oa A Review of Geophysical Methods used for Near Surface Exploration in Seismic Microzonation Studies and their Application

Seismic microzonation studies are an essential tool for the mitigation of seismic risk, especially in developing countries with rapidly increasing population moving to big urban areas with increasing earthquake risk. Besides regional seismic hazard due to proximity to active faults, local soil conditions are a first order feature that controls shaking. Urban planning and development that considers local soil conditions helps preventing building damage and loss of lives. Geophysical investigations, coordinated with geological observations and geotechnical studies, are a key for addressing the subsoil configuration. The application of the different geophysical methods depends strongly on the geological characteristics and addresses the shape and depth of sedimentary basins and quality of the uppermost layer (Vs30) regarding shaking amplification and induced effects as soil liquefaction and landslide hazard. Due to restricted space and city regulations, methods based on ambient noise recently take the lead over traditional geophysical methods, as they are cost-effective and environmentally friendly. Basin shape and depth can effectively be addressed with a combination of gravimetric surveys (for basin geometry) and seismic noise measurements (for the generation of 1-D shear wave soil profiles) from individual measurements for determination of fundamental soil periods to array measurements. In both cases, depending on the record time, inversion of Rayleigh wave dispersion and ellipticity curves allows to derive 1-D profiles down to the seismic basement. For near surface exploration, passive methods have widely replaced seismic acquisition with active sources as sledgehammer or explosives. Acquisition geometry and logistics is similar to active methods with standard 48 channel seismographs, but a wide variety of processing options are available as ReMi (Refraction Microtremor), MASW and iMASW (interferometric Multi-channel Analysis of Surface Waves). Density of measurement grid depends strongly on geological conditions and urban dimensions but should not exceed 300-500 m. Data might be completed with available geotechnical information or specially designed geotechnical drillings.