Radar measurement of the effect of boundary-layer saturation on mountain-wave amplitude

Meso-Strato-Troposphere and weather radars are used to show the effect of saturated air near the ground on mountain waves in the troposphere and lower stratosphere, at 52.4°N, 4.0°W. Mountain waves are observed above scattered precipitation; however, long-term observations confirm here that wave amplitude is reduced above extensive precipitation, as predicted from numerical models. Ceilometer measurements of average cloud base near the mountain tops suggest that saturated air could be reducing the generation of mountain waves, in addition to trapping or absorbing waves.     

Diagnostic hydrogeologic characteristics of a karst aquifer (Kentucky, USA)

Laboratory experiments and numerical modeling have shown that dissolution in carbonate aquifers results in high-permeability channel networks. However, the lack of techniques to assess the extent and significance of these channel networks presents a major problem in characterizing carbonate aquifers. This problem was addressed by identifying the differences between two simulations (with and without channels) of the intensely studied limestone aquifer at Mammoth Cave (Kentucky, USA). Long-distance tracer-test results and spring discharges were used for assessing model accuracy as well as head measurements in wells. The channel simulation provided a much better calibration than the homogeneous porous-medium simulation and revealed five important differences: (1) convergent flow to large springs, (2) equipotentials forming troughs, (3) decreases in hydraulic gradient and (4) increases in hydraulic conductivity in a downgradient direction, and (5) substantial scaling effects. These five characteristics are also common in other carbonate aquifers and provide a means of identifying whether a carbonate aquifer is more similar to porous-medium or to karst-aquifer end members.     

Calculating the azimuth of mountain waves, using the effect of tilted fine-scale stable layers on VHF radar echoes

A simple method is described, based on standard VHF wind-profiler data, where imbalances of echo power between four off-vertical radar beams, caused by mountain waves, can be used to calculate the orientation of the wave pattern. It is shown that the mountain wave azimuth (direction of the horizontal component of the wavevector), is given by the vector are radar echo powers, measured in dB, in beams pointed away from vertical by the same angle towards north, south, east and west respectively, and W is the vertical wind velocity. The method is applied to Aberystwyth MST radar data, and the calculated wave vector usually, but not always, points into the low-level wind direction. The mean vertical wind at Aberystwyth, which may also be affected by tilted aspect-sensitive layers, is investigated briefly using the entire radar output 1990–1997. The mean vertical-wind profile is inconsistent with existing theories, but a new mountain-wave interpretation is proposed.     

Test methods for characterizing contaminant transport in a glaciated carbonate aquifer

It is widely recognized that karstification can substantially influence flow and transport characteristics in carbonate aquifers. Surface features such as sinkholes are widely used to diagnose the presence of a karst aquifer, but specific borehole tests for karst have not been well defined. Such tests are especially important in glaciated areas where karst features have been eroded or buried by till. One such area is Smithville, Ontario, where more than 60 boreholes at a PCB-contaminated dolostone site provided an opportunity for a wide range of downhole tests and monitoring to be carried out. It was found that there were a number of useful tests for indicating karstification. These included (1) the order of magnitude differences between pump, slug and packer test results, (2) the presence of water table troughs, (3) rapid water level response following recharge events, (4) the rapid changes in water quality following recharge events, (5) water undersaturated with respect to calcite following recharge events, and (6) a wide range in fracture apertures along major bedding planes. Most parameters vary over a similar range at Smithville as they do at Mammoth Cave, Kentucky, indicating that the Smithville aquifer behaves as a typical karst aquifer.     

A comparison of cross-hole electrical and seismic data in fractured rock

Cross‐hole anisotropic electrical and seismic tomograms of fractured metamorphic rock have been obtained at a test site where extensive hydrological data were available. A strong correlation between electrical resistivity anisotropy and seismic compressional‐wave velocity anisotropy has been observed. Analysis of core samples from the site reveal that the shale‐rich rocks have fabric‐related average velocity anisotropy of between 10% and 30%. The cross‐hole seismic data are consistent with these values, indicating that observed anisotropy might be principally due to the inherent rock fabric rather than to the aligned sets of open fractures. One region with velocity anisotropy greater than 30% has been modelled as aligned open fractures within an anisotropic rock matrix and this model is consistent with available fracture density and hydraulic transmissivity data from the boreholes and the cross‐hole resistivity tomography data. However, in general the study highlights the uncertainties that can arise, due to the relative influence of rock fabric and fluid‐filled fractures, when using geophysical techniques for hydrological investigations.     

Predicting horizontal velocities from well data

The Imperial College borehole test site consists of four boreholes with depths lying between 260 and 280 m. The boreholes intersect several cyclical sequences of sandstones, mudstones and limestones. The formations are highly laminated and ultrasonic measurements on preserved core have shown that the mudstones are intrinsically anisotropic. Little or no anisotropy is associated with the sandstones and limestones. A scheme is proposed to predict synthetic vertical and horizontal P- and S-wave logs. Combining (an)isotropic effective medium theories, the Gassmann equation and Backus averaging, the scheme extends previous sand-shale models to transversely isotropic rock formations. The model assumes that the anisotropy is due to layering and due to the preferred horizontal orientation of the clay minerals, pores and cracks within the mudstones. The pores and cracks within the sandstones and limestones are randomly orientated. After fitting the model to the ultrasonic data to obtain the unknown parameters, the model successfully predicts the sonic log and the direct arrival times from a cross-hole survey.     

The characteristics of mountain waves observed by radar near the west coast of Wales

Radar observations at 46.5 MHz of vertical-velocity perturbations at Aberystwyth (52.4^○N, 4.1^○W) have been used to examine the incidence of mountain waves and their dependence on local topography and the wind vector at low heights. A contrast is drawn between the effects of easterly winds passing over major topographical features to the east of the radar site and those of westerly winds crossing low coastal topographical features to the west. Estimates are made of the vertical flux of horizontal momentum associated with mountain waves, and the general influence of mountain-wave activity on vertical-velocity measurements at the site is assessed.     

Estimating permeability from field measurements of seismic attenuation in fractured chalk

Broadband (100–4000 Hz) cross‐hole seismic data have been acquired at a borehole test site where extensive hydrological investigations have previously been performed, including in situ estimates of permeability. The rock type is homogeneous chalk and fractures and bedding planes have been identified from well logs. High values of seismic attenuation, Q= 22 ≤ 27 ≤ 33, were observed over a 10 m depth interval where fracture permeability values of 20–50 darcy had been recorded. An attempt has been made to separate the attenuation due to scattering and intrinsic mechanisms. The estimated values of intrinsic attenuation, Q= 31 ≤ 43 ≤ 71, have been reproduced using a number of current theories of seismic‐wave propagation and fluid‐flow‐induced seismic attenuation in cracked and fractured media. A model that considers wavelength‐scale pressure gradients is the preferred attenuation mechanism. Model parameters were obtained from the hydro‐geological and seismic data. However, we conclude that it is not possible to use seismic Q to measure rock permeability remotely, principally because of the inherent uncertainties arising from model parameterisations.