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Delineating the near‐surface geometry of the fracture system affecting the Valley of Querétaro, Mexico: Correlation of GPR signatures and physical properties of sediments
Author(s) -
CarreonFreyre Dora C.,
Cerca Mariano
Publication year - 2006
Publication title -
near surface geophysics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.639
H-Index - 39
eISSN - 1873-0604
pISSN - 1569-4445
DOI - 10.3997/1873-0604.2005031
Subject(s) - geology , ground penetrating radar , fracture (geology) , hydrogeology , sedimentary rock , fault (geology) , stratigraphy , lithology , bedrock , subsoil , geomorphology , petrology , seismology , geotechnical engineering , tectonics , radar , soil science , geochemistry , soil water , telecommunications , computer science
ABSTRACT We present a combined ground‐penetrating radar (GPR) and geotechnical investigation of a fault‐fracture system that affects the Valley of Querétaro. The main fracture, known as Falla Central (FC), is aligned with N–S‐orientated regional faults suggesting that the buried fault scarps influence the geometry and propagation of fractures. Although the origin of fracturing is closely related to geological factors, in the urbanized area of Querétaro the mechanical and hydraulic equilibrium in the subsoil is also perturbed by anthropogenic activities, such as overexploitation of groundwater and overloading of compressible ground. In order to delineate the fracturing geometry, we obtained several vertical GPR profiles, perpendicular and oblique to the main trace of the fracture, with two different prospecting frequencies, 900 and 300 MHz. Processing of radar profiles consisted of background removal, topographic correction and AGC amplitude correction to enhance stratigraphic‐related records. The near‐surface stratigraphy consists of partially saturated fluvio‐lacustrine granular and pyroclastic deposits. Detailed measurements of specific gravity, grain‐size, plasticity, water content, and electrical conductivity were performed on samples collected from two shallow trenches in order to relate physical changes in the sedimentary sequence to the recorded electrical contrasts. The analysis of the GPR profiles, plus the stratigraphic record, enabled us to identify variations in the deformation of layers, and changes in direction, width and vertical displacement of fractures. Our results suggest that the propagation of the fracture is influenced by the conditions of deposition of the geological materials and by the anthropogenic activity.