bluuebunny/crossref_metadata_2025 huggingface dataset:
Abundance of zinc isotopes as a marine biogeochemical tracer
Authors: Maréchal Chloé N. , Nicolas Emmanuel , Douchet Chantal , Albarède Francis
Published: February 2004

The Zn isotopic compositions of ferromanganese nodules, sediment trap samples, sediments, and organic reference samples have been analyzed by multiple collector inductively coupled plasma–mass spectrometry (ICP‐MS). The range of isotopic variations (1‰ for 66Zn/64Zn) is significant with respect to the analytical precision (0.04‰ at the 95% confidence level). Marine argillaceous sediments show rather constant values consistent with those of continental shales and loess and with the value of a basalt from Réunion Island. We infer that the depletion of the light Zn isotopes in marine particles and in ferromanganese nodules (presumably equilibrated with seawater) results from biological activity in the upper water column. The δ66Zn values of sediment trap samples collected near the upwelling off the coast of Mauritania (central Atlantic) show a seasonal isotopic fluctuation consistent with biological pumping during the high‐productivity period. Higher values of δ66Zn in nodules appear to be associated with the amplitude of seasonal variations rather than with the mean values of the biological productivity.
Atmospheric organochlorine pesticides in the western Canadian Arctic: Evidence of transpacific transport
Authors: Bailey R. , Barrie L. A. , Halsall C. J. , Fellin P. , Muir D. C. G.
Published: February 2004

Concentrations of hexachlorocyclohexanes (HCHs), chlordane, and dichlorodiphenyltrichloroethane (DDT) were measured in ambient air samples on a weekly basis between December 1992 and January 1995 at Tagish Yukon, Canada. In winter, unusually high air concentrations of HCHs, DDT, and chlordanes at Tagish were predominantly influenced by transpacific long‐range atmospheric transport from eastern Asia that generally occurred within 5 days. HCH and heptachlor epoxide concentrations were correlated with the time that air spent over eastern Asia prior to arrival at Tagish. Chlordane and DDT, which also increase with transpacific transport, do not show a correlation with the time the upwind airshed included Asia as the composition of these pesticides in the atmosphere is affected by differences in usage patterns, application methods, variable composition of parent pesticides and metabolites in the soil, and rates of volatilization. Air masses originating from North America had the highest concentrations of HCHs and chlordanes when the 5‐day upwind airshed included the western United States. Concentrations of HCHs may also be influenced by lindane usage in Canada.
Simulated degradation of lunar impact craters and a new method for age dating farside mare deposits
Authors: Craddock Robert A. , Howard Alan D.
Published: February 2004

With the advent of Clementine data it is now possible to determine the lithology and extent of geologic materials on the Moon, particularly the farside mare deposits. However, traditional crater counting techniques do not provide reliable age estimates of these materials owing to their small surface areas. To support such studies, we present a model for estimating their age by analyzing the morphometry of degraded craters 1–3 km in diameter. A photoclinometric model was adapted for use with monoscopic 0.750‐μm ultraviolet‐visible and high‐resolution images where we extracted the topography of fresh craters. A two‐dimensional computer model simulating linear diffusional creep was applied to fresh craters at a variety of diameters. The resulting profiles were then compared to photoclinometric profiles of degraded craters of known ages for calibration. Application of the resulting model to degraded craters in the mare deposit of the central Apollo basin (∼36.5° latitude, 208.0° longitude) indicates that this material was emplaced during the early Imbrian period (∼3.85 Ga). By calculating the amount of material eroded from each of the degraded craters observed in this unit, the average erosion rate is estimated to be 2.0±0.1 × 10−7 mm/yr on the Moon since the Imbrian. The estimated amount of material eroded during any given period suggests that the erosion rate has decreased with time, implying that the flux of larger impactors has as well.
Distributions of CO2 and SO2 on the surface of Callisto
Authors: Hibbitts C. A. , McCord T. B. , Hansen G. B.
Published: February 2004

Absorption bands in the infrared reflectance spectra from the Galileo Near‐Infrared Mapping Spectrometer (NIMS) which are attributed to the presence of CO2 and SO2 on the surface of Callisto have been analyzed and mapped in detail. CO2 of varying concentrations appears to exist everywhere on Callisto, except at higher latitudes, where it may be masked by frost. The CO2 concentration on the trailing hemisphere has a longitudinal distribution largely consistent with a sinusoid centered on the equator near 270° longitude. The approximately sinusoidal pattern suggests that exogenic effects related to Jupiter's corotating magnetic field are involved. Closer inspection of both hemispheres reveals that in many cases, visibly bright and ice‐rich impact craters have high CO2 concentrations within or near them. The CO2 sometimes appears to be associated more with dark material near the craters than with the water ice. These correlations suggest impact processes may also affect the distribution of CO2 on the surface of Callisto. The center of the absorption band has been refined to be at 4.258±0.004 μm. The presence of a single band shape and band minimum wavelength position in all data sets for the CO2 absorption implies the physical state of CO2 is similar over the surface of Callisto. The distribution of SO2 on the surface is less well defined owing to characteristically shallower band depths, but it appears generally mottled, with some areas of high concentrations correlated with ice‐rich impact craters. Large‐scale patterns include the depletion of SO2 in the polar regions and a depletion of SO2 on the trailing side relative to the leading side. There is no sinusoidal pattern to this depletion. The center of the SO2 band is determined to be between 4.01 and 4.02 μm.
Tyre and Pwyll: Galileo orbital remote sensing of mineralogy versus morphology at two selected sites on Europa
Authors: Fanale Fraser P. , Granahan James C. , Greeley Ronald , Pappalardo Robert , Head James , Shirley James , Carlson Robert , Hendrix Amanda , Moore Jeffrey , McCord Thomas B. , Belton Michael
Published: February 2004

Observational data from the Galileo Orbiter's remote‐sensing instruments, including solid‐state imaging (SSI), the Near‐Infrared Mapping Spectrometer (NIMS), and the ultraviolet spectrometer (UVS), are analyzed and interpreted in terms of the history of the Tyre and Pwyll impact sites and the nature of Europa's crustal zonal structure. Material characterized by asymmetric 1.4 and 2.0 μm bands and visible coloration characterizes each site where morphologic evidence suggests disruption of the topmost crust. The material is not H2O ice, is endogenic, and is common to the linea, impact basins, and dark trailing side terrain. Differences between Tyre and Pwyll are interpreted in terms of Europa's peculiar energy history, while other evidence may suggest flooding of the endogenic material as a liquid. The pure ice appearance of most of Europa's crust is probably superficial, while beneath a thin patina of sputtered H2O molecules the crust is everywhere laced with numerous generations of intrusions and extrusions of an aqueous phase. Our results, together with numerous laboratory experiments and theoretical analysis, suggest that the aqueous phase is dominated by the SO4= anion.
Imaging of lunar surface maturity
Authors: Lucey Paul G. , Blewett David T. , Taylor G. Jeffrey , Hawke B. Ray
Published: February 2004

The physical evolution of the lunar surface with exposure to the space environment is termed maturation, and maturity is the degree to which a particular lunar soil possesses quantitative characteristics consistent with that exposure. Several quantitative measures or indices of maturity have been proposed and employed, including the abundance of solar wind gas, abundance of various types of agglutinates, various measures of grain size, and Is/FeO. Among the changes attendant with space exposure are striking changes in the optical characteristics of soils. Mature lunar soils are dark red and exhibit reduced spectral contrast relative to immature soils. This paper presents an optical maturity index that quantifies the spectral effects of maturation. We show that this optical maturity index correlates with other maturity indices about as well as the accepted maturity indices correlate among themselves and is only weakly coupled to composition. The modest correlations among maturity indices suggest important controls on individual maturity indices other than age, uncorrelated variations in the rates of accumulation of individual indicators, or variations in depths over which indicators are emplaced. In addition to mixing, these effects conspire to reduce the equivalence of maturity and duration of surface exposure. Optical maturity illustrates some of these effects, showing that ejecta of large and small craters mature at different rates and that the interiors and ejecta of large craters exhibit systematically different optical maturities. The same or analogous effects are likely to influence other maturity indices.
The MVACS tunable diode laser spectrometers
Authors: May Randy D. , Forouhar Siamak , Crisp David , Woodward W. Stephen , Paige David A. , Pathare Asmin , Boynton William V.
Published: February 2004

Two independent tunable diode laser spectrometers are resident aboard the Mars Polar Lander as part of the Mars Volatiles and Climate Surveyor payload. One spectrometer is located on the meteorological mast for measurements of H2O and CO2 in the free atmosphere, and the other serves as the H2O and CO2 analyzer for the Thermal and Evolved Gas Analyzer. Water vapor is measured using a tunable diode laser operating at 1.37 μm, while CO2 is measured using a second laser operating near 2.05 μm. The 2.05 μm laser also has isotopic analysis capability. In addition to the major CO2 isotopomer (12C16O16O), analyses of 13C16O16O and 12C18O16O in the atmosphere and in the Thermal and Evolved Gas Analyzer are possible under certain conditions. The spectrometers were designed and built at the Jet Propulsion Laboratory and have their heritage in a series of tunable diode laser spectrometers developed for Earth atmospheric studies using high‐altitude aircraft and balloon platforms. The 1.37 μm diode laser on the meteorological mast will provide the first in situ measurements of water vapor in the Martian boundary layer, with a detection sensitivity an order of magnitude greater than the water vapor abundances inferred from the remote‐sensing observations by the Viking Orbiters.
Distribution of strike‐slip faults on Europa
Authors: Hoppa Gregory , Greenberg Richard , Tufts B. Randall , Geissler Paul , Phillips Cynthia , Milazzo Moses
Published: February 2004

Study of four different regions on Europa imaged by the Galileo spacecraft during its first 15 orbits has revealed 117 strike‐slip faults. Europa appears to form preferentially right‐lateral faults in the southern hemisphere and left‐lateral faults in the northern hemisphere. This observation is consistent with a model where diurnal tides due to orbital eccentricity drive strike‐slip motion through a process of “walking,” in which faults open and close out of phase with alternating right‐and left‐lateral shear. Lineaments that record both left‐and right‐lateral motion (e.g., Agave Linea) may record the accommodation of compression in nearby chaotic zones. Nearly all identified strike‐slip faults were associated with double ridges or bands, and few were detected along ridgeless cracks. Thus the depth of cracks without ridges does not appear to have penetrated to the low‐viscosity decoupling layer, required for diurnal displacement, but cracks that have developed ridges do extend down to such a level. This result supports a model for ridge formation that requires cracks to penetrate to a decoupling layer, such as a liquid water ocean.
Autonomous image analyses during the 1999 Marsokhod rover field test
Authors: Gulick Virginia C. , Morris Robert L. , Ruzon Mark A. , Roush Ted L.
Published: February 2004

A Martian rover capable of analyzing images autonomously could traverse greater path lengths and return data with greater scientific content. A more intelligent rover could, for example, automatically select targets of interest (e.g., rocks, layers), return spectral or high‐resolution image data of these targets at the same time, remove less interesting or redundant parts of images before transmitting them, and provide compact information or representations of its environment. Three prototype algorithms, a horizon detector, a rock detector, and a layer detector have been developed and tested during the 1999 Marsokhod rover field test in Silver Lake, California. The results are encouraging and demonstrate the potential savings in time as well as the potential increase in the amount of relevant science data returned in each command cycle.
Spectral anomalies in the 11 and 12 μm region from the Mariner Mars 7 Infrared Spectrometer
Authors: Kirkland Laurel E. , Herr Kenneth C.
Published: February 2004

Two hundred‐forty infrared spectra acquired by the 1969 Mariner Mars 7 Infrared Spectrometer (IRS), spanning the wavelength region 1.8–14.4 μm (5550–690 cm−1), have recently been recovered and calibrated in both wavelength and intensity. An examination of these IRS spectra has revealed absorptions at 11.25 and 12.5 μm that have not previously been reported for Mars. A search of the literature and spectral data bases shows that materials that exhibit a doublet at 11.25 and 12.5 μm are rare. In this paper we examine potential causes for these features and include a detailed discussion of carbonates, goethite, CO2 ice, and water ice. CO2 ice and water ice measured in transmission do not exhibit bands that match those recorded at 11.25 and 12.5 μm for Mars, which indicates that CO2 or water ice clouds are not the source of these features. Since these bands show no clear correlation with atmospheric path length, they are most likely caused by a surface material. In the IRS database they appear to be exceptionally intense in the western part of the Hellas basin. Goethite exhibits bands that are a good spectral match, but confirming whether goethite causes the features will require additional studies of the 20–50 μm region. These studies will require laboratory measurements of weathering coatings and an examination of spectra recorded of Mars by the 1971 Mariner Mars Infrared Interferometer Spectrometer (IRIS; 5–50 μm, 2000–200 cm−1) and the 1996 Thermal Emission Spectrometer (TES; 6–50 μm, 1667–200 cm−1).
Implications from stereo‐derived topography of Venusian impact craters
Authors: Herrick Robert R. , Sharpton Virgil L.
Published: February 2004

Using radargrammetry we have created high‐resolution topographic maps of 74 Venusian craters, including all bright‐floored craters over 12 km in diameter covered by Magellan stereo imagery. Our trend for rim‐floor depths RF as a function of diameter D for bright‐floored craters in the volcanic plains is RF = (0.345±0.05) D0.235±0.05, and for dark‐floored craters in the plains, RF = (0.5±0.1) D0.0035±0.07. Rim heights RH for bright‐floored craters in the plains are RH = (0.06±0.03) D0.4±0.2 (D>15 km), and for dark‐floored craters in the plains, RH = (0.027±0.015) D0.4±0.2. These trends indicate that bright‐floored craters 30 km in diameter are, on average, deeper than dark‐floored craters by 180 m from rim to floor and have a 140 m higher rim, and at 90 km in diameter they are 380 m deeper from rim to floor and have a 220 m higher rim. The bright‐ and dark‐floored populations are different at a 99% confidence level for both rim‐floor depths and rim heights. The interpretation most consistent with our data and previous work by others is that Venusian craters with radar‐dark floors have been partially filled and had their ejecta blankets embayed by regional‐scale lava flooding. When the topography is examined in conjunction with the imagery, it is clear that many dark‐floored craters have been surrounded by lavas that rose nearly to the crater rim even though a substantial portion of the crater's ejecta blanket was retained. Most previous analyses of the resurfacing history of Venus have relied on past interpretations that only a small percentage of Venusian craters are embayed by exterior volcanism. Because most craters on Venus have dark floors, our data indicate that the majority of Venusian craters have been surrounded and partially filled by postimpact lavas, and consequently, those previous analyses may have significantly underestimated the amount of volcanism on the Venusian surface over the past few hundred million years. Rim‐floor depths for Venusian craters are consistent with the inverse gravity trend observed for the terrestrial planets, and they are ∼50% deeper than current estimates for complex craters on the Earth. Unlike the other terrestrial planets, neither terrain‐floor depths nor central structure heights increase with increasing crater diameter. An interesting trend for which we have no explanation is that on Venus, the Moon, Mars, and Ganymede, central peaks generally rise to within a constant elevation relative to the surrounding terrain, but that elevation is lower on the Moon and Mars than on Venus and Ganymede.
Evidence for stronger El Niño‐Southern Oscillation (ENSO) Events in a Mid‐Holocene massive coral
Authors: Corrège Thierry , Delcroix Thierry , Récy Jacques , Beck Warren , Cabioch Guy , Le Cornec Florence
Published: February 2004

We present a 47‐year‐long record of sea surface temperature (SST) derived from Sr/Ca and U/Ca analysis of a massive Porites coral which grew at ∼ 4150 calendar years before present (B.P.) in Vanuatu (southwest tropical Pacific Ocean). Mean SST is similar in both the modern instrumental record and paleorecord, and both exhibit El Niño‐Southern Oscillation (ENSO) frequency SST oscillations. However, several strong decadal‐frequency cooling events and a marked modulation of the seasonal SST cycle, with power at both ENSO and decadal frequencies, are observed in the paleorecord, which are unprecedented in the modern record.
Glacial‐interglacial variability in denitrification in the World's Oceans: Causes and consequences
Authors: Ganeshram Raja S. , Pedersen Thomas F. , Calvert Stephen E. , McNeill Gavin W. , Fontugne Michel R.
Published: February 2004

The late Quaternary history of water‐column denitrifcation of the eastern Pacific margins and the Arabian Sea is reconstructed using sedimentary δ15N measurements. The δ15N values in six piston cores raised from these regions show remarkably similar cyclic variations, being heavy (9–10.5‰) during the interglacials and 2–3‰ lighter during the glacials. This implies that denitrification in these regions decreased substantially during the glacial periods. The glacial decline in denitrification is attributed to reduced upwelling and flux of organic material through the oxygen minimum zone. Since water‐column denitrification in these areas accounts for about half of the fixed‐nitrogen loss in the modern ocean, the inferred decrease in denitrification should have increased the oceanic nitrate inventory during glacial periods. Because nitrate is a limiting nutrient, oceanic productivity and attendant changes in CO2 may therefore have been modulated on glacial‐interglacial timescales by variations in the oceanic NO3 content.
Evaluation of Mg/Ca thermometry in foraminifera: Comparison of experimental results and measurements in nature
Authors: Toyofuku Takashi , Kitazato Hiroshi , Kawahata Hodaka , Tsuchiya Masashi , Nohara Masato
Published: February 2004

We studied Mg/Ca in high‐Mg, shallow‐water benthic foraminifera in culture and in samples from natural environments, in order to evaluate the expression of latitudinal and seasonal temperature variability in Mg/Ca in their tests. We culturedPlanoglabratella opercularis(d'Orbigny) andQuinqueloculina yabeiAsano under controlled temperature (10°–25°C) and salinity (30–38) conditions. Both species show a linear correlation between Mg/Ca and temperature, but they differ in temperature sensitivity. Salinity does not significantly influence Mg/Ca. In the samples collected in nature, Mg/Ca and seawater temperatures are positively correlated, but there are more complexities than in the records for cultured specimens due to such factors as seasonal fluctuations in temperature. We conclude that Mg/Ca ratios in monospecific benthic foraminiferal samples may be used as a reliable temperature proxy, if the lifetime of the species is taken into account.
A coral oxygen isotope record from the northern Red Sea documenting NAO, ENSO, and North Pacific teleconnections on Middle East climate variability since the year 1750
Authors: Felis Thomas , Pätzold Jürgen , Loya Yossi , Fine Maoz , Nawar Ahmed H. , Wefer Gerold
Published: February 2004

A 245‐year coral oxygen isotope record from the northern Red Sea (Ras Umm Sidd/Egypt, ∼28°N) in bimonthly resolution is presented. The mean annual coral δ18O signal apparently reflects varying proportions of both sea surface temperature and δ18Oseawater variability. In conjunction with instrumental observations of climate the coral record suggests for interannual and longer timescales that colder periods are accompanied by more arid conditions in the northern Red Sea but increased rainfall in the southeastern Mediterranean, whereas warmer periods are accompanied by decreased rainfall in the latter and less arid conditions in the northern Red Sea. A ∼70‐year oscillation of probably North Atlantic origin dominates the coral time series. Interannual to interdecadal variability is correlated with instrumental indices of the North Atlantic Oscillation (NAO), the El Niño‐Southern Oscillation (ENSO), and North Pacific climate variability. The results suggest that these modes contributed consistently to Middle East climate variability since at least 1750, preferentially at a period of ∼5.7 years.
Millennial‐scale depositional cycles related to British Ice Sheet variability and North Atlantic paleocirculation since 45 kyr B.P., Barra Fan, U.K. margin
Authors: Knutz Paul C. , Austin William E. N. , Jones E. John W.
Published: February 2004

Lithology, lithic petrology, planktonic foraminiferal abundances, and clastic grain sizes have been determined in a 30 m‐long core recovered from the Barra Fan off northwest Scotland. The record extends back to around 45 kyr B.P., with sedimentation rates ranging between 50 and 200 cm/kyr. The abundance of ice‐rafted debris indicates 16 glacimarine events, including temporal equivalents to Heinrich events 1–4. Enhanced concentrations of basaltic material derived from the British Tertiary Province suggest that the glacimarine sediments record variations in a glacial source on the Hebrides shelf margin. Glacimarine zones are separated by silty intervals with high planktonic foraminifera concentrations that reflect an interstadial circulation regime in the Rockall Trough. The results suggest that the last British Ice Sheet fluctuated with a periodicity of 2000–3000 years, in common with the Dansgaard‐Oeschger climate cycle.
On the late Pleistocene‐Holocene sapropel record of climatic and oceanographic variability in the eastern Mediterranean
Authors: Calvert S. E. , Fontugne M. R.
Published: February 2004

Sapropels and intercalated marls in a piston core from the eastern Mediterranean are chemically and mineralogical distinct: kaolinite, smectite, and total S contents, Fe/A1, Ba/A1, Co/A1, Mo/A1, and V/A1 are higher, whereas quartz, Mg calcite, illite contents, Si/A1, Ti/A1, K/A1, Rb/A1, and Zr/A1 are lower in the sapropels. Missing and “ghost” sapropels are identified by mineralogical and chemical properties that are not prone to diagenesis. Primary production was higher (Ba/A1) and bottom water and/or interstitial oxygenation was lower (Co/A1, Mo/A1, Ni/A1, and V/A1) during sapropel formation. Wind speeds (quartz/clay, Si/A1, and Zr/A1) and bottom water salinities (Mg calcite/calcite) were higher during periods of marl formation. Sapropels represent a fundamentally different sedimentary facies whose formation is linked to changes in the hydrological balance in the basin, driven by precessionally modulated changes in monsoon strength and subtropical precipitation changes, which altered circulation, production, and sediment source areas.
Ozone trends: A review
Authors: Staehelin J. , Harris N. R. P. , Appenzeller C. , Eberhard J.
Published: February 2004

Ozone plays a very important role in our atmosphere because it protects any living organisms at the Earth's surface against the harmful solar UVB and UVC radiation. In the stratosphere, ozone plays a critical role in the energy budget because it absorbs both solar UV and terrestrial IR radiation. Further, ozone in the tropopause acts as a strong greenhouse gas, and increasing ozone trends at these altitudes contribute to climate change. This review contains a short description of the various techniques that provided atmospheric ozone measurements valuable for long‐term trend analysis. The anthropogenic emissions of substances that deplete ozone (chlorine‐ and bromine‐containing volatile gases) have increased from the 1950s until the second half of the 1980s. The most severe consequence of the anthropogenic release of ozone‐depleting substances is the “Antarctic ozone hole.” Long‐term observations indicate that stratospheric ozone depletion in the southern winter‐spring season over Antarctica started in the late 1970s, leading to a strong decrease in October total ozone means. Present values are only approximately half of those observed prior to 1970. In the Arctic, large ozone depletion was observed in winter and spring in some recent years. Satellite and ground‐based measurements show no significant trends in the tropics but significant long‐term decreasing trends in the northern and southern midlatitudes (of the order of 2–4% per decade in the period from 1970 to 1996 and an acceleration in trends in the 1980s). Ozone at northern midlatitudes decreased by −7.4±2% per decade at 40 km above mean sea level, while ozone loss was small at 30 km. Large trends were found in the lower stratosphere, −5.1±1.8% at 20 km and −7.3±4.6% at 15 km, where the bulk of the ozone resides. The possibility of a reduction in the observed trends has been discussed recently, but it is very hard to distinguish this from the natural variability. As a consequence of the Montreal Protocol process, the emissions of ozone‐depleting substances have decreased since the late 1980s. Chlorine is no longer increasing in the stratosphere, although the total bromine amount is still increasing. Considering anthropogenic emissions of substances that deplete ozone, the turnaround in stratospheric ozone trends is expected to take place in the coming years. However, anthropogenic climate change could have a large influence on the future evolution of the Earth's ozone shield.
Scaling of fracture systems in geological media
Authors: Bonnet E. , Bour O. , Odling N. E. , Davy P. , Main I. , Cowie P. , Berkowitz B.
Published: February 2004

Scaling in fracture systems has become an active field of research in the last 25 years motivated by practical applications in hazardous waste disposal, hydrocarbon reservoir management, and earthquake hazard assessment. Relevant publications are therefore spread widely through the literature. Although it is recognized that some fracture systems are best described by scale‐limited laws (lognormal, exponential), it is now recognized that power laws and fractal geometry provide widely applicable descriptive tools for fracture system characterization. A key argument for power law and fractal scaling is the absence of characteristic length scales in the fracture growth process. All power law and fractal characteristics in nature must have upper and lower bounds. This topic has been largely neglected, but recent studies emphasize the importance of layering on all scales in limiting the scaling characteristics of natural fracture systems. The determination of power law exponents and fractal dimensions from observations, although outwardly simple, is problematic, and uncritical use of analysis techniques has resulted in inaccurate and even meaningless exponents. We review these techniques and suggest guidelines for the accurate and objective estimation of exponents and fractal dimensions. Syntheses of length, displacement, aperture power law exponents, and fractal dimensions are found, after critical appraisal of published studies, to show a wide variation, frequently spanning the theoretically possible range. Extrapolations from one dimension to two and from two dimensions to three are found to be nontrivial, and simple laws must be used with caution. Directions for future research include improved techniques for gathering data sets over great scale ranges and more rigorous application of existing analysis methods. More data are needed on joints and veins to illuminate the differences between different fracture modes. The physical causes of power law scaling and variation in exponents and fractal dimensions are still poorly understood.
Electrical parameters of soils in the frequency range from 1 kHz to 1 GHz, using lumped‐circuit methods
Authors: Sternberg Ben K. , Levitskaya Tsylya M.
Published: February 2004

For studying electrical properties of soil samples with various compositions and water contents, we applied a lumped‐circuit approach. The extension of this method up to 1GHz was made possible by using a coaxial sample holder. The complex electrical parameters of soils, such as the relative permittivity
, conductivity
, and resistivity
, were obtained by measuring the magnitude Z and phase ϕ of the sample impedance
. The experimental setup is described in our previous paper [Levitskaya and Sternberg, 2000]. The relative real permittivity ε′ and imaginary permittivity (dielectric losses) ε″ for high‐loss soils from Arizona decrease with frequency and increase with water content. Regression equations, derived for the relative permittivity ε′ versus water content at a given frequency, can be used to determine the water content in soil from ε′ data. The third‐degree polynomial equations, which relate the relative permittivity to the volumetric soil moisture content, are different for various frequencies. The complex electrical resistivity components ρ′ and ρ″ reveal a time‐dependent polarization process at frequencies above 1 MHz, which shifts to higher frequencies with increasing water content. The propagation parameters, such as attenuation constant α, phase velocity Vp, and penetration depth P, which we calculated from the electrical parameters, also depend on soil wetness. Our comparison of the electrical and propagation parameters for different soils shows that the high‐loss soil samples from Avra Valley, Arizona, have higher values ε′ and ε″, higher attenuation constant α, and lower penetration depth P than the low‐loss soils from Brookhaven, New York. For example, at 500 MHz, a high‐loss soil (Avra Valley) with volumetric moisture content of ∼10%, exhibits an attenuation of 43 dB/m, whereas for a low‐loss soil (Brookhaven) with the same wetness the attenuation constant is only 4 dB/m. We also note that very dry, clean sand in a sheltered “sand box,” which is a favorite medium for testing ground‐penetrating radar (GPR), is usually not representative of natural conditions. Therefore, GPR data from such “sand box” experiments must be used with considerable caution because they yield unrealistically large penetration depths and unnatural target responses.
Evidence of tilted layers in angle of arrival and Doppler beam steering power measurements
Authors: Hobbs Bridget G. , Reid Iain M. , Holdsworth David A.
Published: February 2004

The results of an experiment combining Doppler beam steering (DBS) and angle of arrival (AOA) measurements are presented. Nonzero vertical beam AOAs and off zenith DBS power measurements have been attributed by various authors to the tilting of horizontal layers in the atmosphere. This study aims to directly compare vertical beam AOAs and off zenith beam power data to determine whether the relationship between the two sets of measurements is consistent with tilted layers. The results indicate that the power differences in beams at 3.6° off zenith are related to the vertical beam AOAs and that the variation of the two quantities is consistent with tilted layers being observed in all of these beams. In contrast, the power differences at 10.8° off zenith do not appear to have the same relationship to the vertical beam AOAs.
A three‐dimensional matching method for tropospheric features
Authors: Fast Stephen A. , Young George S. , Bode Jason N. , Pelman Kenneth
Published: February 2004

In order for high‐fidelity radio wave propagation models to make accurate predictions a correct model of the refractivity is required. To this end, several methods have been developed to match refractivity profile structures between multiple range‐dependent profiles. There is broad agreement that using refractivity to match features dismisses relevant information that should be used. The method presented here takes a different approach. When the parabolic equation methods are used to calculate propagation loss, the refractivity is only required at each range step. Hence any method that can correctly predict refractivity at the range step is appropriate. Previous methods match refractive modulus, M unit, features for a given vertical cut of the atmosphere, thus dismissing relevant information found in the temperature, pressure, and humidity profiles. Here we match features in three dimensions using the virtual potential temperature, a conserved quantity derived from temperature, pressure, and specific humidity. The variables used to compute refractivity are then interpolated in this matched coordinate system. This approach results in correct mapping of features over a region, resolving sampling issues and modeling refractivity with a smooth function in three dimensions. Therefore, as a parabolic equation steps in range, the refractivity is derived from the interpolated data. This paradigm eliminates the burden of refractivity interpolation/extrapolation from the propagation prediction software. Examples of matched atmospheres are presented, accompanied by propagation predictions based on the matched atmosphere.
Experimental validation of a stable method of moments procedure in time domain for the scattering from arbitrarily shaped conducting bodies
Authors: Manara Giuliano , Monorchio Agostino
Published: February 2004

In this paper, numerical results obtained by the application of a stable marching‐on‐in‐time solution scheme of the electric field integral equation are compared against experimental data, assuming as a testing case that of electromagnetic scattering from perfectly electrically conducting bodies. The experiments have been performed in an electromagnetic pulse hardware simulator, reproducing plane wave illumination conditions. The surface current densities induced on the metallic bodies have been measured by means of probes sensitive to the tangential magnetic fields. The measured current densities have therefore been processed in order to eliminate the effects of the measuring instrument chain. Near‐field measurements have been performed by using probes sensitive to the electric field. Experimental data show a good agreement with numerical predictions.
Ice crystals and raindrop canting angle affecting the performance of a satellite system suffering from differential rain attenuation and cross‐polarization
Authors: Kanellopoulos J. D. , Panagopoulos A. D.
Published: February 2004

For frequencies above 10 GHz, which are of high importance in current satellite systems, interference is mainly aggravated because of the following reasons: potentially existing differential rain attenuation along the wanted and interfering paths as well as depolarization induced by the rainfall medium. The latter source concerns, of course, satellite systems using the frequency‐sharing technique. In the present paper an already proposed method to predict the degradation of the carrier‐to‐interference (C/I) ratio due to the above sources is properly modified by taking into account ice crystals and raindrop canting angle effects. The novel assumptions reflect upon the more accurate estimation of cross‐polarization discrimination and thus contribute to the reliable design of the system being interfered with. The present results are compared with the so far existing ones. The sensitivity of various parameters affecting the interference performance of the system is investigated. As a general conclusion, the inclusion of the ice crystals along with the raindrop canting angle effects may be of importance in some cases, such as the operation in the K band combined with the circular incident polarization, for the accurate estimation of the degradation of the total (C/I) ratio.
Vertical electron content from ionograms in real time
Authors: Huang Xueqin , Reinisch Bodo W.
Published: February 2004

A new technique for calculating the vertical total electron content (TEC) from ground‐based ionosonde measurements is introduced. The ionogram provides the information to directly calculate the vertical electron density profile up to the peak of the F2 layer. The profile above the peak is approximated by an α‐Chapman function with a scale height that is derived from the profile shape around the F2 peak. The ionosonde TEC, or ITEC, is then calculated as the integral from 0 to ∞ over the entire profile. ITEC values from Digisonde observations at Millstone Hill, Wallops Island, and Jicamarca are compared with incoherent scatter radar and with Faraday and TOPEX satellite TEC measurements, showing very good agreement at middle latitudes and the magnetic equator.
GPS data analysis near Puerto Rico for World Day campaigns
Authors: Reilly Michael H. , Singh Malkiat
Published: February 2004

Experiments near Puerto Rico around World Ionosphere Days in June 1998 and September 1999 were carried out to develop and test techniques for nowcasting and forecasting ionospheric electron density and space weather during quiet and disturbed geomagnetic conditions. We investigate the use of GPS data for this purpose. We use the Raytrace/ ICED‐Bent‐Gallagher (RIBG) ionospheric model, line‐of‐sight propagation, and the assumption of constant differential hardware biases to model two‐frequency GPS data. Discrete inverse theory is used to extract model parameters from the data. We avoid the traditional simplifying approximation of a fixed‐height spherical shell model for mapping slant to vertical TEC. We show that our method is practical, provides accuracy superior to the traditional methods, and is robust against geomagnetic disturbances.
Validation of electron density profiles derived from oblique ionograms over the United Kingdom
Authors: Heaton J. A. T. , Cannon P. S. , Rogers N. C. , Mitchell C. N. , Kersley L.
Published: February 2004

Inversion algorithms are available to derive the vertical electron density profile at the midpoint of an oblique sounder path. The techniques open up the possibility of monitoring the ionosphere at otherwise inaccessible locations, such as over sea or inhospitable terrain. A new method of monitoring the ionosphere based on radio tomography can be used to create two‐dimensional images of electron density. The results in this paper compare midpoint profiles derived from oblique ionograms with corresponding profiles obtained from tomographic images of electron density and from a vertical ionospheric sounder. The comparisons illustrate the oblique sounder inversion technique and its inherent limitations. The results provide useful information on the complementary nature of the separate ionospheric measurement techniques and have implications for the use of these measurements as inputs to real‐time ionospheric models.
Analysis of the interference due to differential rain attenuation induced by an adjacent terrestrial microwave system on a double‐site diversity Earth‐space system
Authors: Kanellopoulos J. D. , Livieratos S. N.
Published: February 2004

One of the main propagation effects on interference between a satellite system and an adjacent microwave terrestrial path is the differential rain attenuation. In this paper a method to predict rain differential attenuation statistics, used for single‐site interfered Earth‐space systems, is extended to include double‐site diversity systems. The modified method is again based on a model of convective rain cells as well as on the lognormal assumption for the point rainfall rate distribution at the location under consideration. Numerical results are presented concerning a realistic interfered satellite link using double‐site diversity and located in Japan. Because of a lack of available experimental data, our attention has mainly been concentrated on the examination of the optimum coordination between the adjacent systems. Some very useful conclusions are deduced.
Relations between hinterland and foreland shortening: Sevier orogeny, central North American Cordillera
Authors: Taylor Wanda J. , Bartley John M. , Martin Mark W. , Geissman John W. , Walker J. Douglas , Armstrong Phillip A. , Fryxell Joan E.
Published: February 2004

The tectonic relations between foreland and hinterland deformation in noncollisional orogens are critical to understanding the overall development of orogens. The classic central Cordilleran foreland fold‐and‐thrust belt in the United States (Late Jurassic to early Tertiary Sevier belt) and the more internal zones to the west (central Nevada thrust belt) provide data critical to understanding the development of internal and external parts of orogens. The Garden Valley thrust system, part of the central Nevada thrust belt, crops out in south‐central Nevada within a region generally considered to be the hinterland of the Jurassic to Eocene Sevier thrust belt. The thrust system consists of at least four principal thrust plates composed of strata as young as Pennsylvanian in age that are unconformably overlain by rocks as old as Oligocene, suggesting that contraction occurred between those times. New U/Pb dates on intrusions that postdate contraction, combined with new paleomagnetic data showing significant tilting of one area prior to intrusion, suggest that regionally these thrusts were active before ∼85–100 Ma. The thrust faults are characterized by long, relatively steeply dipping ramps and associated folds that are broad and open to close, upright and overturned. Although now fragmented by Cenozoic crustal extension, individual thrusts can be correlated from range to range for tens to hundreds of kilometers along strike. We correlate the structurally lowest thrust of the Garden Valley thrust system, the Golden Gate‐Mount Irish thrust, southward with the Gass Peak thrust of southern Nevada. This correlation carries the following regional implications. At least some of the slip across Jurassic to mid‐Cretaceous foreland thrusts in southern Nevada continues northward along the central Nevada thrust belt rather than northeastward into Utah. This continuation is consistent with age relations, which indicate that thrusts in the type Sevier belt in central Utah are synchronous with or younger than the youngest thrusts in southern Nevada. This in turn implies that geometrically similar Sevier belt thrusts in Utah must die out southward before they reach Nevada, that slip along the southern Nevada thrusts is partitioned between central Nevada and Utah thrusts, or that the Utah thrusts persist into southeastern Nevada but are located east of the longitude of the central Nevada thrust belt. As a result of overall cratonward migration of thrusting, the central Nevada thrust belt probably formed the Cordilleran foreland fold‐thrust belt early in the shortening event but later lay in the hinterland of the Sevier fold‐thrust belt of Idaho‐Wyoming‐Utah.
Geodynamics of flat subduction: Seismicity and tomographic constraints from the Andean margin
Authors: Gutscher Marc‐André , Spakman Wim , Bijwaard Harmen , Engdahl E. Robert
Published: February 2004

The cause and geodynamic impact of flat subduction are investigated. First, the 1500 km long Peru flat slab segment is examined. Earthquake hypocenter data image two morphologic highs in the subducting Nazca Plate which correlate with the positions of subducted oceanic plateaus. Travel time tomographic images confirm the three‐dimensional slab geometry and suggest a lithospheric tear may bound the NW edge of the flat slab segment, with possible slab detachment occurring down dip as well. Other flat slab regions worldwide are discussed: central Chile, Ecuador, NW Colombia, Costa Rica, Mexico, southern Alaska, SW Japan, and western New Guinea. Flat subduction is shown to be a widespread phenomenon, occuring in 10% of modern convergent margins. In nearly all these cases, as a spatial and temporal correlation is observed between subducting oceanic plateaus and flat subduction, we conclude that flat subduction is caused primarily by (1) the buoyancy of thickened oceanic crust of moderate to young age and (2) a delay in the basalt to eclogite transition due to the cool thermal structure of two overlapping lithospheres. A statistical analysis of seismicity along the entire length of the Andes demonstrates that seismic energy release in the upper plate at a distance of 250–800 km from the trench is on average 3–5 times greater above flat slab segments than for adjacent steep slab segments. We propose this is due to higher interplate coupling and the cold, strong rheology of the overriding lithosphere which thus enables stress and deformation to be transmitted hundreds of kilometers into the heart of the upper plate.