An improved method of determining near-surface Q

Abstract

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 Q p and Q s 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 Q s 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 n p approximately 1.1 and k<<1 in Q = kf n . This may confirm that the weathered unconsolidated layer is saturated partially, and Q s (super Q p stresses that attenuation in our study is physically a local compressional mechanism.