| Abstract: | This paper presents a methodology in geophysics applied to the study of porous aquifers. This methodology is based on an improved “very low frequency-resistivity” equipment, on greater importance being attached to the local geological heterogeneity of the environment, and also on a method of data processing which is simple but well adapted to a large number of studies concerning prospecting, protection and management of underground water.The radio magnetotelluric-resistivity equipment (RMT-R) is derived from the classical VLF-resistivity equipment. Its increased reception band (12–240 kHz) allows us to realize rapid frequency soundings.In the example to be discussed, a geostatistical analysis of semivariance, made on a series of more than 600 measurements, shows a spatially structured geophysical heterogeneity, both for the values of resistivity and for those of phase. The correlation between this geophysical heterogeneity and the lithological heterogeneity of the environment is confirmed by mechanical soundings (geological test holes). The indirect relations between this geophysical heterogeneity and the hydraulic behaviour of the aquifer system can be estimated and are confirmed, on this specific site, by the observation of the piezometric levels.The results of our study illustrate the positive contribution RMT-R can make to hydrogeology. Above all they show that those porous environments, frequently considered as homogeneous, cannot be considered as such either hydrogeologically or geophysically. |
