A HIGH-RESOLUTION MODELING TECHNIQUE OF IRREGULARSUBSURFACE STRUCTURES USING H/V SPECTRAL RATIOOF LONG-PERIOD MICROTREMORS

摘要

One of the key problems in modeling the three-dimensional (3D) subsurface structure ofsedimentary basins is extrapolation and/or interpolation from two-dimensional (2D) modelsconstructed 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 forthis purpose. The validity and usefulness of long-period (>1s) microtremor H/V spectral ratiosare investigated as a possible tool for extrapolation in 3D basin structure modeling.Three-component microtremor observations were conducted at 51 stations located on threesurvey lines in the north of the Osaka basin, which encompasses the most complex geologicalstructures in the basin. To reproduce these observations, 2D numerical simulations have beendeveloped that account for the irregular shape of subsurface structures deduced from seismicreflection studies, microtremor array studies and deep drilling data. The 3D model obtained bycombining the interpreted 2D subsurface structures is consistent with geological information forthe area, such as surface geology and the location of active faults. Compared with the basementstructure derived from the gravity prospecting, the proposed 3D model is more complicated,indicating a higher resolution than by the gravitational method.The 3D numerical simulation of microtremors for the proposed 3D model is also conducted.It is indicated that in the synthetic H/V spectral ratios, it doesn't affect the predominantfrequency though the wave-arrival direction of the microtremors affects the peak value. It isconcluded that the proposed technique based on the microtremor H/V spectral ratio is a practicaland useful means of extrapolating and interpolating irregular subsurface structures in 3D basinstructure modeling.