An improved method of determining near‐surfaceQ

Abstract

We performed field measurements using the modified method of spectral ratios to estimate shallow seismic Q. Three component seismograms from artificial sources were recorded to determine [Formula: see text] and [Formula: see text] in the unconsolidated sedimentary layer at the experimental site. This modified spectral ratio method was assumed to be frequency dependent, and the amplitude ratios then were plotted against the arrival‐time difference of any two receivers for one particular frequency. The slope of the regression line in the log‐amplitude‐time space yields a Q for each frequency. Results show that Q is a function of frequency in the frequency range (below 300 Hz) we tested. A simple mathematical derivation with experimental data strongly suggests that the Q of shallow seismic waves is frequency dependent. Corrections for geometric spreading are used; however, the original and corrected Qs show no significant difference in our data, and therefore the geometric factor may be ignored in this problem. The conventional frequency‐independent spectral ratio method is easier and faster to apply, but it gives less stable results than this modified method. The unstable Q is attributed to geometric amplification effects in the conventional frequency‐independent spectral ratio method. The source factor can have an effect on the estimates of Q; however, different seismic sources give about the same Q over the dominant frequency band. We established the frequency function by assuming a simple power law regression model, where [Formula: see text] and k ≪ 1 in [Formula: see text]. This may confirm that the weathered unconsolidated layer is saturated partially, and [Formula: see text] stresses that attenuation in our study is physically a local compressional mechanism.

Estimation of near-surface quality factors by constrained inversion of Rayleigh-wave attenuation coefficients

“…In his work, Q was independent of frequency. Although some authors suggest that near-surface Q may be frequency dependent (Jeng et al, 1999), Xia et al (2002) followed the laboratory results (Johnston, 1981) and Mitchell's work (1975) that Q is independent of frequency, allowing determination of Q as a function of depth based on amplitude attenuation of Rayleigh-wave data. The quality factor Q as a function of depth, which is directly related to the material damping ratio D (= 0.5Q − 1 ) (Lai and Rix, 1998;Rix et al, 2000), is of fundamental interest in earthquake engineering (Kramer, 1996), geotechnical engineering, ground-water, and environmental studies, as well as in oil exploration and earthquake seismology.…”

Stable Q analysis on vertical seismic profiling data

“…The Q model is assumed to be frequency independent (Kjartansson, 1979;Wang and Guo, 2004), but could be frequency dependent in practice (Jeng et al, 1999). Defining a single variable χ ≡ ωτ, equation 1 can be rewritten asĀ…”

Relationships between geotectonic and seismodynamic characteristics of the crust in the Eastern Anatolia

“…Vassiliou et al (1982) found the Q S /Q P ratio equal to unity for air dry rocks and less than unity for fully saturated rocks. Although some authors suggest that near-surface Q may be frequency dependent (Jeng et al 1999), Xia et al (2002) followed the laboratory results (Johnston 1981) and Mitchell's work (1975) that Q is independent of frequency, allowing determination of Q as a function of depth based on the amplitude attenuation of Rayleigh-wave data.…”