Near Surface Geophysics, 2006, 49-55 © 2006 European Association of Geoscientists & Engineers 49 Delineating the near-surface geometry of the fracture system affecting the Querétaro valley, Mexico: Correlation of GPR signatures and physical properties of sediments Dora C. Carreon-Freyre 1* and Mariano Cerca1, 2 1 Centro de Geociencias, Iniversidad Nacional Autónoma de México, Campus Juriquilla, Apartado Postal 1-742, Querétaro, Qro. 76001, Mexico 2 Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, México, D.F. 04510, Mexico Received January 2005, revision accepted May 2005 ABSTRACT We present a combined ground-penetrating radar (GPR) and geotechnical investigation of a fault- fracture system that affects the Querétaro valley. 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 geo- logical 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 sev- eral vertical GPR profiles, perpendicular and oblique to the main trace of the fracture, with two dif- ferent prospecting frequencies, 900 and 300 MHz. Processing of radar profiles consisted of back- ground 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 vari- ations 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. al. 2002; Carreón-Freyre et al. 2005) and overloading of highly compressible ground. The rapid development of the urban infra- structure in the Querétaro valley has caused an increase in the demand for groundwater. Furthermore, the problem is increased because the near-surface clay-bearing sediments of this valley fracture easily when dry, and the lateral variations of the geolog- ical properties of sediments can influence the distribution and behaviour of the near-surface fractures that affect urban areas. The aim of this paper is to provide a non-destructive identifica- tion of the structure of volcanic-lacustrine sediments in order to correlate physical properties with changes in the fracture geom- etry. Detailed determinations of physical properties, i.e. water content, electrical conductivity, grain-size, and variation of com- paction with depth, performed in two trenches excavated perpen- dicular to the trace of the fractures, allowed a correlation between the electromagnetic response and the physical-mechan- ical properties of clayey-sandy lacustrine deposits. INTRODUCTION Major urban areas in central Mexico, and specifically within the Trans Mexican Volcanic Belt (TMVB) area, were built on the plane surface of sedimentary basins bounded by volcanoes and affected by regional geological faults. In the last three decades an increasing problem for these cities is that fractures in near-sur- face sediments, caused by subsidence, have considerably affect- ed the urban infrastructure. Thus, understanding the causes of generation and the geometry of propagation of this type of frac- ture and fault has become a major focus of study since it can help to predict where new fractures will appear. Although the origin of fracturing in these recent sedimentary basins is closely relat- ed to regional geological factors, in some urbanized areas, such as the city of Querétaro, the mechanical and hydraulic equilibri- um in the subsoil is also affected by anthropogenic activities, such as overexploitation of groundwater (Holzer 1984; Rojas et * freyre@geociencias.unam.mx