An Adaptive Wavenumber Sampling Strategy for 2.5D Seismic-Wave Modeling in the Frequency Domain     

Bing Zhou  Stewart Greenhalgh 《Pure and Applied Geophysics》2006,163(7):1399-1416

Simulation of seismic waves from a 3D point-source in a 2D medium may be performed in the frequency-wavenumber domain (called 2.5D modelling). It involves computing the Fourier-transformed Green's function for a number of frequency (ω) and strike direction wavenumber (k_y) values and doubly inverse transforming to convert to the traveltime and distance space. Such modeling produces a wavefield with 3D features but the computation becomes pseudo 2D (i.e., in the xz-plane) rather than 3D (in the xyz-frame). The common sampling strategy for the wavenumber is inefficient for 2.5D wave modeling because it employs a large number of wavenumbers (k_y). This leads to a high cost of computer time in the linear-equation-solving processing, which detracts from the advantages of 2.5D modeling. In this paper, we use two analytic frequency-wavenumber-domain solutions for seismic waves in a homogeneous medium and an inhomogeneous media (two semi-infinite media in contact) to investigate the properties of the solutions and an efficient sampling strategy for choosing the wavenumbers. We have carried out analytic and numerical experiments with these solutions, and present adaptive Gauss–Legendre abscissae for the wavenumber sampling in terms of a modeling situation. We show that the effective range and the number of sampling points of the wavenumber define the adaptive sampling strategy, and they can be estimated in terms of the wavelength and the maximum source-receiver offset. We apply this sampling strategy to the finite-element method and demonstrate that the range and number of sampling points may be adapted for obtaining significant computational efficiency and satisfactory accuracy for every frequency component. Such 2.5D wave modeling can be readily applied for frequency-domain full-waveform inversion for seismic surface measurements and crosshole seismic waveform tomography.  相似文献   

Effects of an anisotropic overburden on azimuthal amplitude analysis in horizontal transverse isotropic media     

Sonja Maultzsch  Steve Horne  Stewart Archer  Hans Burkhardt 《Geophysical Prospecting》2003,51(1):61-74

Azimuthal amplitude variation in fractured media, commonly used to characterize fracture systems, is a function not only of reflection at the target but also of transmission through the overburden. This study investigates the sensitivity of amplitudes to various anisotropic overburden effects in horizontal transverse isotropic (HTI) media. Issues considered here are geometric spreading, transmission coefficients and attenuation due to fluid flow. Their influence on the azimuthal amplitude variation is evaluated quantitatively over a wide model space.
Only the variation of transmission coefficients with azimuth proves to be negligible. Geometric spreading alters the amplitude signature significantly over a relatively narrow range of models, and its influence increases with layer thickness. The most severe effect of an anisotropic fractured overburden is attenuation due to fluid flow between the cracks or the cracks and pores in the surrounding matrix. The relative changes in amplitudes between the symmetry directions due to anisotropic absorption are of the same order of magnitude as the changes in the reflection coefficient. The effect is significant over a very wide range of petrophysical parameters. Thus it leads to considerable problems in the amplitude analysis for almost any case of an overburden that contains cracks and pores. A correct amplitude analysis at the target will not be possible unless the effect of attenuation is removed.  相似文献   

Are we all environmentalists now? Rhetoric and reality in environmental action   总被引：1，自引：0，他引：1  

Stewart Barr 《Geoforum》2004,35(2):231-249

Throughout the western world, public support for environmental protection measures has grown consistently during the past three decades. Endorsement for a wide array of measures to reduce the human impact on nature and conserve resources has been high among individuals in the UK, relating to a variety of environmental issues, such as transport, waste management, water quality and green consumerism. These stated concerns are somewhat in contrast to the behavioural responses which similar research projects have reported. This attitude-behaviour inconsistency, or more colloquially, the ‘value-action gap’, has become a major area of soul searching for policy makers who have used techniques such as contingent valuation and willingness to pay models to construct their policy measures. Accordingly, this paper presents an alternative means of investigating the attitude-behaviour inconsistency, with the use of research by social psychologists, sociologists and geographers. A framework of environmental behaviour is presented, so as to provide environmental researchers with a means of examining which variables modify the relationship between stated intention and action. The results indicate that whilst there is a moderately strong relationship between stated intention and behaviour, the factors influencing these two constructs are significantly different so as to suggest that public rhetoric towards environmental action may be influenced by different antecedents to that of actual behaviour. As such, the paper provides a fresh perspective on the difference between rhetoric and reality in environmental action.  相似文献   

Dating recent colluvial sequences with Pb and Cs along an active fault scarp, the Eliki Fault, Gulf of Corinth, Greece     

Andrew B. Cundy  Iain S. Stewart 《Tectonophysics》2004,386(3-4):147-156

Reliable dating is an essential element of palaeoseismological studies, yet whilst a suite of geochronological methods can now provide late Quaternary age control it remains very difficult to date modern events (i.e., those occurring within the last 150 years). This is significant because the starting point for many palaeoseismological investigations is a modern surface-rupturing event, whose geological effects need to be disentangled in trench stratigraphies from palaeoseismic ruptures. Two dating methods which, in combination, can provide robust dating control in recently deposited sediments are the ²¹⁰Pb and ¹³⁷Cs dating methods. Here, we test the applicability of using ²¹⁰Pb and ¹³⁷Cs to date colluvial sediments exposed in three trenches excavated across an earthquake fault—the Eliki fault, Gulf of Corinth, Greece—which ruptured in an earthquake in 1861. The ²¹⁰Pb and ¹³⁷Cs profiles observed in these colluvial sequences are relatively erratic due to the mixed nature of the sediments, i.e., their deposition in an environment where the supply of slope sediments is driven by seasonal rainfall, causing non-uniform sediment accretion and sediment reworking. In one trench, however, ²¹⁰Pb dating, corroborated by ¹³⁷Cs dating, indicates that a proposed post-1861 surface colluvial unit has been deposited over the period 1950 AD–present (at a rate of ca. 9 mm/year), and overlies a significantly older unit (>120 years old). The dating control provided here by ²¹⁰Pb and ¹³⁷Cs dating corroborates the published interpretation of the trench stratigraphy, and refines the ¹⁴C-based estimated dates for the upper unit. At two other trenches ²¹⁰Pb and ¹³⁷Cs dating only provided minimum ages (based on the presence or absence of ²¹⁰Pb_excess and ¹³⁷Cs). Such approximate ages, however, may still useful in corroborating interpretations made using the trench stratigraphy, or, at sites which have long earthquake recurrence intervals, determining which earthquake event was responsible for a particular bed offset.  相似文献   

Organic- and carbonate-rich soil formation ∼2.6 billion years ago at Schagen, East Transvaal district, South Africa     

Yumiko Watanabe  Brian W. Stewart 《Geochimica et cosmochimica acta》2004,68(9):2129-2151

A ∼17-m paleosol sequence at Schagen, South Africa, which developed on a serpentinized dunite intrusion in a granite-gneiss terrain ∼2.6 Ga ago, is characterized by an alternating succession of thick (∼1-3 m) carbonate-rich (dolomite and calcite) zones and silicate-rich (serpentines, talc, and quartz) zones; the upper ∼8 m section is especially rich in organic C (up to ∼1.4 wt.%). Petrologic and geochemical data suggest the upper ∼8 m section is composed of at least three soil profiles that developed on: (i) silicate-rich rock fragments (and minerals) that were transported from local sources (serpentinite and granite) by fluvial and/or eolian processes; and (ii) dolomite and calcite zones that formed by locally discharged groundwater. The Mg and Fe in the paleosol sequence were largely supplied from local sources (mostly serpentinite), but the Ca, Sr, and HCO₃⁻ were supplied by groundwater that originated from a surrounding granite-gneiss terrain. In the uppermost soil profile, the (Fe is retained, the Fe³⁺/Fe²⁺ ratio increases, and ferri-stilpnomelane is abundant. These data suggest the atmospheric pO₂ was much greater than ∼10^−3.7 atm (>0.1% present atmospheric level [PAL]).The carbonaceous matter in the soils is intimately associated with clays (talc, chlorite, and ferri-stilpnomelane) and occurs mostly as seams (20 μm to 1 mm thick) that parallel the soil horizons. These occurrences, crystallographic structures, H/C ratios, and δ¹³C_org values (−17.4 to −14.4‰ PDB) suggest that the carbonaceous matter is a remnant of in situ microbial mats, originally ∼1 to ∼20 mm thick. The microbial mats developed: (a) mostly on soil surfaces during the formation of silicate-rich soils, and (b) at the bottom of an evaporating, anoxic, alkaline pond during the precipitation of the Fe-rich dolomite. These δ¹³C_org values are difficult to be explained by a current popular idea of a methane- and organic haze-rich Archean atmosphere (Pavlov et al., 2001); these values, however, can be easily explained if the microbial mats were composed of cyanobacteria and heterotrophs that utilized the remnants of cyanobacteria in a strongly evaporating environment.  相似文献   

Behaviour of a trial embankment on hydraulically placed pfa     

T. W. Cousens  D. I. Stewart 《Engineering Geology》2003,70(3-4):293-303

The paper describes the performance of a 5.3-m trial embankment constructed on approximately 45 m of hydraulically placed pulverised fuel ash (pfa). It is planned to redevelop the 17-ha lagoon containing the pfa as a landfill. There is little variation in the particle size distribution of the uniformly graded silt sized pfa over the lagoon. However, the density of the pfa varies with depth with loose material underlying a denser surface layer, in a pattern that probably results from the water level in the lagoon during pfa deposition.

Settlement under the trial embankment was apparently largely complete by the end of the construction period (17 days), with approximately 300 mm of settlement under the crest of the embankment. The embankment settlement is significantly affected by compression of the loose layers within the deposit. Analysis of the problem using the conventional one-dimensional settlement method, and an m_v profile determined by CPT calibrated against laboratory tests gave a reasonable prediction of the embankment crest settlement.  相似文献   

Reply to Commentary     

Dona J. Stewart 《The Professional geographer》2003,55(2):277-278

  相似文献   

Rare earth element sources and modification in the Lower Kittanning coal bed, Pennsylvania: implications for the origin of coal mineral matter and rare earth element exposure in underground mines   总被引：1，自引：0，他引：1  

Steven J. Schatzel  Brian W. Stewart   《International Journal of Coal Geology》2003,54(3-4):223

In this study, we examine the variations in rare earth elements (REE) from the Lower Kittanning coal bed of eastern Ohio and western Pennsylvania, USA, in an attempt to understand the factors that control mineral matter deposition and modification in coal, and to evaluate possible REE mixed exposure hazards facing underground mine workers. The results of this study suggest that the Lower Kittanning coal mineral matter is derived primarily from a clastic source similar to that of the shale overburden. While highly charged cations like silicon, aluminum, and titanium remained relatively immobile within the coal mineral matter, iron (primarily as pyrite) was added from nonclastic sources, either during deposition of the coal mire vegetation or subsequent to burial. Other mobile cations (e.g., alkali and alkaline earth elements) appear to have been added to and/or leached from the originally deposited clastic mineral matter. Most of the sulfur in the Lower Kittanning coal bed is bound as FeS₂ in the mineral matter, but a majority of samples contain a small excess of S that is most likely organically bound.In general, the total rare earth element content (TREE) in coal ash is greater than that in the shale overburden. If the primary source of mineral matter is the same as that for the overlying shale, then REE must have been enriched in the coal mineral matter subsequent to deposition. The total rare earth element content of Lower Kittanning coals correlates strongly with Si concentration ([TREE]≈0.0024 [Si]), which provides a threshold for evaluating possible mixed exposure health effects. Chondrite-normalized REE patterns reveal a shale-like light rare earth element (LREE) enrichment for the coal, similar to that of the shale overburden, again suggesting a primarily clastic REE source. However, when normalized to the shale overburden, most of the coal ash samples display a small but distinct heavy rare earth element (HREE) enrichment. We surmise that the HREE were added and/or preferentially retained during epigenesis, possibly associated with groundwater flow through the coal unit, but not necessarily in close association with the addition of iron. At least some of the “excess” HREE could be organically bound within the Lower Kittanning coal.  相似文献   

A discrimination test procedure for ambiguity resolution on-the-fly   总被引：18，自引：2，他引：16  

J. Wang  M. P. Stewart  M. Tsakiri 《Journal of Geodesy》1998,72(11):644-653

Ambiguity validation tests include the acceptance test and discrimination test, which are both important steps in the Global Positioning System ambiguity resolution process. An ambiguity discrimination test procedure based on a test statistic which is constructed by the difference (not the ratio as used in current procedures) between the minimum and second minimum quadratic form of the residuals in ambiguity identification, and its standard deviation, is proposed. The distribution function of the proposed test statistic is theoretically identified as a standard normal distribution when the known a priori variance factor is used, or as a Student's t distribution when the estimated variance factor is used. With this procedure, the ambiguity discrimination test is based on a more rigorous test statistic whose critical value can be calculated with any chosen level of significance. Test results indicate that the proposed ambiguity discrimination test procedure is reliable for use in ambiguity resolution on-the-fly. Received: 17 December 1997 / Accepted: 30 June 1998  相似文献   

GLONASS Broadcast Orbit Computation     

Mike Stewart  Maria Tsakiri 《GPS Solutions》1998,2(2):16-27

The potential 48-satellite constellation offered by the combination of observations from both the GPS and GLONASS positioning systems has created considerable interest among existing GPS users. In the published literature, a considerable amount of work has been devoted to the theoretical issue of algorithm design for combined GPS/GLONASS positioning solutions. Little work has been published, however, on the practical conversion of existing GPS software to include GLONASS observations. This paper considers the computation issues pertaining to the GLONASS broadcast ephemeris for inclusion of GLONASS observations into existing GPS software. The format of the GLONASS broadcast ephemeris is discussed and theory of satellite orbits and their stepwise numerical integration is reviewed. Finally, a strategy for GLONASS broadcast ephemeris computation is proposed to facilitate combination of GPS and GLONASS observations. ? 1998 John Wiley & Sons, Inc.  相似文献