MEASUREMENT OF SEISMIC ATTENUATION FROM HIGH-RESOLUTION CROSSHOLE DATA

摘要

High-resolution crosshole seismic data have been acquired at the Imperial College borehole test site in Northumberland, England. Raypaths between source and receiver were horizontal and parallel to plane layered beds of sandstone, mudstone, and limestone. Shots were fired at 25 cm intervals between the depths of 50 and 72 m. Estimates of the total attenuation using the spectral ratio and centroid frequency shift methods show Q values that correlate well with the frequency content of the first arrivals. Synthetic modeling studies with and without intrinsic attenuation show that the contribution of scattering attenuation is significant and must be accounted for to recover the intrinsic Q values used in the modeling. At some depths, there appears to be an increase in the frequency content of the compressional wave with distance, implying a negative scattering e. Snapshots of the wave as it travels across the velocity structure at this depth show that the high frequencies are preserved in the wavefield and the low frequencies are scattered to other depths. Intrinsic Q values of less than 20 were obtained at all depths. Within experimental error, there is no evidence to suggest any significant variation of attenuation with lithology. This is consistent with estimates of attenuation from core samples. However, there is a significant difference in the median of Q estimates from the crosshole and core data. [References: 27]