Design of a 3D Chirp Sub-bottom Imaging System

Chirp sub-bottom profilers are marine acoustic devices that use a known and repeatable source signature (1–24 kHz) to produce decimetre vertical resolution cross-sections of the sub-seabed. Here the design and development of the first true 3D Chirp system is described. When developing the design, critical factors that had to be considered included spatial aliasing, and precise positioning of sources and receivers. Full 3D numerical modelling of the combined source and receiver directivity was completed to determine optimal source and receiver geometries. The design incorporates four source transducers (1.5–13 kHz) that can be arranged into different configurations, including Maltese Cross, a square and two separated pairs. The receive array comprises 240 hydrophones in 60 groups whose group-centres are separated by 25 cm in both horizontal directions, with each hydrophone group containing four individual elements and a pre-amplifier. After careful consideration, it was concluded that the only way to determine with sufficient accuracy the source–receiver geometry, was to fix the sources and receivers within a rigid array. Positional information for the array is given by a Real Time Kinematic GPS and attitude system incorporating four antennas to give position, heading, pitch and roll. It is shown that this system offers vertical positioning accuracy with a root-mean-square (rms) error less than 2.6 cm, while the horizontal positioning rms error was less than 2.0 cm. The system is configured so that the Chirp source signature can be chosen by software aboard the acquisition vessel. The complete system is described and initial navigational and seismic data results are presented. These data demonstrate that the approach of using fixed source-receiver geometry combined with RTK navigation can provide complete 3D imaging of the sub-surface.     

Sediment deposition from flow at low gradients into a buffer strip—a critical test of re-entrainment theory

The spatial and size distribution of sediment deposited from short periods of overland flow due to the effect of a simulated grass buffer strip was measured for low slopes of 1.6, 3.4 and 5.1%. These data were analysed so as to critically evaluate two alternative models of the process of re-entrainment of recently deposited sediment. A model of re-entrainment, previously thought to be appropriate only for a steady-state or equilibrium situation, was found to give better agreement with experiments than did a model previously used in the literature on this subject.     

Lapis lazuli from Baffin island — a precambrian meta-evaporite

On Baffin Island, the major minerals in lapis lazuli are calcite, lazurite, diopside, nepheline and phlogopite in the Main Occurrence and calcite, lazurite, diopside, plagioclase and scapolite in the North Occurrence. The abundances of Na, K, S, Cl, Br, F and Fe, the well-developed layering parallel to the regional foliation, and the scarcity of intrusive rocks, support the hypothesis that the deposit evolved from an evaporite parent during regional metamorphism. After sedimentation, considerable reduction of sulfur was effected by CO, perhaps aided by H₂, Cl^? and anaerobic bacteria. Chemographic analysis suggests that the observed phase assemblages represent local equilibrium within small volumes, where SiO₂-Al₂O₃-MgO-Na₂O-K₂O behaved as inert components at the peak of metamorphism. The lack of systematic zoning of phase assemblages, and the uniformity of mineral compositions within the deposit, also argue against a metasomatic origin. Most phases equilibrated at or near the peak of metamorphism (granulite facies).     

Unsteady soil erosion due to rainfall impact: a model of sediment sorting on the hillslope

A new method is presented for predicting sediment sorting associated with soil erosion by raindrop impact for non-equilibrium conditions. The form of soil erosion considered is that which results from raindrop impact in the presence of shallow overland flow itself where the flow is not capable of eroding sediment. The method specifically considers early time runoff and erosion when sediment leaving an eroding area is generally finer and thus may have a higher potential for transport of sorbed pollutants. The new mechanism described is the formation of a deposited layer on the soil surface, which is shown to lead to sediment sorting during an erosion event. The deposited layer is taken to have two roles in this process: to temporarily store sediment on the surface between successive trajectories, and to shield the underlying soil from erosive stresses. Equations describing the dynamics of the suspended sediment mixture and the deposited layer are developed. By integrating these equations over the length of eroding land element and over the duration of the erosion event, an event-based solution is proposed which predicts total sediment sorting over the event. This solution is shown to be consistent with experimentally observed trends in enrichment of fine sediment. Predictions using this approach are found to only partly explain measured enrichment for sets of experimental data for two quite different soils, but to be in poor agreement for an aridsol of dispersive character. It is concluded that the formation of the deposited layer is a significant mechanism in the enrichment of fine sediment and associated sorbed pollutants, but that processes in the dispersive soil are not as well described by the theory presented.     

A simple method for the rapid determination of exponential soil-water diffusivity in the laboratory

We present a simple method in two dimensions for the determination of exponential soil-water diffusivity. This determination is made by measuring the wetting front position when water is injected in the soil at different fluxes. The results are exact and represent an improvement over previous techniques both in terms of ease and accuracy.¹     

Longer distance commuting as a substitute for migration in Britain: a review of trends,issues and implications

"Drawing on analyses of commuting data from secondary sources as well as on selected results from a research project on long-distance commuting in Britain, this article investigates the extent to which, and why, households may choose to substitute longer distance commuting for migration. Reasons for long-distance weekly commuting, and associated advantages and disadvantages from individual, household and employer perspectives, are outlined. The evidence points to increasing complexity in home and working lives, with important implications for housing, transport and human resource management policies, as well as for family life."     

Soil erosion due to rainfall impact with no inflow: a numerical solution with spatial and temporal effects of sediment settling velocity characteristics

Dynamic changes take place in the nature of sediment eroded from bare soil at low slopes by rainfall impact when there is no inflow of water at the top of the eroding slope. This relates initially to fine soil sediment not settling back onto the soil after the rainfall impact. Coupled partial differential equations describing such dynamic changes have been solved numerically for a bed of soil, bounded at its upper end, and subject to a constant rainfall rate. This solution allows prediction of the change with time and downslope distance in the concentration and settling velocity (or size) characteristics of eroding sediment, allowing critical evaluation of the assumption of space-independent sediment characteristics made in prior approximate analytical solutions of the equations involved. Following the determination of as yet unpredictable soil-related parameters in the equations, the solution was tested by comparison with experimented data on two soils of contrasting structural stability, namely a vertosol [The Australian Soil Classification (1996)] and a aridisol. Investigations included the determination of a minimum number of sediment size classes required to adequately describe the settling velocity characteristics, based on the shape of the underlying basic settling velocity characteristic, which is used to predict the dynamics of sediment deposition. The effect on the solution of observed structural breakdown in soil aggregation due to rainfall impact was investigated, leading to more accurate predictions of the settling velocity characteristics of eroded sediment. Other sources of discrepancy between theory and observation remain to be determined.     

A comparison of brachyuran crab community structure at four mangrove locations under different management systems along the Melaka Straits-Andaman Sea Coast of Malaysia and Thailand

Brachyuran crab community structure was compared between mangrove sites under different management systems from four locations along the Melaka Straits-Andaman Sea Coast. Klong Ngao, a mangrove estuary in Ranong Province of southern Thailand, lies within a Biosphere Reserve designated in 1997. Sites were positioned in plantations at a former charcoal concession forest, a disused tin mine, and an abandoned shrimp pond along this estuary. The Merbok estuary in Kedah, Malaysia, is partially managed: the mangroves are cut for charcoal and poles on a small scale and the forests are left to regenerate naturally. The Matang Mangrove Forest Reserve in Perak, Malaysia, is heavily exploited but well managed, forRhizophora wood to produce charcoal, and has been for 100 years. Sites were positioned in plantations of different ages. Kuala Selangor Nature Park, Selangor, Malaysia, was established as a nature reserve in 1987 and contains mature mangrove forest regenerating naturally from previous selective felling. At Klong Ngao and Matang, mature reserve forest sites were also studied for comparison with plantation sites. The sites included both upstream and downstream locations and were of similar area, minimizing effects from possible species-area relationships. Sites were chosen with similar environmental conditions and with a dominance ofRhizophora spp. At each site per location, the brachyuran crabs were sampled quantitatively in 100 m² quadrats by three independent 15-min timed crab catches. The crab community recorded was analyzed by univariate and multivariate statistical techniques. Management history plays an important role in moderating the crab community structure. The crab community also changes with the age of the mangrove forest stand. Sesarmid crabs consistently dominated in mature forests, whereas young plantations were colonized mainly by ocypodid crabs. The findings show that heavily effected sites—e.g., disused tin mining areas, former concession forests, and abandoned shrimp ponds—can be rehabilitated by planting mangroves and that the crab community is a useful ecological indicator of habitat status.     

Effects of spatial variations in source areas upon dust concentration profiles during three wind erosion events in Australia

Dust storms are a major contributor to soil erosion in inland Australia, and the Simpson Desert–Channel Country region is one of the most active wind erosion regions. While information is available on wind erosion rates at the land‐type level, little is known about the influence that spatial variations in the erodibility within a land type have on the resulting dust concentration profile. A Gaussian plume model, DSIS, is presented along with tower‐based dust data, to describe the influence of different spatial combinations of dust source areas, during three dust events on the Diamantina River floodplain in Western Queensland, Australia. Copyright © 2001 John Wiley & Sons, Ltd.