Discussion on ‘Locating potential borehole sites in a discordant flow regime in the Chalk aquifer at Lulworth using integrated geophysical surveys’ by J. T. F. Houston, J. C. Eastwood & T. K. P. Cosgrove

Abstract

C. R. C. Jones and S. Beeson write: Mr Houston and his co-authors are to be congratulated on a valuable paper which describes the integrated use of the Geonics EM 34 and the ABEM SAS 300 Terrameter to locate potential borehole sites. A similar procedure was developed in Kano State, Nigeria for the siting of 429 boreholes in the Basement Complex which were drilled between 1984 and 1986. Much of the experience from Kano is relevant to the discussion of the Lulworth case history: Fractures at depths of up to 60 m can be detected by using a 40 m coil spacing in the vertical dipole orientation. In areas of interest the EM 34 traverse should comprise both horizontal and vertical dipoles in order to define the depth and nature of conductivity features more fully. The vertical dipole orientation is insensitive to near-surface variations, which include the root zone, and the need for smoothing the data is reduced. The ratio of the results for the two orientations can be incorporated directly in the site selection criteria. Traversing with a range of coil spacings is possible but slower. The specific electrical conductance (EC) of groundwater is as important as moisture content and rock type in governing overall geoelectric properties. In Kano State where EC values ranged from 0.02 to 1.0 ms cm −1 , interpretation of the geophysical results required that the hydrochemistry be taken into account. No details are given for the Lulworth area; could EC differences (between pore water and fissure