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1.
As deduced from the data with high spatial resolution obtained at the radio heliographs of the Siberian Solar Radio Telescope
(SSRT, 5.7 GHz) and the Nobeyama radio heliograph (NoRH, 17 GHz), radio brightness centers in the distribution of the Stokes
parameter I are shifted relative to the distribution of the parameter V 1–2 days before an intense flare. It has been shown that this phenomenon can be related to the behavior of quasi-stationary
sources over the inversion line of the radial component of the magnetic field (neutral-line associated sources (NLSs)). These
sources have a brightness temperature up to 106 K and a circular polarization up to 90%. The origination of NLSs is associated
with the outflow of a new magnetic flux into the atmosphere of an active region that is a classical factor of the flare activity.
Therefore, an NLS is a precursor of power solar flares and can be used as a forecast factor. Owing to the high resolution
of the SSRT, the deviation of the observed polarization distribution of microwave radiation of the active region from the
normal one within the solar disk zone containing the active region can be used as a precursor of the preflare state of the
active region. As a result, the single-frequency Tanaka-Enome criterion is modified. The use of the data from two radio heliographs
(SSRT and NoRH) allows us to propose a two-frequency criterion of normal longitudinal zones that is more efficient for short-term
forecasting of solar flares. Preflare features associated with the displacement of brightness centers in I and V, which is manifested as the transformation of NLSs into spot sources, are fine attributes added to forecast according to
the two-frequency criterion. This is illustrated by an example of active region 10930, which produced power proton flares
on December 6 and 13, 2006. 相似文献
2.
3.
Broadband strong motion simulation in layered half-space using stochastic Green’s function technique
Y. Hisada 《Journal of Seismology》2008,12(2):265-279
The stochastic Green’s function method, which simulates one component of the far-field S-waves from an extended fault plane
at high frequencies (Kamae et al., J Struct Constr Eng Trans AIJ, 430:1–9, 1991), is extended to simulate the three components of the full waveform in layered half-spaces for broadband frequency
range. The method firstly computes ground motions from small earthquakes, which correspond to the ruptures of sub-faults on
a fault plane of a large earthquake, and secondly constructs the strong motions of the large earthquake by superposing the
small ground motions using the empirical Green’s function technique (e.g., Irikura, Proc 7th Japan Earthq Eng Symp, 151–156, 1986). The broadband stochastic omega-square model is proposed as the moment rate functions of the small earthquakes,
in which random and zero phases are used at higher and lower frequencies, respectively. The zero phases are introduced to
simulate a smooth ramp function of the moment function with the duration of 1/fc s (fc: the corner frequency) and to reproduce
coherent strong motions at low frequencies (i.e., the directivity pulse). As for the radiation coefficients, the theoretical
values of double couple sources for lower frequencies and the theoretical isotropic values for the P-, SV-, and SH-waves (Onishi
and Horike, J Struct Constr Eng Trans AIJ, 586:37–44, 2004) for high frequencies are used. The proposed method uses the theoretical Green’s functions of layered half-spaces
instead of the far-field S-waves, which reproduce the complete waves including the direct and reflected P- and S-waves and
surface waves at broadband frequencies. Finally, the proposed method is applied to the 1994 Northridge earthquake, and results
show excellent agreement with the observation records at broadband frequencies. At the same time, the method still needs improvements
especially because it underestimates the high-frequency vertical components in the near fault range. Nonetheless, the method
will be useful for modeling high frequency contributions in the hybrid methods, which use stochastic and deterministic methods
for high and low frequencies, respectively (e.g., the stochastic Green’s function method + finite difference methods; Kamae
et al., Bull Seism Soc Am, 88:357–367, 1998; Pitarka et al., Bull Seism Soc Am 90:566–586, 2000), because it reproduces the full waveforms in layered media including not only random characteristics at
higher frequencies but also theoretical and deterministic coherencies at lower frequencies. 相似文献
4.
The effect of horizontal grid resolution on the horizontal relative dispersion of particle pairs has been investigated on
a short time scale, i.e. one tidal M
2 cycle. Of particular interest is the tidal effect on dispersion and transports in coastal waters where small-scale flow features
are important. A three-dimensional ocean model has been applied to simulate the tidal flow through the Moskstraumen Maelstrom
outside Lofoten in northern Norway, well known for its strong current and whirlpools (Gjevik et al., Nature 388(6645):837–838,
1997; Moe et al., Cont Shelf Res 22(3):485–504, 2002). Simulations with spatial resolution down to 50 m have been carried out. Lagrangian tracers were passively advected with
the flow, and Lyapunov exponents and power law exponents have been calculated to analyse the separation statistics. It is
found that the relative dispersion of particles on a short time scale (12–24 h) is very sensitive to the grid size and that
the spatial variability is also very large, ranging from 0 to 100 km2 over a distance of 100 m. This means that models for prediction of transport and dispersion of oil spills, fish eggs, sea
lice etc. using a single diffusion coefficient will be of limited value, unless the models actually resolves the small-scale
eddies of the tidal current. 相似文献
5.
Calculation of lava effusion rates from Landsat TM data 总被引:1,自引:0,他引:1
Andrew J. L. Harris Luke P. Flynn Laszlo Keszthelyi Peter J. Mouginis-Mark Scott K. Rowland Joseph A. Resing 《Bulletin of Volcanology》1998,60(1):52-71
We present a thermal model to calculate the total thermal flux for lava flowing in tubes, on the surface, or under shallow
water. Once defined, we use the total thermal flux to estimate effusion rates for active flows at Kilauea, Hawaii, on two
dates. Input parameters were derived from Landsat Thematic Mapper (TM), field and laboratory measurements. Using these parameters
we obtain effusion rates of 1.76±0.57 and 0.78±0.27 m3 s–1 on 23 July and 11 October 1991, respectively. These rates are corroborated by field measurements of 1.36±0.14 and 0.89±0.09 m3 s–1 for the same dates (Kauahikaua et al. 1996). Using weather satellite (AVHRR) data of lower spatial resolution, we obtain
similar effusion rates for an additional 26 dates between the two TM-derived measurements. We assume that, although total
effusion rates at the source declined over the period, the shut down of the ocean entry meant that effusion rates for the
surface flows alone remained stable. Such synergetic use of remotely sensed data provides measurements that can (a) contribute
to monitoring flow-field evolution, and (b) provide reliable numerical data for input into rheological and thermal models.
We look forward to being able to produce estimates for effusion rates using data from high-spatial-resolution sensors in the
earth observing system (EOS) era, such as Landsat 7, the hyperspectral imager, the advanced spaceborne thermal emission spectrometer,
and the advanced land imager.
Received: 25 July 1997 / Accepted: 26 February 1998 相似文献
6.
We present thermal measurements made by high spatial resolution ground-based (a hand-held thermal camera) and low spatial
resolution space-based (MODIS) instruments for a lava flow field active during the last phase of the May–July 2003 eruption
at Piton de la Fournaise (La Réunion). Multiple oblique ground-based thermal images were merged to provide full coverage of
the flow-field. These were then corrected for path length attenuation and orthorectified, allowing the at-surface radiance
emitted by the flow-field to be estimated. Comparison with the radiance recorded by the MODIS sensors during the eruption
reveals that, for clear-sky conditions and moderate-to-low viewing angles (satellite zenith <40°), the satellite measurements
represent ∼90% of the at-surface radiance, and thus represent valuable data for quantifying volcanic thermal anomalies. Nevertheless,
extreme viewing geometries and the presence of clouds strongly affect the radiance reaching the sensor and affected data from
94% of the overpasses. Ground-based thermal data were used to investigate an empirical relationship between the radiant heat
flux and lava discharge rate during the emplacement of pahoehoe flows. While the average radiation temperature for flow surface
that were 6–24 h old ranged between 500 K and 625 K, the ratio between radiative heat flux and Time-Averaged lava Discharge
Rate (TADR) ranged between 1.5 × 108 J m−3 and 3.5 × 108 J m−3. This relationship was used to estimate TADR values from optimal MODIS data and produced results in line with those obtained
from GPS surveys (Coppola et al. 2005). Our results underscore the importance of ground-based thermal analysis for the interpretation of satellite measurements,
particularly in terms of calculating discharge rate trends. 相似文献
7.
K. Kh. Truong T. S. Vo Yu. M. Mikhailov A. L. Kharitonov O. V. Kapustina 《Geomagnetism and Aeronomy》2010,50(5):690-694
The preliminary results of observations of VLF electromagnetic signals (atmospherics) in the mountains of North Vietnam (Dien
Bien Phu, 21°23′50″ N, 103°0′28″ E) are presented. Primarily, signals of typical atmospherics with a maximum in the frequency
range of 4–9 kHz were observed; sometimes they were accompanied by so-called “tails” at frequencies less than 1 kHz, and also
tweeks of usually short duration (about 10 ms) were observed. Several parameters of the ionosphere, as well as the distance
to sources of atmospherics (in accordance to the data of the World Wide Lightning Location Network (TOGA)), were estimated
by spectral-temporal characteristics of tweeks. 相似文献
8.
R. B. Teplitskaya O. A. Ozhogina I. P. Turova R. A. Sych 《Geomagnetism and Aeronomy》2010,50(7):831-842
The dynamic regime of eight regions in the quiet solar chromosphere was analyzed. Observations in CaII lines were performed
with a moderate spatial resolution of around 2″. The purpose of this study was to compare the diagnostic capabilities of these
observations with high-precision measurements conducted in recent years by different authors with new-generation instruments.
Being probably of importance in the chromospheric heating, the recently revealed structural formations (magnetoacoustic portals,
magnetic aureoles, and magnetic shadows) were found to manifest themselves at moderate resolutions too. These findings are
important in the context of investigations of coronal hole bases conducted by the authors with the same observational data. 相似文献
9.
We address possibilities of minimising environmental risks using statistical features of current-driven propagation of adverse
impacts to the coast. The recently introduced method for finding the optimum locations of potentially dangerous activities
(Soomere et al. in Proc Estonian Acad Sci 59:156–165, 2010) is expanded towards accounting for the spatial distributions of probabilities and times for reaching the coast for passively
advecting particles released in different sea areas. These distributions are calculated using large sets of Lagrangian trajectories
found from Eulerian velocity fields provided by the Rossby Centre Ocean Model with a horizontal resolution of 2 nautical miles
for 1987–1991. The test area is the Gulf of Finland in the northeastern Baltic Sea. The potential gain using the optimum fairways
from the Baltic Proper to the eastern part of the gulf is an up to 44% decrease in the probability of coastal pollution and
a similar increase in the average time for reaching the coast. The optimum fairways are mostly located to the north of the
gulf axis (by 2–8 km on average) and meander substantially in some sections. The robustness of this approach is quantified
as the typical root mean square deviation (6–16 km) between the optimum fairways specified from different criteria. Drastic
variations in the width of the ‘corridors’ for almost optimal fairways (2–30 km for the average width of 15 km) signifies
that the sensitivity of the results with respect to small changes in the environmental criteria largely varies in different
parts of the gulf. 相似文献
10.
P. V. Vatagin Yu. E. Charikov A. V. Stepanov I. V. Kudryavtsev 《Geomagnetism and Aeronomy》2012,52(8):1015-1020
Unique measurements by a solar submillimeter radio telescope (SST) have been carried out in the sub-THz radiation at 212 and 405 THz over the past decade. The spectrum of RF radiation in this region increased with frequency for the three flares of November 2 and 4, 2003, and December 6, 2006, and the flux value reached 5 × 103?2 × 104 sfu at 405 GHz (Kaufman et al., 2009). In this work, we consider a set of nonlinear equations for an accelerated electrons beam and the Langmuir wave energy density. The distribution functions of the accelerated electron beam and wave energy density are calculated taking into account Coulomb collisions, electron scattering by waves, and wave scattering by plasma ions. In addition, the source of accelerated particles and the heat level of the Langmuir turbulence are specified. The beam and plasma parameters are chosen based on the aims of a problem. The plasma concentration varies from n = 1013 to 1015 cm?3, the electron plasma frequency f p = (3 × 1010?3 × 1011) Hz in this case. The ratio of plasma and beam concentrations, sufficient to explain the value of the radio flux at a frequency of 300 GHz, is n b/n = 10?3. The Langmuir turbulence is excited due to the instability of the accelerated electron beam with an initial distribution function of the ??bump-in-tail?? type. Then, the parameters of radiowaves are calculated in the sub-THz range under the assumption of coalescence of two plasma waves. The calculation results show that a sub-THz radio flux can be obtained under the condition of injection of accelerated electrons. The fine time structure of radio flux observed is easily simulated based on this statement by the pulsed time structure of electron beams and their dynamics in overdense plasma. X-ray and gamma radiation was recorded during the events under study. Hard X-ray radiation is bremsstrahlung radiation from accelerated electron beams. 相似文献
11.
A reappraisal of shear wave splitting parameters from Italian active volcanic areas through a semiautomatic algorithm 总被引:1,自引:0,他引:1
Shear wave splitting parameters represent a useful tool to detail the stress changes occurring in volcanic environments before
impending eruptions. In the present paper, we display the parameter estimates obtained through implementation of a semiautomatic
algorithm applied to all useful datasets of the following Italian active volcanic areas: Mt. Vesuvius, Campi Flegrei, and
Mt. Etna. Most of these datasets have been the object of several studies (Bianco et al., Annali di Geofisica, XXXXIX 2:429–443,
1996, J Volcanol Geotherm Res 82:199–218, 1998a, Geophys Res Lett 25(10):1545–1548, 1998b, Phys Chem Earth 24:977–983, 1999, J Volcanol Geotherm Res 133:229–246, 2004, Geophys J Int 167(2):959–967, 2006; Del Pezzo et al., Bull Seismol Soc Am 94(2):439–452, 2004). Applying the semiautomatic algorithm, we confirmed the results obtained in previous studies, so we do not discuss in much
detail each of our findings but give a general overview of the anisotropic features of the investigated Italian volcanoes.
In order to make a comparison among the different volcanic areas, we present our results in terms of the main direction of
the fast polarization (φ) and percentage of shear wave anisotropy (ξ). 相似文献
12.
Lascar Volcano (22°22'S, 67°44'W) is the most active volcano of the central Andes of northern Chile. Activity since 1984
has been characterised by periods of lava dome growth and decay within the active crater, punctuated by explosive eruptions.
We present herein a technique for monitoring the high-temperature activity within the active crater using frequent measurements
of emitted shortwave infrared (SWIR) radiation made by the spaceborne along-track scanning radiometer (ATSR). The ATSR is
an instrument of low spatial resolution (pixels 1 km across) that shares certain characteristics with the MODIS instrument,
planned for use as a volcano monitoring tool in the NASA EOS Volcanology Project. We present a comprehensive time series of
over 60 cloud- and plume-free nighttime ATSR observations for 1992–1995, a period during which Lascar experienced its largest
historical eruption. Variations in short wavelength infrared flux relate directly to changes in high-temperature surfaces
within the active crater. From these data, interpretations can be made that supplement published field reports and that can
document the presence and status of the lava dome during periods where direct, ground-based, observations are lacking. Our
data agree with less frequent information collected from sensors with high spatial resolution, such as the Landsat thematic
mapper (Oppenheimer et al. 1993) and are consistent with field observations and models that relate subsidence of the dome
to subsequent explosive eruptions (Matthews et al., 1997). Most obviously, Lascar's major April 1993 eruption follows a period
in which the magnitude of emitted shortwave infrared radiation fell by 90%. At this time subsidence of the 1991–1992 lava
dome was reported by field observers and this subsidence is believed to have impeded the escape of hot volatiles and ultimately
triggered the eruption (Smithsonian Institution 1993a). Extrapolating beyond the period for which field observations of the
summit are available, our data show that the vulcanian eruption of 20 July 1995 occurred after a period of gradual increase
in short wavelength infrared flux throughout 1994 and a more rapid flux decline during 1995. We attribute this additional,
otherwise undocumented, cycle of increasing and decreasing SWIR radiance as most likely representing variations in degassing
through fumaroles contained within the summit crater. Alternatively, it may reflect a cycle of dome growth and decay. The
explosive eruption of 17 December 1993 appears to have followed a similar, but shorter, variation in SWIR flux, and we conclude
that large explosive eruptions are more likely when the 1.6-μm signal has fallen from a high to a low level. The ATSR instrument
offers low-cost data at high temporal resolution. Despite the low spatial detail of the measurements, ATSR-type instruments
can provide data that relate directly to the status of Lascar's lava dome and other high-temperature surfaces. We suggest
that such data can therefore assist with predictions of eruptive behaviour, deduced from application of physical models of
lava dome development at this and similar volcanoes.
Received: 1 October 1996 / Accepted: 13 January 1997 相似文献
13.
Hongjoon Shin Younghun Jung Changsam Jeong Jun-Haeng Heo 《Stochastic Environmental Research and Risk Assessment (SERRA)》2012,26(1):105-114
An important problem in frequency analysis is the selection of an appropriate probability distribution for a given sample
data. This selection is generally based on goodness-of-fit tests. The goodness-of-fit method is an effective means of examining
how well a sample data agrees with an assumed probability distribution as its population. However, the goodness of fit test
based on empirical distribution functions gives equal weight to differences between empirical and theoretical distribution
functions corresponding to all observations. To overcome this drawback, the modified Anderson–Darling test was suggested by
Ahmad et al. (1988b). In this study, the critical values of the modified Anderson–Darling test statistics are revised using simulation experiments
with extensions of the shape parameters for the GEV and GLO distributions, and a power study is performed to test the performance
of the modified Anderson–Darling test. The results of the power study show that the modified Anderson–Darling test is more
powerful than traditional tests such as the χ2, Kolmogorov–Smirnov, and Cramer von Mises tests. In addition, to compare the results of these goodness-of-fit tests, the
modified Anderson–Darling test is applied to the annual maximum rainfall data in Korea. 相似文献
14.
Esther Hintersberger Frank Scherbaum Sebastian Hainzl 《Bulletin of Earthquake Engineering》2007,5(1):1-16
Scherbaum et al. [(2004) Bull Seismolo Soc Am 94(6): 2164–2185] proposed a likelihood-based approach to select and rank ground-motion
models for seismic hazard analysis in regions of low-seismicity. The results of their analysis were first used within the
PEGASOS project [Abrahamson et al. (2002), In Proceedings of the 12 ECEE, London, 2002, Paper no. 633] so far the only application
of a probabilistic seismic hazard analysis (PSHA) in Europe which was based on a SSHAC Level 4 procedure [(Budnitz et al.
1997, Recommendations for PSHA: guidance on uncertainty and use of experts. No. NUREG/CR-6372-V1). The outcome of this project
have generated considerable discussion (Klügel 2005, Eng Geol 78:285–307, 2005b) Eng Geol 78: 285–307, (2005c) Eng Geol 82:
79–85 Musson et al. (2005) Eng Geol 82(1): 43–55]; Budnitz et al. (2005), Eng Geol 78(3–4): 285–307], a central part of which
is related to the issue of ground-motion model selection and ranking. Since at the time of the study by Scherbaum et al. [(2004.)
Bull Seismolo Soc Am 94(6): 2164–2185], only records from one earthquake were available for the study area, here we test the
stability of their results using more recent data. Increasing the data set from 12 records of one earthquake in Scherbaum
et al. [(2004) Bull Seismolo Soc Am 94(6): 2164–2185] to 61 records of 5 earthquakes, which have mainly occurred since the
publication of the original study, does not change the set of the three top-ranked ground-motion models [Abrahamson and Silva
(1997) Seismolo Res Latt 68(1): 94–127; Lussou et al. (2001) J Earthquake Eng 5(1):13–33; Berge-Thierry et al. (2003) Bull
Seismolog Soc Am 95(2): 377–389. Only for the lower-ranked models do we obtain modifications in the ranking order. Furthermore,
the records from the Waldkirch earthquake (Dec, 5th, 2004, M
w = 4.9) enabled us to develop a new stochastic model parameter set for the application of Campbell’s [(2003) Bull Seismolo
Soc Am 93(3): 1012–1033] hybrid empirical model to SW Germany and neighbouring regions. 相似文献
15.
We present a simple and efficient hybrid technique for simulating earthquake strong ground motion. This procedure is the combination
of the techniques of envelope function (Midorikawa
et al. Tectonophysics 218:287–295, 1993) and composite source model (Zeng
et al. Geophys Res Lett 21:725–728, 1994). The first step of the technique is based on the construction of the envelope function of the large earthquake by superposition
of envelope functions for smaller earthquakes. The smaller earthquakes (sub-events) of varying sizes are distributed randomly,
instead of uniform distribution of same size sub-events, on the fault plane. The accelerogram of large event is then obtained
by combining the envelope function with a band-limited white noise. The low-cut frequency of the band-limited white noise
is chosen to correspond to the corner frequency for the target earthquake magnitude and the high-cut to the Boore’s f
max or a desired frequency for the simulation. Below the low-cut frequency, the fall-off slope is 2 in accordance with the ω2 earthquake source model. The technique requires the parameters such as fault area, orientation of the fault, hypocenter,
size of the sub-events, stress drop, rupture velocity, duration, source–site distance and attenuation parameter. The fidelity
of the technique has been demonstrated by successful modeling of the 1991 Uttarkashi, Himalaya earthquake (Ms 7). The acceptable
locations of the sub-events on the fault plane have been determined using a genetic algorithm. The main characteristics of
the simulated accelerograms, comprised of the duration of strong ground shaking, peak ground acceleration and Fourier and
response spectra, are, in general, in good agreement with those observed at most of the sites. At some of the sites the simulated
accelerograms differ from observed ones by a factor of 2–3. The local site geology and topography may cause such a difference,
as these effects have not been considered in the present technique. The advantage of the technique lies in the fact that detailed
parameters such as velocity-Q structures and empirical Green’s functions are not required or the records of the actual time
history from the past earthquakes are not available. This method may find its application in preparing a wide range of scenarios
based on simulation. This provides information that is complementary to the information available in probabilistic hazard
maps. 相似文献
16.
Anomalous high frequency PKKPBC signals (displaying a large amount of energy around 2.5 Hz), recorded globally for deep and intermediate depth earthquakes,
are compared to PKKPAB signals. The attenuation difference
t\textAB* - t\textBC* t_{\text{AB}}^{*} - t_{\text{BC}}^{*} is evaluated from spectral amplitudes in the range 96–111°, being approximately twice the results provided by full-wave theory
and PREM (with no low Qμ zone in the lowermost mantle and a nearly infinite QK in the outer core). Most ray paths for such recordings are piercing the D″ region in the proximity of regions where ultra-low
velocity zones (ULVZ) have been previously reported beneath the North Atlantic Ocean, the Southwest Pacific and the southwestern
part of South America. If BC amplitudes around 2.5 Hz and at low frequencies (0.5–1.5 Hz) are comparable, the observed attenuation
difference (in the frequency range 0.2–2.5 Hz) is small (around 0.25 s) and close to the PREM value. The particle motion of
the high-frequency PKKPBC at 2.5 Hz is quite similar to that of the raw recording, suggesting a deep source. An explanation for this might be scattering
of the BC branch in some very restricted areas of the lowermost mantle. Alternately, the presence of a thin layer with high
attenuation in the D″ region would most likely be associated with either the ultra-low velocity zone (ULVZ) or light sediments
on the underside of the core-mantle boundary (CMB). Correlated to other methods to investigate the lowermost mantle, the high-frequency
PKKPBC can be used to map lateral variations of attenuation above the CMB, possibly associated with the boundary of the superplumes,
especially when PKKPAB is observed. 相似文献
17.
The spectra of the coronal hole radio emission in solar cycles 23 and 24 have been studied based on RATAN-600 data in the 4–16.5 GHz range at frequencies of 5.7 and 17 GHz and 327 MHz. It has been found that bright features of coronal hole microwave emission at 17 GHz and dark features at 5.7 GHz can exist in coronal holes when the spectral index is 1.25–1.5 in the 6.5–16.5 GHz range; the radio spectrum in this range is flat when coronal holes are indiscernible against the background of a quiet Sun. The possible vertical scale of the solar atmosphere over coronal holes is discussed. 相似文献
18.
Further improvement of temporal resolution of seismic data by autoregressive (AR) spectral extrapolation 总被引:1,自引:0,他引:1
Hakan Karsl 《Journal of Applied Geophysics》2006,59(4):324-336
Seismic data have still no enough temporal resolution because of band-limited nature of available data even if it is deconvolved. However, lower and higher frequency information belonging to seismic data is missing and it is not directly recovered from seismic data. In this paper, a method originally applied by Honarvar et al. [Honarvar, F., Sheikhzadeh, H., Moles, M., Sinclair, A.N., 2004. Improving the time-resolution and signal–noise ratio of ultrasonic NDE signals. Ultrasonics 41, 755–763.] which is the combination of the most widely used Wiener deconvolution and AR spectral extrapolation in frequency domain is briefly reviewed and is applied to seismic data to improve temporal resolution further. The missing frequency information is optimally recovered by forward and backward extrapolation based on the selection of a high signal–noise ratio (SNR) of signal spectrum deconvolved in signal processing technique. The combination of the two methods is firstly tested on a variety of synthetic examples and then applied to a stacked real trace. The selection of necessary parameters in Wiener filtering and in extrapolation are discussed in detail. It is used an optimum frequency windows between 3 and 10 dB drops by comparing results from these drops, while frequency windows are used as standard between 2.8 and 3.2 dB drops in study of Honarvar et al. [Honarvar, F., Sheikhzadeh, H., Moles, M., Sinclair, A.N., 2004. Improving the time-resolution and signal–noise ratio of ultrasonic NDE signals. Ultrasonics 41, 755–763.]. The results obtained show that the application of the purposed signal processing technique considerably improves temporal resolution of seismic data when compared with the original seismic data. Furthermore, AR based spectral extrapolated data can be almost considered as reflectivity sequence of layered medium. Consequently, the combination of Wiener deconvolution and AR spectral extrapolation can reveal some details of seismic data that cannot be observed in raw signal or which lost during the previous processing. 相似文献
19.
Investigation of topographic reductions and aliasing effects on gravity and the geoid over Greece based on various digital terrain models 总被引:2,自引:0,他引:2
The reduction of gravity-field related quantities (e.g., gravity anomalies, geoid heights) due to the topography plays a crucial
role in both geodetic and geophysical applications, since in the former it is an intermediate step towards geoid prediction
and in the latter it reveals lateral as well as radial density contrasts and infers the geology of the area under study. The
computations are usually carried out by employing a DTM and/or a DBM, which describe the topography and bathymetry, respectively.
Errors in these DTMs/DBMs will introduce errors in the computed topographic effects, while poor spatial resolution of the
topography and bathymetry models will result in aliasing effects to both gravity anomalies and geoid heights, both influencing
the accuracy of the estimated solutions. The scope of this work is twofold. First, a validation and accuracy assessment of
the SRTM 3″ (90 m) DTM over Greece is performed through comparisons with existing global models as well as with the Greek
450 m national DTMs. Whenever a misrepresentation of the topography is identified in the SRTM data, it is “corrected” using
the local 450 m DTM. This process resulted in an improved SRTM DTM called SRTMGr, which was then used to determine terrain
effects to gravity field quantities. From the fine-resolution SRTMGr DTMs, coarser models of 15″, 30″, 1′, 2′ and 5′ have
been determined in order to investigate aliasing effects on both gravity anomalies and geoid heights by computing terrain
effects at variable spatial resolutions. From the results acquired in two test areas, it was concluded that SRTMGr provides
similar results to the local DTM making the use of other older global DTMs obsolete. The study for terrain aliasing effects
proved that when high-resolution and accuracy gravity and geoid models are needed, then the highest possible resolution DTM
should be employed to compute the respective terrain effects. Based on the results acquired from two the test areas a corrected
SRTMGr DTM has been compiled for the entire Greek territory towards the development of a new gravimetric geoid model. Results
from that analysis are presented based on the well-known remove-compute-restore method, employing land and marine gravity
data, EGM08 as a reference geopotential model and the SRTMGr DTM for the computation of the RTM effects. 相似文献
20.
T.B. Veronese R.R. Rosa M.J.A. Bolzan F.C. Rocha Fernandes H.S. Sawant M. Karlicky` 《Journal of Atmospheric and Solar》2011,73(11-12):1311-1316
High temporal resolution solar observations in the decimetric range (1–3 GHz) can provide additional information on solar active regions dynamics and thus contribute to better understanding of solar geoeffective events as flares and coronal mass ejections. The June 6, 2000 flares are a set of remarkable geoeffective eruptive phenomena observed as solar radio bursts (SRB) by means of the 3 GHz Ondrejov Observatory radiometer. We have selected and analyzed, applying detrended fluctuation analysis (DFA), three decimetric bursts associated to X1.1, X1.2 and X2.3 flare-classes, respectively. The association with geomagnetic activity is also reported. DFA method is performed in the framework of a radio burst automatic monitoring system. Our results may characterize the SRB evolution, computing the DFA scaling exponent, scanning the SRB time series by a short windowing before the extreme event. For the first time, the importance of DFA in the context of SRB monitoring analysis is presented. 相似文献