HYDROGEOLOGICAL INVESTIGATIONS IN NORTHWESTERN YUCATAN, MEXICO, USINGRESISTIVITY SURVEYS

Eight Schlumberger soundings and four Wenner anisotropy measurements w ere conducted in the northwestern section of the Yucatan Peninsula for hydrogeological investigations of a karst aquifer. This system is inf luenced by a circular high permeability zone (Ring of Cenotes) probabl y related to the Chicxulub Impact Crater. Schlumberger soundings and W enner anisotropy measurements show that the karst aquifer can be model ed as an electrically anisotropic medium. Anisotropy is related to pre ferential permeability directions channeling ground-water flow within the aquifer. Directions of maximum permeability were determined using Wenner anisotropy measurements. Electrical soundings were conducted at different sites near the Ring of Cenotes. Resistivity values decrease toward the Ring of Cenotes supporting the hypothesis that selected se gments of the Ring have high permeability. Several soundings were cond ucted in order to study lateral permeability variations along the Ring . A high permeability section can be identified by low resistivity mod els and is related to a zone of high cenote density. A low permeabilit y section of the Ring was found showing high resistivity models. This zone overlaps with an area of low cenote density. Electrical soundings were used to determine the depth of the fresh-water lens; the interfa ce was detected along two profiles perpendicular and parallel to the R ing of Cenotes resulting in a depth that ranged from 18 m near the coa st up to 110 m in the southeastern part of the study area. The predict ed depths of the interface using electrical methods showed a good corr elation with Ghyben-Herzberg and measured interface depths at some sit es. Discrepancies between calculated and interpreted interface depths at two sites may be explained by horizontal-to-vertical permeability a nisotropy.