THE TRANSIENT ELECTROMAGNETIC RESPONSE OF A POLARIZABLE SPHERE IN A CONDUCTING HALF SPACE1     

TERRY LEE  LINDSAY THOMAS 《Geophysical Prospecting》1992,40(5):541-563

A polarizable sphere embedded in a conducting half-space can give rise to negative voltage transients in a coincident-loop time-domain electromagnetic system. Such transients have been observed in field situations. Our results are obtained from a model in which the contributions of the host rock, the currents in the sphere, and the interaction between the sphere and the host rock are separated and superposed. This model uses approximations to the integral equation solutions rather than finite-element or finite-difference approximations, and so allows very rapid computation. The theoretical demonstration suggests that interpretation of the negative voltage transients as a polarization response is valid, but more detailed interpretation of polarization properties may not be possible, because the superposition of the polarization response on the normal response depends strongly on the position of the target.  相似文献   

INVERSION OF FIRST-BREAK TRAVELTIME DATA OF DEEP SEISMIC REFLECTION PROFILES1     

THOMAS RÜHL  EWALD LÜSCHEN 《Geophysical Prospecting》1990,38(3):247-266

First breaks of 2D deep reflection data were used to construct velocity-depth models for improved static corrections to a deeper datum level and for geological interpretations. The highly redundant traveltime data were automatically picked and transformed directly into a velocity-depth model by maximum depth methods such as the Giese- and the Slichter-method. Comparisons with the results of synthetic calculations and a tomographic approach using iterative inversion methods (ART, SIRT) showed that maximum depth methods provide reliable velocity models as a basis for the computation of static corrections. These methods can economically be applied during data acquisition in the field. They provide particularly long-period static anomalies, which are of the order of 20–40 ms (0.5-1 wavelength) within CMP gathers of an example of a deep reflection profile in SW-Germany sited on crystalline basement. Reprocessing of this profile, which was aimed at the comparison between the effects of the originally used and the new statics, did not result in dramatically improved stacking quality but showed a subtle influence on the detailed appearance of deep crustal events.  相似文献   

Interpretation of moment measures derived from water-lain sediments   总被引：3，自引：0，他引：3  

P. G. SLY  R. L. THOMAS  B. R. PELLETIER† 《Sedimentology》1983,30(2):219-233

ABSTRACT Although mean size provides a useful value for characterizing sediments, it is an inadequate expression of the interaction between sediments and hydraulic regimes and many authors have explored the significance of other statistical moment measures in different parts of the particle size spectrum. Few publications, however, have described such relationships over a very wide range of particle sizes. Using a combined data base (of marine and lacustrine sediments) we have been able to demonstrate the following: The distributions of standard deviation (and entropy) values are repeated in similar form on either side of the sand size class, describing mixtures between sands and gravel and sands and clay. Skewness is modified by the hydraulic response of the sediment and a skewness divide is present at about 2.7 φ, where it coincides with the minimum shear velocity required to initiate particle motion. This divide may be used to distinguish between high and low energy regimes. Kurtosis, like standard deviation, is a measure of the mixing of end member populations but it is more sensitive to changes in the tails of distribution curves. The maximum positive peak of kurtosis, at about 2.5 φ, lies close to the skewness divide (in our data). By using skewness/kurtosis plots it is possible to define separate field distribution for sediments which, to a greater or lesser degree, appear to conform to hydraulic equilibrium. By using sediments which are close to equilibrium (and lie at or close to our skewness/kurtosis boundary curves, it should be possible to define hydraulic shear velocities, at the time of formation. Values may be established by empirical relationships or by defining the break-point (traction and intermittent suspension) between linear components of individual sample φ size/probability distribution plots. Such relationships should be enhanced by using hydraulic equivalent diameters instead of sieve diameters. In fine sediments, the use of fractional (silt: clay) ratios offers sensitivity at a level comparable to that of skewness/kurtosis plots and may be more easily computed.  相似文献   

ESSAI DE DETERMINATION AUTOMATIQUE DES VITESSES SISMIQUES PAR MESURES ENTRE PUITS*     

P. BOIS  M. LA PORTE  M. LAVERGNE  G. THOMAS 《Geophysical Prospecting》1971,19(1):42-83

Large memory computers create new possibilities for seismic interpretation in well-drilled areas, and more generally in problems involving deep sources. Records obtained by well-to-surface, surface-to-well and/or well-to-well propagation contain a large amount of information, owing to the great number of seismic rays travelling throughout the subsurface. This paper considers two successive steps: — direct case: computation of the first arrival times of seismic rays propagating through a two-dimensional subsurface with given vertical and horizontal velocity variations; — inverse case: computation of a two-dimensional velocity distribution, knowing the travel times of seismic rays through the subsurface. A computer program has been elaborated to solve both direct and inverse problems. There are no difficulties with the direct case; a certain number of uncertainties remain with the inverse case.  相似文献   

Lithified carbonate sediment and zeolitic tuff in basalts, Mid-Atlantic Ridge   总被引：1，自引：0，他引：1  

ROBERT E. GARRISON  JAMES R. HEIN  THOMAS F. ANDERSON 《Sedimentology》1973,20(3):399-410

Basaltic boulders dredged from the Mid-Atlantic Ridge contain lithified coccolith-foraminiferal ooze in fractures and small pockets. Textural and isotopic studies of this sediment provide no evidence for high temperature metamorphism. The lithified carbonate sediment occurs together with palagonitized basaltic glass and zeolitic tuff, and appears to have been squeezed into cracks and other voids within the congealed margins of partly-cooled basalt pillows which intruded soft carbonate ooze. Thermal metamorphism probably was precluded by rapid heat dissipation in convecting pore waters and by the thermal stability of calcareous microplankton. Low temperature alteration of basaltic glass appears to have provided the chemical milieu for precipitation of calcite cement as well as zeolites in the sediment.  相似文献   

A coarse-grained turbidite system with morphotectonic control (Middle Albian, Ondarroa, northern Iberia)     

THOMAS PLETSCH  MARTIN MESCHEDE 《Sedimentology》1995,42(3):523-523

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Trace fossils as diagnostic indicators of deep-marine environments, Middle Eocene Ainsa-Jaca basin, Spanish Pyrenees   总被引：1，自引：0，他引：1  

THOMAS G. HEARD†  KEVIN T. PICKERING 《Sedimentology》2008,55(4):809-844

A quantitative study of trace fossil abundance in the Middle Eocene deep-marine clastic systems, Ainsa-Jaca basin, Spanish Pyrenees, shows that they are powerful discriminators of submarine fan and related environments. Sixteen fan and related environments are recognized from upper-slope gully to the distal basin-floor. For each environment, the degree of bioturbation (density), trace-fossil diversity, number of pre-depositional and post-depositional trace fossils, as well as the number of graphoglyptid ichnospecies were quantified. In the more laterally confined and channel-dominated Ainsa basin, there is a trend of increasing bioturbation intensity and trace-fossil diversity away from channel-axis to off-axis environments. In the more unconfined and distal Jaca basin, there is a trend of increasing trace-fossil diversity and number of pre-depositional trace fossils including graphoglyptids from the channel-lobe transition to the fan-fringe. The trace-fossil assemblages of the Ainsa-Jaca basin are characteristic of a number of sub-ichnofacies of the Nereites ichnofacies. In the distal Jaca basin, the Paleodictyon sub-ichnofacies occurs in the lobe-fringe and fan-fringe, whereas the distal basin-floor has a trace-fossil assemblage typical of the Paleodictyon sub-ichnofacies, but with a high proportion of post-depositional fodinichnia . Trace-fossil assemblages of proximal basin, axial, environments are characteristic of the Ophiomorpha rudis sub-ichnofacies, whilst proximal off-axis environments, have a mixed Paleodictyon-Ophiomorpha rudis sub-ichnofacies trace-fossil assemblage.  相似文献   

Time-scales of Differentiation from Mafic Parents to Rhyolite in North American Continental Arcs   总被引：4，自引：4，他引：4  

REAGAN  M. K.; SIMS  K. W. W.; ERICH  J.; THOMAS  R. B.; CHENG  H.; EDWARDS  R. L.; LAYNE  G.; BALL  L. 《Journal of Petrology》2003,44(9):1703-1726

Young rhyolites and associated lavas and magmatic enclaves fromthe Katmai–Novarupta volcanic system (Alaskan Peninsula),and the Crater Lake and Medicine Lake volcanic system (Cascades)were analyzed for U and Th isotope abundances, as well as majorand trace element concentrations, to investigate the time-scalesof the processes that lead to rhyolite generation in continentalarcs. Basalts and basaltic andesites typically migrate fromthe mantle to the surface within several thousand years. Variationsin (²³⁰Th)/(²³²Th) and (²³⁸U)/(²³²Th) ratios with SiO₂ concentrationsin intermediate lavas appear to result from crystal fractionationcombined with assimilation of recently crystallized magmas.These data also suggest that  相似文献   

THE DRIVING FORCES OF LAND CHANGE IN THE NORTHERN PIEDMONT OF THE UNITED STATES     

ROGER F. AUCH  DARRELL E. NAPTON  STEVEN KAMBLY  THOMAS R. MORELAND JR.  KRISTI L. SAYLER 《Geographical review》2012,102(1):53-75

Driving forces facilitate or inhibit land‐use / land‐cover change. Human driving forces include political, economic, cultural, and social attributes that often change across time and space. Remotely sensed imagery provides regional land‐change data for the Northern Piedmont, an ecoregion of the United States that continued to urbanize after 1970 through conversion of agricultural and forest land covers to developed uses. Eight major driving forces facilitated most of the land conversion; other drivers inhibited or slowed change. A synergistic web of drivers may be more important in understanding land change than individual drivers by themselves.  相似文献   

THE INFLUENCE OF RIPARIAN VEGETATION COVER ON DIFFUSE LATERAL SEDIMENT CONNECTIVITY AND BIOGEOMORPHIC PROCESSES IN A MEDIUM‐SIZED AGRICULTURAL CATCHMENT,AUSTRIA     

RONALD E. POEPPL  MARGRETH KEILER  KIRSTEN VON ELVERFELDT  IRENE ZWEIMUELLER  THOMAS GLADE 《Geografiska Annaler: Series A, Physical Geography》2012,94(4):511-529

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