Use of a bubble tiltmeter as a horizontal seismometer

Summary. A bubble tiltmeter has been used as a horizontal seismometer. With the appropriate filters, the bubble system has good response for displacement over the passband of conventional seismometers (from about 10 Hz to 200 s), and for tilt from about 1 Hz to DC. The accuracy of the response is confirmed by comparing the filtered bubble output to conventional seismic instruments. The agreement between the filtered bubble records and broad band ( T ₀= 1 s, T _g= 90 s) and short period ( T ₀= 1 s, T_g= 0.2 s) conventional records is extremely good in every case.
The small size, broad-band response, and lack of moving parts make the bubble ideal as an instrument for remote environments. In particular, the instrument seems ideal for the ocean bottom, land and marine boreholes and planetary missions.     

The characteristics of turbulence in a shear zone downstream of a channel constriction in a coarse-grained forced pool

A field-based project was initiated to characterize the influence of varying discharge and bed topography along a pool exit-slope on the strength of turbulence generated by vortex shedding. Velocities were measured with an ADV downstream of a boulder constriction within a shear zone of a large pool. Measurements were repeated for four flows that varied from 20% to 50% of bankfull discharge. An additional longitudinal profile was conducted along the pool thalweg. Plots of velocity demonstrate high near-bed velocities and turbulence levels in the pools. Turbulence levels were highest closer to the constriction in the shear zone. Turbulent kinetic energy decayed in the downstream direction in response to the bed topography and at lower discharges. Instantaneous velocities are large enough to temporarily lift pebbles at this depth. It appears that instantaneous forces created by vortex shedding may play an important role in scouring pools downstream of constrictions. The turbulence may also respond to changes in bed topography in a manner that encourages feedbacks among bed topography, characteristics of turbulence, and sediment transport.     

Medium-term evolution of a gully developed in a loess-derived soil

Field surveys in the Belgian loess belt revealed the presence in many forested areas of large, permanent gully systems, most of which are currently inactive. In cultivated areas, such gullies can only be observed in cross-sectional soil profiles through hollows, as virtually all such large gullies are currently infilled with colluvium. Little is known about the spatial distribution, initiation and temporal evolution of these large, permanent gully systems on loess-derived soils. Therefore, the medium-term evolution of a gully initiated in a cultivated area on loess-derived soils southwest of Leuven (Belgium) in May–June 1986, was studied over 13 years. Two intense rainfall events created this (ephemeral) gully, which was not erased by subsequent tillage. Between June 1986 and the December 1999, eight field surveys were conducted to measure gully dimensions. During two surveys, topographic indices (e.g., slope and drainage area) were also measured. Daily rainfall for the measuring period were obtained from a rainfall station located some 10 km southwest of the gully. Analysis of rainfall data showed that no extreme rainfall event was required to initiate such large (permanent) gullies, as observed in forested areas and through cross-sectional profiles in cultivated fields in the Belgian loess belt. Return periods of the event that caused the gully varied between <1 year and 25 years, depending on the assumptions used for defining event rain intensity. Once established, length, surface area and volume of the studied gully evolved with time, cumulative rainfall or cumulative runoff, following a negative exponential relation. This accords with observations reported for gullies in Australia and the USA. This study shows that a degressive increase of gully extension, can be largely explained by the evolution of a “slope–drainage area” factor (S×A, which is proportional to stream power) with time. While gully length and gully surface area asymptotically evolve towards a final value, gully volume decreased at a given point in time. From this, it is inferred that sediment deposition will potentially infill the gully to such an extent that the farmer can drive across it. From this moment on, the combined effect of water and tillage erosion in the gully drainage area, will lead towards rapid infilling. This expected evolution of a gully in cultivated fields accords with observations of large infilled gully systems in cultivated areas in eastern Belgium. The permanent gullies observed under forest are attributed to the fact that after severe gully erosion, this area was reforested or abandoned. Therefore, the sediment source was cut off and the gully was not filled in by sediment deposition.     

Ground deformation in a viscoelastic medium composed of a layer overlying a half-space: a comparison between point and extended sources

We obtain and compare analytical and numerical solutions for ground displacement caused by an overpressurized magma chamber placed in a linear viscoelastic medium composed of a layer over a half-space. Different parameters such as size, depth and shape of the chamber, crustal rheology and topography are considered and discussed. Numerical solutions for an axisymmetric extended source are computed using a finite element method (FEM). Analytical solutions for a point source are obtained using the dislocation theory and the propagator matrix technique. In both cases, the elastic solutions are used together with the correspondence principle of linear viscoelasticity to obtain the solution in the Laplace transform domain. Viscoelastic solutions in the time domain are derived inverting the Laplace transform using the Prony series method. The differences between the results allow us to constrain the applicability of the point source and the flat surface hypothesis, which are usually implicitly assumed when analytical solutions are derived. The effect of the topography is also considered. The results obtained show that neglecting the topographic effects may, in some cases, introduce an error greater than that implicit in the point-source hypothesis. Therefore, for an adequate modelling and interpretation of the time-dependent displacements, topography must be considered.     

Velocity profile of a sand cloud blowing over a gravel surface

Particle dynamic analyzer (PDA) measurement technology was used to study the turbulent characteristics and the variation with height of the mean horizontal (in the downwind direction) and vertical (in the upward direction) particle velocity of a sand cloud blowing over a gravel surface. The results show that the mean horizontal particle velocity of the cloud increases with height, while the mean vertical velocity decreases with height. The variation of the mean horizontal velocity with height is, to some extent, similar to the wind profile that increases logarithmically with height in the turbulent boundary layer. The variation of the mean vertical velocity with height is much more complex than that of the mean horizontal velocity. The increase of the resultant mean velocity with height can be expressed by a modified power function. Particle turbulence in the downwind direction decreases with height, while that in the vertical direction is complex. For fine sands (0.2–0.3 mm and 0.3–0.4 mm), there is a tendency for the particle turbulence to increase with height. In the very near-surface layer (<4 mm), the movement of blown sand particles is very complex due to the rebound of particles on the bed and the interparticle collisions in the air. Wind starts to accelerate particle movement about 4 mm from the surface. The initial rebound on the bed and the interparticle collisions in the air have a profound effect on particle movement below that height, where particle concentration is very high and wind velocity is very low.     

On flow of a heat conducting fluid through a porous medium

Summary . A two-space singular perturbation technique is employed to derive approximate governing equations for flow of a viscous heat-conducting fluid through a rigid porous solid. It is assumed that buoyancy forces are significant, and it is shown that standard approximations used in the study of flow through a porous medium are valid provided that Gr ≫ 1, where Gr is a Grashof number calculated using a typical pore radius as the length scale. Results previously derived in the literature for flow through an isotropic random array of spherical particles are used to show how the permeability and conductivity tensors can be calculated for a problem of interest in planetary science.     

Education as a determinant of fertility: a case study of Kullu Town

Using a sample of 885 females (1/2 of all married women of reproductive age), this study examines the role of education as a determinant of fertility among couples in Kullu town of Himachol Pradesh, India. Of the 885 respondents, only 149 were illiterate. The average family size was 2.88. Findings reveal that 1) the average family size was largest for illiterate respondents (3.57) and for illiterate males (3.76); 2) average family size declined consistently after the middle level of education to 1.29 for post graduate respondents and 2.33 for post graduate males; 3) a negative correlation exists between fertility and education of both husband and wife, with the wife's education having a stronger negative correlation with fertility; and 4) couples with an educational level of matriculation and above have a distinctly smaller family size than those less educated.     

某公路滑坡成因机制及稳定性

对某公路滑坡的变形特征、成因机制及稳定性进行了详细研究,并提出了相应的防治措施.研究表明:此滑坡为推移式滑坡,坡体的地形及物质组成是滑坡发生的地质基础,大量的降雨是滑坡发生的主要诱发因素;滑坡在自然状态下是稳定的,而在暴雨和地震工况下不稳定;此滑坡防治应以加强排水措施为主,并在前缘修筑挡墙,同时加强监测.查明滑坡的变形特征及成因机制是滑坡灾害评价和防治的基础,研究思路及其成果可供类似滑坡稳定性分析及灾害防治参考.     

Magnetotelluric response of a uniformly stratified earth containing a magnetized layer

Summary The magnetotelluric (MT) response is studied of a uniformly stratified earth which contains a magnetized layer. The impedance as a function of the layer parameters (resistivity, ρ permeability, μ and thickness h ) is discussed. The MT response from a layer (μ, ρ, h ) is equivalent to that from a layer (μ/μ_r=μ₀, μ_rρ, μ_r h ) where μ_r is the relative permeability of the layer. Thus the effect of a magnetized layer is to make it apppear μ_r times more resistive and μ_r times thicker than an unmagnetized equivalent layer. Master curves of apparent resistivity and phase are computed for three-layer models with varying permeability associated with varying resistivity in each layer. An example of MT field data is presented in which the most reasonable interpretation is that a magnetized layer exists beneath the observatory site.     

Towards a GeoEd Research Agenda: Observations of a Concerned Professional

This Focus section concerns Geographic Education as a PhD specialty. As an introduction, and to establish a theme, the present paper advocates that GeoEd research should move towards a more central position in human geography and social science, and contribute broadly to geographic knowledge. This involves expanding the GeoEd focus well beyond K-12 classroom concerns. The argument involves setting out dimensions of a GeoEd research agenda, describing the observable GeoEd research agenda, and articulating a vision of what GeoEd research in the future might encapsulate. The central message is that, as a PhD specialty, Geographic Education must be solid social science, embody the most current concepts and ideas, and at its best, may not be distinguishable from research in other aspects of human geography.