The measurement of velocity dispersion and frequency-dependent intrinsic attenuation in sedimentary rocks

A series of experiments to determine the elastic properties of a sequence of saturated sedimentary rocks over as wide a frequency range as possible was carried out at the Imperial College borehole test site. These experiments fall into four categories: vertical seismic profiles (VSPs) within the frequency range 30-280 Hz, crosshole surveys (0.2-2.3 kHz), sonic logging (8-24 kHz), and laboratory measurements (300-900 kHz). The intrinsic attenuation and velocity of compressional and shear waves were measured whenever possible. Velocity dispersion is observed for both compressional and shear waves. The intrinsic attenuation of compressional waves is frequency dependent with a peak in the attenuation in the sonic frequency band. The data were modeled assuming the attenuation is caused by local fluid flow in pores of small aspect ratio. The modeling indicates that the intrinsic attenuation may be dominated by cracks with aspect ratios of around 10(-3) to 10(-4).