A FIELD-STUDY OF SEISMIC ATTENUATION IN LAYERED SEDIMENTARY-ROCKS .2. CROSSHOLE DATA

A series of crosshole seismic experiments have been performed at the Imperial College borehole test site in Northumberland, England using a detonator source and hydrophone receivers. The holes have been drilled through a horizontally layered sequence of Carboniferous limestones, shales and sandstones. Estimates of intrinsic Q were obtained for frequencies between approximately 500 Hz and 1.5 kHz. A comparison with the results of Part I suggests evidence of frequency-dependent intrinsic Q with attenuation at 1 kHz being significantly greater than at 100 Hz. No change of intrinsic attenuation with ray direction from horizontal (0-degrees) to 60-degrees was observed. However, since no high-frequency crosshole data for ray angles between 60-degrees and vertical is available, we cannot entirely discount the possibility that the difference in intrinsic Q estimates from the vertical VSP data (Part I) and the crosshole data is due, at least in part, to anisotropic intrinsic attenuation.