveform algorithm is applied to shallow seismic reflection data using vertical and radial components. Using an elastic formulation the desired parameters were P and S wave velocities, densities and the depths of layers. A restriction of flat layers with constant parameters was applied. To solve FWI we evaluated the sensitivity and behaviour of the objective function with respect to the model parameters. The results reveal a good sensitivity for the velocity parameters above the semi space and the stochastic algorithm should be improved for the estimation of the P wave velocity. This research is in the initial stage and more tests will be performed considering different offsets windows and about the mapping of the objective function.
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Approach to 3D Seismic Data Interpretation for Drilling Geohazard Assessment
Authors Y.E. Terekhina, M.Y. Tokarev, A.S. Pirogova, A.G. Roslyakov, N.V. Shalaeva and A.A. SemenovaSummaryIn the present there is an active offshore oil & gas shelf fields development. The standard methods applied for investigation of near surface geological conditions for site survey goals are high resolution seismic (HRS) for top hole drilling hazard assessment and ultrahigh resolution seismic (UHRS) mutually used with hydrographic surveying for sub-bottom geohazard assessment. On the other hand, application of 3D seismic technology for site survey investigations requires improvements both in data processing and interpretation. This paper focuses on the generalization of experience in the geohazards assessment in offshore fields based on the 3D seismic data. Involving of modern interpretation procedures standard for prospecting and exploration: inversion, wavelet analysis, pore pressure prediction based on spectral time analysis, Poisson ratio evaluation can significantly improve the accuracy and efficiency of 3D seismic data interpretation for site survey investigation. But still remain unsolved problems related to resolution of sub-bottom part, which caused by insufficient sources frequency range and absence of near offsets.
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Near-surface Voids in the Neapolitan Volcanic Tuff (Italy) Detected by Seismic Refraction Tomography
Authors S. Maraio and P.P.G. BrunoSummaryThe Neapolitan area (Italy) is affected by the presence of shallow man-made cavities in volcanic tuffs that were used in the past as building material. Therefore, the identification of these voids is a fundamental prerequisite for the minimization of subsurface collapse risks. In the past few years, several authors have shown that the geophysics prospecting methods can provide a most convenient solution to detecting underground voids. In this work we show how the seismic refraction tomography represents a powerful tool for the detection of near-surface tuff-cavities located in Casamarciano, Naples. The interpolation of the velocity models, allowed us to build a three-dimensional model of the tuff basement top and to show the lateral extend of the voids. The cavity location detected by seismic refraction tomography has been subsequently confirmed by four core-drilling made in the area.
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On the Stationarity of the Horizontal to Vertical Noise Spectral Ratio
Authors A. D’Alessandro, R. Martorana, P. Capizzi and D. LuzioSummaryThe Horizontal to Vertical Noise Spectral Ratio (HVNSR) method is nowadays widely used to estimate the resonance frequencies of geological structures. In the HVNSR method, seismic noise is considered as a stationary stochastic process. However, especially in industrialized/urbanized area, this is a very strict assumption seldom occurred. Several sources of noise can generate non stationary and anisotropic microtremor fields. To investigate the stationarity of microtremor, we have carried out several long-term measures of seismic noise with broad-band seismic sensors, in areas where the main source of anthropogenic noise is well known. The signals acquired have been analyzed both in frequency and in time domain. Our analysis have showed as the shape of the HVNSR curves can be strongly conditioned by the presence of near anthropogenic sources of noise. Useful information can be derived by the analysis of the data spectral density and by checking the azimuthal dependence of the HVNSR. The first one allows to easy identify different sources of seismic noise while the second one, in particular its time variability, is a useful instrument to evaluate the reliability of the data to estimate resonance frequencies of geological structures.
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First Attempt to Characterize Neoproterozoic Stromatolites at Chapada Diamantina (Brazil) by Geophysical Methods
SummarySince microbialites can be important reservoir rock, it is important to understand the paleoenvironmental conditions that favour the formation. Nevertheless, few studies have provided a detailed understanding of the strong lateral heterogeneity of these reservoirs and their complex development using near surface geophysical methods. The aim of this research is elucidate the internal architecture of a well exposed colonial stromatolite bioherms located at the Chapada Diamantina basin NE Brazil. Electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) sections were acquired in order to parameterize their geophysical signatures.
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Characterization GPR and FDTD Modeling of Asphaltic Sandstones of the Paraná Basin, Brazil - Preliminary Results
Authors L. Bertolla, J.L. Porsani and V.R. ElisSummaryThis paper shows the preliminary results of GPR profiles and numerical simulation by the finite difference method (FDTD) of asphaltic sandstones in the municipality of Anhembi, East Central study of the state of São Paulo, Brazil. Situated on the edge of the Paraná Basin, the study area has been the subject of great interest by Petrobrás (Brazilian Oil Exploration Company), that due to the existence of bitumen impregnated sandstones. With the results of the GPR profiles was observed shadow zones that could be simulated by FDTD modeling, although preliminary results suggest the presence of bacterial action, detailed studies are underway to test the hypothesis biodegradation.
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Introducing Particle Swarm Optimization to Invert Multimode Rayleigh Wave Dispersion Curve
Authors A. Zarean, R. Pourmirzaee and S. ChaychizadehSummaryRayleigh waves have been used increasingly as an attractive tool to obtain near-surface shear wave velocity profiles. Inversion of Rayleigh wave dispersion curves is demanding for most local-search methods due to its high nonlinearity and to its multimodality. Among many approaches which have been proposed for surface wave inversion so far, Intelligence Swarm algorithms effectively applied to solve it, and attempt to avoid trapping in local minima. In this paper, we proposed a new approach (Particle Swarm Optimization PSO) to overcome above disadvantages. To evaluate calculation efficiency and stability of PSO to inversion of surface wave data, we first inverted a broadband (1–30 Hz) synthetic dispersion curves from a synthetic model. After that we added first high mode to improve non- uniqueness of models. Results from all synthetic data demonstrate that PSO can be used for quantitative analysis of Rayleigh wave dispersion curves.
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Basic Role of the Electrical Tomography in the Exploration of Building Stones
Authors F. Šumanovac, S. Kolar and J. OreškovicSummaryIn the framework of exploration of building stones electrical tomography is an essential method. It is particularly effective in three types of geological models: boundary between carbonates and clastic rocks (1), boundary between rather compact igneous rocks and clastites (2) and fault and fracture zones in hard rocks (3). These are the most common models in surveying of building stones and great potentials of electrical tomography are presented on examples of two quarry explorations.
Model 1 and model 3 are shown on Vojnovec quarry where electrical tomography enabled precise definition of boundaries between carbonates and clastic rocks, as well as determination of margins of fractured zone within carbonates. Model 2 is presented by Hruškovec quarry where diabase bodies are in contact with clastites. Since high resistivities similar to the resistivity of diabase, also may be caused by dry sandstones magnetic method has been applied to distinguish between rocks of similar resistivity but different magnetic properties. In one case magnetic method was helpful in determination of the edge of diabase bodies, whereas in the second case the edge could not be determined due to changes in magnetic properties of diabases. Electrical tomography, in both cases, enabled determination of reliable two-dimensional model.
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A Comparison of DCIP Inversion Software
Authors M. Wennermark, P.I. Olsson, S. Johansson and K. HellmanSummaryElectrical resistivity tomography (ERT) is a commonly used geophysical method, which can conveniently be combined with measuring time-domain induced polarization. The combined measurement can be termed DCIP, and can be employed in a wide range of situations such as pre-investigations for construction, or contaminant monitoring to name a few. An integral part of a DCIP survey is the software used for processing and interpretation. There exists many such software packages. We compare the ubiquitous, commercial software, Res2DInv and two other common alternatives; the semi-academic AarhusInv and the academic, open source, BERT/GIMLi. The comparison is done by simulating measurements of resistivity and IP with a multiple gradient protocol over a model that represents a waste pond setting. The results show that there are quite a few similarities between the different softwares, but also some notable differences that one should be aware of during interpretation. This work is the first step towards an exhaustive comparison.
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Impact of Time-domain IP Pulse Length on Measured Data and Inverted Models
Authors P.-I. Olsson, G. Fiandaca, T. Dahlin and E. AukenSummaryThe duration of time domain (TD) induced polarization (IP) current injections has significant impact on the acquired IP data as well as on the inversion models, if the standard evaluation procedure is followed. However, it is still possible to retrieve similar inversion models if the waveform of the injected current and the IP response waveform are included in the inversion. The on-time also generally affects the signal-to-noise ratio (SNR) where an increased on-time gives higher SNR for the IP data.
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Identification of Possible Weakness Zones in Limestone Bedrock by Resistivity and Induced Polarization
Authors S. Johansson, C. Sparrenbom and T. DahlinSummaryIn connection to an environmental investigation of a former industrial site, the cretaceous sandy limestone bedrock in the surrounding area were mapped with resistivity and time domain induced polarization (DCIP). The aim of the DCIP survey was to delineate geological units and possible preferential pathways of DNAPL contaminated groundwater. Although IP anomalies corresponding to fractured crystalline bedrock have been observed previously and are commonly interpreted as results of clay weathering, there are not many studies of the IP behavior of limestone in previous research. In contrast to silicates, the weathering of calcite minerals in pure limestone results only in dissolution, without any weathering products in form of minerals. In this study, varying resistivity values and large IP anomalies were found in the sandy limestone bedrock, which are probable indications of weakness zones. The observed strong IP anomalies may be caused by calcite dissolution and precipitation processes or by clay mineral precipitation from chemical weathering of the silica grains contained in the sandy limestone. Another possibility may be sporadic presence of glauconitic sand in the limestone. More research is needed to confirm the sources of the observed anomalies.
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Diffraction Imaging for Edge Detection
Authors F. Alonaizi and R. PevznerSummaryIn this work a robust imaging algorithm based on detecting edge diffraction has been developed. The algorithm is based on measuring the coherency of the data using the semblance function computed along the traveltime curve of the diffracted wave. Based on the fact that waveforms on two sides of edge diffraction have opposite polarity, the proposed method exploit the phase-reversal phenomenon of edge diffractions in the computation process and use it as a criterion to separate edge diffractions from specular reflections. Utilizing the phase-reversal property of edge diffraction allows the detection of small features in the subsurface, and image hidden diffracted waves that could be masked by strong reflections. Two examples are provided to illustrate the validity of the method, the first example is a physical model of an edge diffractor, and the second example is real GPR data acquired over a lake.
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Imaging and Tomographic Interpretations of Seismic Refraction Data for Near Surface Basement Assessment in Somaliland
Authors R. De Franco, G. Caielli, A. Morrone and B. PetrucciSummaryWe present a comparison between the results of the processing of seismic profiles acquired in Somaliland, near the town of Boroma, in the frame of an UNESCO project for groundwater research. The main objective of the work was to support the siting of drilling water wells. The aim of geophysical survey was the identification of the geophysical parameters of the basement and overlying limestone unit.
The data processing was performed with a classical approach based on the tomographic inversion of P first arrivals. Whereas the picking of first arrival times is sometimes difficult, we present the application of the multi refractor imaging technique (MRI) based on the stack of refraction convolution sections of the refracted signals.
The results show that MRI is a suitable technique to processing and interpret the refracted data and it allows to obtain an accurate image of refracting interfaces. Further it complements and integrates the interpretation of the results obtained with tomographic approach also in presence of complex structures.
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Detection of Underground Hollow Objects Using Seismoacoustic Noise (On the Example of Buried Pipes Physical Model)
Authors Yu.I. Kolesnikov and K.V. FedinSummaryThe possibility of underground hollow objects detection using passive seismoacoustic measurements is demonstrated on the example of buried pipes physical model. The proposed method is based on the extraction of standing waves that can be generated by seismoacoustic noise in the space between the day surface and the hollow object. The experimental results showed that this method make it possible to detect a hollow objects at depths up to 5–8 their smallest horizontal sizes.
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Fully Non-linear Self-organizing Inversion of Cross-borehole Tomographic Data
By H. PaascheSummaryGeophysical model reconstruction by data inversion is usually ill-posed and suffers ambiguity due to limited number and accuracy of the available observations. Here, we formulate a new concept for an almost completely data-driven fully-nonlinear inversion to achieve an ensemble of different tomographic models fitting the underlying data set equally well. Since we parameterize the model reconstruction area using regular grids with grid cell dimensions not exceeding the spatial resolution of the tomographic data set we can find models of any reasonable complexity. To avoid the finding of models characterized by numerous anomalies below the spatial resolution limit of the data set we concurrently address a data misfit objective and a model structure objective formulated in fuzzy domain. The final ensemble of models addresses the data objective equally well but the model structure objective only to different degrees and allows for the finding of model ensembles illustrating the tomographic reconstruction ambiguity reasonably.
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Staggered Grids as a Tool to Improve the Model Resolution in Seismic Tomography for Elliptical Anisotropic Media
Authors E. Cardarelli and A. CerretoSummaryIn this study we present the use of staggered grid as a tool to improve the resolution of inverse model in seismic tomography in the case of elliptical anisotropic media. We analyze two synthetic models, both showing elliptical anisotropic characteristics, with different complexity: the first consists of a body with rectangular shape having inside an anomaly with higher velocity and the second one is characterized by morphological variation to simulate a watershed. This second case is studied for both poor and high ray coverage configuration. Successively, a previous field survey is analyzed introducing staggered grids and the results are compared with the old ones, solved using a constant cells’ size model. Staggered grid give enhanced results in terms of the velocity model and can be used as a tool to improve the knowledge of the studied domain by determining cells with variable shape and size, useful to infer a good initial model for a further inversion. This conclusion applies also to isotropic media.
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