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41.
Ian Kraucunas Leon Clarke James Dirks John Hathaway Mohamad Hejazi Kathy Hibbard Maoyi Huang Chunlian Jin Michael Kintner-Meyer Kerstin Kleese van Dam Ruby Leung Hong-Yi Li Richard Moss Marty Peterson Jennie Rice Michael Scott Allison Thomson Nathalie Voisin Tristram West 《Climatic change》2015,129(3-4):573-588
42.
A nonlinear regression method is developed that can be used to estimate parameters of a ground waterflow model from a combination of observations of hydrological variables and observations of geophysical properties that are functionally related with the hydraulic conductivity. The procedure estimates: parameters characterizing the hydraulic conductivity field (e.g., zonal or pilot point values); geophysical properties that have been observed and that are functionally related with the hydraulic conductivity parameters; and a few parameters of the function that relates the hydraulic conductivity parameters with the geophysical properties (the type of function is assumed known). A fidelity factor, sigma(r)2, of a term of the minimized objective function reflects the faith one has in the validity of this functional relationship. The estimation methodology has been tested by means of synthetic models. The experimental results demonstrate that the number of estimated hydraulic conductivity parameters can be increased by adding geophysical observations to the set of hydrological observations that are traditionally used for model calibration. The improvement of the estimated hydraulic conductivity field and the simulated hydraulic head field can be significant but is dependent on the number, the locations, and the uncertainty of geophysical observations. The sensitivity of the estimation results to the value of sigma(r) is small for the studied problems except when the uncertainty of geophysical observations is high. In the latter case, a large sigma(r) value was found to be optimal to avoid that hydraulic conductivity estimates are closely tied to corresponding but highly uncertain geophysical observations. 相似文献
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Anomalous harmonics in the spectra of GPS position estimates 总被引:22,自引:3,他引:19
Prior studies of the power spectra of GPS position time series have found pervasive seasonal signals against a power-law background
of flicker noise plus white noise. Dong et al. (2002) estimated that less than half the observed GPS seasonal power can be explained by redistributions of geophysical fluid mass
loads. Much of the residual variation is probably caused by unidentified GPS technique errors and analysis artifacts. Among
possible mechanisms, Penna and Stewart (2003) have shown how unmodeled analysis errors at tidal frequencies (near 12- and 24-hour periods) can be aliased to longer periods
very efficiently. Signals near fortnightly, semiannual, and annual periods are expected to be most seriously affected. We
have examined spectra for the 167 sites of the International GNSS (Global Navigation Satellite Systems) Service (IGS) network
having more than 200 weekly measurements during 1996.0–2006.0. The non-linear residuals of the weekly IGS solutions that were
included in ITRF2005, the latest version of the International Terrestrial Reference Frame (ITRF), have been used. To improve
the detection of common-mode signals, the normalized spectra of all sites have been stacked, then boxcar smoothed for each
local north (N), east (E), and height (H) component. The stacked, smoothed spectra are very similar for all three components.
Peaks are evident at harmonics of about 1 cycle per year (cpy) up to at least 6 cpy, but the peaks are not all at strictly
1.0 cpy intervals. Based on the 6th harmonic of the N spectrum, which is among the sharpest and largest, and assuming a linear
overtone model, then a common fundamental of 1.040 ± 0.008 cpy can explain all peaks well, together with the expected annual
and semiannual signals. A flicker noise power-law continuum describes the background spectrum down to periods of a few months,
after which the residuals become whiter. Similar sub-seasonal tones are not apparent in the residuals of available satellite
laser ranging (SLR) and very long baseline interferometry (VLBI) sites, which are both an order of magnitude less numerous
and dominated by white noise. There is weak evidence for a few isolated peaks near 1 cpy harmonics in the spectra of geophysical
loadings, but these are much noisier than for GPS positions. Alternative explanations related to the GPS technique are suggested
by the close coincidence of the period of the 1.040 cpy frequency, about 351.2 days, to the “GPS year”; i.e., the interval
required for the constellation to repeat its inertial orientation with respect to the sun. This could indicate that the harmonics
are a type of systematic error related to the satellite orbits. Mechanisms could involve orbit modeling defects or aliasing
of site-dependent positioning biases modulated by the varying satellite geometry. 相似文献
46.
Astronomical forcing in Upper Miocene continental sequences: implications for the Geomagnetic Polarity Time Scale 总被引:1,自引:0,他引:1
Hayfaa Abdul Aziz Jan van Dam Wout Krijgsman 《Earth and Planetary Science Letters》2004,222(1):243-258
We present an integrated stratigraphic study of cyclically bedded distal alluvial fan to lacustrine deposits in the late Miocene continental sections of Cascante and Cañizar (Teruel basin, NE Spain). The cyclostratigraphic analysis reveals that different scales of sedimentary cyclicity are present with a thickness ratio of about 1:2:5. Spectral analysis of colour records in the depth domain indicates the presence of a significant peak at ∼2.2 m, which corresponds to the average thickness of the basic, mudstone-carbonate, cycle. Other peaks correspond to the large-scale cycle, which consists of clusters of five basic cycles, and to a cycle twice the average thickness of the basic cycle. Magnetostratigraphic results, in combination with small-mammal biostratigraphy, indicate that the three normal polarity intervals recorded in our sections correspond to C5n.2n, C5n.1n and C4Ar.2n. Assigning the ages of CK95 to the polarity reversals implies significant changes in sedimentation rate, which is not in agreement with the regularity of the sedimentary cyclicity. Thus, spectral analysis of high-resolution colour records in the time domain only produce a spectrum that is consistent with Milankovitch climate forcing, if several of the assigned age tie points are excluded. This indicates that the sedimentary cyclicity in these sections is related to astronomical variations in climate and that the ages of reversal boundaries are in error. Hence, we have calculated the astronomical durations for C4Ar.2n (87 ky), C4Ar.3r (54 ky), C5n.1n (141 ky), and C5n.1r (33 ky), which indeed show significant discrepancies with CK95. The duration pattern of our polarity intervals is confirmed by seafloor anomaly profiles and magnetostratigraphic records of deep-sea cores. Consequently, this study demonstrates that astronomically forcing in continental sequences can be a powerful tool to improve the fundamental dating of the geological record. 相似文献
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An experimental study of the preservation of cross-sets during the migration of current ripples under aggrading and non-aggrading conditions was conducted in order to test the modified Paola–Borgman theory for distribution of cross-set thickness as a function of distribution of bed-wave height. In a series of flume experiments, the geometry and migration characteristics of the ripples did not vary systematically with aggradation rate and are comparable to other flume and river data.
Mean cross-set thickness/mean formative bed-wave height is less than 0·4, and mean cross-set thickness/mean bed-wave height is less than 0·53. In the present experiments, the primary control of cross-set thickness is the variability of ripple height. Aggradation rate accounts for only 1–7% of the total cross-set thickness.
A two-parameter gamma density function was fitted to histograms of ripple height to determine the value of parameter a needed for the modified Paola–Borgman model. This model underestimates cross-set thickness because of its assumption that bed-form height spreads evenly above and below the mean bed level, which is not the case in reality. Mean cross-set thickness is predicted quite well if the model constant is increased to 1·3. 相似文献
Mean cross-set thickness/mean formative bed-wave height is less than 0·4, and mean cross-set thickness/mean bed-wave height is less than 0·53. In the present experiments, the primary control of cross-set thickness is the variability of ripple height. Aggradation rate accounts for only 1–7% of the total cross-set thickness.
A two-parameter gamma density function was fitted to histograms of ripple height to determine the value of parameter a needed for the modified Paola–Borgman model. This model underestimates cross-set thickness because of its assumption that bed-form height spreads evenly above and below the mean bed level, which is not the case in reality. Mean cross-set thickness is predicted quite well if the model constant is increased to 1·3. 相似文献
50.
Matt A. King Zuheir Altamimi Johannes Boehm Machiel Bos Rolf Dach Pedro Elosegui François Fund Manuel Hernández-Pajares David Lavallee Paulo Jorge Mendes Cerveira Nigel Penna Riccardo E. M. Riva Peter Steigenberger Tonie van Dam Luca Vittuari Simon Williams Pascal Willis 《Surveys in Geophysics》2010,31(5):465-507
The provision of accurate models of Glacial Isostatic Adjustment (GIA) is presently a priority need in climate studies, largely due to the potential of the Gravity Recovery and Climate Experiment (GRACE) data to be used to determine accurate and continent-wide assessments of ice mass change and hydrology. However, modelled GIA is uncertain due to insufficient constraints on our knowledge of past glacial changes and to large simplifications in the underlying Earth models. Consequently, we show differences between models that exceed several mm/year in terms of surface displacement for the two major ice sheets: Greenland and Antarctica. Geodetic measurements of surface displacement offer the potential for new constraints to be made on GIA models, especially when they are used to improve structural features of the Earth’s interior as to allow for a more realistic reconstruction of the glaciation history. We present the distribution of presently available campaign and continuous geodetic measurements in Greenland and Antarctica and summarise surface velocities published to date, showing substantial disagreement between techniques and GIA models alike. We review the current state-of-the-art in ground-based geodesy (GPS, VLBI, DORIS, SLR) in determining accurate and precise surface velocities. In particular, we focus on known areas of need in GPS observation level models and the terrestrial reference frame in order to advance geodetic observation precision/accuracy toward 0.1 mm/year and therefore further constrain models of GIA and subsequent present-day ice mass change estimates. 相似文献