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81.
Effects of linking up of discontinuities on fracture growth and groundwater transport 总被引:3,自引:1,他引:2
Agust?GudmundssonAgust.Gudmundsson@gwdg.de" title="agust.gudmundsson@geol.uib.no Agust.Gudmundsson@gwdg.de" itemprop="email" data-track="click" data-track-action="Email author" data-track-label="">Email author Otilie?Gjesdal Sonja?L.?Brenner Ingrid?Fjeldskaar 《Hydrogeology Journal》2003,11(1):84-99
It is proposed that the growth of fractures is the basic process for generating and maintaining permeability in solid rock
(bedrock). Many extension fractures grow as hydrofractures, whereas many shear (and extension) fractures grow through the
formation of transverse fractures that connect the adjacent tips of existing fractures. In a boundary-element analysis, the
hydrofractures are modeled as being driven open by a fluid overpressure that varies linearly from 10 MPa at the fracture centre
to 0 MPa at the fracture tip. The host rock has a uniform Young's modulus of 10 GPa, a Poisson's ratio of 0.25, and is dissected
by vertical joints and horizontal contacts, each of which is modeled as an internal spring of stiffness 6 MPa m−1. The number of joints and contacts, and their location with respect to the hydrofracture tip are varied in different model
runs. The results of the analyses indicate that the tensile stresses generated by overpressured hydrofractures open up joints
and contacts out to considerable distances from the fracture tip, so that they tend to link up to form a hydraulic pathway.
Using the same Young's modulus, Poisson's ratio, and internal spring constant for joints as in the hydrofracture models, boundary-element
models were made to study the interaction stresses that cause neighbouring joints to become interconnected through the growth
of linking transverse fractures that, ultimately, may evolve into shear fractures. The models were subjected to tensile stress
of 6 MPa acting normal to the joint planes as the only loading. The offset (horizontal distance) and underlap (vertical distance)
between the adjacent tips of the joints were varied between model runs. The results show a concentration of tensile and shear
stresses in the regions between the neighbouring tips of the joints, but these regions become smaller when the underlap of
the joints decreases and changes to overlap. These stress-concentration regions favour the development of transverse (mostly
shear) fractures that link up the nearby tips of the joints, so as to form a segmented shear or extension fracture. Analytical
results on aperture variation of a hydrofracture in a homogeneous, isotropic rock are compared with boundary-element results
for a hydrofracture dissecting layered rocks. The aperture is larger where the hydrofracture dissects soft (low Young's modulus)
layers than where it dissects stiff layers. Aperture variation may encourage subsequent groundwater-flow channeling along
a pathway generated by a hydrofracture in layered rocks.
Electronic Publication 相似文献
82.
Willy Sarlet 《Celestial Mechanics and Dynamical Astronomy》1978,17(3):299-311
In this paper it is shown that the well-known averaging method (of Krylov, Bogoliubov-Mitropolski) and the two-timescale method, applied to periodic first-order ordinary differential equations, can be derived from one common principle, as two more or less complementary special cases. The uniformity of this treatment includes the proof of asymptotic convergence of both methods, since a single proof can be given under certain hypotheses, which are verifieda posteriori in both cases. 相似文献
83.
84.
Single receiver phase ambiguity resolution with GPS data 总被引:26,自引:12,他引:14
Willy Bertiger Shailen D. Desai Bruce Haines Nate Harvey Angelyn W. Moore Susan Owen Jan P. Weiss 《Journal of Geodesy》2010,84(5):327-337
Global positioning system (GPS) data processing algorithms typically improve positioning solution accuracy by fixing double-differenced
phase bias ambiguities to integer values. These “double-difference ambiguity resolution” methods usually invoke linear combinations
of GPS carrier phase bias estimates from pairs of transmitters and pairs of receivers, and traditionally require simultaneous
measurements from at least two receivers. However, many GPS users point position a single local receiver, based on publicly
available solutions for GPS orbits and clocks. These users cannot form double differences. We present an ambiguity resolution
algorithm that improves solution accuracy for single receiver point-positioning users. The algorithm processes dual- frequency
GPS data from a single receiver together with wide-lane and phase bias estimates from the global network of GPS receivers
that were used to generate the orbit and clock solutions for the GPS satellites. We constrain (rather than fix) linear combinations
of local phase biases to improve compatibility with global phase bias estimates. For this precise point positioning, no other
receiver data are required. When tested, our algorithm significantly improved repeatability of daily estimates of ground receiver
positions, most notably in the east component by approximately 30% with respect to the nominal case wherein the carrier biases
are estimated as real values. In this “static” test for terrestrial receiver positions, we achieved daily repeatability of
1.9, 2.1 and 6.0 mm in the east, north and vertical (ENV) components, respectively. For kinematic solutions, ENV repeatability
is 7.7, 8.4, and 11.7 mm, respectively, representing improvements of 22, 8, and 14% with respect to the nominal. Results from
precise orbit determination of the twin GRACE satellites demonstrated that the inter-satellite baseline accuracy improved
by a factor of three, from 6 to 2 mm up to a long-term bias. Jason-2/Ocean Surface Topography Mission precise orbit determination
tests results implied radial orbit accuracy significantly below the 10 mm level. Stability of time transfer, in low-Earth
orbit, improved from 40 to 7 ps. We produced these results by applying this algorithm within the Jet Propulsion Laboratory’s
(JPL’s) GIPSY/OASIS software package and using JPL’s orbit and clock products for the GPS constellation. These products now
include a record of the wide-lane and phase bias estimates from the underlying global network of GPS stations. This implies
that all GIPSY–OASIS positioning users can now benefit from this capability to perform single-receiver ambiguity resolution. 相似文献
85.