Characterization of the barquisimeto subsoil –state of lara, venezuela-, from applying geophyisical and geothecnical methods

Abstract

Between 1998 and 2002, ambient noise measurements, geotechnical evaluations, seismic refraction and gravity measurements were carried out in Barquisimeto –Lara State, in order to obtain information about soil conditions in the city. The predominant periods derived from H/V analyses vary between 0.3 and 1.2 s, with values ranging between 0.3 and 0.6 s in the east of the city. Further to the west, predominant periods increase to average values between 0.6 and 0.9 s, corresponding to a thickness increase of the sedimentary cover. The Bouguer gravity anomaly values vary between -30.6 y -13.4 mGal, being the minimum values associated to an increasing thickness of the sedimentary cover in the Barquisimeto terrace. The seismic velocities of the terrace sediments, derived from seismic refraction measurements, range between 900 and 1200 m/s for P-wave and between 400 and 600 m/s for S-wave velocities. For deeper layers the P-wave velocities vary between 2200 and 2500 m/s, and the S-wave velocities reach 1000 m/s. From the analysis of 32 geotechnical drillings, we have obtained an image of the thickness of the terrace in the western part of city. From the joint interpretation of ambient noise, seismic refraction measurements and geotechnical drillings, we infer that the thickness of the terrace is between 30 to 50 m in the center of the city, increasing in thickness to the west and decreasing to the east.