Reconnaissance electrical resistivity and self-potential studies of the Atotonilco-Jonacatepec region, Morelos State, Mexico.

Abstract

Preliminary results of a geophysical study of the Atotonilco-Jonacatepec area (Morelos State) using DC-resistivity and self-potential (SP) soundings, are used to outline the shallow underground structure of the volcanoclastic and lahar units and the intrusive bodies. Fifty-one 300 to 440 m-AB, six 600 m-AB and ten 1000 m-AB vertical soundings with the Schlumberger configuration were completed along three profiles. Most of the soundings could be interpreted by 3 to 4 layer models, but half of the 1000 m-AB soundings suggest more complex (multi-layered) structures. Maximum depths are of 150 to 250 meters. Deeper layers 4 and 5 may correspond to limestone; layers 2 and 3 may correspond to lahar deposits and sediments of the Tlayecac and Cayuca formations; and the surficial layer corresponds to unconsolidated material. The subsurface continuation of the Jantetelco granodiorites seems to be deeper than 150 meters. The SP anomalies are negative and appear to form part of larger dipolar anomalies. They may result from upwelling where regional southward flow approaches the intrusive bodies. Limited studies conducted over a limestone cave near Cocoyoc feature high apparent resistivities, which outline the surface projection of the cave entrance; parts of the cave deeper than 3-4 m proved difficult to detect. Soundings in a fault zone near Cafion de Lobos above volcanoclastic and alluvial sediments feature very low resistivities and almost flat Schlumberger apparent resistivity curves.