Inversion for Range-Dependent Sediment Compressional-Wave-Speed Profiles From Modal Dispersion Data

A perturbative inversion method for estimating sediment compressional-wave-speed profiles from modal travel-time data is extended to include range-dependent environments. The procedure entails dividing a region into range-independent sections and obtaining estimates of the sediment properties for each region. Inversion results obtained using synthetic data show that range-dependent properties can be obtained if an experiment is designed to include multiple source/receiver combinations. This approach is applied to field data collected during the 2006 Shallow Water Experiment (SW06). The sediment compressional-wave-speed profiles resulting from analysis of the field data are evaluated by comparing acoustic fields predicted based on the inversion to acoustic fields measured during a different experiment conducted in the same region. The model is also compared to seismic reflection survey data collected during SW06. Resolution and variance estimated for the inversion results are also presented.      

Biomarkers in sedimentary sulfides of precambrian age

Samples of the Mt Isa formation (Australia, c, 1.5 Ga), and the Shungit formation (U.S.S.R., c. 2.3 Ga) were studied by organic geochemical means. All samples were freed of any low-molecular-weight solvent soluble organic material in order to insure the authenticity of the analysed material. The isolation of biochemical compounds entrapped in sulfides was the major goal of this work.Organic compounds that were entrapped during the early stages of sulfide formation may obviously survive extended periods of time and can be released by a mild hydrogenation of the sulfides. Preliminary investigations of the hydrocarbon fraction indicate n-alkanes, monomethylalkanes, cycloalkanes, and an unknown series of branched alkanes as major constituents. Their distribution patterns show great selectivity with respect to structures and chain lengths of individual compounds. Differences between the hydrogenation reaction product and the sample extract may arise from the release of a different kind of lipid material through dissolution of the sulfides.     

Gas phase reactions of alkyl nitrates with hydroxyl radicals under tropospheric conditions in comparison with photolysis

Rate constants for the reaction of OH radicals with some branched alkyl nitrates have been measured applying a competitive technique. Methyl nitrite photolysis in synthetic air was used as OH radical source at 295±2 K and 1000 mbar total pressure. Using a rate constant of 2.53×10^-12 cm³ s^-1 for the reaction of OH radicals with n-butane as reference, the following rate constants were obtained (units: 10^-12 cm³ s^-1): isopropyl nitrate, 0.59±0.22; isobutyl nitrate, 1.63±0.20; 3-methyl-2-butyl nitrate, 1.95±0.15; 2-methyl-1-butyl nitrate, 2.50±0.15; 3-methyl-1-butyl nitrate, 2.55±0.35. These values have been combined with the literature data to recalculate the substituent factors F(X) for the different nitrate groups which can be used to predict OH rate constants for organic nitrates for which experimental data are not available.Preliminary measurements of the photolysis frequency of isopropyl nitrate have shown that for this nitrate as a model substance, OH reactions and direct photolysis are of equal importance under tropospheric conditions.     

Observations of mine seismicity in the eastern Wasatch Plateau,Utah, U.S.A.: A possible case of implosional failure

In the summer of 1984, a three-dimensional, high-resolution microearthquake network was operated in the vicinity of two coal mines beneath Gentry Mountain in the eastern Wasatch Plateau, Utah. During a six-week period, approximately 3,000 seismic events were observed of which the majority were impulsive, higher frequency (>10 Hz), short duration (<2–3 sec) events probably associated with the caving of the roof from a longwall operation. In contrast, 234 of the largest located events appeared to occur predominantlybeneath the mines to a depth of 2 to 3 km consistent with previous studies. The magnitudes of these events ranged from less thanM _c 0 to 1.6. In addition to the unusual depths of these latter events, an anomalous aspect displayed by the events was an apparent dilatational focal mechanism suggesting a non-double-couple, possibly implosional source. Implosional events have been observed in other studies of mine seismicity; however, the generally inadequate instrumental coverage of the focal sphere has cast some doubt on the validity of such mechanisms. Previously suggested source mechanisms for such implosional events have included tensional failure through strata collapse, and a shear-implosional displacement mechanism. Shear failure must be involved in the failure process of the Gentry Mountain implosional events as evidenced by well-defined shear waves in the observed seismograms. Simultaneous monitoring in the East Mountain coal mining area to the south by the University of Utah revealed typical shear failure events mixed with implosional events. The observed double-couple, reverse focal mechanisms at East Mountain were similar to mechanisms determined in previous studies and a composite focal mechanism determined in this study for a sequence outside the mining areas. This suggested that the shear events within the mining areas are being influenced by the regional tectonic stress field. Thus in addition to the seismic events associated with caving of the roof from the longwall operation, there appear to be at least two other types of mining-induced seismic events occurring in the eastern Wasatch Plateau, both submine in origin: (1) events characterized by apparent non-double-couple possibly implosional focal mechanisms and well-defined shear waves; and (2) shear events, which are indistinguishable from tectonic earthquakes and may be considered mining triggered earthquakes. The small mining-induced stress changes that occur beyond a few hundred meters from the mine workings suggest both types of seismic events are occurring on critically stressed, pre-existing zones of weakness. Topography, overburden, method of mining, and mine configuration also appear to be significant factors influencing the occurrence of the implosional submine events.     

Significance of the remotely sensed thermal infrared measurements obtained over a citrus orchard

In this work we have developed a theoretical model that helps the interpretation of the remotely sensed thermal infrared measurements carried out over citrus orchards. A detailed analysis of the different factors which take part in the definition of the effective emissivity and temperature (observation height, viewing angle, type of soil, dimensions and separation between orange trees) is made. The model was validated under vertical observation in a citrus orchard during seven nights. In this situation we have determined that the model performs to an accuracy of about 1%.     

Chemical structural units of macromolecular coal components

Chemical degradations of coal resins, coal asphaltenes and coal residues have been performed with selective, non-oxidative reagents. The coal comprise samples a rank interval 0.30–1.21 %R_m. Degraded low-molecular-weight compounds were analyzed by GC-MS. The distributions of pentacyclic terpanes, isoprenoids, n-alkanes, acids and alcohols obtained as degradation products illustrate a considerable variability between the macromolecular coal fractions. A structural study on the molecular level of the degraded compounds from coal resins and coal asphaltenes show their potential to generate hydrocarbons during coalification process.     

Structure and origin of terpenoid hydrocarbons in some German coals

Hydrocarbon extracts of coal samples from major coal fields in West Germany have been investigated by gas chromatography-mass spectrometry (GC/MS). The samples are of Carboniferous and Tertiary age and derive from limnic and paralic environments, the Saar District, the Ruhr Area and the Lower Rhine Basin.Differences in facies, maturity and age reflected in the distribution patterns of the extracted alkylcyclohexanes. An unexpected occurrence of cyclic diterpenoid hydrocarbons in some samples of Carboniferous age has been found. The tetracyclic compounds mainly show phyllocladane type structures and can serve as source indicators.Hydrocarbons obtained from chemical degradation of the macromolecular material of the samples also revealed phyllocladane-type compounds which are normally associated to resins of modern and extinct familes of conifers. Origin and structural variations of this class of hydrocarbons are discussed in terms of maturity and palaeobotanical implications.     

The accessibility of nodes in the Belgian road network

This paper presents an analysis of the accessibility of nodes in the Belgian road network. The intersection of roads are considered as the nodes and their connecting roads as the edges of a network. By means of the Dijkstra algorithm accessibility values are computed for each node by summing shortest travel times to a reference system of 158 nodes spread evenly over the Belgian territory. A comparison of the measured actual accessibility with that determined from an optimal situation provides an indication of the quality of directness of the Belgian road system. Accessibility and directness are shown cartographically. An isochronal map illustrates the access to Brussels.