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31.
Niels Grobbe Joost van der Neut Evert Slob Kees Wapenaar Carlos Almagro Vidal Guy Drijkoningen 《Geophysical Prospecting》2016,64(2):361-391
Wavefield decomposition forms an important ingredient of various geophysical methods. An example of wavefield decomposition is the decomposition into upgoing and downgoing wavefields and simultaneous decomposition into different wave/field types. The multi‐component field decomposition scheme makes use of the recordings of different field quantities (such as particle velocity and pressure). In practice, different recordings can be obscured by different sensor characteristics, requiring calibration with an unknown calibration factor. Not all field quantities required for multi‐component field decomposition might be available, or they can suffer from different noise levels. The multi‐depth‐level decomposition approach makes use of field quantities recorded at multiple depth levels, e.g., two horizontal boreholes closely separated from each other, a combination of a single receiver array combined with free‐surface boundary conditions, or acquisition geometries with a high‐density of vertical boreholes. We theoretically describe the multi‐depth‐level decomposition approach in a unified form, showing that it can be applied to different kinds of fields in dissipative, inhomogeneous, anisotropic media, e.g., acoustic, electromagnetic, elastodynamic, poroelastic, and seismoelectric fields. We express the one‐way fields at one depth level in terms of the observed fields at multiple depth levels, using extrapolation operators that are dependent on the medium parameters between the two depth levels. Lateral invariance at the depth level of decomposition allows us to carry out the multi‐depth‐level decomposition in the horizontal wavenumber–frequency domain. We illustrate the multi‐depth‐level decomposition scheme using two synthetic elastodynamic examples. The first example uses particle velocity recordings at two depth levels, whereas the second example combines recordings at one depth level with the Dirichlet free‐surface boundary condition of zero traction. Comparison with multi‐component decomposed fields shows a perfect match in both amplitude and phase for both cases. The multi‐depth‐level decomposition scheme is fully customizable to the desired acquisition geometry. The decomposition problem is in principle an inverse problem. Notches may occur at certain frequencies, causing the multi‐depth‐level composition matrix to become uninvertible, requiring additional notch filters. We can add multi‐depth‐level free‐surface boundary conditions as extra equations to the multi‐component composition matrix, thereby overdetermining this inverse problem. The combined multi‐component–multi‐depth‐level decomposition on a land data set clearly shows improvements in the decomposition results, compared with the performance of the multi‐component decomposition scheme. 相似文献
32.
Overcoming Squeezing in the Yacambú-Quibor Tunnel,Venezuela 总被引:2,自引:2,他引:0
Summary The 5 m diameter 23.3 km long Yacambú-Quibor tunnel is designed to carry water through the Andes from the Yacambú dam in the
wet tropical Orinoco basin to the semi-arid but fertile Quibor basin in western Venezuela. The tunnel is excavated in silicified
and graphitic phyllites at depths of up to 1270 m below surface and extreme squeezing problems have been encountered. Construction
involved 8 contracts extending over 32 years with breakthrough being achieved in July 2008. Several excavation methods and
various lining designs were used over the years until the adoption of yielding support permitted the Owner and the Contractor
to agree that only a circular section would be used and emphasis was placed on developing a routine construction procedure,
irrespective of the rock conditions encountered at the face. This paper describes some of the rock engineering issues that
were faced during the construction of this tunnel.
Correspondence: E. Hoek, Consulting Engineer, Vancouver, Canada 相似文献
33.
Summary A simple triaxial cell is described together with details of the technique employed for triaxial testing. Triaxial strength results for 254 specimens of eight rock types are tabulated.
With 2 Figures 相似文献
Zusammenfassung Ein einfaches Triaxial-Prüfgerät wird, zusammen mit Einzelheiten des experimentellen Verfahrens, beschrieben. Ergebnisse dreiachsiger Festigkeitsprüfung sind für 254 Proben aus acht verschiedenen Gesteinsarten in Tabellen wiedergegeben.
Résumé Un appareil triaxial simple est décrit avec des détails sur la technique expérimentale. Des résultats de résistance triaxiale pour 254 échantillons pris parmi huit types de roche sont donnés.
With 2 Figures 相似文献
34.
35.
Ying Zhu Jingjing Liu Omanjana Goswami Ashaki A. Rouff Evert J. Elzinga 《Geochemical transactions》2018,19(1):3
We studied the effects of humic substances (HS) on the sorption of Fe(II) onto Al-oxide and clay sorbents at pH 7.5 with a combination of batch kinetic experiments and synchrotron Fe K-edge EXAFS analyses. Fe(II) sorption was monitored over the course of 4 months in anoxic clay and Al-oxide suspensions amended with variable HS types (humic acid, HA; or fulvic acid, FA) and levels (0, 1, and 4 wt%), and with differing Fe(II) and HS addition sequences (co-sorption and pre-coated experiments, where Fe(II) sorbate was added alongside and after HS addition, respectively). In the Al-oxide suspensions, the presence of HS slowed down the kinetics of Fe(II) sorption, but had limited, if any, effect on the equilibrium aqueous Fe(II) concentrations. EXAFS analyses revealed precipitation of Fe(II)–Al(III)-layered double hydroxide (LDH) phases as the main mode of Fe(II) sorption in both the HA-containing and HA-free systems. These results demonstrate that HS slow down Fe(II) precipitation in the Al-oxide suspensions, but do not affect the composition or stability of the secondary Fe(II)–Al(III)-LDH phases formed. Interference of HS with the precipitation of Fe(II)–Al(III)-LDH was attributed to the formation organo-Al complexes HS limiting the availability of Al for incorporation into secondary layered Fe(II)-hydroxides. In the clay systems, the presence of HA caused a change in the main Fe(II) sorption product from Fe(II)–Al(III)-LDH to a Fe(II)-phyllosilicate containing little structural Al. This was attributed to complexation of Al by HA, in combination with the presence of dissolved Si in the clay suspension enabling phyllosilicate precipitation. The change in Fe(II) precipitation mechanism did not affect the rate of Fe(II) sorption at the lower HA level, suggesting that the inhibition of Fe(II)–Al(III)-LDH formation in this system was countered by enhanced Fe(II)-phyllosilicate precipitation. Reduced rates of Fe(II) sorption at the higher HA level were attributed to surface masking or poisoning by HA of secondary Fe(II) mineral growth at or near the clay surface. Our results suggest that HS play an important role in controlling the kinetics and products of Fe(II) precipitation in reducing soils, with effects modulated by soil mineralogy, HS content, and HS properties. Further work is needed to assess the importance of layered Fe(II) hydroxides in natural reducing environments. 相似文献
36.
A Comparison of Strategies for Seismic Interferometry 总被引:2,自引:0,他引:2
Roel Snieder Masatoshi Miyazawa Evert Slob Ivan Vasconcelos Kees Wapenaar 《Surveys in Geophysics》2009,30(4-5):503-523
The extraction of the response from field fluctuations excited by random sources has received considerable attention in a variety of different fields. We present three methods for the extraction of the systems response that are based on cross-correlation, deconvolution, and the solution of an integral equation, respectively. For systems that are invariant for time-reversal the correlation method requires random sources on a bounding surface only, but when time-reversal invariance is broken, for example by attenuation, a volume distribution of sources is needed. For this reason the correlation method is not useful for diffusive or strongly attenuating systems. We provide examples of the three methods and compare their merits and drawbacks. We show that the extracted field may satisfy different boundary conditions than does the physical field. This can be used, for example, to suppress surface-related multiples in exploration seismology, to study the coupling of buildings to the subsurface, and to remove the airwave in controlled source electromagnetics (CSEM). 相似文献
37.
Review paper: Virtual sources and their responses,Part II: data‐driven single‐sided focusing 下载免费PDF全文
Kees Wapenaar Jan Thorbecke Joost van der Neut Evert Slob Roel Snieder 《Geophysical Prospecting》2017,65(6):1430-1451
In Part I of this paper, we defined a focusing wave field as the time reversal of an observed point‐source response. We showed that emitting a time‐reversed field from a closed boundary yields a focal spot that acts as an isotropic virtual source. However, when emitting the field from an open boundary, the virtual source is highly directional and significant artefacts occur related to multiple scattering. The aim of this paper is to discuss a focusing wave field, which, when emitted into the medium from an open boundary, yields an isotropic virtual source and does not give rise to artefacts. We start the discussion from a horizontally layered medium and introduce the single‐sided focusing wave field in an intuitive way as an inverse filter. Next, we discuss single‐sided focusing in two‐dimensional and three‐dimensional inhomogeneous media and support the discussion with mathematical derivations. The focusing functions needed for single‐sided focusing can be retrieved from the single‐sided reflection response and an estimate of the direct arrivals between the focal point and the accessible boundary. The focal spot, obtained with this single‐sided data‐driven focusing method, acts as an isotropic virtual source, similar to that obtained by emitting a time‐reversed point‐source response from a closed boundary. 相似文献
38.
Sivajini Gilchrist Alexander E. Gates Matthew Gorring Evert Jan Elzinga 《Environmental Earth Sciences》2011,63(5):1029-1041
Organic material in metal contaminated soils around an abandoned magnetite mine–smelter complex in the critical Highlands
watershed protects the groundwater and surface water from contamination. Metals in these waters were consistently below local
and national water standards. Two groups of soil types cover the area: (1) Group A disturbed metal-rich soils, and (2) Group
B undisturbed organic soils. Chromium and nickel were more elevated than other metals with Cr more widespread than Ni. In
Group A, Cr correlated strongly with sesquioxides in the lower horizons (Fe2O3: r = 0.74, p < 0.025; Al2O3: r = 0.92, p < 0.005). In Group B, Cr correlated strongly (r = 0.96, p < 0.005) with soil organic matter (SOM) in the O-horizons. Ni–Cr (Group A: 52 and 70% in O- and lower horizons, respectively;
Group B: ~100% in both horizons) and V–Cr correlations (78% only in Group A lower horizons) suggest similar retention mechanisms
for these elements. Average soil
\textpH\textCaCl2 {\text{pH}}_{{{\text{CaCl}}_{2} }} for both groups ranged between 3.65 and 5.91, suggesting that soil acidity is determined by organic acids and solubility
of Al3+ releasing H+ ions. SOM and sesquioxides contribute significantly to creating naturally occurring filtration systems, removing metals,
and protecting water quality. High Ca, Fe, and Ti in Group A soils suggest slag and ash were mixed into the soils. Some low-Cr
sources include magnetite, slag, and ash (100, 100 and 200 mg/kg, respectively). Constant ZrO
2
:TiO
2
ratios in the lower soils indicate soil formation from breakdown of underlying tailing rocks, contributing Cr to these layers. 相似文献
39.
Kenneth F. Rijsdijk Julian P. Hume Frans Bunnik F.B. Vincent Florens Claudia Baider Beth Shapiro Johannes van der Plicht Anwar Janoo Owen Griffiths Lars W. van den Hoek Ostende Holger Cremer Tamara Vernimmen Perry G.B. De Louw Assenjee Bholah Salem Saumtally Nicolas Porch James Haile Mike Buckley Matthew Collins Edmund Gittenberger 《Quaternary Science Reviews》2009,28(1-2):14-24
Although the recent history of human colonisation and impact on Mauritius is well documented, virtually no records of the pre-human native ecosystem exist, making it difficult to assess the magnitude of the changes brought about by human settlement. Here, we describe a 4000-year-old fossil bed at Mare aux Songes (MAS) in south-eastern Mauritius that contains both macrofossils (vertebrate fauna, gastropods, insects and flora) and microfossils (diatoms, pollen, spores and phytoliths). With >250 bone fragments/m2 and comprising 50% of all known extinct and extant vertebrate species (ns = 44) of Mauritius, MAS may constitute the first Holocene vertebrate bone Concentration-Lagerstätte identified on an oceanic volcanic island. Fossil remains are dominated by extinct giant tortoises Cylindraspis spp. (63%), passerines (~10%), small bats (7.8%) and dodo Raphus cucullatus (7.1%). Twelve radiocarbon ages [four of them duplicates] from bones and other material suggest that accumulation of fossils took place within several centuries. An exceptional combination of abiotic conditions led to preservation of bones, bone collagen, plant tissue and microfossils. Although bone collagen is well preserved, DNA from dodo and other Mauritian vertebrates has proved difficult. Our analysis suggests that from ca 4000 years ago (4 ka), rising sea levels created a freshwater lake at MAS, generating an oasis in an otherwise dry environment which attracted a diverse vertebrate fauna. Subsequent aridification in the south-west Indian Ocean region may have increased carcass accumulation during droughts, contributing to the exceptionally high fossil concentration. The abundance of floral and faunal remains in this Lagerstätte offers a unique opportunity to reconstruct a pre-human ecosystem on an oceanic island, providing a key foundation for assessing the vulnerability of island ecosystems to human impact. 相似文献
40.
Wim Z. Hoek 《《幕》》2008,31(2):226-229
The Last Glacial-Interglacial Transition is one of the most intensively studied periods in Earth History. The rapid climate and environmental changes that occurred during the transition can be used to test ideas about the functioning of our climate system. The stratigraphy of this period has been thoroughly investigated and, in particular, the recently proposed event stratigraphy for the Last Glacial-Interglacial Transition based on the Greenland ice core records serves as a tool for synchronisation of records from the ice, marine and terrestrial environment. 相似文献