Two-dimensional electrical imaging surveys are now widely used in engineering and environmental surveys to map moderately complex structures. In order to adequately resolve such structures with arbitrary resistivity distributions, the regularised least-squares optimisation method with a cell-based model is frequently used in the inversion of the electrical imaging data. The L 2 norm based least-squares optimisation method that attempts to minimise the sum of squares of the spatial changes in the model resistivity is often used. The resulting inversion model has a smooth variation in the resistivity values. In cases where the true subsurface resistivity consists of several regions that are approximately homogenous internally and separated by sharp boundaries, the result obtained by the smooth inversion method is not optimal. It tends to smear out the boundaries and give resistivity values that are too low or too high. The blocky or L 1 norm optimisation method can be used for such situations. This method attempts to minimise the sum of the absolute values of the spatial changes in the model resistivity. It tends to produce models with regions that are piecewise constant and separated by sharp boundaries. Results from tests of the smooth and blocky inversion methods with several synthetic and field data sets highlight the strengths and weaknesses of both methods. The smooth inversion method gives better results for areas where the subsurface resistivity changes in a gradual manner, while the blocky inversion method gives significantly better results where there are sharp boundaries. While fast computers and software have made the task of interpreting data from electrical imaging surveys much easier, it remains the responsibility of the interpreter to choose the appropriate tool for the task based on the available geological information.

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Chemical weathering is the dominant process in the development of a weathering profile on crystalline basement rocks. In the saturated lower part of the weathering profile, groundwater is the principal chemical reagent and the rate of groundwater flow determines the rate of weathering reactions. The search for successful borehole sites in this environment also depends upon finding locations where the rate of groundwater flow is at an optimum. This search could therefore be facilitated by an appreciation of the part which groundwater flow plays in the chemical weathering process. The principal factors which control the quantity of groundwater flow through a unit cross sectional area of the weathering system are the availability of recharge, the permeability of the weathered material and the hydraulic gradient between the recharge and discharge areas. Of these three, the importance of recharge is clear as this determines the input to the groundwater system. The permeability of the weathered material is a function of the extent of chemical weathering and is therefore intrinsically linked to the history of groundwater flow through the system. The remaining independent variable is the hydraulic gradient. This is normally developed in what is generally a relatively thin weathered layer which closely follows the surface morphology. The hydraulic gradient is therefore a function of the average surface slope over the groundwater flow path and therefore related to the surface morphology. A plot of recharge against surface morphology therefore becomes a valid projection upon which the various groundwater provinces can be plotted and provides a useful means of classifying the various occurrences of groundwater within the weathered profile.