Transmissivity and morphological features in a chalk aquifer: A geostatistical approach of their relationship

Whether for the management of an aquifer or to locate new wells, hydrogeologists have always tried to establish transmissivity maps of regional aquifers with the highest possible accuracy. Classically, the transmissivity is obtained from well tests and/or from calibrating a groundwater flow model on the piezometric data, In this paper, we investigate the possibility of adding new information to improve the estimates of the transmissivity in a shallow chalk aquifer, i.e. the characteristics of certain morphological features, here referred to as 'lynchets', which are short lineaments that can be seen on aerial photographs or on topographic maps and represent a sudden breach in slope of the ground surface. We implicitly associate the density of these features with the density of vertical fracturing of the chalk, which is linked to the transmissivity, We selected the chalk aquifer of Northern France to test this approach. Using numerical and geostatistical techniques to construct maps of the density of the lynchets, we compared them with transmissivity maps produced either by calibrating a groundwater flow model with piezometric data or by kriging the transmissivity values obtained with well tests, We conclude that the lynchet information is clearly correlated with the transmissivity, and that this information can best be used by cokriging as a second variable associated with the well test data. Incidentally, to fit our geostatistical models of spatial variability (variogram), we use a new technique called the 'integral semi-variogram', which is ideally suited to cases such as this where the spatial distribution of the data is such that the data points cannot easily be grouped into pairs of increasing lag distance. This new technique is described briefly. (C) 1997 Elsevier Science B.V.