Pseudo-N-value from the S-wave velocity: a proposal for communication between engineers and geophysicists

摘要

The S-wave velocity is a one of the physical parameters to describe competence or hardness of thernground. It is estimated by the seismic survey particularly Multichannel Analysis of Surface Wavesrn(MASW). In geotechnical engineering, competence of the ground is traditionally described by the Nvaluerndirectly measured by the standard penetration test (SPT). While the physical significance of thernN-value is tenuous, it has long established in the geotechnical field. Soil classification is based on thisrnparameter and authorities use it as a standard means of description of strength of soil.rnWhile the SPT provides point data at a drilling location measured with a large rig, the seismic surveyrncan achieve data collection from the surface along a line; and cover an area by several lines. Thisrnallows mapping of S-wave velocity distribution in the 2-dimensional and 3-dimensional spaces. If thernS-wave velocity is converted to the N-value, soil classification map can be generated easily with muchrnhigher sampling density.rnNearly thirty authors in the past attempted to relate the S-wave velocity and the N-value in greatrndetails. However, these attempts inevitably had considerable error bars. This paper propose a simplernformula: Vs =60N~(0.4) or its reciprocal N=(Vs/60)~(2.5) and to call it “pseudo-N-value”. The validity of thisrnformula is examined in reference to the actual measurements.rnThe pseudo-N-values are expressed in the 2-dimensional cross sections or 3-dimensional datarnvolumes. These displays help geotechnical engineers understand geophysical data.