Extrapolation of irregular subsurface structures using the horizontal-to-vertical spectral ratio of long-period microtremors

One of the key problems in modeling the 3D subsurface structure of sedimentary basins is extrapolation and/or interpolation from 2D models constructed from reflection and refraction surveys. The use of microtremor horizontal-to-vertical (H/V) spectral ratios, allowing simple and quick microtremor measurements, may be suitable for this purpose. The validity and usefulness of long-period microtremor H/V spectral ratios are investigated as a possible tool for extrapolation in 3D basin structure modeling. Three-component microtremor observations were conducted at 51 stations in the northern section of the Osaka basin, which encompasses the most complex geological structures in the basin. The microtremor HN spectral ratios were measured in the range I to 5 sec. To reproduce these observations, three 2D finite-element models have been developed that account for the irregular shape of subsurface structures deduced from seismic reflection studies, microtremor array studies, and deep-drilling data. The 3D model obtained by combining the 2D models is consistent with geological information for the area, such as surface geology and the location of active faults. Furthermore, compared with the basement structure derived from the gravity prospecting, this 3D model is more detailed and offers higher resolution than the gravitational method. Therefore, the microtremor H/V spectral ratio represents a useful technique of extrapolation and interpolation for modeling 3D basin structures.