bluuebunny/crossref_metadata_2025_split huggingface dataset:
Effects of biogenic silica on sediment compaction and slope stability on the Pacific margin of the Antarctic Peninsula
Authors: Volpi V. , Camerlenghi A. , Hillenbrand C.‐D. , Rebesco M. , Ivaldi R. 
Published: August 2003

AbstractAnalysis of physical properties measured on cores and on discrete samples collected by the Ocean Drilling Programme (ODP) Leg 178 on the Pacific margin of the Antarctic Peninsula reveals anomalous down‐hole curves of porosity, density, water content, and P‐wave velocity. These indicate an overall trend of increasing porosity with depth and suggest that the drifts are mostly undercompacted. In one of the two boreholes analysed, a sharp decrease in porosity, matching increasing bulk sediment density and increasing compressional velocity occurs towards the base of the hole, which corresponds to a bottom‐simulating reflector in the seismic section. Analysis of seismic reflection, down‐hole logging, geotechnical and mineralogical data from two drilling sites indicates that the observed anomalous consolidation trends are a consequence of the presence of biogenic silica (diatom and radiolarian skeletons) even with a small to moderate amount. Above the bottom‐simulating reflector, intergranular contacts among whole or broken siliceous microfossils prevent normal sediment consolidation. Diagenetic alteration of biogenic opal‐A to opal‐CT causes a dramatic reduction of intra‐ and interskeletal porosity allowing sediments to consolidate at depth. This results in overpressuring and a decrease in the effective stress. Excess fluids are expelled towards the sediment surface through near vertical, small throw normal faults extending from the diagenetic front to the seafloor and affecting the stability of the submarine slope in the form of gravitational creep along a weakened surface.This work shows how physical properties of shallow fine‐grained marine sediments can be analysed as basin‐wide indicators of biogenic silica abundance. The diagenetic alteration of siliceous microfossils is a possible cause of slope instability along world continental margins where bottom‐simulating reflectors related to silica diagenesis are present at a regional scale.
Modelling detrital cooling‐age populations: insights from two Himalayan catchments
Authors: Brewer I. D. , Burbank D. W. , Hodges K. V. 
Published: August 2003

AbstractThe distribution of detrital mineral cooling ages in river sediment provides a proxy record for the erosional history of mountain ranges. We have developed a numerical model that predicts detrital mineral age distributions for individual catchments in which particle paths move vertically toward the surface. Despite a restrictive set of assumptions, the model permits theoretical exploration of the effects of thermal structure, erosion rate, and topography on cooling ages. Hypsometry of the source‐area catchment is shown to exert a fundamental control on the frequency distribution of bedrock and detrital ages. We illustrate this approach by generating synthetic 40Ar/39Ar muscovite age distributions for two catchments with contrasting erosion rates in central Nepal and then by comparing actual measured cooling‐age distributions with the synthetic ones. Monte Carlo sampling is used to assess the mismatch between observed and synthetic age distributions and to explore the dependence of that mismatch on the complexity of the synthetic age signal and on the number of grains analysed. Observed detrital cooling ages are well matched by predicted ages for a more slowly eroding Himalayan catchment. A poorer match for a rapidly eroding catchment may result from some combination of large analytical uncertainties in the detrital ages and inhomogeneous erosion rates within the basin. Such mismatches emphasize the need for more accurate thermal and kinematic models and for sampling strategies that are adapted to catchment‐specific geologic and geomorphic conditions.
Use of VFSA for resolution, sensitivity and uncertainty analysis in 1D DC resistivity and IP inversion
Authors: Bhattacharya Bimalendu B. , Shalivahan  , Sen Mrinal K. 
Published: August 2003

ABSTRACTWe present results from the resolution and sensitivity analysis of 1D DC resistivity and IP sounding data using a non‐linear inversion. The inversion scheme uses a theoretically correct Metropolis–Gibbs' sampling technique and an approximate method using numerous models sampled by a global optimization algorithm called very fast simulated annealing (VFSA). VFSA has recently been found to be computationally efficient in several geophysical parameter estimation problems. Unlike conventional simulated annealing (SA), in VFSA the perturbations are generated from the model parameters according to a Cauchy‐like distribution whose shape changes with each iteration. This results in an algorithm that converges much faster than a standard SA. In the course of finding the optimal solution, VFSA samples several models from the search space. All these models can be used to obtain estimates of uncertainty in the derived solution. This method makes no assumptions about the shape of an a posteriori probability density function in the model space. Here, we carry out a VFSA‐based sensitivity analysis with several synthetic and field sounding data sets for resistivity and IP. The resolution capability of the VFSA algorithm as seen from the sensitivity analysis is satisfactory. The interpretation of VES and IP sounding data by VFSA, incorporating resolution, sensitivity and uncertainty of layer parameters, would generally be more useful than the conventional best‐fit techniques.
3D wavepath migration
Authors: Sun Hongchuan , Schuster Gerard T. 
Published: August 2003

ABSTRACT3D Kirchhoff migration (KM) smears a trace's time sample along a quasi‐ellipsoid in the model space. This is a costly and sometimes noisy process as reflection energy is smeared far away from the actual reflector position, introducing far‐field migration artefacts. As a reduced form of 3D KM, 3D wavepath migration (WM) smears a picked reflection arrival to a small Fresnel zone portion centred about the specular reflection point, leading to fewer migration artefacts and reduced computation time. Both the traveltime and the angle of incidence are required by WM for locating the specular reflection point. Our results with 3D prestack synthetic data show that WM generates fewer migration artefacts and can sometimes define complex structure better than KM. Our results with 3D prestack field data show that WM can mostly suppress migration artefacts and can sometimes resolve reflection interfaces better than KM. The CPU comparison shows that, for both the synthetic and field data examples, WM can be more than an order of magnitude faster than KM. The limitation with 3D WM is that the angle of incidence calculation is sensitive to the recording geometry and the signal‐to‐noise (S/N) ratio, which can lead to blurred images.
Reflection and transmission responses of layered transversely isotropic viscoelastic media
Authors: Stovas Alexey , Ursin Bjorn 
Published: August 2003

ABSTRACTWe consider a layered heterogeneous viscoelastic transversely isotropic medium with a vertical symmetry axis (a viscoelastic TIV medium) and parameters that depend on depth only. This takes into account intrinsic attenuation, anisotropy and thin layering. The seismic wavefield is decomposed into up‐ and downgoing waves scaled by the vertical energy flux. This scaling gives important symmetry relationships for both reflection and transmission (R/T) responses. For a stack of homogeneous layers, the exact reflection response can be computed in a numerically stable way by a simple layer‐recursive algorithm. We derive exact plane‐wave R/T coefficients and several linear and quadratic approximations between two viscoelastic TIV media, as functions of the real‐valued horizontal slowness. The approximations are valid for pre‐ and post‐critical values of horizontal slowness provided that the proper complex square roots are used when computing the vertical slowness. Numerical examples demonstrate that the quadratic approximations can be used for large differences in medium parameters, while the linear approximations can be used for small differences. For weak anisotropy it is sufficient to use an isotropic background medium, while for strong anisotropy it is necessary to use a weak TIV or TIV background medium. We also extend the O'Doherty–Anstey formula to the P‐ and SV‐wave transmission responses of a stack of viscoelastic TIV layers, taking into account intrinsic attenuation, anisotropy and thin layering.
The Wilson–Burg method of spectral factorization with application to helical filtering
Authors: Fomel Sergey , Sava Paul , Rickett James , Claerbout Jon F. 
Published: August 2003

ABSTRACTSpectral factorization is a computational procedure for constructing minimum‐phase (stable inverse) filters required for recursive inverse filtering. We present a novel method of spectral factorization. The method iteratively constructs an approximation of the minimum‐phase filter with the given autocorrelation by repeated forward and inverse filtering and rearranging of the terms. This procedure is especially efficient in the multidimensional case, where the inverse recursive filtering is enabled by the helix transform.To exemplify a practical application of the proposed method, we consider the problem of smooth two‐dimensional data regularization. Splines in tension are smooth interpolation surfaces whose behaviour in unconstrained regions is controlled by the tension parameter. We show that such surfaces can be efficiently constructed with recursive filter preconditioning and we introduce a family of corresponding two‐dimensional minimum‐phase filters. The filters are created by spectral factorization on a helix.
Using a non‐integer moment of the impulse response to estimate the half‐space conductivity
Authors: Lee Terry J. , Smith Richard S. , Hyde Christophe S.B. 
Published: August 2003

ABSTRACTThe nth‐order moments of the electromagnetic impulse response are useful for interpreting electromagnetic data. We have derived an analytic expression for the half‐order moment of a conductive half‐space. By inverting this expression, the measured half‐order moment can be used to estimate an apparent conductivity of the ground. The first‐order moment can also be used to estimate the half‐space conductivity. A sensitivity analysis indicates that for an airborne EM configuration, the half‐order moment will be most sensitive to material in the top 26–48 m, while the first‐order moment will be sensitive to deeper material (down to depths between 66 and 127 m).
Frequency‐dependent anisotropy due to meso‐scale fractures in the presence of equant porosity
Authors: Chapman Mark 
Published: August 2003

ABSTRACTFractured rock is often modelled under the assumption of perfect fluid pressure equalization between the fractures and equant porosity. This is consistent with laboratory estimates of the characteristic squirt‐flow frequency. However, these laboratory measurements are carried out on rock samples which do not contain large fractures. We consider coupled fluid motion on two scales: the grain scale which controls behaviour in laboratory experiments and the fracture scale. Our approach reproduces generally accepted results in the low‐ and high‐frequency limits. Even under the assumption of a high squirt‐flow frequency, we find that frequency‐dependent anisotropy can occur in the seismic frequency band when larger fractures are present. Shear‐wave splitting becomes dependent on frequency, with the size of the fractures playing a controlling role in the relationship. Strong anisotropic attenuation can occur in the seismic frequency band. The magnitude of the frequency dependence is influenced strongly by the extent of equant porosity. With these results, it becomes possible in principle to distinguish between fracture‐ and microcrack‐induced anisotropy, or more ambitiously to measure a characteristic fracture length from seismic data.
Traveltime computation for 3D anisotropic media by a finite‐difference perturbation method
Authors: Soukina S.M. , Gajewski D. , Kashtan B.M. 
Published: August 2003

ABSTRACTThe first‐order perturbation theory is used for fast 3D computation of quasi‐compressional (qP)‐wave traveltimes in arbitrarily anisotropic media. For efficiency we implement the perturbation approach using a finite‐difference (FD) eikonal solver. Traveltimes in the unperturbed reference medium are computed with an FD eikonal solver, while perturbed traveltimes are obtained by adding a traveltime correction to the traveltimes of the reference medium. The traveltime correction must be computed along the raypath in the reference medium. Since the raypath is not determined in FD eikonal solvers, we approximate rays by linear segments corresponding to the direction of the phase normal of plane wavefronts in each cell. An isotropic medium as a reference medium works well for weak anisotropy. Using a medium with ellipsoidal anisotropy as a background medium in the perturbation approach allows us to consider stronger anisotropy without losing computational speed. The traveltime computation in media with ellipsoidal anisotropy using an FD eikonal solver is fast and accurate. The relative error is below 0.5% for the models investigated in this study. Numerical examples show that the reference model with ellipsoidal anisotropy allows us to compute the traveltime for models with strong anisotropy with an improved accuracy compared with the isotropic reference medium.
Modelling frequency‐dependent seismic anisotropy in fluid‐saturated rock with aligned fractures: implication of fracture size estimation from anisotropic measurements
Authors: Maultzsch Sonja , Chapman Mark , Liu Enru , Li Xiang Yang 
Published: August 2003

ABSTRACTMeasurements of seismic anisotropy in fractured rock are used at present to deduce information about the fracture orientation and the spatial distribution of fracture intensity. Analysis of the data is based upon equivalent‐medium theories that describe the elastic response of a rock containing cracks or fractures in the long‐wavelength limit. Conventional models assume frequency independence and cannot distinguish between microcracks and macrofractures. The latter, however, control the fluid flow in many subsurface reservoirs. Therefore, the fracture size is essential information for reservoir engineers. In this study we apply a new equivalent‐medium theory that models frequency‐dependent anisotropy and is sensitive to the length scale of fractures. The model considers velocity dispersion and attenuation due to a squirt‐flow mechanism at two different scales: the grain scale (microcracks and equant matrix porosity) and formation‐scale fractures. The theory is first tested and calibrated against published laboratory data. Then we present the analysis and modelling of frequency‐dependent shear‐wave splitting in multicomponent VSP data from a tight gas reservoir. We invert for fracture density and fracture size from the frequency dependence of the time delay between split shear waves. The derived fracture length matches independent observations from borehole data.
CD25+, interleukin‐10‐producing CD4+ T cells are required for suppressor cell production and immune privilege in the anterior chamber of the eye
Authors: Skelsey Molly E. , Mayhew Elizabeth , Niederkorn Jerry Y. 
Published: August 2003

SummaryAn important factor in the establishment of ocular immune privilege is the dynamic down regulation of T helper 1 (Th1) immune responses that occurs in response to antigens delivered intraocularly; a phenomenon that has been termed anterior chamber‐associated immune deviation (ACAID). ACAID is characterized by the generation of splenic regulatory cells that inhibit the expression of delayed‐type hypersensitivity. Previous studies have shown that antigens introduced into the anterior chamber of the eye induce the generation of a CD4+ T‐cell population that suppress the induction of Th1 immune responses and the appearance of a second population of CD8+ T regulatory cells that suppresses the expression of Th1 inflammatory responses (= efferent suppressor cells). Experiments described here characterized the function of the CD4+ ACAID suppressor cell population and its effect on the generation of CD8+ efferent suppressor cells that inhibit the expression of DTH in situ. Both in vivo and in vitro experiments demonstrated that CD4+ T cells are required for the generation of CD8+ efferent suppressor cells. CD4+ T cells do not require cell contact with CD8+ T cells; instead they produce soluble IL‐10 that is sufficient for the generation of ACAID suppressor cells. Finally, the CD4+ afferent T suppressor cells are not natural killer T cells, but do express the CD25 cell surface marker.
Interleukin‐12 and interleukin‐2‐induced invariant natural killer T‐cell cytokine secretion and perforin expression independent of T‐cell receptor activation
Authors: Hou Runhua , Goloubeva Olga , Neuberg Donna S. , Strominger Jack L. , Wilson S. Brian 
Published: August 2003

SummaryHuman invariant natural killer (iNK) T cells expressing an invariant Vα24‐Jα15 T‐cell receptor (TCR) are thought to be important regulators of autoimmunity and tumour surveillance. Two major subsets of iNK T cells, CD4+ or CD4− CD8− are known to exist, but the in vivo importance of CD4 expression is unclear. Since interleukin‐12 (IL‐12) is a key iNK T‐cell‐activating cytokine, the effect of IL‐12 plus or minus the T‐cell growth factor IL‐2 on a large panel of CD4+ versus CD4− CD8− iNK T‐cell clones was examined. Strikingly, IL‐12 and IL‐2 significantly activated iNK T cells to secrete IL‐4, interferon‐γ and granulocyte–macrophage colony‐stimulating factor, and up‐regulated perforin expression in the absence of TCR stimulation. Furthermore, IL‐2 and IL‐12 treatment resulted in a preferential increase in apoptosis of CD4− CD8− clones. Thus, independent of TCR activation, IL‐2 and IL‐12 can directly activate iNK T cells and provide a selective advantage to the CD4+ iNK T‐cell population.
Interferon‐α disables dendritic cell precursors: dendritic cells derived from interferon‐α‐treated monocytes are defective in maturation and T‐cell stimulation
Authors: Dauer Marc , Pohl Katrin , Obermaier Bianca , Meskendahl Tobias , Röbe Julian , Schnurr Max , Endres Stefan , Eigler Andreas 
Published: August 2003

SummaryDendritic cells (DC) can be derived from monocytes in vitro by culture with granulocyte–macrophage colony‐stimulating factor (GM‐CSF) and interleukin‐4 (IL‐4). It is unknown whether this regimen reflects DC differentiation from blood precursors under physiological conditions. Induction of DC development from monocytes by interferon‐α (IFN‐α) may occur in vivo during infection or inflammation and thus may represent a more physiological approach to DC differentiation in vitro. Here, we show that incubation of GM‐CSF‐cultured monocytes with IFN‐α does not induce DC differentiation: cells maintain their original phenotype and cytokine secretion pattern. Even after stimulation with pro‐inflammatory or T‐cell‐derived activation signals, IFN‐α‐treated monocytes do not develop DC characteristics. Addition of IL‐4 during stimulation of IFN‐α‐treated monocytes results in the rapid development of DC‐like cells expressing co‐stimulatory molecules, CD83 and chemokine receptor CCR7, indicating that some degree of developmental plasticity is preserved. However, DC pre‐activated with IFN‐α are less effective in inducing allogeneic or antigen‐specific autologous T‐cell proliferation, produce less IL‐12 and express lower levels of CCR7 compared to DC generated by culture with GM‐CSF and IL‐4. Incubating GM‐CSF‐cultured monocytes simultaneously with IFN‐α and IL‐4 does not affect phenotypic maturation of DC, but reduces IL‐12 production upon pro‐inflammatory activation. We conclude that: (1) IFN‐α fails to induce DC differentiation and thus cannot replace IL‐4 in generating DC from monocytes in vitro; and (2) the presence of IFN‐α prior to or during differentiation of DC from monocyte precursors alters their response to maturation stimuli and may affect their capacity to stimulate T helper type 1 immune responses in vivo.
Influence of oestrogen receptor α and β on the immune system in aged female mice
Authors: Islander U. , Erlandsson M. C. , Hasséus B. , Jonsson C. A. , Ohlsson C. , Gustafsson J‐Å. , Dahlgren U. , Carlsten H. 
Published: August 2003

SummaryOestrogen has a dichotomous effect on the immune system. T and B lymphopoiesis in thymus and bone marrow is suppressed, whereas antibody production is stimulated by oestrogen. In this study the importance of the oestrogen receptors (ER) ER‐α and ER‐β in the aged immune system was investigated in 18 months old‐wild type (WT), ER‐α (ERKO), ER‐β (BERKO) and double ER‐α and ER‐β (DERKO) knock‐out mice, and compared with 4 months old WT mice. Cell phenotypes in bone marrow, spleen and thymus, and the frequency of immunoglobulin (Ig) spot forming cells (SFC) were determined. We show here that the 17‐β‐oestradiol (E2)‐induced downregulation of B lymphopoietic cells in bone marrow of young ovariectomized mice can be mediated through both ER‐α and ER‐β. However, only ER‐α is required for the age‐related increased frequency of immunoglobulin M (IgM) SFC in the bone marrow, as well as for the increased production of interleukin‐10 (IL‐10) from cultured splenocytes in aged mice. Furthermore, increased age in WT mice resulted in lower levels of both pro‐ and pre‐B cells but increased frequency of IgM SFC in the bone marrow, as well as increased frequency of both IgM and IgA SFC in the spleen. Results from this study provide valuable information regarding the specific functions of ER‐α and ER‐β in the aged immune system.
The role of antigen‐presenting cells and interleukin‐12 in the priming of antigen‐specific CD4+ T cells by immune stimulating complexes
Authors: Robson Neil C. , Beacock‐Sharp Helen , Donachie Anne M. , Mowat Allan McI. 
Published: August 2003

SummaryImmune stimulating complexes (ISCOMs) containing the saponin adjuvant Quil A are vaccine adjuvants that promote a wide range of immune responses in vivo, including delayed‐type hypersensitivity (DTH) and the secretion of both T helper 1 (Th1) and Th2 cytokines. However, the antigen‐presenting cell (APC) responsible for the induction of these responses has not been characterized. Here we have investigated the role of dendritic cells (DC), macrophages (Mφ) and B cells in the priming of antigen‐specific CD4+ T cells in vitro by ISCOMs containing ovalbumin (OVA). OVA ISCOMs pulsed bone marrow (BM)‐derived DC but not BM Mφ, nor naïve B cells prime resting antigen‐specific CD4+ T cells, and this response is greatly enhanced if DC are activated with lipopolysaccharide (LPS). Of the APC found in the spleen, only DC had the capacity to prime resting antigen specific CD4+ T cells following exposure to OVA ISCOMs in vitro, while Mφ and B cells were ineffective. DC, but not B cells purified from the draining lymph nodes of mice immunized with OVA ISCOMs also primed resting antigen‐specific CD4+ T cells in vitro, suggesting that DC are also critical in vivo. Using DC and T cells from interleukin (IL)‐12 p40−/− mice, we also identified a crucial role for IL‐12 in the priming of optimal CD4+ T cell responses by OVA ISCOMs. We suggest that DC are the principal APC responsible for the priming of CD4+ T cells by ISCOMs in vivo and that directed targeting of these vectors to DC may enhance their efficancy as vaccine adjuvants.