V.S. Hope

An assessment of the effectiveness of atmospheric correction algorithms through the remote sensing of some reservoirs

Although satellite remote sensing techniques have been widely implemented for a variety of applications, using either single or time-series images, few studies have explicitly considered atmospheric effects, and how they can most effectively be minimized. Despite the considerable number of available atmospheric correction algorithms, there is little literature concerning their relative merits. Over water bodies, atmospheric effects account for the majority of the at-satellite measured radiance in the visible bands, and therefore targets of this type provide an opportunity for assessing the effectiveness of the different methods available. This paper reports a study of atmospheric effects and their correction, using multi-spectral satellite remote sensing data for an area to the west of London that includes eight large water reservoirs and a major international airport (Heathrow). Through comparisons of reflectance within a time series of 12 Landsat-5 Thematic Mapper (TM) images, the overall impact of atmospheric contributions is shown. The available atmospheric correction literature is then reviewed, and the methods applied to the satellite imagery of the reservoirs. The results are compared with reflectances acquired using a field spectro-radiometer. A critical appraisal of the results, and of problems encountered in applying the various methods, leads to an evaluation of their value in practice.

The geotechnical value of ground stiffness determined using seismic methods

Abstract Geotechnical design routinely requires that the in situ strength, stiffness and permeability of the ground be determined. Most satisfactory designs for constructions such as buildings, excavations and tunnels ensure that an adequate margin of safety is maintained, and, under these conditions, measurements of the stiffness of the ground are required so that movements in the ground, both during and after construction, can be calculated. Over the past two decades the careful back-analysis of the behaviour of the ground around constructions such as tunnels and excavations has repeatedly shown that the in situ stiffness of soils and rocks is much higher than was previously thought, and that stress—strain behaviour of these materials is non-linear in most cases. Numerical analyses, using finite element and finite difference computations and field observations, have demonstrated that when margins of safety are adequate the strain levels in the ground around retaining walls, foundations and tunnels are small, and typically of the order of 0.0-l%–0.1%. Improved measurements in the laboratory have confirmed the non-linear stress-strain behaviour of soil, and shown that stiffness is much higher when measured locally and at small strain levels than when determined using conventional laboratory techniques. The realization that strain levels around construction are small, and that field stiffnesses are much higher than previously measured in the laboratory, has led us to re-appraise the value of stiffnesses derived from field seismic geophysical methods. Such methods allow stiffnesses to be determined on representative volumes of the ground, and at the in situ stress state, and for this reason may provide valuable data. This paper reviews the importance of stiffness in geotechnical design and how seismic methods are used in ground stiffness investigations.

The importance of accounting for atmospheric effects in satellite remote sensing: a case study from the Lower Thames Valley area, UK

Solar radiation reflected by the Earths surface to satellite sensors is modified by its interaction with the atmosphere. The objective of atmospheric correction is to determine true surface reflectance values by removing atmospheric effects from satellite images. Atmospheric correction is arguably the most important part of the pre-processing of satellite remotely sensed data and any omission produces erroneous results. The effects of the atmosphere are more severe for dark targets such as water reservoirs. The paper presents two methods of assessing the need for atmospheric correction, and addresses the importance of removing atmospheric effects in the satellite remote sensing of large reservoirs.

In situ determination of Ghh at Bothkennar using a novel seismic method

Abstract The paper describes a novel seismic method for determining the variation of velocity with depth of horizontally propagating, horizontally polarized shear waves (Sh,h). The technique exploits the curved paths of seismic waves in deposits with increasing velocity properties with depth. The new method has been applied at the soft clay test site at Bothkennar, and the results of this survey are compared with Sh,h and Sv,h wave velocity profiles obtained using conventional crosshole survey techniques at the site. The seismic data are further compared with the results of recently reported bender element tests on unconfined samples from the bedded and mottled Bothkennar clay. It is shown that, at the site investigated, the new method presents results comparable to those from the established field survey techniques. Neither the seismic nor the bender element data indicate significant stiffness anisotropy at the site.