共查询到20条相似文献,搜索用时 31 毫秒
1.
D. H. Boteler 《Pure and Applied Geophysics》1990,134(4):511-526
A new approach to the theory of electromagnetic induction is developed that is applicable to moving as well as stationary sources. The source field is considered to be a standing wave generated by two waves travelling in opposite directions along the surface of the earth. For a stationary source the incident waves have velocities of the same magnitude, however for a moving source the velocities of the two incident waves are respectively increased and decreased by the velocity of the source. Electromagnetic induction in the earth is then considered as refraction of these waves and gives, for both stationary and moving sources, the magnetotelluric relation: $$\frac{{ - E_y }}{{H_x }} = \left( {\frac{{i\omega \mu }}{\sigma }} \right)^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} \left( {1 - i\frac{{v^2 }}{{\omega \mu \sigma }}} \right)^{ - {1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} $$ where ν is the wavenumber of the source, μ is the permeability (4π·10?7) and σ is the conductivity of the earth. ω is the angular frequency of the variation observed on the earth. For a stationary source the observed frequency is the same as the source frequency, however the effect of moving a time-varying source is to make the observed frequency different from the frequency of the source. Failure to recognise this in previous studies led to some erroneous conclusions. This study shows that a moving source isnot “electromagnetically broader” than a stationary source as had been suggested. 相似文献
2.
This article deals with the right-tail behavior of a response distribution \(F_Y\) conditional on a regressor vector \({\mathbf {X}}={\mathbf {x}}\) restricted to the heavy-tailed case of Pareto-type conditional distributions \(F_Y(y|\ {\mathbf {x}})=P(Y\le y|\ {\mathbf {X}}={\mathbf {x}})\), with heaviness of the right tail characterized by the conditional extreme value index \(\gamma ({\mathbf {x}})>0\). We particularly focus on testing the hypothesis \({\mathscr {H}}_{0,tail}:\ \gamma ({\mathbf {x}})=\gamma _0\) of constant tail behavior for some \(\gamma _0>0\) and all possible \({\mathbf {x}}\). When considering \({\mathbf {x}}\) as a time index, the term trend analysis is commonly used. In the recent past several such trend analyses in extreme value data have been published, mostly focusing on time-varying modeling of location or scale parameters of the response distribution. In many such environmental studies a simple test against trend based on Kendall’s tau statistic is applied. This test is powerful when the center of the conditional distribution \(F_Y(y|{\mathbf {x}})\) changes monotonically in \({\mathbf {x}}\), for instance, in a simple location model \(\mu ({\mathbf {x}})=\mu _0+x\cdot \mu _1\), \({\mathbf {x}}=(1,x)'\), but the test is rather insensitive against monotonic tail behavior, say, \(\gamma ({\mathbf {x}})=\eta _0+x\cdot \eta _1\). This has to be considered, since for many environmental applications the main interest is on the tail rather than the center of a distribution. Our work is motivated by this problem and it is our goal to demonstrate the opportunities and the limits of detecting and estimating non-constant conditional heavy-tail behavior with regard to applications from hydrology. We present and compare four different procedures by simulations and illustrate our findings on real data from hydrology: weekly maxima of hourly precipitation from France and monthly maximal river flows from Germany. 相似文献
3.
Attenuation of P,S, and coda waves in Koyna region,India 总被引:1,自引:0,他引:1
The attenuation properties of the crust in the Koyna region of the Indian shield have been investigated using 164 seismograms
from 37 local earthquakes that occurred in the region. The extended coda normalization method has been used to estimate the
quality factors for P waves and S waves , and the single back-scattering model has been used to determine the quality factor for coda waves (Q
c). The earthquakes used in the present study have the focal depth in the range of 1–9 km, and the epicentral distance vary
from 11 to 55 km. The values of
and Q
c show a dependence on frequency in the Koyna region. The average frequency dependent relationships (Q = Q
0
f
n) estimated for the region are , and . The ratio is found to be greater than one for the frequency range considered here (1.5–18 Hz). This ratio, along with the frequency
dependence of quality factors, indicates that scattering is an important factor contributing to the attenuation of body waves
in the region. A comparison of Q
c and in the present study shows that for frequencies below 4 Hz and for the frequencies greater than 4 Hz. This may be due to the multiple scattering effect of the medium. The outcome of this
study is expected to be useful for the estimation of source parameters and near-source simulation of earthquake ground motion,
which in turn are required in the seismic hazard assessment of a region. 相似文献
4.
Wave tank experiments with long internal waves of elevation, of different initial length l, moving in a two-fluid system, interacting with a weak slope of 0.045 rad, show an onshore flow of the dense water, at the undisturbed pycnocline-slope intersection, of duration $11.3\sqrt{l/g'}Wave tank experiments with long internal waves of elevation, of different initial length l, moving in a two-fluid system, interacting with a weak slope of 0.045 rad, show an onshore flow of the dense water, at the
undisturbed pycnocline-slope intersection, of duration 11.3?{l/g¢}11.3\sqrt{l/g'} (g′ reduced gravity). This period corresponds to that of a strong bottom current event measured in the stratified ocean at the
Ormen Lange gas field, at 850 m depth, lasting for 24 hrs, corresponding to 11.2?{l/g¢}11.2\sqrt{l/g'}, using the width l = 300 km of the Norwegian Atlantic Current (NAC) at the site as length scale, suggesting a lateral sloshing motion of the
NAC causing the event. The onshore velocity of the dense fluid has a maximal velocity of 0.4?{g¢h2}0.4\sqrt{g'h_2} in laboratory and 0.5 ms-1=0.3?{g¢h2}^{-1}=0.3\sqrt{g'h_2} in the field (h
2 mixed upper layer thickness). Run-up of the dense fluid, beyond the undisturbed pycnocline-slope intersection, has initially
a front velocity of 0.35?{g¢h2}0.35\sqrt{g'h_2}, corresponding to the velocity of the head of a density current on a flat bottom. Due to disintegration, an initially depressed
pycnocline results in comparatively smaller run-up and velocity. While moving past the turning point, a dispersive wave train
is formed in the back part of the depression wave, developing by breaking into a sequence of up to eight boluses moving by
the undisturbed pycnocline-slope intersection. 相似文献
5.
James W. Telford 《Pure and Applied Geophysics》1980,119(2):278-293
Applications of the entrainment process to layers at the boundary, which meet the self similarity requirements of the logarithmic profile, have been studied. By accepting that turbulence has dominating scales related in scale length to the height above the surface, a layer structure is postulated wherein exchange is rapid enough to keep the layers internally uniform. The diffusion rate is then controlled by entrainment between layers. It has been shown that theoretical relationships derived on the basis of using a single layer of this type give quantitatively correct factors relating the turbulence, wind and shear stress for very rough surface conditions. For less rough surfaces, the surface boundary layer can be divided into several layers interacting by entrainment across each interface. This analysis leads to the following quantitatively correct formula compared to published measurements. 1 $$\begin{gathered} \frac{{\sigma _w }}{{u^* }} = \left( {\frac{2}{{9Aa}}} \right)^{{1 \mathord{\left/ {\vphantom {1 4}} \right. \kern-\nulldelimiterspace} 4}} \left( {1 - 3^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} \frac{a}{k}\frac{{d_n }}{z}\frac{{\sigma _w }}{{u^* }}\frac{z}{L}} \right)^{{1 \mathord{\left/ {\vphantom {1 4}} \right. \kern-\nulldelimiterspace} 4}} \hfill \\ = 1.28(1 - 0.945({{\sigma _w } \mathord{\left/ {\vphantom {{\sigma _w } {u^* }}} \right. \kern-\nulldelimiterspace} {u^* }})({z \mathord{\left/ {\vphantom {z L}} \right. \kern-\nulldelimiterspace} L})^{{1 \mathord{\left/ {\vphantom {1 4}} \right. \kern-\nulldelimiterspace} 4}} \hfill \\ \end{gathered} $$ where \(u^* = \left( {{\tau \mathord{\left/ {\vphantom {\tau \rho }} \right. \kern-0em} \rho }} \right)^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0em} 2}} \) , σ w is the standard deviation of the vertical velocity,z is the height andL is the Obukhov scale lenght. The constantsa, A, k andd n are the entrainment constant, the turbulence decay constant, Von Karman's constant, and the layer depth derived from the theory. Of these,a andA, are universal constants and not empirically determined for the boundary layer. Thus the turbulence needed for the plume model of convection, which resides above these layers and reaches to the inversion, is determined by the shear stress and the heat flux in the surface layers. This model applies to convection in cool air over a warm sea. The whole field is now determined except for the temperature of the air relative to the water, and the wind, which need a further parameter describing sea surface roughness. As a first stop to describing a surface where roughness elements of widely varying sizes are combined this paper shows how the surface roughness parameter,z 0, can be calculated for an ideal case of a random distribution of vertical cylinders of the same height. To treat a water surface, with various sized waves, such an approach modified to treat the surface by the superposition of various sized roughness elements, is likely to be helpful. Such a theory is particularly desirable when such a surface is changing, as the ocean does when the wind varies. The formula, 2 $$\frac{{0.118}}{{a_s C_D }}< z_0< \frac{{0.463}}{{a_s C_D (u^* )}}$$ is the result derived here. It applies to cylinders of radius,r, and number,m, per unit boundary area, wherea s =2rm, is the area of the roughness elements, per unit area perpendicular to the wind, per unit distance downwind. The drag coefficient of the cylinders isC D . The smaller value ofz o is for large Reynolds numbers where the larger scale turbulence at the surface dominates, and the drag coefficient is about constant. Here the flow between the cylinders is intermittent. When the Reynolds number is small enough then the intermittent nature of the turbulence is reduced and this results in the average velocity at each level determining the drag. In this second case the larger limit forz 0 is more appropriate. 相似文献
6.
7.
J. Blower 《Bulletin of Volcanology》2001,63(7):497-504
A model has been developed to investigate the sensitivity of magma permeability, k, to various parameters. Power-law relationships between k and porosity J are revealed, in agreement with previous experimental and theoretical studies. These relationships take the form % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBae % rbd9wDYLwzYbWexLMBbXgBcf2CPn2qVrwzqf2zLnharyavP1wzZbIt % LDhis9wBH5garqqtubsr4rNCHbGeaGqiVCI8FfYJH8sipiYdHaVhbb % f9v8qqaqFr0xc9pk0xbba9q8WqFfeaY-biLkVcLq-JHqpepeea0-as % 0Fb9pgeaYRXxe9vr0-vr0-vqpWqaaeaabiGaciaacaqabeaadaabau % aaaOqaaiqbdUgaRzaajaGaeyypa0Jaem4AaSMaei4la8IaemOCai3a % aWbaaSqabeaacqaIYaGmaaGccqGH9aqpcqWGHbqycqGGOaakcqaHgp % GzcqGHsislcqaHgpGzdaWgaaWcbaGaem4yamMaemOCaihabeaakiab % cMcaPmaaCaaaleqabaGaemOyaigaaaaa!4CE4! [^(k)] = k/r2 = a(f- fcr )b\hat k = k/r^2 = a(\phi - \phi _{cr} )^b where r is the mean bubble radius, Jcr is the percolation threshold below which permeability is zero, and a and b are constants. It is discovered that % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBae % rbd9wDYLwzYbWexLMBbXgBcf2CPn2qVrwzqf2zLnharyavP1wzZbIt % LDhis9wBH5garqqtubsr4rNCHbGeaGqiVCI8FfYJH8sipiYdHaVhbb % f9v8qqaqFr0xc9pk0xbba9q8WqFfeaY-biLkVcLq-JHqpepeea0-as % 0Fb9pgeaYRXxe9vr0-vr0-vqpWqaaeaabiGaciaacaqabeaadaabau % aaaOqaaiqbdUgaRzaajaGaeyOeI0IaeqOXdygaaa!3CDB! [^(k)] - f\hat k - \phi relationships are independent of bubble size. The percolation threshold was found to lie at around 30% porosity. Polydisperse bubble-size distributions (BSDs) give permeabilities around an order of magnitude greater than monodisperse distributions at the same porosity. If bubbles are elongated in a preferred direction then permeability in this direction is increased, but, perpendicular to this direction, permeability is unaffected. In crystal-free melts the greatest control on permeability is the ease of bubble coalescence. In viscous magmas, or when the cooling time-scale is short, bubble coalescence is impeded and permeability is much reduced. This last effect can cause variations in permeability of several orders of magnitude. 相似文献
8.
Global upper ocean heat content and climate variability 总被引:3,自引:2,他引:1
Peter C. Chu 《Ocean Dynamics》2011,61(8):1189-1204
Observational data from the Global Temperature and Salinity Profile Program were used to calculate the upper ocean heat content
(OHC) anomaly. The thickness of the upper layer is taken as 300 m for the Pacific/Atlantic Ocean and 150 m for the Indian
Ocean since the Indian Ocean has shallower thermoclines. First, the optimal spectral decomposition scheme was used to build
up monthly synoptic temperature and salinity dataset for January 1990 to December 2009 on 1° × 1° grids and the same 33 vertical
levels as the World Ocean Atlas. Then, the monthly varying upper layer OHC field (H) was obtained. Second, a composite analysis was conducted to obtain the total-time mean OHC field ([`([`(H)])] \bar{\bar{H}} ) and the monthly mean OHC variability (
[(\textH)\tilde] \widetilde{\text{H}} ), which is found an order of magnitude smaller than
[^(\textH)] \widehat{\text{H}} . Third, an empirical orthogonal function (EOF) method is conducted on the residue data (
[^(\textH)] \widehat{\text{H}} ), deviating from
[(\textH)\tilde] \widetilde{\text{H}} +
[(\textH)\tilde] \widetilde{\text{H}} , in order to obtain interannual variations of the OHC fields for the three oceans. In the Pacific Ocean, the first two EOF
modes account for 51.46% and 13.71% of the variance, representing canonical El Nino/La Nina (EOF-1) and pseudo-El Nino/La
Nina (i.e., El Nino Modoki; EOF-2) events. In the Indian Ocean, the first two EOF modes account for 24.27% and 20.94% of the
variance, representing basin-scale cooling/warming (EOF-1) and Indian Ocean Dipole (EOF-2) events. In the Atlantic Ocean,
the first EOF mode accounts for 49.26% of the variance, representing a basin-scale cooling/warming (EOF-1) event. The second
EOF mode accounts for 8.83% of the variance. Different from the Pacific and Indian Oceans, there is no zonal dipole mode in
the tropical Atlantic Ocean. Fourth, evident lag correlation coefficients are found between the first principal component
of the Pacific Ocean and the Southern Oscillation Index with a maximum correlation coefficient (0.68) at 1-month lead of the
EOF-1 and between the second principal component of the Indian Ocean and the Dipole Mode Index with maximum values (around
0.53) at 1–2-month advance of the EOF-2. It implies that OHC anomaly contains climate variability signals. 相似文献
9.
Lubomír Kubáček Lea Bartalošová Ján Pecár Reviewer F. Charamza 《Studia Geophysica et Geodaetica》1977,21(3-4):227-235
Summary If the condition R(A)=k(n), whereA is the design matrix of the type n × k and k the number of parameters to be determined, is not satisfied, or if the covariance matrixH is singular, it is possible to determine the adjusted value of the unbiased estimable function of the parameters f(), its dispersion D(
(x)) and
2
as the unbiased estimate of the value of
2
by means of an arbitrary g-inversion of the matrix
. The matrix
, because of its remarkable properties, is called the Pandora Box matrix. The paper gives the proofs of these properties and the manner in which they can be employed in the calculus of observations. 相似文献
10.
Bülent Oruç 《Pure and Applied Geophysics》2011,168(10):1769-1780
In this paper the application of an edge detection technique to gravity data is described. The technique is based on the tilt
angle map (TAM) obtained from the first vertical gradient of a gravity anomaly. The zero contours of the tilt angle correspond
to the boundaries of geologic discontinuities and are used to detect the linear features in gravity data. I also present that
the distance between zero and
±p\mathord