Geophysical determination of buried geological structures and their influence on aquifer characteristics

Many coastal plains in the semi-arid regions of Mexico become fertile lands when properly irrigated. In the last thirty years extensive drilling in several places has disturbed the natural equilibrium of the aquifers; this is partly due to poor knowledge of their distribution and properties, as well as lack of adequate exploitation strategies. This study constitutes a case history of the valley of Guaymas in northwestern Mexico, in which three sets of data are considered: (a) a set of 262 wells, (b) four telluric lines of approximate total length of 90 km, and a set of 326 randomly distributed gravimetric stations. The valley dimensions are 20 km by 50 km; two aquifers have been located, one above 160 m and the other below 320 m. Models have been computed for the four telluric lines and four gravimetric sections. They suggest that sediments on the south-central portion of the valley have a thickness of 800 m. The basement becomes shallow toward the north and south portions of the valley, reaching depths ranging from 200 to 300 m. The valley is flanked by two buried depressions oriented in NNE-SSW direction; these regions reach depths of over 1000 m in some places and apparently constitute reservoirs in which the surface recharge waters are maintained relatively free of contamination from hydrothermal fluids. Such fluids are extracted from shallow wells (under 200 m) in some areas in which the basement approaches the surface. It is concluded that performing geophysical studies on the aquifer's location, in order to determine its regional geological characteristics, is a cost-effective procedure, that allows the establishment of timely extraction strategies.