Saline intrusion in Guaymas Valley, Mexico from time-domain electromagnetic soundings

Abstract

A severe salt water intrusion in Guaymas Valley, Sonora is mapped by means of 44 time–domain electromagnetic soundings. In the southern part of the valley the resistivity distribution is simple. A conducting layer associated with the saline intrusion in the upper aquifer shows a systematic increase of resistivity inland. Three soundings feature a low– resistivity layer within this conductor, suggesting higher salinity at the bottom. The northern zone has a complex electrical structure, but a conductor with increasing resistivity can be recognized. The results suggest that the lower aquifer is not affected by the intrusion.
When plotted against salinity the conductivity is biased with respect to the predictions of Archie's Law. Clay lenses in the unsaturated sediments may account for the bias and dispersion of conductivities.