EMPIRICAL CORRELATION OF SEISMIC CONE PENETROMETER AND REFRACTION MICROTREMOR DATA FOR CONSTRUCTING SHEAR WAVE VELOCITY PROFILES

Methods of quantification of the shear wave (S-wave) velocity of near-surface materials have been determined to address the fundamental interest in environmental and engineering studies. Shear wave velocity is an important parameter in numerical modeling of local site effects in any probabilistic seismic hazard analysis. Traditionally, the seismic cone penetrometer test was conducted to evaluate in-situ shear wave velocities. However, the refraction microtremor technique may be used in the consulting field as a non-intrusive means to obtain shear wave velocities.

A probabilistic seismic hazard analysis in the Knox Formation in northern Tennessee was completed utilizing both the traditional downhole seismic cone penetrometer test and the non-intrusive refraction microtremor method to obtain shear wave velocities. This project focuses on the empirical correlation between downhole and non-intrusive methodologies to obtain shear wave velocities of near-surface materials.

A compression model electronic piezo cone penetrometer, with a 15 cm² tip and a 225 cm² friction sleeve was used to perform seismic cone penetration soundings in the project area. Shear wave measurements were taken on approximately 1-meter intervals in the soundings to construct a shear wave velocity profile down to the bedrock surface. Due to shallow bedrock in the project area, a conservative estimation of competent rock (per IBC Table 1613.5.3) was used to complete the average weighted shear wave velocity profile to 100 feet. The average weighted shear wave velocity profile to 100 feet using the downhole cone penetrometer method was determined to be 2,211 ft/s. The refraction microtremor technique was also conducted in the same area to measure the ambient shear wave movements using geophones at the near-surface. Shear wave velocities for each soil and rock layer in the upper 100 feet of the subsurface soil column were calculated from the geophone data. Based on results of the refraction microtremor method, the average weighted shear wave velocity profile to 100 feet was determined to be 2,333 ft/s.

Data gathered during this study indicate the refraction microtremor technique may be used to calculate the average weighted shear wave velocity for use in probabilistic seismic hazard analyses within the Knox Formation in northern Tennessee.