摘要:

Analytical orbit perturbation theory suggests that the errors in the ephemerides of the Global Positioning System (GPS) satellites should be mostly resonant effects that can be corrected by adjusting a few parameters in a simple empirical acceleration formula, despite the complexity of their causes (mismodeling of gravity, radiation pressure, etc.), at least for arcs free from orbit maneuvers. This theoretical conclusion has been tested with simulations and actual data analyses. For these analyses, data from the Spring 1985 Experiment have been used to calculate improved ephemerides; in turn, these ephemerides have been used in the estimation of the coordinates of stations within the continental United States, previously positioned with very long baseline interferometry (VLBI). The agreement between the VLBI and the GPS results gives a measure of orbit quality. The outcome of this test supports the idea that the errors are mostly of a resonant nature. The principles described here are potentially applicable to the computation of the precise ephemerides of other spacecraft, such as geosynchronous relay satellites and oceanographic satellites, which are usually in highly resonant, nearly circular, repeating orbits.

ISSN:0148-0227    

DOI:10.1029/JB094iB07p09167

年代:1989

数据来源: WILEY

摘要:

A technique that uses synthetic aperture radar (SAR) images to measure very small (1 cm or less) surface motions with good resolution (10 m) over large swaths (50 km) is presented along with experimental results. The method could be used for accurate measurements of many geophysical phenomena, including swelling and buckling in fault zones, residual displacements from seismic events, and prevolcanic swelling. The method is based on SAR interferometry, where two images are made of a scene by simultaneously flying two physically separated antennas. Then the phases of corresponding pixels are differenced, and altitude formation is deduced from some simple computation and image rectification. It is also possible to use one antenna flown twice over the same scene; then, if the second flight exactly duplicates the track of the first, an interesting possibility occurs. There would be no phase changes between the images at all unless there was a physical change in the scene, such as ground swelling, that would alter the distance from some resolution element to the antenna. Since the phase changes all occur at the short carrier wavelength, the basic limitation on sensitivity is only the phase noise in the system. When the two imaging passes are made from flight tracks that are separated (which is the case with the Seasat images used here), it is no longer possible to distinguish surface changes from the parallax caused by topography. However, with some additional computation, a third image made at some other baseline may be used to remove the topography and leave only the surface changes. This method was applied using Seasat data to an imaging site in Imperial Valley, California, where motion effects were observed that were ascribed to the expansion of water‐absorbing clays. Phase change images of this area are shown, along with associated ground truth about the presence of water. Problems with the technique are explored, along with a discussion of future experimental possibilities on upcoming SAR missions like Earth Observing System (EOS), Earth Resources Satellite (ERS 1), SIR‐C, and the Venus imaging radar, Magel

ISSN:0148-0227    

DOI:10.1029/JB094iB07p09183

年代:1989

数据来源: WILEY

摘要:

Fundamental molecular vibration bands are significantly diminished by scattering. Thus such bands in spectra of fine particulate regoliths (i.e., dominated by<5‐μm particles), or regoliths displaying a similar scale of porosity, are difficult to use for mineralogical or rock type identification. Consequently, other spectral features have been sought that may be more useful in spectroscopic remote sensing of composition. We find that mineralogical information is retained in overtones and combination tones of the fundamental molecular vibrations in the 3.0‐ to 7.0‐μm region, but that relatively few minerals have a sufficiently distinctive band structure to be unambiguously identified with currently available techniques. More significantly, identification of general rock type, as defined by the SCFM chemical index (SCFM = SiO2/SiO2+ CaO + FeO + MgO), is possible using spectral features associated with the principal Christiansen frequency and with a region of relative transparency between the Si‐O stretching and bending bands. However, environmental factors may affect the appearance and wavelengths of these features. Finally, prominent absorption bands may result from the presence of relatively small amounts of water, hydroxyl or carbonate, because absorption bands exhibited by these materials in the 2.7‐ to 4.0‐μm region, where silicate spectra are otherwise featureless, increase strongly in spectral contrast with decreasing particle size. Such materials are thus detectable in very small amounts in a particulate regolith composed predominantly of sil

ISSN:0148-0227    

DOI:10.1029/JB094iB07p09192

年代:1989

数据来源: WILEY

摘要:

Variation in the spectral response of silicates in the thermal infrared (TIR) from 8 to 12 μm is the foundation for potentially important applications in the lithologic characterization of terrestrial and extraterrestrial surfaces. The purpose of this study is to elucidate the crystallo‐chemical basis for the variation in spectral behavior in terms which may be useful in remote sensing. This has led to the definition of a new parameter, SCFM, which is defined as the ratio SiO2/(SiO2+ CaO + FeO + MgO) in minerals and rocks. This parameter reflects the degree of depolymerization of the silica tetrahedra and is therefore highly correlated with the structure of TIR reflectance spectra of major silicate minerals in both fine‐ and coarse‐grained igneous rocks. It is also a good numerical descriptor of igneous rock composition. Thus SCFM is proposed as a very useful determinant for spectral identification of igneous rocks. The SCFM parameter was used to assess the effect of variations in location, number, and width of bands in discriminating rock composition. (An unfortunate ambiguity exists in the use of the term “band” which in spectroscopy indicates a feature in a spectrum and in remote sensing signifies a specific wavelength increment or channel. The latter definition is used in this paper.) Spectra of coarse‐particulate mineral and solid rock samples were acquired in the laboratory. A regression analysis of bands varying in width from 0.2 to 1.4 μm versus SCFM results in coefficients of determination (correlation coefficients),R2, ranging from 0.88 (for two 1.2‐μm‐wide bands) to 0.97 (for seven 0.2‐μm‐wide bands). Deletion of bands in the ozone absorption region (9.4–10.2 μm) results in a decrease inR2by only 0.01. Stimulation of system noise by addition of ±5% random counts to the bands decreasesR2by only 0.03. Because spectra structure is modified by weathering, alteration, and texture of the samples, the validity of these results is limited to fresh igneous rocks. With this constraint, it is concluded that accurate spectral identification of igneous rock composition is not greatly affected by bandwidth or system noise. It is the overall spectral envelope rather than the fine structure in the TIR which contains most of the information diagnostic of igneous rock composition. This permits design of remote sensing systems with broad bands and corresponding higher signal‐to‐noise, which can be traded off against the important requiremen

ISSN:0148-0227    

DOI:10.1029/JB094iB07p09203

年代:1989

数据来源: WILEY

摘要:

Considerable geochemical and petrographic evidence suggests that magma mixing phenomena are important in producing the chemical heterogeneity commonly observed in plutonic and volcanic rocks on a variety of scales in both space and time. Simulations of time‐dependent, variable viscosity, double‐diffusive convection have been carried out to quantitatively investigate the mixing dynamics of magma in melt‐dominated magma bodies. Two distinct measures of the “goodness of mixing” are used to quantify magma mixing: (1) the linear scale of segregation (L) which corresponds to the length scale of a typical compositional anomaly; and, (2) the intensity of segregation (I) which is a measure of the deviation of compositional anomalies from the mean. Nondimensionalization of the governing conservation equations shows that the style and time scale of mixing depend on the flux Rayleigh number (Rq= αgqd4/kκνm), the buoyancy ratio (Rr= βΔCk/αqd), the Lewis number (Le= κ/D), the silicic to mafic melt viscosity ratio (νr= νS/νm), and the aspect ratio (A=w/d) of the chamber. Simulations of magma mixing were carried out by solving the conservation equations for parameter ranges 105

ISSN:0148-0227    

DOI:10.1029/JB094iB07p09215

年代:1989

数据来源: WILEY

摘要:

Evidence of large‐scale episodic venting of hydrothermal fluids was initially discovered in August 1986 in the form of a 130‐km3radially symmetric “megaplume” over the southern Juan de Fuca Ridge. We report here on the discovery in September 1987 of a second, smaller megaplume about 45 km north of the location of the first megaplume. The3He/heat,3He/dissolved Mn, and3He/dissolved silica ratios in both megaplumes were typical of high‐temperature vent fluids. Evidence from long‐term records of current flow over the southern Juan de Fuca Ridge, and from the mineralogy and Mn chemistry of megaplume particles, makes it unlikely that the second megaplume was a reencounter of the first. A plume model that relates the heat flux to the observed plume rise height of ∼1000 m finds that the total heat content of the fluids that formed the megaplumes was 1016–1017J, or equivalently a fluid volume of 3–8 × 107m3at 350°C. The geometry and suspended particle population of the first megaplume imply that such features are formed within a few days time. The extraordinary heat and volume fluxes associated with megaplumes (102–103greater than ordinary vent fields), as well as their typical hydrothermal chemistry, suggest that they resulted from tectonic or hydraulic fracturing that suddenly increased the permeability of the hydrothermal fluid reservoir in the axial crust. The flux of hydrothermal heat from continuous venting and episodic megaplumes on the southern Juan de Fuca Ridge is presently 4 − 10 × 109W, a factor of 5–10 greater than various geophysical model calculations for this ridge segment. This imbalance may be symptomatic of a recent surge in the local

ISSN:0148-0227    

DOI:10.1029/JB094iB07p09237

年代:1989

数据来源: WILEY

摘要:

One‐atmosphere experiments conducted on a synthetic glass similar to Galapagos Spreading Center (GSC) FeTi basalt (POO.82N2), (Byerly et al., 1976) define liquid lines of descent atfO2values between the quartz‐fayalite‐magnetite (QFM) buffer and 2 log units more oxidizing than the nickle‐nickle oxide (NNO) buffer. The experiments provide a framework for understanding the development of FeTi basalts by fractionation at near‐ocean floor conditions. GSC lavas from near 85°W initially follow a compositional trend, distinguished by FeO° (= FeO + 0.9Fe2O3) enrichment and SiO2depletion, which is nearly identical to the trend observed in experiments at QFM to which olivine seeds were added. This compositional trend can be produced by crystallization along an olivine → pigeonite reaction boundary in a shallow crystal‐rich magma reservoir. In contrast, GSC lavas from 95°W do not mimic the 1‐atm liquid line of descent, but appear to have fractionated at somewhat higher pressure. Basaltic liquids from 95°W underwent fractional crystallization at 1–2 kbar, did not experience FeO° enrichment along an olivine → low‐Ca pyroxene reaction boundary, and developed FeO° enrichment concomitant with SiO2enrichment. This compositional variation is consistent with a differentiation process in which crystals are continually removed from contact with liquid. Rhyodacites from 95°W cannot be related to the basalts and FeTi basalts recovered at 95°W by shallow‐level crystal fractionation. Instead, rhyolite liquids were formed either by fractionation of similar parents at greater depth and higher PH2O, or formed by fractionation of different parents. Andesite formed by mixing between basaltic and rhyodacitic liquids. As a consequence, mixed andesites define a trend of decreasing P2O5which has been previously interpreted to represent apatite saturation at approximately 0.22 wt% P2O5, significantly earlier than at 85°W (where P2O5decreases at approximately 0.7 wt% P2O5). Our experiments suggest that thefO2when titanomagnetite first saturates at the GSC was approximately at the NNO buffer. Together with the Fe2O3/FeO data of Byers et al. (1983, 1984) and Christie et al. (1986), this requires an increase infO2during crystallization in excess of that produced during closed‐system fractional crystallization. We suggest that this increase infO2results from interaction with oxidizing surr

ISSN:0148-0227    

DOI:10.1029/JB094iB07p09251

年代:1989

数据来源: WILEY

摘要:

At least part of the inside corner uplift observed at slow ridge‐transform intersections is shown to be a consequence of the flexure of an elastic plate caused by a twisting moment exerted along the transform fault. The frictional drag exerted along the transform increases with depth (overburden) down to the depth where plastic flow predominates; this depth‐dependent drag results in a moment applied to the edge of the plate. An uplift of an inside corner of several hundred meters can be obtained by this mechanism. This can be increased to roughly 1 km (the observed uplift at some intersections) if we assume the elastic thickness of the plate is effectively thinner because of relaxation by long‐term creep of the stresses resisting plate flexure. Simple models formulated to test this hypothesis show that the observed inside comer uplift of the Vema and Oceanographer transforms can be explained if the “effective elastic thickness” of the plate is about half of the thickness over which drag increases with depth along the transform fault

ISSN:0148-0227    

DOI:10.1029/JB094iB07p09275

年代:1989

数据来源: WILEY

摘要:

We present an analysis of wide‐angle reflection/refraction data collected in the immediate vicinity of Deep Sea Drilling Project hole 504B in the Panama Basin, currently the deepest drill hole (1.288 km) into oceanic crust. The data were acquired with a 1785 inch3air gun array and fixed‐gain sonobuoy receivers and consist of four intersecting profiles shot along three different azimuths. Near‐normal‐incidence, multichannel seismic (MCS) reflection data were acquired simultaneously. Observed P and S wave arrivals out to maximum ranges of 30 km provide constraints on the velocity structure of the middle and lower crust and on total crustal thickness. Comparison of the travel times and amplitudes of the P and S wave arrivals on all four profiles revealed important similarities which were modeled using the reflectivity synthetic seismogram method. Forward modeling shows that in contrast to standard oceanic velocity models, a velocity‐depth profile that better explains the observed data is characterized by high‐velocity gradients (up to 0.6 km/s/km) in the middle crust, a 1.8‐km‐thick low‐velocity zone (Vp= 7.1–6.7 km/s) immediately above Moho, and a total crustal thickness of only 5 km. Interpretation of the high‐velocity gradients in the middle crust is constrained by the observation of P wave amplitude focusing at ranges of 16–19 km. Although not as well developed in comparison to the P wave arrivals, S wave arrivals show similar focusing. Total crustal thickness is constrained by the combined interpretation of a P wave, wide‐angle reflection event observed at a range of 16–28 km and an MCS reflection event with a crustal travel time of 1.4–1.5 s. Although these events cannot be directly correlated, their travel times are consistent with the assumption that both have a common origin. Amplitude modeling of the wide‐angle event demonstrates that these e

ISSN:0148-0227    

DOI:10.1029/JB094iB07p09283

年代:1989

数据来源: WILEY

摘要:

We carried out a borehole seismic experiment at Deep Sea Drilling Project hole 418A in order to study the vertical and lateral velocity variations in 100‐m.y.‐old oceanic crust. We clamped a seismometer successively at five depths between 41 m and 430 m within the extrusive basalt layer while air gun and explosive sources were fired on eight radial lines and four concentric circles to 8 km range. Our travel time data do not give consistent evidence of anisotropy. However, our experimental error may mask anisotropy of up to 0.2–0.3 km/s. Velocities from inflection point analysis, sonic log, and reflectivity synthetic seismograms support results of τ‐ζ inversion of radial line P arrival times. Velocity increases with an almost linear gradient of 1.3 s−1from 4.6 km/s at the top of basement to 1.8 km depth. The τ‐ζ velocity profile at 418A does not differ significantly at the 95% confidence level from that at 417D located ∼7.5 km away. Seismic velocities at shorter‐length scales do vary laterally. Travel time anomalies indicate that seismic velocity in the upper 0.5 km increases laterally northwest to 3 km range. Other data indicate that this velocity anomaly in the extrusive layer may be due to alteration controlled by topography or to primary

ISSN:0148-0227    

DOI:10.1029/JB094iB07p09303

年代:1989

数据来源: WILEY