Ray tomography: Errors and error functions

Tomography is the inversion of boundary projections to reconstruct the internal characteristics of the medium between the source and detector boreholes. Tomography is used to image the structure of geological formations and localized inhomogenieties. This imaging technique may be applied to either seismic or electromagnetic data, typically recorded as transmission measurements between two or more boreholes. Algebraic algorithms are error-driven solutions where the goal is to minimize the error between measured and predicted projections. The purpose of this study is to assess the effect of the ray propagation model, the measurement errors, and the error functions on the resolving ability of algebraic algorithms. The problem under consideration is the identification of a two-dimensional circular anomaly surveyed using crosshole measurements. The results show that: (1) convergence to the position of the circular anomaly in depth between vertical boreholes is significantly better than for convergence in the horizontal direction; (2) error surfaces may not be convex, even in the absence of measurement and model errors; (3) the distribution of information content significantly affects the convexity of averaging error functions; (4) measurement noise and model inaccuracy manifest in increased residuals and in reduced convergence gradients near optimum convergence; (5) the maximum ray error function increases convergence gradients compared with the average error function, and is unaffected by the distribution of information content; however, it has higher probability of local minima. Therefore, inversions based on the minimization of the maximum ray error may be advantageous in crosshole tomography but it requires smooth projections. These results are applicable to both electromagnetic and seismic data for wavelengths significantly smaller than the size of anomalies.     

Chemical differences between minerals from mineralizing and barren intrusions associated with molybdenum mineralization at Climax,Colorado

Summary At Climax, comagmatic igneous intrusions can be subdivided into two groups, mineralizing stocks which are parent intrusions for Mo orebodies and barren stocks. Magmatic biotites in mineralizing stocks are similar to hydrothermal biotites in that they contain a greater proportion of Si-enriched and Ti-depleted compositional domains than do magmatic biotites in barren stocks. A similar trend of Si-enrichment correlated with Ti-depletion is also recorded in biotites from the Bingham porphyry copper deposit. Such trends are attributed to vapour exsolution associated with mineralization. Mo concentrations of magmatic biotites, as determined by ion-probe analysis, from mineralizing stocks (av. 40 gg/g) and barren stocks (av. 33 /g) at Climax are similar, being in the same order of magnitude as magmatic biotite Cu concentrations in mineralizing and barren (type A) intrusions in North American porphyry copper deposits (Hendry et al., 1985). These Cu and Mo values are more than a factor of 10 lower than Cu concentrations commonly recorded in magmatic mafic phases in barren (type B) South West Pacific and Australian granitic systems that are temporally distinct from mineralizing events, and are consistent with the magmatic-hydrothermal origin for the Climax deposits proposed by White et al. (1981).

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Chemische Charakteristika von Mineralen aus erzführenden und erzfreien Intrusionen im Bereich der Molybdänlagerstätte Climax, Colorado

Zusammenfassung Zwei Gruppen komagmatischer Intrusivkörper können im Bereich der Molybdän-Lagerstätte Climax, Colorado, unterschieden werden: Mineralisierende Intrusiva, die Stammagmen für Molybdän-Erzkörper darstellen, und erzfreie Intrusiva. Magmatische Biotite in mineralisierenden Intrusiva sind hydrothermalen Biotiten insofern ähnlich, als sie einen größeren Anteil von Si-angereicherten und Ti-verarmten Bereichen enthalten als magmatische Biotite in erzfreien Intrusiva. Ein ähnlicher Trend von Si-Anreicherung, die mit Ti-Verarmung korreliert werden kann, ist auch in Biotiten aus der Porphyry-copper-Lagerstätte Bingham bekannt geworden. Derartige Trends werden auf Entmischung von Dampfphasen in Zusammenhang mit der Mineralisation zurückgeführt.Molybdän-Konzentrationen von magmatischen Biotiten in Climax wurden mit der Ionensonde bestimmt und zeigen in mineralisierenden und erzfreien Intrusiva ähnliche Werte, i.e., 40 p/g bzw. 33 gg/g. Diese Werte liegen in derselben Größenordnung wie Kupferkonzentrationen in magmatischen Biotiten aus vererzten und erzfreien (Typ A) Intrusionen im nordamerikanischen Porphyry-copper-Lagerstätten (Hendry et al., 1985). Diese Kupfer- und Molybdänwerte sind um den Faktor 10 niedriger als Kupferkonzentrationen, die man gewöhnlich in magmatischen mafischen Phasen in erzfreien (Typ B) Granitsystemen im südwestlichen Pazifik und Australien nachgewiesen hat. Diese sind zeitlich einer anderen Entstehungsperiode zuzuordnen als die erzbildenden Vorgänge, und dies stimmt wiederum mit der magmatisch hydrothermalen Entstehung der Lagerstätten von Climax überein, wie sie vonWhite et al. (1981) vorgeschlagen worden ist.

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With 4 Figures     

Urban irrigation in the modeling of a semi-arid urban environment: Ballona Creek watershed,Los Angeles,California

ABSTRACT

Ballona Creek watershed in Los Angeles, California provides a unique combination of heterogeneous urban land cover, a semi-arid environment, and a large outdoor water-use flux that presents a challenge for physically-based models. We ran simulations using the Noah Land Surface Model and Parflow-Community Land Model and compared to observations of evapotranspiration (ET), runoff, and land surface temperature (LST) for the entire 11-year study period. Both models were systematically adjusted to test the impact of land cover and urban irrigation on simulation results. Monthly total runoff and ET results are greatly improved when compared to an in-situ stream gauge and meteorological tower data: from 0.64 to 0.81 for the Nash–Sutcliffe efficiency (NSE) for runoff and from a negative NSE to 0.82 for ET. The inclusion of urban irrigation in semi-arid urban environments is found to be vital, but not sufficient, for the accurate simulation of variables in the studied models.     

Barium content of marine sediments near drilling sites: A potential pollutant indicator

Barite is commonly used as a weighting agent in oil- and gas-well drilling mixture; therefore, the distribution pattern of barium in marine sediments near drilling sites may provide an indicator of anthropogenic chemical contamination from these operations. Present average content of barium in benthic sediments from the Southern California Bight was determined to be 637 ppm, with a range from 43 to 1899 ppm, and that of the mainland and the channel island intertidal sediments averaged 879 and 388 ppm, respectively.     

A microstructural study of the Tishomingo meteorite

Metallography, electron microscopy and X-ray diffraction techniques were employed to study a fragment of the Tishomingo iron meteorite. The results suggest the following thermal-mechanical history: The fragment was originally a large crystal of taenite (γ). Cooling through the α + γ phase boundary did not result in accompanying precipitation of kamacite (α). Transformation to a martensitic structure initiated between ? 25 and ?65°C. Transformation continued as the temperature fell to ? 75 to ? 115°C, resulting in approx 80% martensite (α′). Subsequent shock deformation and thermal aging processes substantially modified the taenite and martensite microstructures. Twins in the retained taenite phase are attributed to shock deformation at a pressure estimated for a single event at ~170 kbar. The existing complex, altered martensite structure containing both taenite and kamacite (3–15% Ni) particles was apparently the product of both shock deformation and thermal aging processes. The maximum temperature reached during thermal aging is estimated to be less than 400°C, and perhaps below 310°C.     

Molecular compositions of weathered petroleum and comparison with its possible source

Two surface outcrop samples and three drill core samples of oil-impregnated sandstone from the P.R. Spring Seep, Uinta Basin, Utah were analyzed. These oils exhibit in common the entire homologous series of tricyclic diterpane hydrocarbons, which have been previously described only in extracts from the Mahogany Ledge Member of the Green River Formation. Hopane and a series of degraded hopanes are also present in the oil samples, while steranes are conspicuously absent. Aliphatic alkanes are present only in the deepest core sample.The cycloalkanes are apparently not readily utilized by the petroleum oxidizing microbes, and therefore survive the weathering process. Tricyclic and tetracyclic alkylated diterpanes are unchanged, whereas the series from norhopane through tetrakisnorhopane may be interpreted as progressive bacterial degradation of the hopane molecule.The occurrence and distribution of tricyclic diterpanes, of tetracyclic diterpanes and pentacyclic triterpanes is similar to the stratigraphically nearby Mahogany Ledge Member, suggesting that the oil shales were the source rocks for the oil now exposed as the P.R. Spring Seep. The absence of steranes, which are abundant in the oil shales, is puzzling.     

Experimental determination of the partitioning of gallium between solid iron metal and synthetic basaltic melt: Electron and ion microprobe study

The distribution of Ga between solid Fe metal and synthetic basaltic melt has been investigated experimentally at two temperatures over a limited range of oxygen fugacities. Reversal experiments were conducted, indicating a close approach to equilibrium. Analysis of run products was performed using an electron and an ion microprobe. At 1 bar total pressure, the solid metal/silicate melt partition coefficient. D(Ga), is given by: 1190°C: logD(Ga) = ?0.92 logfO₂ ? 11.91330°C: logD(Ga) = ?0.77 logfO₂ ? 8.8. For the common assumption of a valence state for Ga of 3 in silicates and assuming ideal solution of Ga in both phases, a slope of ?0.75 is predicted. The slope obtained at 1300°C (?0.77) is indistinguishable from this value, while the slope at 1190°C (?0.92) is somewhat higher. Henry's law is obeyed over the concentration range 0.007 wt.% to 0.15 wt.% Ga in metal, a factor of 20 in concentration. These partition coefficients may be utilized to evaluate metal/silicate fractionation processes in the Earth, Moon and Eucrite Parent Body. The lunar mantle appears to be depleted in Ga by a factor of 20–40 relative to CI abundances. This depletion is consistent with extraction of Ga into a geophysically plausible lunar core if Ga was initially present at a subchondritic concentration. A similar explanation probably accounts for the depletion of Ga in the Eucrite Parent Body. The upper mantle of the Earth appears to be depleted in Ga by a factor of 4– 7 relative to CI abundances. This depletion is far smaller than would be expected as a result of core formation, but is consistent with two quite different hypotheses: (i) a small amount of solid metal and sulfur-bearing metallic liquid was retained in the mantle after core formation; or (ii) addition of a late-stage chondritic component involving 5% to 10% of the upper mantle following core formation.