Multiple geophysical techniques are used to recognize unexposed geological materials and their spatial correlation. However, the perforation of one or several boreholes most of the times is needed. In these cases, the geological formations can be identified from the drill cutting and also through geophysical well loggings using parameters such as Gamma-Ray or Resistivity. In variable density aquifers, such as coastal aquifers, the water salinity is stratified from freshwater on the top to saltwater at the bottom. An electrical conductivity log run in a water well permits determining the exact depth and thickness of these fresh, salt and interface water bodies, but the geological characteristic of the traversed materials can´t be characterized. In cases where there are several sub-aquifers, this picture is more complicated and this technique would not be applicable.
Nevertheless, pumping of the saline water with large discharge nearby the boreholes cause the distribution of salinity to change dramatically, exhibited in a salinity decrease landward. Once the pumping stops, the salinity distribution tends to adopt the previous configuration. The speed of this recovery depends on the permeability of the different materials in the aquifer. Thus, in a multilayer aquifer, the electrical conductivity logs obtained during the aquifer recovery will show larger salinity peaks in those more permeable rock formations and lower values in lithology associated with a low permeability. The resolution of these fluctuations can be metric, and it is possible to correlate them with logs measured in others boreholes affected by the pumping.
This technique has been used in Andarax coastal aquifer (SE Spain) allowing to correlate materials several hundred meters away. These correlations have been compared with the stratigraphic logs from the drill cuttings and with others geophysical logs, being more accurate the first of the indicated tools. Moreover, using this technique, it is possible to get qualitative information about the permeability of different materials.
This work takes part of the general research lines promoted by the CEI-MAR Campus of International Excellence and it was supported by MINECO and FEDER, through Project CGL2015-67273-R.