Gabor domain analysis of Q in the nearsurface

Robert J. Ferguson, Gary F. Margrave, and Kevin W. Hall

ABSTRACT

We present a Q estimation method based on traveltime derivatives applied to multilevel vertical seismic profile (VSP) acquisition. Long, narrowband vibroseis sweeps provide approximately monochromatic wavefields over short time windows, and analysis is done in the Gabor domain. There, attenuation β is estimated as a function of frequency f , and Q(f) is computed through inversion of β .

We provide an example based on a 7-level, near-offset VSP. Each VSP level consists of a 3-component receiver with inline, crossline, and vertical sweeps. Eight, narrowband sweeps are used to span 10 Hz - 250 Hz for each source orientation, and they are found to help reduce noise in the data due to baseplate harmonics.

Two subunits are identified within the local formation that correspond to an expected 50 m of unsaturated media underlain by an aquifer. Our results demonstrate that β varies near-linearly with fre Q uency for the 95 m depth range, Strong values of β that increase with f are found in the unsaturated unit, and weaker, decreasing β is found in the saturated unit.

Q for the formation is estimated only approximately due to excessive powerline noise and and strong harmonics in the well. For both units and the overall formation, Q increases linearly with f until about 100 Hz.

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