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21.
William S. Cassata Brad S. Singer Joseph C. Liddicoat Robert S. Coe 《Quaternary Geochronology》2010,5(5):533-543
Lacustrine sediments of the Wilson Creek Formation in the Mono Basin, California, record a paleomagnetic field excursion constrained by 14C and 40Ar/39Ar geochronology to have occurred within the last 50 ka. However, 14C and 40Ar/39Ar ages are discordant, making it difficult to distinguish which of two possible excursions during this period, the Mono Lake or Laschamp, is recorded in the Mono Basin. New 40Ar/39Ar age determinations from sanidine, as well as the first biotite and obsidian ages, for three of the nineteen rhyolitic ashes intercalated with these sediments are presented and compared to previous 14C and 40Ar/39Ar data sets. Although the sanidine ages of the three ashes are stratigraphically consistent with each other and previously determined 40Ar/39Ar ages for other ashes in the Wilson Creek Formation, each is significantly older than 14C ages obtained from stratigraphically equivalent beds, relative paleointensity field correlations, oxygen isotope records, and glacial histories. These data indicate an absence of juvenile, eruptive crystals and most likely reflect the incorporation of crystals from older volcanic centers or underlying sediment. We examine the strengths and weaknesses of all available geochronologic data for the section exposed at Wilson Creek to arrive at an internally consistent set of age constraints. Using these constraints we propose two new relative paleointensity correlations for the section, both of which indicate that the excursion recorded in the Mono Basin occurred at ~30–34 ka on the Greenland Ice Sheet Project 2 (GISP2) ice core time scale. 相似文献
22.
N. Ardjmandpour C. Pain J. Singer J. Saunders E. Aristodemou J. Carter 《Geophysical Prospecting》2011,59(4):721-748
An artificial neural network method is proposed as a computationally economic alternative to numerical simulation by the Biot theory for predicting borehole seismoelectric measurements given a set of formation properties. Borehole seismoelectric measurements are simulated using a finite element forward model, which solves the Biot equations together with an equation for the streaming potential. The results show that the neural network method successfully predicts the streaming potentials at each detector, even when the input pressures are contaminated with 10% Gaussian noise. A fast inversion methodology is subsequently developed in order to predict subsurface material properties such as porosity and permeability from streaming potential measurements. The predicted permeability and porosity results indicate that the method predictions are more accurate for the permeability predictions, with the inverted permeabilities being in excellent agreement with the actual permeabilities. This approach was finally verified by using data from a field experiment. The predicted permeability results seem to predict the basic trends in permeabilities from a packer test. As expected from synthetic results, the predicted porosity is less accurate. Investigations are also carried out to predict the zeta potential. The predicted zeta potentials are in agreement with values obtained through experimental self potential measurements. 相似文献
23.
24.
Paul R. Renne Alan L. Deino Willis E. Hames Matthew T. Heizler Sidney R. Hemming Kip V. Hodges Anthony A.P. Koppers Darren F. Mark Leah E. Morgan David Phillips Brad S. Singer Brent D. Turrin Igor M. Villa Mike Villeneuve Jan R. Wijbrans 《Quaternary Geochronology》2009,4(5):346-352
Data reported in 40Ar/39Ar geochronology studies are commonly insufficient to allow computation of ages. This deficiency renders it difficult to compare ages based on different standards or constants, and often hinders critical evaluation of the results. Herein are presented an enumeration of the data that should be reported in all 40Ar/39Ar studies, including a discussion in support of these requirements. The minimum required data are identified and distinguished from parameters that are useful but may be derived from them by calculation. Finally, recommendations are made for metadata needed to document age calculations (e.g., from age spectrum or isochron analyses). 相似文献
25.
Nataa J. Vidic Jeff D. TenPas Kenneth L. Verosub & Michael J. Singer 《Geophysical Journal International》2000,142(2):551-562
Magnetic susceptibility variations in the Chinese loess/palaeosol sequences have been used extensively for palaeoclimatic interpretations. The magnetic signal of these sequences must be divided into lithogenic and pedogenic components because the palaeoclimatic record is primarily reflected in the pedogenic component. In this paper we compare two methods for separating the pedogenic and lithogenic components of the magnetic susceptibility signal: the citrate-bicarbonate-dithionite (CBD) extraction procedure, and a mixing analysis. Both methods yield good estimates of the pedogenic component, especially for the palaeosols. The CBD procedure underestimates the lithogenic component and overestimates the pedogenic component. The magnitude of this effect is moderately high in loess layers but almost negligible in palaeosols. The mixing model overestimates the lithogenic component and underestimates the pedogenic component. Both methods can be adjusted to yield better estimates of both components. The lithogenic susceptibility, as determined by either method, suggests that palaeoclimatic interpretations based only on total susceptibility will be in error and that a single estimate of the average lithogenic susceptibility is not an accurate basis for adjusting the total susceptibility. A long-term decline in lithogenic susceptibility with depth in the section suggests more intense or prolonged periods of weathering associated with the formation of the older palaeosols.
The CBD procedure provides the most comprehensive information on the magnitude of the components and magnetic mineralogy of loess and palaeosols. However, the mixing analysis provides a sensitive, rapid, and easily applied alternative to the CBD procedure. A combination of the two approaches provides the most powerful and perhaps the most accurate way of separating the magnetic susceptibility components. 相似文献
The CBD procedure provides the most comprehensive information on the magnitude of the components and magnetic mineralogy of loess and palaeosols. However, the mixing analysis provides a sensitive, rapid, and easily applied alternative to the CBD procedure. A combination of the two approaches provides the most powerful and perhaps the most accurate way of separating the magnetic susceptibility components. 相似文献
26.
Abandoned mining operations continue to severely degrade many ecosystems worldwide by releasing acidic water and/or heavy metals into surface and groundwater. Contaminant concentrations in affected streams vary with discharge in patterns that reflect both geochemical reactions and variable mixing of contaminated and non-contaminated waters. However, controls on concentration-discharge (C-Q) patterns remain unclear, particularly for constituents that experience changing solubility across redox and pH gradients. Understanding the C-Q behaviour of contaminants aids in predicting both downstream transport and effects on aquatic life under variable flow. Here, we examined the C-Q behaviours of non-reactive (Na, K, Ca, Mg, Cl−) and reactive (Fe, Mn, Al, H+, SO42−) solutes in a stream contaminated with acid mine drainage in northeastern Ohio, USA. Concentration-discharge patterns at the watershed outlet primarily reflected mixing of contaminated baseflow with intermittent inputs of high pH water draining from a passive limestone treatment system into the stream. The treatment system acted as an ephemeral tributary that mitigated contamination in the stream by diluting solutes, raising pH, and driving metal precipitation, but only when flow was present during wet seasons. Consequently, AMD-derived reactive solutes (H+, Fe, Mn, Al) decreased with increasing stream discharge while relatively conservative solutes (e.g., Ca, Mg, K, Na) decreased only slightly or were chemostatic. This study highlights both the unique C-Q patterns of reactive solutes when compared to those of non-reactive solutes and the potential for intermittent streams to control C-Q behaviour in headwater catchments. 相似文献
27.
Summary. Asymptotic expressions for components of the electromagnetic field of a grounded electric dipole are considered for the model consisting of a thin surface-layer overlapping a stratified medium with a highly resistive screen on the roof. It is shown that the method of spatial derivatives makes it possible to obtain proper estimates of the impedance at distances of r ≥|λ0 | from the nearest edge of the surface anomaly (|λ0 | being the effective depth of the field penetration in the underlying section). the magnetotelluric methods allow one to obtain the true values of impedance, provided r ≥ max {|λ0 |, |/( S −1 + Z − 0 |1/2 } where S is the integrated conductivity of the surface layer, is the transverse resistance of the screen, and Z − 0 is the Tikhonov—Cagniard impedance for the medium underlying the surface layer. 相似文献
28.
Mineral Deposit Densities for Estimating Mineral Resources 总被引:1,自引:0,他引:1
Donald A. Singer 《Mathematical Geosciences》2008,40(1):33-46
Estimates of numbers of mineral deposits are fundamental to assessing undiscovered mineral resources. Just as frequencies
of grades and tonnages of well-explored deposits can be used to represent the grades and tonnages of undiscovered deposits,
the density of deposits (deposits/area) in well-explored control areas can serve to represent the number of deposits. Empirical
evidence presented here indicates that the processes affecting the number and quantity of resources in geological settings
are very general across many types of mineral deposits. For podiform chromite, porphyry copper, and volcanogenic massive sulfide
deposit types, the size of tract that geologically could contain the deposits is an excellent predictor of the total number
of deposits. The number of mineral deposits is also proportional to the type’s size. The total amount of mineralized rock
is also proportional to size of the permissive area and the median deposit type’s size. Regressions using these variables
provide a means to estimate the density of deposits and the total amount of mineralization. These powerful estimators are
based on analysis of ten different types of mineral deposits (Climax Mo, Cuban Mn, Cyprus massive sulfide, Franciscan Mn,
kuroko massive sulfide, low-sulfide quartz-Au vein, placer Au, podiform Cr, porphyry Cu, and W vein) from 108 permissive control
tracts around the world therefore generalizing across deposit types. Despite the diverse and complex geological settings of
deposit types studied here, the relationships observed indicate universal controls on the accumulation and preservation of
mineral resources that operate across all scales. The strength of the relationships (R
2=0.91 for density and 0.95 for mineralized rock) argues for their broad use. Deposit densities can now be used to provide
a guideline for expert judgment or used directly for estimating the number of most kinds of mineral deposits. 相似文献
29.
L.G. Medaris Jr. R.H. Dott Jr. B.S. Singer W.R. Van Schmus D.K. Holm 《Precambrian Research》2008,164(3-4):236-238
30.
Singer Donald A U. S. Geological Survey Middlefield Road Menlo Park California USA 《中国地质大学学报(英文版)》2001,12(1)
Like most journeys, success depends critically on wherewe are going to be in the end. Thus, if we are to have someideas about future directions of quantitative assessments ofmineral resources, we need some basic understanding of whatthe assessments will be used for. What will be expected ofquantitative resource assessments in the future? It would behelpful to identify who will use these future assessments, howthe assessments will be used, and what are acceptable forms ofproducts. The purpose o… 相似文献