Geophysical Techniques Improving Geological Mapping in Urban Areas at Two Different Scales

We present a geophysical study where different geophysical techniques are combined to support an urban geological mapping project in NE of Spain. The study has been carried out at two different scales. Seismic and electrical resistivity tomography methods have provided a subsoil image up to 30/40 m depth supported by surface-wave and H/V microtremor analysis. The Quaternary/Neogene contact is related to a sharp velocity contrast in one of the profiles producing a significant secondary peak in the H/V microtremor ratio. Therefore, the identification of this second peak could be used for Quaternary thickness estimation in urban areas. Furthermore, large scale study has been carried out combining CSAMT/MT and H/V microtremor methods. The resistivity model obtained from electromagnetic data allows imaging Neogene/Paleogene contact and Paleozoic bedrock. Combination of this model and bedrock depth estimation obtained from H/V technique has led to identify the geological contact linked to the soil fundamental frequency. Therefore, H/V measurements extended to the whole urban areas can be used to assess bedrock depth. The methodology introduced in this study will improve geological mapping at 1:5000 scale in other urban areas of Catalonia region.