Application of geophysical surveys to clarify subsurface geologic conditions of man-made lands

The geo-environmental damages in the man-made land depend on not only the facies and structures of artificial strata but also on the contrast between artificial strata and the basements. Results that reflect precisely the strength and motion characteristics and the chemical properties are important. Therefore, investigating methods adapted for the above purposes were examined and then the relationship between such investigating results and subsurface geology was clarified. Short-period microtremor observation was carried out in reclaimed lands and filled lands in valleys, and electric prospecting in the waste and surplus soil deposits in valleys. The following are the main results of this study. (1) Reclaimed lands in bay area Subsurface geology on the bottom of Holocene alluvial sediments in the reclaimed land is classified into three groups by the sedimentary facies. Microtremor characteristics reflect these subsurface geologic groups. Basic characteristics of microtremors which are predominant in soft sediments on buried valleys are different from those in other sediments. Motion characteristics at the same point vary widely before and after large scale soil improvements. Microtremors observed in bay areas notably reflect the subsurface geology. (2) Filled land in valley plains Ratios of the average amplification on filled zones to those on cut zones are related to the thickness of fills, including Holocene alluvial sediments. When the strength and shear wave velocity differ significantly between fills, including Holocene alluvial sediments and the basements, the variation in thickness of the fills are reflected notably in microtremor characteristics. (3) Waste and surplus soil deposits Resistivity of waste deposits consisting mainly of ashes is a few Ω-m. Those of surplus soil deposits depend on included materials. Conductivities of groundwater and seepage water in waste deposits are ten to hundred fold of those in filled strata with sand and silt. These agree well with the resistivity of waste strata. Temporal changes of resistivity characteristics of waste deposits could be used as one of the deciding factors for closing landfill sites.