Matched-field inversion for geoacoustic model parameters in shallowwater

Matched-field inversion is used to, estimate geoacoustic properties from data obtained in an experiment with a vertical line array (VLA). The experiment was carried out using broad-band sources (shots) in water depths of about 200 m on the continental shelf off Vancouver Island. The data were processed to obtain spectral components of the field for frequencies near the bubble frequency for the shot. The ocean bottom in this region consists of a layer of mainly sandy sediments (about 100 m thick) overlying older consolidated material. Consequently, the inversion was designed to estimate the parameters of a two-layer elastic sediment model. In the inversion, an adaptive global search algorithm was used to investigate the multidimensional space of geoacoustic models in order to determine the set of values corresponding to the best replica field. Convergence is driven by adaptively guiding the search to regions of the parameter space associated with above-average values of the matched field correlation between the measured and replica fields. The geoacoustic profile estimated by the inversion consisted of a 125-m layer with compressional speed ~1700 m/s and shear speed ~400 m/s, overlying a layer with compressional speed ~1900 m/s. This model is consistent with the results from conventional seismic experiments carried out in the same region     

Measurement and analysis of depression storage on a hillslope

A precise photogrammetric technique was used to determine the microtopography of seven 2.6 m × 1.2 m experimental units located on a hillslope. Surface elevations were determined with an accuracy of better than 1 mm, from which contours at 2mm intervals were interpolated. These contour plots were then manually interpreted to define depressions and associated storage volumes. Analysis of the results highlighted the complex variability of depression storage over the hillslope, for example there being little relation between depression storage volumes and unit slope. This study also highlights the sampling problem for the measurement of depression storage on natural surfaces, which appears not to have been formally recognized previously, and also emphasizes the practical difficulty of achieving depression storage estimates with coefficients of variation less than ten per cent to 50 per cent, much of this variability being due to problems of interpretation rather than measurement of the surface.     

A digital analysis of sunspot areas

Full-disk white light images of the Sun have been digitized, calibrated, and examined to determine objective sunspot areas for the early part of the operation of the Solar Maximum Mission satellite. We find that published sunspot areas determined from synoptic programs compare favorably with our digital areas. The mean residual between published areas and our digital areas is approximately 80 millionths of a hemisphere. The largest residual found is 642 millionths on April 1980 for Hale No. 16752.     

The application of borehole logging to characterize the hydrogeology of the faultless nuclear test site,Nevada, USA

The Central Nevada Test Area was the site of the “Faultless” underground nuclear test, a potential source of radio nuclide contamination to aquifers in Hot Creek Valley, Nevada, USA. Field studies in 1992 and 1993 used hydrologic logging and water sampling to determine the adequacy of the current groundwater monitoring network. New data from the monitoring wells and an abandoned postshot hole raise questions about the ability of the current monitoring network to detect migration. Logging and sampling at monitoring wells HTH-1 and HTH-2 reveal that the construction of HTH-1 may be responsible for its elevated water level (as compared to pre-nuclear test levels) and may also create a local groundwater mound in the alluvium that accounts for higher post-test water levels at HTH-2. This mound would serve to divert flow around the monitoring wells, so that only migration of contaminants through the underlying, higher-pressure volcanic units is currently monitored. A hydraulic high observed in an abandoned hole between the nuclear test and the monitoring wells further reduces the likelihood that HTH-1 and HTH-2 can intercept contaminant migration. Hydrologic logs from a postshot well, drilled into the nuclear detonation zone, reveal inflow at about 485 m below land surface that drives strong vertical flow both up and down the well to outflow zones at about 350 m and 600 m. Although hydrologic relationships between the cavity, the rubble chimney, and the surrounding alluvium are unknown, migration from the site has probably not yet occurred, because regional hydraulic pressures are greater than those observed in this postshot hole.     

The hydrodynamic and sedimentary setting of nearshore coral reefs, central Great Barrier Reef shelf, Australia: Paluma Shoals, a case study

The Great Barrier Reef (GBR) shelf contains a range of coral reefs on the highly turbid shallow inner shelf, where interaction occurs with terrigenous sediments. The modern hydrodynamic and sedimentation regimes at Paluma Shoals, a shore‐attached ‘turbid‐zone’ coral reef, and at Phillips Reef, a fringing reef located 20 km offshore, have been studied to document the mechanisms controlling turbidity. At each reef, waves, currents and near‐bed turbidity were measured for a period of ≈1 month. Bed sediments were sampled at 135 sites. On the inner shelf, muddy sands are widespread, with admixed terrigenous and carbonate gravel components close to the reefs and islands, except on their relatively sheltered SW side, where sandy silty clays occur. At Paluma Shoals, the coral assemblage is characteristic of inner‐shelf or sheltered habitats on the GBR shelf (dominated by Galaxea fascicularis, up to >50% coral cover) and is broadly similar to that at Phillips Reef, further offshore and in deeper water. The sediments of the Paluma Shoals reef flats consist of mixed terrigenous and calcareous gravels and sands, with intermixed silts and clays, whereas the reef slope is dominated by gravelly quartz sands. The main turbidity‐generating process is wave‐driven resuspension, and turbidity ranges up to 175 nephelometric turbidity units (NTU). In contrast, at Phillips Reef, turbidity is <15 NTU and varies little. At Paluma Shoals, turbidity of >40 NTU probably occurs for a total of >40 days each year, and relatively little time is spent at intermediate turbidities (15–50 NTU). The extended time spent at either low or high turbidities is consistent with the biological response of some species of corals to adopt two alternative mechanisms of functioning (autotrophy and heterotrophy) in response to different levels of turbidity. Sedimentation rates over periods of hours may reach the equivalent of 10 000 times the mean global background terrigenous flux (BTF) of sediment to the sea floor, i.e. 10 000 BTF, over three orders of magnitude greater than the Holocene average for Halifax Bay of <3 BTF. As elsewhere along the nearshore zone of the central GBR, dry‐season hydrodynamic conditions form a primary control upon turbidity and the distribution of bed sediments. The location of modern nearshore coral reefs is controlled by the presence of suitable substrates, which in Halifax Bay are Pleistocene and early Holocene coarse‐grained (and relatively stable) alluvial deposits.     

Rapid determination of canyon geometry parameters for use in surface radiation budgets

Summary  The parameterisation of canyon geometry as a sky-view factor is common practice when studying surface radiation budgets. However, the accurate computation of sky-view factors can prove to be slow and has often resulted in studies involving the sky-view factor to be limited to just a few study sites. A fully automated digital approach to calculating sky-view factors direct from digital imagery is presented along with a simple methodology to determine the impact of surface geometry on the nature of incoming shortwave radiation. The validity of these new automated techniques is tested by a comparative study with alternative methodologies, producing nearly identical results. It is hoped that the rapidity of the techniques should allow for the impacts of canyon geometry on surface radiation budgets to be studied on a scale not previously possible. Received July 6, 2000 Revised December 11, 2000     

Conductivity as a predictor of a total cations and salinity in Ethiopian lakes and rivers: revisiting earlier models

We used regression analyses of water samples from 18 lakes, nine rivers, and one spring in Ethiopia to (a) test the hypothesis that water bodies of relatively higher salinity (K₂₅>1000 μS cm⁻¹) have a different conductivity to salinity relationship than waters of lower salinity (K₂₅ < 1000 μS cm⁻¹), and (b) develop models to predict total cations and salinity from conductivity that can be used for Ethiopian waters and other African aquatic systems of similar chemical composition. We found no statistical difference in the bilogarithmic relationships (total cations vs. conductivity; salinity vs. conductivity) for waters of higher salinity (K₂₅ > 1000 μS cm⁻¹) and waters of lower salinity (K₂₅ < 1000 μS cm⁻¹). However, comparison among our models and models from the literature suggests that developing separate equations for low and high salinity water bodies has some merit. We believe that the equations developed in this study can be used for Ethiopian waters and other African waters within the range of conductivity in this study.     

Urban-rural relationships in India: a macro-scale approach using population potentials

The urban system in India is the product of a long and complex history and the urban-rural dichotomy is marked. While urban India is often viewed as ‘modern’ and ‘commercial’, rural India is generally characterised as myopic, backward and traditional. The diffusion of a modern commercial sector into traditionally agricultural areas is seen to be slow because of the lack of small sized towns, and many of the major cities are viewed as the centres of introverted economic systems. This paper seeks to examine these aspects of urban-rural relationships and levels of development in India at two scales, the national and the district. The national scale study analyses the urban and rural components of population potential surfaces. The study at the district level uses similar data, but at a finer scale, and in addition considers potentials of retail trade which gives a clearer illustration of ‘importance’ than population data alone. Several innovations in technique are discussed. The paper concludes with some general methodological comments about the nature of location as perceived by geographers.