Offshore Microtremor Array Measurement in Southwest Coast of Hokkaido for the Construction of Offshore Wind Turbines

In seismic design for the construction of bottom-mounted offshore wind turbines, it is important to understand the depth of the engineering bedrock(S-wave velocity (Vs) ≧400m/sec).

The Vs has been measured by PS logging in boreholes, but it may be difficult to carry out long-term borehole surveys due to oceanographic conditions.

Therefore, we have developed the offshore microtremor array measurements that easily measures the Vs beneath the seafloor, and have carried out measurements at many sites across Japan.

This paper reports the results of a microtremor array measurements conducted off the southwest coast of Hokkaido. At this site, hard rock with high Vs is distributed in the shallow underground area, which may make it difficult to apply microtremor array measurements. The Vs of the hard rock determined by microtremor array measurements was 740 m/s, but the PS logging conducted at the same location showed that the Vs of the hard rock exceeded 2000 m/s. Numerical experiments revealed that this discrepancy is due to the array size of measurements, but that the depth at which high-velocity rock appears can be determined correctly. Therefore, it was confirmed that microtremor array measurements is effective in understanding the depth of the engineering bedrock.