Microtremor Array Method Used for Certification of Offshore Wind Farms in Japan

Seismic design is very important when building offshore wind farms in Japan, a country prone to earthquakes. Shear wave velocity (Vs) structure below the seabed is one of the most important parameters in the seismic design. Especially for bottom-mounted offshore wind farms, the process of estimating seismic motions by determining the engineering bedrock depth (Vs≥400m/s) is essential. Conducting suspension PS logging in offshore boreholes is the most reliable method to obtain Vs structure, but it is expensive and time-consuming, making it difficult to conduct at many points. We have developed an offshore microtremor array method that can measure Vs structure without drilling and is used, along with suspension PS logging data, CPT and UHRS depth sections, to characterize sites for offshore wind farm certification.