1,721,010 research outputs found
Imaging and characterization of heterogeneous landfills using geophysical methods
No full textNowadays many countries use landfilling for the management of their waste or for treating old landfills. Emissions from landfills can be harmful to the environment and to human health, making the stabilization of landfills a priority for the landfill communities. Estimation of the emission potential for determination of the aftercare period and improvement of the treatment technologies for the minimization of the aftercare period are examples of problems landfill research groups are now facing. For handling these problems, the degradation processes inside the landfill, which highly depend on the heterogeneity of the landfill, must be well understood. Geophysical methods can be used to image and characterize the heterogeneity of the landfill body non-intrusively. Researchers have earlier used geophysical methods for landfills. However, till now, artifacts, uncertainties and low resolution remained problematic in imaging and characterizing the landfill body in detail. In addition, the results so far have been rather qualitative. In this thesis, we aim to improve the imaging and characterization of landfill bodies using seismic and electrical methods, by proposing new ways to deal with some major problems that were previously encountered. A landfill body is very heterogeneous with many high-density areas that act as obstructions to the fluid flow. Characterization of these high-density areas is important for the understanding of the preferential flow paths inside the landfill body and hence of the degradation processes. These high-density areas manifest themselves as scatterers in the recorded seismic wavefield. Till now, research works using the seismic method have had difficulties in imaging a landfill body mainly because of the presence of very strongly scattered seismic energy. We investigate the use of seismic interferometry (SI), by performing a number of modeling studies, to improve the imaging. The additional amount of traces and sources computed by SI provides more information, as an increased number of rays penetrate into the earth and are recorded. Furthermore, scatterers act as secondary sources illuminating the landfill from below, thus reducing possible artifacts due to the location of the seismic receivers and sources only at the earth's surface. We discuss in detail the concept of using SI for landfill application and its advantages. The SI approach is compared with the conventional reflection seismic survey (CRSS), considering different acquisition geometries and processing and acquisition errors. Our results have established the merits of SI in landfill applications. The next step was to test our modeling results on field data. For this, we acquire seismic datasets at two different landfill sites. Before applying SI to the CRSS data, we attempt to image the landfill body solely by CRSS. We have found that this is possible when special processing steps are adopted. Special care has been taken in velocity analysis. The developed methodology is proposed to be used for very heterogeneous subsurfaces in order to image the higher-density areas (scatterers) in detail. We apply this procedure in two fieeld datasets. We have succeeded to image higher-density areas, the top and bottom of the landfill, and the geological subsurface below the landfill body. The geometry used in acquiring the second field dataset was better for the purpose than the first one, resulting in an improved imaging of the landfill. SI is applied to the first acquired field dataset for validation of the modeling studies. Not only have we been able to improve the imaging of the landfill body, but we have also proposed a new method for the removal of surface waves that dominate the field seismic data. In our first field dataset, surface-wave signal from another source was recorded; that was not possible to be removed using conventional processing/filtering techniques. We use the method of adaptive subtraction (AS) involving SI to remove the surface waves and have, thereby, shown the improved imaging of reflections inside the landfill body. Finally, we explore the improvement of the characterization of the landfill body when using the velocity analysis results from all three approaches (CRSS, SI and AS). Besides imaging, the characterization of the higher-density areas and of the leachate- and gas/air-bearing zones inside the landfill is very important for understanding the degradation processes. Using a landfill-specific empirical relationship between the unit weight and shear (S)-wave velocity of the landfill materials, we have calculated the density distribution inside the landfill. These values can be used in models that predict the emission potential. For further characterization, we use electrical methods in conjunction with the seismic methods. In the first study, we are able to determine the heterogeneities (wet and dry pockets) through combined use of S-wave velocity and electrical resistivity (ER). S-wave velocity can resolve the high-density areas (scatterers) that act as an obstruction to the fluid flow. ER helps to identify the pockets that represent accumulation of water (low-resistivity values) above the high-density areas. In the second field study, we acquire S- and compressional (P)-wave reection seismic datasets. By jointly interpreting the results, leachate and gas/air bearing zones are distinguished. The conditions that must be met for interpreting leachate or gas/air (relatively dry) zones are thoroughly discussed, taking into account the P- and S-wave velocities. Interpreting results of ER and induced polarization (IP) measurements acquired at the same location as the seismic surveys, the structure of the landfill is better defined, wet and gas pockets are further characterized, and an indication for the type of the waste is obtained. The velocity analysis of extremely heterogeneous seismic data is challenging. We check the velocity distribution obtained from analysis of seismic reflection data by comparing it with the results of multichannel analysis of surface waves (MASW) and early-arrival waveform tomography. In the second field study, additional measurements (gas concentration and mechanical resistance to waste deformation) are used for validation of results from seismic and electrical methods. The methodology developed in this research presents a way for improved imaging and characterization of a heterogeneous landfill body through use of specially adapted seismic and electrical methods. New approaches are presented to overcome some problems that were encountered in the past and to provide more reliable, quantitative results.Geoscience & EngineeringCivil Engineering and Geoscience
Soil properties from seismic intrinsic dispersion
No full textTheoretical and experimental studies in the past have shown the sensitivity of seismic waves to soil/rock properties, such as composition, porosity, pore fluid, and permeability. However, quantitative characterization of these properties has remained challenging. In case of unconsolidated soils, the inherently loose and heterogeneous nature complicates the task of obtaining the in situ properties and spatial variations. In this thesis, we investigate the possibility of exploiting the information of seismic intrinsic dispersion in the low frequency band (10-200 Hz), which is relevant to onshore field data, in order to quantify these physical properties, with special focus on soil porosity and permeability. In situ values of these properties are crucial in many different projects. We first investigate the frequency-dependent seismic velocity and attenuation caused by inelastic losses at grain contacts and wave-induced fluid flow at different scales (from grain size to seismic wavelength), using the theory of poroelasticity first proposed by Biot and many subsequent extensions and modifications. Several pertinent models of poroelasticity are looked at in order to find out their applicability in explaining the observed seismic dispersion. The observed dispersion can vary greatly between various unconsolidated, fully-saturated soils. Further, we develop a stress-dependent Biot (SDB) model in order to study the behaviour of seismic waves propagating through a fully-saturated porous medium subjected to different stress conditions. This is achieved by combining the mechanics of granular soils with the effective-stress laws, finally coupling with Biot's theory. Careful analyses of the underlying soil/rock physics that relate geophysical observations to the physical properties reveal an interesting feature in the property domain among several different measurements. This is an extention to some recent work done by others. We have found that it is possible to find two or more measured quantities, showing contrasting (sometimes quasi-orthogonal) behaviour in the common parameter space, such that a combination of those measured quantities leads to a physics-based uniqueness in the property estimation. This quasi-orthogonality in the common property domain among different measured quantities is advantageously used for estimation of porosity, permeability, water saturation, and effective stress. Several numerical examples are presented where P- and S-wave velocity and attenuation are efficiently integrated in order to obtain soil properties. In addition to seismic waves, electromagnetic waves are briefly considered for extracting extra soil properties. In this research, considerable attention has been paid to the investigation of S waves travelling through a porous medium, since S waves have well-known significance in the context of shallow subsurface characterization. Twelve selected datasets of frequency-dependent S-wave velocity and attenuation from various soft-soil sites are used in this study. Data for fully-saturated, unconsolidated soils from land/onshore environment are only considered. It is found that the behaviour of seismic intrinsic dispersion can vary greatly with the soil-type. One of the main challenges in property estimation using intrinsic dispersion relates to reliable extraction of the information of intrinsic dispersion from the recorded seismic data. The difficulty lies in the quantification of scattering attenuation, the effect of which is always present in the recorded seismograms due to the wavelength-scale and smaller heterogeneities in the subsurface. Scattering has an absorption-like effect on the transmitted seismic energy. Accordingly, determining and subtracting the scattering attenuation from the total (or apparent) attenuation is critically important. We have discussed and successfully tested an approach, achieving this goal. Several shallow vertical seismic profiling (VSP) measurements are conducted in the field using a recently developed digital, array-seismic cone penetrometer (CPT) system. CPT provides information on cone-tip resistance, sleeve friction, and pore pressure, thus offering direct, additional knowledge on geological layering, that is used to calculate the scattering attenuation. To obtain the soil properties, an inversion algorithm is presented based on simulated annealing and the poroelasticity theory. We study the sensitivity of different parameters involved in the cost function to be minimized. The most and the least sensitive parameters are discriminated based on the eigenanalyses of the covariance matrix of the gradient of the cost function. The eigenvectors and the corresponding eigenvalues of the covariance matrix are used to navigate efficiently the search algorithm in the multidimensional space and find a relatively stable, global solution of the cost function. Finally, we apply the methodology developed in this research to a VSP dataset acquired in a layered sequence of siltstone, shale and sandstone. The porous sandstone contains hydrocarbon accumulations. The influence of fluid mobility (permeability-to-viscosity ratio) on the estimated P-wave intrinsic dispersion is distinctly observed. Using optimization by simulated annealing together with VSP and well-log measurements, the Biot and squirt flow (BISQ) model is found to provide one possible mechanism for the observed dispersion. The layer-specific fluid mobility values are estimated using our approach; they are found to be close to the independent measurements of mobility using Stoneley waves and from dynamic formation-tests carried out at the same borehole. The depth distribution of fluid mobility matches well between our estimate and the independent measurements. The methodology developed and the results obtained in this research pave the way to a new direction for in situ, quantitative soil/rock characterization using seismic waves.Geoscience & EngineeringCivil Engineering and Geoscience
Analysis of 9-component SCPT data to obtain shallow subsoil structure
No full textA 9-component seismic cone penetration test (SCPT) was conducted at a site near Delft. A sledge hammer striking horizontally a wooden plate was used to generate shear (S) waves polarized in the crossline and inline directions. A vertical hammer hitting a metal plate generated the P wave. The data were acquired using a recently developed array (7-level) 3-component SCPT tool. In addition to downhole SCPT data, surface seismic data were collected using 24 3-component geophones planted at 0.75 m interval on the ground. The most explicit layer boundaries are found at 4.5 m, 5.5 m and 10 m depths. The following velocity model for the shallow subsoil could be obtained. 2.5-4.5 m: Vs=82.23 m/s, Vp=951.77 m/s; 4.5-5.5 m: Vs=33.01 m/s, Vp=488.33 m/s; 5.5-10 m: Vs= 92, 52 m/s, Vp=1483.89 m/s; 10-14.5 m: Vs=221.80 m/s, Vp=796.58 m/s.Applied Geophysics and PetrophysicsGeoscience & EngineeringCivil Engineering and Geoscience
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Seismic interferometry as a tool for improved imaging of the heterogeneities in the body of a landfill
No full textAccepted Author ManuscriptGeo-engineeringApplied Geophysics and Petrophysic
Frequency-dependent reflection response of a laterally varying fracture, represented as a nonwelded interface
No full textDisplacement across the interface is not continuous in the case of a nonwelded interface. The stiffness of a nonwelded interface is a function of fracture properties, e.g., roughness, asperities, aperture and fracture infill. In this research, the fracture compliance has been estimated by AVO inversion of P-P reflected waves from a nonwelded interface. Laboratory experiments were carried out in order to measure the reflection response of dry and wet fractures. CMP data were acquired for homogeneous and heterogeneous fluid-fillled fractures. The objectives of the research are: 1) to capture in the laboratory and study the reflection response of a laterally varying heterogeneous fracture, 2) to investigate the effectiveness of linear slip theory in predicting the AVO response of a heterogeneous fracture and estimate the fracture properties, 3) to observe in laboratory and utilize in characterization the scattered seismic waves from a laterally varying heterogeneous fracture, and 4) to utilize the time-lapse seismic reflection response for monitoring a fluid-filled laterally heterogeneous fracture. AVO inversion was carried out to obtain the fracture compliances. In order to illuminate a fracture which is significantly thinner than the predominant seismic wavelength, one promising approach is to incorporate the linear slip theory in the AVO inversion. This has been successfully implemented in this research. As a result, we could characterize a fracture which is nearly 65 times thinner than the seismic wavelength that we availed in our laboratory experiments. Fracture compliance values obtained from AVO inversion coupled with the linear slip theory can distinguish between wet and dry surfaces within a fracture. The obtained values of fracture compliance show the reliability of the AVO inversion in characterizing heterogeneous fractures. The obtained P-P reflection coefficients for each CMP gather closely match the expected values calculated from the linear slip theory. The value of the average compliance for the fluid-filled fracture in our experiment, estimated from AVO inversion, is 7:4x10^-14m/Pa. This value is slightly higher than the theoretical value of 6:4x10^-14m/Pa. This difference can be explained by the presence of minute quantity of air bubbles that got trapped in the infill fluid in the fracture during our experiment. We calculated that the volume of trapped air that can cause this deviation is 0.001%, which agrees with earlier observations. The AVO inversion of a fluid-filled nonwelded interface can be successfully used to monitor the fractured reservoir properties, using the dry fracture response as a reference. The scattered seismic waves from a heterogeneous fracture, observed for the first time in laboratory, show that these events affect the primary reflections, and therefore, there is a need for inversion that takes into consideration both specular and nonspecular reflection events.Civil Engineering and GeosciencesGeoscience & EngineeringIDEA League Applied Geophysic
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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