DIFFERENT METHODS OF TIME-DEPTH CONVERSION WITH AND WITHOUT MIGRATION*

The increasing use of velocity analysis programs in seismic processing in addition to direct application to normal move out corrections often makes it possible to study the variation of the average velocity versus time and distance. Usually, it is even feasible to compute interval velocities with a good accuracy. We intend to use these velocities to convert the usual time domain displays to the depth domain. An exact conversion requires the knowledge of all interval velocities and all the velocity interfaces. However, when the dips are small, the conversion can be done without considering the dip of all these interfaces, and in some cases migration can even be completely neglected. Three different programs will be described and their use discussed in view of the type of problems to which they are applied. Practical examples of the various methods will be presented.     

UTAH'S GREAT SALT LAKE: A COMPLEX ENVIRONMENTAL‐SOCIETAL SYSTEM*

ABSTRACT. The Great Salt Lake in Utah is a highly complex system, consisting of physical and chemical lake properties and environmental and societal subsystems. Components of these subsystems interact in many nonintuitive ways, with the result that changes and management decisions in one part of the lake system can produce unexpected changes elsewhere. The management history of the lake does not show serious consideration of these interconnections, although recent efforts suggest that this may be changing. Legislative and financial support will be required if integrated management of this unique resource is to succeed.     

Incorporating Cellular Automata simulators as analytical engines in GIS

Contemporvy GIS suffer from a variety of problems. These include poor ability to handle dynamic spatial models, poor performance for many operations, and poor handling of the temporal dimension. Cellular Automata (CA) are dynamic mathematical systems based on discrete time and space. While CA have many similarities to GIS, and indeed excel in many areas in which GIS are deficient, they cannot themselves be considered as GIS since they lack the necessary sophisticated Capabilities in data input, data storage and retrieval, and data output. However, the advantages of CA in data analysis and modelling are significant. In order to exploit fully the advantages of each system - GIS and CA - we have developed a methodology and prototype system for coupling GIS and CA, in which the CA serves as an alternative analytical engine for the GIS. In the prototype system, spatial, temporal (i.e. time series), and spatio-temporal filters as well as spatial diffusion operators are developed. In addition, the integrated system provides a flexible framework for the programming and running of dynamic spatial models. We conclude that CA represent a viable alternate analytical engine for GIS and provide increased ability for dynamic spatial modelling within GIS.     

A Fluid-Dynamical Study of Crystal Settling in Convecting Magmas

Thermal convection in magma chambers is believed to be almostalways highly time-dependent, or ‘turbulent’, andpredicted convective velocities are commonly orders of magnitudelarger than settling velocities for typical crystals calculatedfrom Stokes' Law. To understand crystal settling in magma chamberswe have therefore undertaken a theoretical and experimentalstudy of particle settling in a turbulently convecting fluid. The regime of interest is where the ratio, S, of the Stokes'Law settling velocity, v_s, to the root mean square verticalcomponent of convective velocity, W, at mid-depth in the fluidis less than unity. Although v_s < W, settling is still possiblebecause convective velocities are height-dependent and mustdecrease to zero at the boundaries of the fluid. Particles immediatelyadjacent to the bottom boundary settle out with their full Stokes'settling velocities. At the same time, convection is vigorousenough to ensure that the distribution of particles in the fluidis uniform. It follows that the number of particles in suspensiondecays with time according to an exponential law, and the decayconstant is simply the ratio of v_s to h, the depth of the fluid.Experiments confirm this relationship, at least for low particleconcentrations, provided S < 0.5 and there is no re-entrainmentof particles from the floor of the tank. We apply this relationship to crystals in magma chambers andso calculate residence times for typical crystals. We find thatfor basaltic magmas the predicted residence times are smallcompared with the many thousands of years that a chamber takesto solidify if cooling is dominated by conduction through thecountry rock. We therefore conclude that crystal settling maybe an efficient differentiation mechanism. Significant magmaticevolution may, however, take place on time-scales that are competitivewith these residence times. If the settling of crystals is the rate-limiting step duringthe crystallization of a magma chamber it is expected that asteady state will be achieved at which the rate of supply ofcrystals into the convecting magma by crystallization balancesthe rate at which crystals settle out. We show how this ideacan explain both the lack of hydraulic equivalence in cumulaterocks and the commonly observed discrepancy between the relativeproportions of phenocrysts of various phases in fractionatedbasaltic lavas and the calculated relative proportions of thesemineral phases in the fractionating assemblage. Finally, anattempt is made to calculate the steady-state crystal contentof convecting magma chambers. Comparison of the predicted crystalcontents with the observed phenocryst contents of typical basalticlavas suggests that magma chambers may often cool more rapidlythan would be expected for conduction through the country rockalone.     

Aluminous Granulites of the Archean Craton of Kasai (Zaire): Petrology and P-T Conditions

Aluminous granulites of the Archean (2?8 Ga) Kasai craton (Zaire)consist of two main mineral assemblages: Grt-Opx and Sil?Grt?Crdrocks. The high-grade metamorphic conditions as deduced from Grt-Opxand Grt-Opx-Pl-Qtz equilibria are 720?C-6?7 kb. Consideringthe zoning of the same minerals, the slope of the P-T path isestimated at 15 b/?C. Thermobarometry involving Crd is consistentwith those P-T conditions. Three cordierite-forming reactions have been observed petrographically: These equilibria are continuous reactions; end-member reactionshave slopes less than 15 b/?C; they are decompression reactionsoccurring after the metamorphic climax. Using available thermodynamic data, (R3) fixes the oxygen fugacityto a value below the QFM buffer (log₁₀f_O2 = – 17?6 at720?C, 6?7 kb and in the graphite stability field. The absence of graphite in the rocks showsthat the end of the granulite facies metamorphism did not occurunder important CO₂ streaming. The polymetamorphic history of this Archean craton is considered.     

High-Grade Fluid Metasomatism on both a Local and a Regional Scale: the Seward Peninsula, Alaska, and the Val Strona di Omegna, Ivrea-Verbano Zone, Northern Italy. Part I: Petrography and Silicate Mineral Chemistry

K-feldspar–plagioclase–quartz mineral textures aswell as biotite and hornblende compositions are compared forsuites of metamorphosed mafic rocks from two widely separatedtraverses. A portion of either traverse has experienced a high-gradedehydration event transforming it from an H₂O-rich, hornblende-bearingzone to an H₂O-poor, hornblende-free, orthopyroxene-bearing,‘granulite facies’ zone at 700–800°C and7–8 kbar. In the Kigluaik Mountains, Seward Peninsula,Alaska, dehydration took place over an 85 cm thick layer ofmetatonalite in contact with a marble during regional metamorphismand involved a CO₂-rich fluid, whereas for the Val Strona diOmegna traverse, Ivrea–Verbano Zone, northern Italy, dehydrationtook place over a 3–4 km thick sequence of metabasitesinterlayered with metapelites in a contact metamorphic eventinvolving basaltic magmas intruded at the base of the sequence.Orthopyroxene-bearing samples from both dehydration zones showmicro-veins of K-feldspar along quartz and plagioclase grainboundaries as well as replacement antiperthite in plagioclase.K came primarily from the breakdown of hornblende + quartz toorthopyroxene ± clinopyroxene, feldspar and fluid. Biotiteeither was stabilized or formed in the dehydration zones andis enriched in Ti, Mg, F and Cl relative to biotite in the amphibolitefacies zone. KEY WORDS: KCl–NaCl brines; metasomatism; granulite facies metamorphism; charnockite–enderbite; orthopyroxene; K-feldspar; biotite; hornblende     

MEASUREMENT OF FROST-INDUCED SNOWMELT RUNOFF IN A FOREST SOIL

Snow interception in a coniferous stand leads to considerable short-range variability in snowcover depth, which in turn affects the water and heat regime of the soil. To study the coupling between snow accumulation, frost penetration, and hydrological response, plot-scale experiments were conducted in a subalpine spruce forest. The stony, sandy–loamy Spodosol was highly permeable and had an organic layer of 5–15 cm thickness. Within two plots, one underneath a tree crown and one in a canopy gap, we measured near-surface runoff, soil temperature, and liquid water content. Snow and frost depths varied more in space than between two winter periods at given locations. Frost penetration was greater near the trunk, where a higher portion of snowmelt water drained downslope close to the surface than in the gap due to frost-induced reduction of infiltration. In both years, the spring snowmelt occurred over two distinct periods. During the first snowmelt, the water percolated primarily through the frozen layer and part of it probably refroze within the frozen layer, thereby raising the total water and ice content. During the second event, near-surface runoff was more pronounced.     

Quaternary glaciations of the Rongbuk Valley,Tibet

Randomisation tests on boulder weathering data distinguish moraines of four different ages in the Rongbuk Valley, all deposited by valley glaciers flowing northward into Tibet from the Himalaya. Lichenometry utilising subgenus Rhizocarpon distinguishes two groups of moraines, those <100 yr old and those older than several thousand years. The degree of soil development has a similar, limited utility in relative-age dating these moraines. The radiocarbon ages of calcium carbonate coatings in the lower horizons of moraine soils provide minimum-limiting ages of 1900 yr BP for the penultimate advance of the Rongbuk glacier (Samdopo moraine) and 9500 yr BP for the Rongbuk moraine, the moraine suggested by previous workers to represent the last glacial maximum. Equilibrium-line depression associated with the Rongbuk moraine probably was slight, <200 m. The small magnitude of this depression relative to glaciers in other mountain ranges could relate to a weakening of the monsoon in full glacial times, recent tectonic uplift, and/or to the insensitivity of these high-altitude glaciers to lowering temperatures in the rain shadow of Mount Everest.