## 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 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 n_{p}∼1.1 and k ≪ 1 in Q = kf^{n}. This may confirm that the weathered unconsolidated layer is saturated partially, and Q_{s} > Q_{p} stresses that attenuation in our study is physically a local compressional mechanism.

Original language | English |
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Pages (from-to) | 1608-1617 |

Number of pages | 10 |

Journal | Geophysics |

Volume | 64 |

Issue number | 5 |

DOIs | |

Publication status | Published - 1999 Jan 1 |

## ASJC Scopus subject areas

- Geophysics
- Geochemistry and Petrology