Performance of Active and Passive Methods for Measuring Low-Frequency Surface Wave Dispersion Curves

This paper examines the consistency between surface wave dispersion curves measured at wavelengths of up to 600 m using active and passive methods at sites in the Mississippi Embayment. Large-diameter (200 m) circular receiver arrays were deployed at five deep soil sites located in Tennessee, Arkansas, and Missouri to record ambient ground vibrations at low frequencies. Measurements were performed at the same locations using linear receiver arrays and actively generated low-frequency energy using the recently developed Network for Earthquake Engineering Simulation (NEES) field vibrator (termed Liquidator). Characteristics of the ambient wavefield measured at the five sites in the Mississippi Embayment are presented along with comparisons between the surface wave dispersion curves obtained from the active and passive measurements at each site. The ambient wavefield measurements exhibited peak levels in the frequency range of 1-5 Hz. Surface wave dispersion curves developed from frequency-wavenumber (f-k) processing of the active and passive methods were in good agreement at four of the sites, with phase velocities from the passive measurements within 5-10 % of the active-source measurements out to wavelengths of about 550 m. Improved comparisons were obtained at the fifth site by applying high-resolution f-k processing.