An improved method of determining near-surface Q

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 Qp and Qs 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 np∼1.1 and k ≪ 1 in Q = kfn. This may confirm that the weathered unconsolidated layer is saturated partially, and Qs > Qp stresses that attenuation in our study is physically a local compressional mechanism.