A Field Study of Seismic Attenuation In Layered Sedimentary Rocks-I.VspData

Abstract

S U M M A R YA coherence matching technique was used to estimate Q from both real and log-derived synthetic VSP data to try to determine the relative contributions of apparent and intrinsic attentuation in layered sedimentary rocks. The field experiment was carried out at the Imperial College borehole test site in Northumberland, England. The holes have been drilled through a cyclical sequence of the Namurian Upper Limestone Group. The Q analysis indicates that for frequencies from 40 to 200 Hz, scattering attenuation due to interbed multiples within the horizontally layered sequences is the dominant mechanism and any contribution from intrinsic attenuation is relatively insignificant. Confirmation of this result is obtained from an analysis of the frequency content of scattered energy in the coda of the downgoing compressional wave. Although uncertainty bounds on the Q values are large, these VSP data are not consistent with an intrisic Q value as low as 20 which has been estimated from 500 Hz to 1.5 kHz crosshole data at the same site as described in Part 11.