Linear Velocity Gradient Research Articles

GPS measured rates of deformation in the northern San Francisco Bay Region, California, 1990–1993

A 100‐km‐long, 13‐station profile extending across the San Andreas fault system north of San Francisco Bay was measured 7 times between March 1990 and January 1993 with the Global Positioning System (GPS). The data have been processed using the Bernese Version 3.2 software. Data from a continental‐scale fiducial network were included in the solutions to aid orbit improvement and provide a consistent reference frame. We find 33 ± 2 mm/yr of fault‐parallel (N33°W) shear evenly distributed southwest and northeast of the Rodgers Creek fault and a near linear velocity gradient across the profile. The profile spans most of the zone of active deformation associated with the San Andreas fault system. Shear is negligible at the east end of the profile near the Great Valley. Additional shear of a few millimeters per year is likely beyond Point Reyes Head, the west end of the profile. We observe no systematic convergence upon the fault. The GPS measured velocities are similar to those derived previously from trilateration. The velocity change across the GPS profile (31–35 mm/yr) plus that west of the profile (0–3 mm/yr) and that observed with VLBI east of the Sierra Nevada Mountains (∼10–12 mm/yr) accounts for the North American‐Pacific plate rate (46–47 mm/yr).

A study of head field gradient and velocity on transition noise

A model of the micromagnetic fluctuation in the writing process is proposed. Both higher linear velocity and high field gradient increase the transition noise in the unsaturated region which is related to a lower transition noise in the saturated region as required. The model is as follows: when writing in a slowly turning off (or decreasing) write field, i.e., low field gradient or low linear velocity, the recording magnetization in the transition is reconstructed to a more relaxed state. On the other hand, magnetization at the transition is not relaxed by rapidly turning off the write field, i.e., high field gradient or high linear velocity.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Slant-stack velocity analysis for one-dimensional upper mantle structure using short-period data from MAJO

In this paper, regionalP-wave upper mantle structure is investigated using slant-stack velocity analysis of short-period earthquake data recorded at station MAJO (Matsushiro, Japan). Shallow earthquakes from 1980–1986 within 35° of MAJO are used to construct a common receiver gather. Processing of the wavefield data includes focal depth and static time corrections, as well as deterministic deconvolution, in order to equalize pulse shapes and align wavelets on the first arrivals. The processed wavefield data are slant stacked and interatively downward continued to obtain a regional upper mantle velocity model. The model includes a low velocity zone between 107 and 220 km. Beneath the LVZ, the velocity increases smoothly down to the discontinuity at 401 km. In the transition zone, the velocity model again increases linearly, although there is some suggestion of further complexity in the downward continued wavefield data. At the base of the transition zone, a second velocity discontinuity occurs at 660 km, with a linear velocity gradient below. In addition to slant-stack analysis, travel times and synthetic seismograms are computed and compared with the processed and unprocessed wavefield data.

Velocity and attenuation profiles in the Monterey Deep-Sea Fan

Data obtained during a marine refraction experiment are used to estimate velocity and attenuation profiles in an area of thick sediments (2.5–3 km) on the Monterey Deep-Sea Fan. A 20-element vertical hydrophone array was deployed at mid-depth in 2800 m of water. Explosive sources, set to detonate at 1830 m, were launched at ranges between 3.5 and 37 km from the array. Estimates of velocity and attenuation as a function of depth are obtained from an analysis of the pressure time series generated by the explosive charges and received at the array. The sediment is modeled as a horizontally layered, laterally homogeneous medium. A least-squares solution is found for linear velocity gradients in each layer of the model. Velocity as a function of depth is obtained from these gradients. Attenuation coefficients in each layer of the model are found using a method of spectral ratios. The results presented here, valid for frequencies between 10 and 100 Hz, show an increase in attenuation to a depth of about 500 m. The method of spectral ratios and equations used to infer velocity structure from various parametrizations of the traveltime data are derived in appendices.

The kinematics of a violent outflow from the nucleus of the SC galaxy NGC 3079

view Abstract Citations (57) References (52) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Kinematics of a Violent Outflow from the Nucleus of the SC Galaxy NGC 3079 Filippenko, Alexei V. ; Sargent, Wallace L. W. Abstract We describe an extensive set of long-slit CCD spectra of the peculiar, nearly edge-on Sc galaxy NGC 3079. The disk is cluttered with H II regions which define a rotation curve of normal appearance. In addition, however, clouds having the optical emission-line characteristics of shock-heated gas are visible near the nucleus and roughly along the minor axis. This gas exhibits very complicated kinematics; we see expanding shells of gas similar to those in planetary nebulae, as well as extremely steep linear velocity gradients and spectacular velocity reversals. Radial velocities of up to ~1200 km s^-1^ are observed at a projected distance of ~700 pc from the nucleus. Despite the complex kinematics, the dominant motions in NGC 3079 appear to be expansion away from the nuclear region along a hollow conelike surface. Deprojected speeds exceeding 2000 km s^-1^ are derived if the opening angle of the cone is 90^deg^, as observed in a narrowband Hα + [N II] image of the galaxy previously published by Ford et al. [ApJ, 311, L7 (1986)]. A majority of the kinematically distinct gas is coincident with the "loop" of Hα+[N II] emission discovered by Ford et al., which is itself aligned with the bridges and loops of radio emission studied by Duric et al. [ApJ, 273, L11 (1983)] and by Duric & Seaquist [ApJ, 326,574 (1988)]. In agreement with Heckman et al. (ApJS, 74,833(1990)], we conclude that NGC 3079 hosts an energetic, bipolar outflow-a "galactic superwind" produced by violent activity in the nuclear region of NGC 3079. Such winds are often seen in other galaxies having very large far-infrared luminosities. However, the anomalously high observed gas velocities, together with evidence from radio observations, suggest that the superwind in NGC 3079 is largely driven (or at least initiated) by accretion onto a compact object, rather than by a vigorous starburst. Most of the kinematically distinct, optically emitting gas along the minor axis of NGC 3079 is probably interstellar gas entrained and shock heated by the wind, perhaps with a smaller contribution of material that cooled directly from plasma in the hot wind. Curiously, the radial velocity of this gas generally increases with increasing projected distance from the nucleus. It is possible that most of the currently observed motion was caused by a relatively isolated explosive event, instead of the more continuous, long-term outflow. A more probable alternative, however, is that the velocity gradient is produced by the gradual acceleration of gas entrained by the wind. On the western side of the galaxy, the outflow is aligned with the direction to NGC 3073 (Mrk 131), a dwarf S0 companion galaxy which has a redshifted H I tail pointing away from the nucleus of NGC 3079. This morphology suggests that the superwind is in its blowout phase, and it strongly supports the hypothesis of Irwin et al. [ApJ, 313, L9l (1987)] that the ram pressure of a wind from NGC 3079 is stripping neutral hydrogen away from NGC 3073. The wind-galaxy interaction may have also induced one or more bursts of star formation in NGC 3073, whose optical spectrum reveals the presence of OB stars as well as A-type start. NGC 3079 demonstrates that galactic superwinds may have a profound effect on their environment. Publication: The Astronomical Journal Pub Date: January 1992 DOI: 10.1086/116038 Bibcode: 1992AJ....103...28F Keywords: Active Galactic Nuclei; Astronomical Spectroscopy; Magnetohydrodynamic Flow; Spiral Galaxies; Charge Coupled Devices; Emission Spectra; H Ii Regions; Star Formation; Astrophysics; GALAXIES: KINEMATICS AND DYMANICS full text sources ADS | data products SIMBAD (3) NED (1)

Incorporation of velocity gradients in the synthesis of complete seismograms by the locked mode method

Abstract Virtually all regional phases can be strongly affected by vertical velocity gradients. The best known effects are on the Pn and Sn , in which small changes in the vertical velocity gradient beneath the Moho produce large changes in the decay of Pn and Sn with distance. Methods of synthesizing complete regional seismograms often inadvertently ignore the effect of crustal gradients by parameterizing the Earth model with thick, planar homogeneous layers. To address this problem, we have modified the locked mode method of synthesizing complete regional seismograms to include the Langer uniform asymptotic approximation to vertical wavefunctions within layers having linear vertical velocity gradients. Synthesis of complete regional seismograms using the Langer-locked mode confirm that the Pn and Sn phases are strongly affected by the magnitude of the velocity gradients beneath the Moho, but that Lg is only weakly affected by the details of crustal layering. Tests were made to quantify the error in the use of the Langer approximation as the magnitude of the vertical gradient increases and/or frequency decreases. At sufficiently small magnitude of gradient and/or high frequency, good argeement can be obtained between synthetics computed using the Langer locked mode method, the colocation method, and the conventional locked mode method in models parameterized by thin homogeneous layers. Errors introduced by the use of the Langer approximation in calculated pole positions, residues, and eigenfunctions are bounded by 5 per cent for frequencies f ≥ 5 | ∇ V |. An upper bound to the error in the time domain can be estimated from this inequality using either the peak frequency in a narrow pass band or the lowest frequency in a broad pass band. When 10 or more thin homogeneous layers are required to represent accurately the seismic wavefield in a gradient layer, it is usually more efficient to represent the gradient layer by continuously varying functions in the vertical direction and employ the Langer approximation, provided the errors in the Langer approximation remain within acceptable limits. By reducing the number of parameters needed to describe an earth model, the Langer locked mode method simplifies the inverse problem of determining structure using observed and synthetic complete seismograms. It also facilitates the use of known relations for the effects of continuously varying pressure and temperature on elastic moduli and density.

Source height determination by ground effect inversion in the presence of a sound velocity gradient

A propagation inversion method has been devised and tested that enables determination of height of a sound source which is above an acoustically soft ground. The horizontal separation between the point source and receivers is assumed to be a priori knowledge. The proposed algorithm is site adaptive since it makes use of the predicted influence of the ground on near-grazing propagation over flat, continuous ground surfaces and of the predicted influence of a stable refracting atmosphere. Stable refraction has been included in the analysis by assuming a linear sound velocity gradient in the otherwise quiescent atmosphere. This represents a first step towards an atmospheric equivalent of the matched field processing technique used in underwater acoustics. Extensive measurements have been carried out over two test sites with different ground impedance characteristics. The spectra of the acoustic signals used in the analysis were between 50 Hz and 2000 Hz and the grazing angle of each measurement varied from 5° to 8°. The algorithm has been shown to allow accurate deduction of the source height from a priori knowledge of the range where the range varies from 50 m to 100 m in one site and from 130 m to 175 m in the other site.

Pn attenuation for a spherically symmetric Earth model

This paper presents the results of a theoretical study of the sensitivity of Pn attenuation to the elastic and anelastic structure of the uppermost mantle for frequencies between 1 and 15 Hz and distances between 200 and 1000 km. Synthetic seismograms are computed using wavenumber integration for an elastic model consisting of a crastal layer over an upper mantle with a slight linear velocity gradient (approximately equal to the earth‐flattening transformation of a spherically symmetric homogeneous upper mantle). These are compared to synthetic seismograms for a crustal layer over a mantle halfspace (pure elastic head wave). We find that earth sphericity alone causes a significant departure in Pn attenuation from that of a canonical head wave (almost two orders of magnitude at 1000 km and 15 Hz), and that high frequencies attenuate less rapidly than low frequencies if the spherical earth velocity gradient is greater than or equal to zero. This implies that methods that assume frequency‐independent Pn geometric spreading will produce biased estimates of the anelasticity of the upper mantle. This is demonstrated using synthetic seismograms computed for an anelastic spherical earth model. These results show that the velocity structure must be carefully considered when relating observed Pn spectral amplitudes to the anelastic structure of the upper mantle.

Turbulence and the diffusive layers around small organisms

We consider whether the diffusion of substances to and from a small spherical organism that is motionless relative to the surrounding water is significantly affected by the turbulent motions in the water. Viscosity, by smoothing out turbulent eddies less than a few millimeters across, dictates that the flow of nutrients and wastes to and from a very small organism must occur by molecular diffusion through a thin surrounding boundary layer. Motion of the organism relative to the water through sinking or swimming distorts this boundary layer. This alters the gradients and causes a fairly well understood increase in diffusive flux. The effect of turbulent motion on the flux, however, is less well understood. We first clarify the relationship between the size of the smallest turbulent eddies and the Kolmogorov or viscous length and go on to contend that turbulent motion over the small distances near small organisms is manifested as a linear velocity gradient whose magnitude is determined by the rate of turbulent energy dissipation. Knowing the shear enables us to calculate, from the experimental results of Purcel (1978, Journal of Fluid Mechanics, 84, 551–559), the quantitative effect of the turbulence on the diffusive flux to and from an idealized spherical organism. For motionless cells 100 μm in diameter, high levels of turbulence (dissipation rate 10 −6 W kg −1_ produce a ⋍2% increase in flux. This escalates to a 100% increase for cells ⋍1 mm in diameter. Our results also lead to the conclusion that the microzones of increased nutrient levels surrounding small organisms, proposed by Mitchell et al. (1985, Nature, 316, 58–59), are much more robust than they suggested.

Ray tracing in azimuthally anisotropic media-I. Results for models of aligned cracks in the upper crust

SUMMARY Ray tracing through gradients in anisotropic materials is complicated by singularities where the two quasi-shear wave slowness sheets cross or touch. Difficulties associated with such points can be removed by explicitly including polarization in the ray tracing equations. Slowness sheet and wavefront plots show the polarization and velocity behavior of various anisotropy models of aligned cracks in the upper crust. A simple scaling of the elastic tensor with depth can be shown to be approximately correct for models of aligned cracks within an isotropic host matrix with a linear velocity gradient. Ray tracing examples for models of aligned cracks within a strong vertical velocity gradient in the uppermost crust demonstrate various features of azimuthal anisotropy, including amplitude and polarization anomalies and shear-wave splitting. Quasi-shear wave polarizations typically twist along ray paths, with stronger twisting near the symmetry axis in hexagonally symmetric media. Strong anisotropy can cause unusual effects, such as ray paths which have three turning points in laterally homogeneous models.

Penetration by stretching projectiles

The hydrodynamic theory of long-rod penetration which takes into account material strength is expanded to include those projectiles which can be modeled as having a linear velocity gradient. This is intended to encompass certain projectiles formed by shaped charges. The theory is then built into a model which includes experimentally determined parameters for penetrator particulation and the model is experimentally tested.

The use of flows with uniform velocity gradient in modelling free expansion of a polytropic gas

The free expansion of a heated mass of uniform gas (e.g. a laser produced plasma) can be modelled by self-similar motion with a linear velocity gradient. Using a series of numerical solutions we have shown that a reasonable representation is obtained by the use of a matching parameter relating the scale lengths in the prototype and its model, and that the representation improves as the ratio of the heating and disassembly times increases. In this paper we re-examine these two inferred results, and re-derive them on an analytic basis. The extension of the theory to multi-structured bodies shows that such systems of symmetric form allow self-similar motion, as does the particular case of an asymmetric one-dimensional foil. The case of isothermal foils is examined in detail to illustrate the derivation of the matching conditions.

New observations of the shallow seismic structure of young oceanic crust

The results are presented of three new experiments carried out on the Mid‐Atlantic Ridge (MAR) near latitude 23°N using a unique method of studying the seismic structure of the uppermost few hundred meters of the oceanic crust. The data were collected using a fixed ocean floor hydrophone receiver and a controllable explosive source that was towed within a few tens of meters of the rugged bottom topography. These 1‐ to 2‐km‐long refraction lines produced for the first time direct observations of the compressional wave velocity structure of the uppermost 200–300 m of the young igneous crust. One experiment was carried out over the site of hole 648B of the Ocean Drilling Program on a small volcano within the median valley of the MAR. The seafloor velocity was observed to be 2.1 km s−1 underlain by an approximately linear velocity gradient of 4 s−1. Given that we know the crust at this location consists of fresh basalt lavas with laboratory‐measured velocities in excess of 5.8 km s−1, porosities at the seafloor of as high as 30–50% are inferred. The two other experiments were located over 7‐m.y.‐old crust near Deep Sea Drilling Project site 395 west of the MAR, separated by only 14 km laterally but by over 1400 m in water depth. The experiment positioned in the ∼100‐m‐thick sediment pond and that located 14 km to the south atop a prominent topographic high produced results that within the data resolution were indistinguishable. The velocity of the uppermost basaltic basement was 4.1 km s−1, and the velocity gradient was less than 0.5 s−1. This ∼2 km s−1 difference in velocity between the two sites is assumed to be a consequence of age‐related modifications to the physical properties of the crust. If the primary change is in only the total porosity, then a 15–20% reduction is required to explain these observations. This seems unlikely because any such pervasive process would have been previously observed in geophysical and sampling data (e.g., off‐axis volcanism, secondary mineral deposition, tectonic compression). A combination of many processes acting in unison would be necessary to produce such a decrease in porosity. If the geometry of the cracks and voids were modified in just the right way, then no change in total porosity is required to explain the velocity difference. These on‐bottom refraction profiles are proven to be practical experiments that can provide precise information on a scale appropriate to many of the ocean floor's geologic features. They produce results that may be correlated in a meaningful way with results of ocean drilling and downhole measurements.

Pressure Recovery of Rotating Diffuser With Distorted Inflows

The pressure recovery and velocity distributions in a two-dimensional rotating curved diffuser have been studied experimentally when even and uneven flows, respectively, were introduced to the diffuser. Two types of uneven flow were adopted; one has a linear velocity gradient on the surface of revolution and the other a linear velocity gradient in the meridian plane. The pressure recovery in the diffuser is improved by the unformalizing process of the uneven inlet velocities in the downstream sections if larger velocities are in the suction side region, but it is deteriorated if larger velocities are introduced in the pressure side region. When an uneven flow with a velocity gradient in the meridian plane is introduced to the diffuser, increased rotation speed and the gradient of the inlet velocity profile deteriorate the pressure recovery.

Truncated asymptotic representation of waves in a one‐dimensional elastic medium

A new time‐domain method has been developed for solving for the stress and displacement of normally incident plane waves propagating in a smoothly varying one‐dimensional elastic medium. Both the Young’s modulus E and the density ρ are allowed to vary smoothly with depth. The restriction of geometrical optics, that the wavelength be much less than the material stratification length, is not required in this new method. We truncate the infinite geometrical‐optics asymptotic expansion after n terms (n = 2 in this paper), which imposes a condition on the acoustic impedance I for exact solutions to exist. The resultant expansion is uniform and exact for three general classes of impedance functions. Results are calculated for the case of a medium with a linear velocity gradient (for which there is an exact solution in the frequency domain); the results are compared with a two‐term WKBJ approximation and the new truncated expansion method. Since a linear velocity gradient is not one of the foregoing classes of impedance functions, a curve‐fitting approach is necessary. The results show that the new method compares favorably to both the WKBJ results and the exact solution and is accurate to within the error of the required curve fit. Two classes of synthetic seismograms are then calculated for smooth velocity and density variations. The same impedance as a function of traveltime is used for both classes. In the first class the principal variation in impedance is due to velocity, while in the second it is mainly due to density. The amplitudes in both classes of synthetic seismograms are very similar, but, as expected, the traveltime curves for each class are widely separated.

Velocity structure of Silent Canyon Caldera, Nevada Test Site

Abstract A laterally averaged, one-dimensional P- and S-wave velocity structure is obtained for Silent Canyon Caldera, located beneath Pahute Mesa at the Nevada Test Site. The velocity models are derived from a linearized damped least-squares travel-time inversion for slowness gradient perturbations. A total of 72 P- and 20 S-wave travel times generated from 13 different explosions, obtained at locations above the caldera, are used in the inversion. The structure is modeled using linear velocity gradients in a series of horizontal layers. The inversion produces a model in which velocity for P waves increases from 1.5 km/sec at the surface to 4.5 km/sec at 2.3 km depth and from 1.0 km/sec at the surface to 2.3 km/sec at 2.0 km depth for S waves. The P- and S-wave models produce travel-time residuals which are typically less than 0.1 and 0.2 sec, respectively, which is within observational error. In an attempt to estimate the degree of lateral velocity variations over which the one-dimensional model averages, waveform data collected above the caldera are examined for indications of a laterally heterogeneous structure.

Effect of divergence, stretching and nonnormal distribution on apparent diffusion of surface drifters

Kawai (1976, 1979) derived two simple relations among the apparent diffusivity (based on the geometric-average variance for principal axes of spreading) for many surface drifters in a patch and horizontal divergence and isotropic turbulent diffusivity for ambient water. By a new term “medley diffusivity” is meant an average of the products of the position coordinates relative to the centroid of drifters and the residual velocity, that is, the velocity left by subtracting the mean velocity and the velocity due to the linear velocity gradient from the velocity of a drifter. Regarding the medley diffusivity as the isotropic turbulent diffusivity that is weighted anisotropically according to drifters spreading along each of the principal axes, this paper derives a new relation with a form intermediate between the above two relations. Using the relation, this paper defines the new critical period within which the effect of horizontal divergence on the apparent diffusion surpasses that of isotropic turbulent diffusion, and discusses the time-scales on which the space-scale dependence of the squared divergence (Kawai, 1985b) is based. Stretching of a patch of drifters and deviation of drifters' position from the bivariate normal distribution enhance the effect of isotropic turbulent diffusion on the apparent diffusion. This “stretching effect” is equivalent to the “shear effect” on a horizontal plane. Remarks on reducing errors in the estimation of isotropic turbulent diffusivity are made.

Measurements of three-dimensional turbulent flow behind a propeller in a shear flow

This paper presents detailed measurements close behind a 0.49 m (1.615 ft), three-bladed propeller tested in the Virginia Polytechnic Institute and State University 2x2 m (6x6 ft) wind tunnel. Tests were run with a uniform approach flow as a baseline, and then the main test series was run with a variable mesh wire grid upstream to produce an approach flow with an essentially linear velocity gradient. The results are compared to elucidate the effects of the nonuniform approach flow. Several types of measurements are reported. First are gross quantities such as overall thrust and torque. The second type of measurements are mean (in the turbulence sense) quantities obtained with a five-port yawhead tube. All three components of the mean velocity and static pressure were obtained. The third type of measurements were made with an x-wire anemometer and an rms meter. These measurements yield all components of the turbulence intensities and stresses at a point averaged over many passes of the propeller blades. The last type of measurements were performed with the x-wire and a microcomputer. The computer is triggered to record many (approximately 160) traces from individual bladepassing events. These measurements are then processed to yield profiles of the mean velocities and turbulence intensities and stresses behind an individual blade.

Influence of bottom refractions on the propagation of underwater sound

Experimental measurements [J. S. Hanna, J. Acoust. Soc. Am. 53, 1686–1690 (1973); R. E. Christensen, J. A. Frank, and W. H. Geddes, J. Acoust. Soc. Am. 57, 1421–1426 (1975)] of the transmission loss of underwater sound propagating over an ocean bottom containing unconsolidated sediments have presented evidence on the presence of bottom‐reflected energy being returned into the water column. The bottom refractions manifest themselves either via their pulse arrival times, or via interference effects in the transmission loss. In a previous study [A. Nagl, H. Überall, A. J. Hang, and G. L. Zarur, J. Acoust. Soc. Am. 63, 739–749 (1978)], we have devised a normal‐mode program of underwater sound propagation containing water layers of linear velocity gradients. This program has been extended to a bottom containing unconsolidated sediment layers of different densities and velocity gradients, and shown to reproduce bottom‐refracted fields interfering with reflected sound. The interference minima can be used to determine the sound speeds and their gradients in the bottom layers. [Supported by the Office of Naval Research.]

Aerodynamic characteristics of an airfoil in a nonuniform wind profile

An experimental wind tunnel investigation was carried out to determine the influence of a nonuniform wind profile on the static longitudinal aerodynamic coefficients (c(, cd, cm) of an airfoil. Force balance and surface pressure measurements were obtained from an NACA 0018 airfoil in a linear velocity gradient. The airfoil was tested in a Reynolds number range from 7.5x I04 to 2.0 x 105, with endplates to simulate an infinite wing, and with the outboard endplate removed to simulate a finite wing. The effect of grit on the surface of the airfoil was also investigated. The influence of the velocity gradient on the aerodynamic characteristics of the airfoil was found to be small, especially in comparison to the effects of the grit.