Resolution limits for crosswell migration and traveltime tomography   总被引：2，自引：1，他引：2  

Gerard T. Schuster 《Geophysical Journal International》1996,127(2):427-440

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A general correspondence principle for time-domain electromagnetic wave and diffusion fields     

Adrianus T. de  Hoop 《Geophysical Journal International》1996,127(3):757-761

A general correspondence principle is presented that relates any time-domain electromagnetic diffusion field to an electromagnetic wavefield in a 'corresponding' configuration. The principle applies to arbitrarily inhomogeneous and anisotropic media and arbitrary transmitters and receivers. For the correspondence between the two types of electromagnetic fields to hold, the electric conductivity in the diffusive case and the permittivity in the wavefield case should have the same spatial variation, while the permeability distributions in space in the two cases are to be identical. Essential steps in the derivation of the correspondence principle are the use of the time Laplace transformation of causal signals, taken at real, positive values of the transform parameter, the Schouten-Van der Pol theorem in the theory of the Laplace transformation, and the reliance upon Lerch's theorem of the uniqueness of the interrelation between causal field quantities and their time-Laplace-transform representations at real, positive values of the transform parameter. Correspondence is then established between the tensorial Green's functions in the two cases, where the Green's functions are the point-receiver responses (either electric or magnetic field) to point-transmitter excitations (either electric- or magnetic-current source).
Through the correspondence principle, all transient electromagnetic wavefields (where losses are neglected) have as a counterpart a transient diffusive electromagnetic field (where the electric displacement current is neglected). The interrelation yields the tool to compare quantitatively the potentialities of the two types of fields in transient electromagnetic geophysical prospecting.
Finally, a general medium-parameter scaling law for time-domain electromagnetic wavefields is presented.  相似文献   

Egg laying, chick growth and food of kittiwakes Rissa tridactyla at Hopen, Svalbard   总被引：1，自引：0，他引：1  

ROBERT T. BARRETT 《Polar research》1996,15(2):107-113

In 1984, kittiwakes at Hopen laid eggs ca. 14 days later than previously found in North Norway and on the Kola Peninsula. The mean clutch size, egg volume, and hatching success were otherwise very similar to those documented on the mainland. Although feeding trips were very long, a rapid chick growth and high rate of chick survival suggested that food availability was not a limiting factor in 1984. In contrast to other sites at Svalbard where polar cod Boreogadus saida and crustaceans dominate kittiwake diets, the diets of chicks at Hopen consisted mainly of capelin Mallotus villosus which the adults probably collected at or beyond the polar front southeast of the island.  相似文献   

Fluvial process and the establishment of bottomland trees   总被引：1，自引：0，他引：1  

Michael L. Scott  Jonathan M. Friedman  Gregor T. Auble 《Geomorphology》1996,14(4):327-339

The effects of river regulation on bottomland tree communities in western North America have generated substantial concern because of the important habitat and aesthetic values of these communities. Consideration of such effects in water management decisions has been hampered by the apparent variability of responses of bottomland tree communities to flow alteration. When the relation between streamflow and tree establishment is placed in a geomorphic context, however, much of that variability is explained, and prediction of changes in the tree community is improved.The relation between streamflow and establishment of bottomland trees is conditioned by the dominant fluvial process or processes acting along a stream. For successful establishment, cottonwoods, poplars, and willows require bare, moist surfaces protected from disturbance. Channel narrowing, channel meandering, and flood deposition promote different spatial and temporal patterns of establishment. During channel narrowing, the site requirements are met on portions of the bed abandoned by the stream, and establishment is associated with a period of low flow lasting one to several years. During channel meandering, the requirements are met on point bars following moderate or higher peak flows. Following flood deposition, the requirements are met on flood deposits ;high above the channel bed. Flood deposition can occur along most streams, but where a channel is constrained by a narrow valley, this process may be the only mechanism that can produce a bare, moist surface high enough to be safe from future disturbance. Because of differences in local bedrock, tributary influence, or geologic history, two nearby reaches of the same stream may be dominated by different fluvial processes and have different spatial and temporal patterns of trees. We illustrate this phenomenon with examples from forests of plains cottonwood (Populus deltoides ssp. monilifera) along meandering and constrained reaches of the Missouri River in Montana.  相似文献   

Distribution and morphology of sinkholes triggered by flooding following Tropical Storm Alberto at Albany, Georgia, USA   总被引：1，自引：0，他引：1  

James A. Hyatt  Peter M. Jacobs 《Geomorphology》1996,17(4):305-316

Flooding of the Flint River in July 1994 triggered the collapse of at least 312 sinkholes in the karstic Dougherty Plain at Albany, Georgia. We examined the distribution and morphology of these new sinkholes to evaluate the mode of formation, to characterize early stages of the evolution of sinkhole form, and to estimate the lowering of the surface associated with the development of new sinkholes.Eighty-eight percent of sinkholes occur inside the limits of flooding, especially in areas of sandy overburden, and they often follow joint-controlled linear trends. Sinkhole dimensions are log-normally distributed with median values of circumference = 5.7 m, length = 1.8 m, width = 1.6 m, and depth = 0.7 m; asymmetry (L:W) = 1.2. Cross-sectional forms range from narrow cylinders to large bowls, with many sinkholes having undercut sides.Flooding triggered the formation of sinkholes by saturating and liquefying overburden, which caused soil arches to collapse and flow into cavities in bedrock. The prevalence of sinkholes near the periphery of flooding suggests that drainage and loss of buoyant support as flood waters subsided may also have contributed to failure. A volume ratio index is used to quantify the three dimensional geometric form of sinkholes. Initially, small cylindrical shafts open over a bedrock joint, followed by progressive slumping that leads to widening and increases in volume to a final bowl form. Estimates of the aggregate volume of overburden transported underground in flooded areas range from 7,990 to 11,130 m3. Averaged over flooded areas, this accounts for 0.26 to 0.37 mm/km² lowering of the surface. Based on a 500 year recurrence interval for the flood event, values for lowering of the surface range from 0.52 to 0.74 mm per 1,000 years. These values are an order of magnitude less than estimates of carbonate dissolution and suggest that transport of overburden underground is limited by triggering events.  相似文献   

Considerations of Scale and Ecological Processes in a Forest Migration Model: A Reply to Roth     

James Dyer 《The Professional geographer》1996,48(1):86-88

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Policy concerns in Bulgarian food distribution     

Rene Mueller Dr.  James Mueller Dr. 《GeoJournal》1996,38(2):167-174

The Bulgarian food distribution system, like those of its Central and East European neighbours, lags behind the West in critical performance areas. Previous research quantified Central and East European food distribution deficiencies on the basis of a five factor model. This study investigates the attitudes of 250 Bulgarian consumers using these same five factors: food processing, quality, cost, availability and retailing. The findings show that the Bulgarian food distribution system does not satisfy consumer wants as well as do systems in other former communist countries in four of the five factors. At the same time, Bulgarian consumers are more satisfied than UK and US consumers in the system's ability to make some foods available. The study also shows that Bulgarian consumers who use privatised shops perceive the system more positively than those who use state and cooperative outlets. The implications of these and other findings are discussed in terms of critical policy concerns for government and industry stakeholders.  相似文献   

Phase relations of osumilite and dehydration melting in pelitic rocks: a simple thermodynamic model for the KFMASH system     

T. J. B. Holland  E. V. S. S. K. Babu  D. J. Waters 《Contributions to Mineralogy and Petrology》1996,124(3-4):383-394

 Thermodynamic modelling of (1) osumilite solid solutions and (2) dehydration melting in pelitic compositions within the KFMASH system is quite successful in reproducing the invariant and univariant reactions determined in experimental studies. Even though rather preliminary, such melt thermodynamic models may be very useful in interpolating and extrapolating the limited information available from a small number of experimental runs. These methods allow the compositions of all phases to be monitored as a function of pressure, temperature and equilibrium phase assemblage for any desired bulk composition. Locating the higher variance phase fields (e.g. quadrivariant, quinivariant) is often difficult or impossible by inspection, but is made relatively easy using thermodynamic software such as thermocalc. In the KFMASH system the calculated partition of Fe and Mg between osumilite, garnet, cordierite, orthopyroxene and biotite are shown to be in good agreement with experimental and natural data and allow reliable calculation of mineral compositions coexisting with quartz-saturated and H₂O-undersaturated melts for a variety of bulk compositions. These phase diagram calculations allow quite tight limits to be placed on the pressure, temperature and water activity conditions which accompanied metamorphism of natural osumilite occurrences in Nain, Namaqualand, and Rogaland. At fixed bulk composition, the initial melting of pelites by dehydration of biotite can occur via univariant, divariant or trivariant equilibria depending upon pressure of metamorphism. Of particular interest is that, for low pressures or more magnesian bulk compositions, fluid-absent melting begins by generating liquid from the high-variance assemblage biotite+cordierite+K-feldspar+ quartz. This type of modelling allows investigation, at least qualitatively, of the fine scale details of melt production as a function of changes in pressure, temperature and bulk composition. Received: 29 November 1995 / Accepted: 22 April 1996  相似文献   

A new, instantaneous aeolian sand trap design for field use   总被引：4，自引：0，他引：4  

D. W. T. JACKSON 《Sedimentology》1996,43(5):791-796

A new aeolian sediment trap is described which can give up to 1 Hz measurement frequency in field conditions. The trap adopts a circular, horizontal trap design with a load cell connection to give continuous, unobtrusive trap measurement of sediment flux. Simultaneous velocity recording is carried out using an anemometer. Trap construction costs are approximately £200. Initial results in field conditions using a direct comparison of wind velocity data, sampled at an equivalent frequency, have given a first-order relationship between sediment flux and velocity. The trap enables simultaneous sampling of wind velocity and sediment flux at a sufficiently short interval to enable investigation of sediment transport dynamics under a variety of field conditions.  相似文献   

Continental break-up and collision in the Neoproterozoic and Palaeozoic — A tale of Baltica and Laurentia     

T.H. Torsvik  M.A. Smethurst  J.G. Meert  R. Van der Voo  W.S. McKerrow  M.D. Brasier  B.A. Sturt  H.J. Walderhaug 《Earth》1996,40(3-4):229-258

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