Comparative Study of Refraction Microtremor (ReMi) and Active Source Methods for Developing Low-Frequency Surface Wave
ABSTRACT:
Obtaining high-quality dispersion curves is a critical step in the development
of reliable shear wave velocity profiles from surface wave measurements. Because
of its limited equipment and space requirements, the refraction microtremor
(ReMi) method has become a popular approach for determining surface wave
dispersion curves, and is increasingly being used for estimating low-frequency
(long wavelength) surface wave velocities that are beyond the range of most
active sources. The recent development of a low-frequency field vibrator as part
of the Network for Earthquake Engineering Simulation (NEES) program has made it
possible to actively generate surface wave energy down to frequencies of less
than 1 Hz. This paper presents a comparative study of the ReMi method and the
active-source frequency-wavenumber (f-k) method (using the NEES vibrator) for
developing low-frequency dispersion curves. Linear arrays of 1-Hz seismometers
were deployed at eight deep soil sites in the Mississippi Embayment. Using both
ambient and active energy, surface wave dispersion curves were determined to
wavelengths of 600 m. The dispersion data from the two methods were in good
agreement (within about ±5%) to wavelengths of 100 to 150 m (3 to 4 Hz) at most
sites. However, at longer wavelengths the dispersion estimates from the ReMi
approach deviated significantly from the active source measurements. The
validity of the f-k dispersion curve was supported by dispersion data obtained
using the SASW method, as well as ambient vibration measurements performed using
a circular array at four of the sites. Analyses of ambient vibrations recorded
using circular arrays show that the poor performance of the ReMi method at long
wavelengths is not because of a lack of ambient surface wave energy at low
frequencies, but can be attributed to invalid assumptions about the nature of
the ambient wavefield.
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