GEOPHYSICAL IDENTIFICATION OF FRACTURES AND KARST CONDUITS ALONG THE RUTA DE LOS CENOTES AND HOLBOX FRACTURE ZONE, NORTHEAST YUCATÁN PENINSULA, MÉXICO

The Yucatán Peninsula is a vast limestone platform with a surface area of about 165,000 km² and contains one of the most developed and complex karst systems in the world. Heavy pumping and rapid economic development threatens the underlying karst limestone aquifer. Between 2012 and 2019 numerous field surveys were made using two-dimensional (2D) resistivity, azimuthal resistivity profiles, ground-penetrating radar (GPR) and spontaneous potential (SP) surveys to identify fractures, generally enlarged by solution, that could be karst conduits and contaminant pathways to the water table. Most of these surveys were performed by students selected for the Research Experiences for Undergraduates (REU) program, sponsored by the National Science Foundation.

Surveys performed during the summers of 2018-2019 focused on two areas: the south Cancún wellfield, about 25 km southwest of Cancún, and sites within the Holbox fracture zone, about 70 km west of Cancún. Resistivity profiles identified a fracture or karst conduit about 100 m north of the south end of the wellfield – this fracture was characterized by lower resistivity (about 200 ohm-m) when compared to the surrounding bedrock (about 3000 ohm-m). Azimuthal resistivity surveys revealed a 40 ohm-m apparent resistivity change over this apparent fracture under dry conditions. A spontaneous potential (SP) anomaly of about 40 milliVolts (mV) was also observed after pouring salt-water on the ground in the general vicinity of this fracture. SP surveys made during 2018 in the south wellfield also showed a 9 mV fluctuation adjacent to a small sinkhole that may be a karst conduit within 100 m of a wellfield pumping well. This area has experienced unregulated waste disposal. Azimuthal resistivity surveys were also performed during 2019 over a soccer field in the town of Vicente Guerrero, within the Holbox fracture zone. Elongation of the azimuthal resistivity ellipse corresponded with regional fracture trends within the Holbox zone, and azimuthal variations were about 50 ohm-m for a rotating Wenner array with an electrode spacing of 10 m. In summary, 2D resistivity, SP and azimuthal resistivity appear to be effective tools for characterizing fractures and karst conduits in the northeast Yucatán.