In linear construction, such as bridges, the spatial variability of soil properties plays an important role in defining their foundations. When it comes to lacustrine deposits of soft clays, such as those located in the Northern Bogotá, active methods have great resolution in the upper few meters, but they don't have a good penetration. In those cases, passive surface wave methods have several advantages over active methods, allowing characterizing deep soil deposits. We use SPAC survey methods in northern Bogotá, to delineate geological structure associated with dipping layers. This paper summarizes data acquisition and analysis of passive surface wave data. Seven 1 Hz geophones, added forty-one 4.5 Hz geophones were used to perform high resolution SPAC at three locations. Data acquisition mainly used an L-shape array with geophone spacing of 4 m. One lineal array was also measured to complete information in a narrow urban space. Recorded ambient noise data were processed using spatial autocorrelation method (SPAC) and clear dispersion curves were obtained at all locations. Minimum frequency from 0.8 Hz and maximum frequency from 20 Hz were clearly identified, that shows a great compromise between penetration depth and resolution. Dispersion curves obtained from the linear array are generally consistent with those obtained from L-shaped arrays. Non-linear inversion was performed and 1D Shear wave velocity (Vs) models were obtained. The method penetrated to a depth of 100 m and provided 100 m cross sections along the site of interest. The interpretation focused on identifying the change of colluvium and bedrock depth, which were found at 45 and 80 meter, respectively and shown a dip angle between 12 and 16 grades in western direction. Resultant Vs profiles are generally consistent with borehole logs and existing geological information.
Tópico:
Landslides and related hazards
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FuenteSymposium on the Application of Geophysics to Engineering and Environmental Problems 2000