Electrical image logs interpretation from Wenchuan earthquake Fault: Scientific Drilling project-hole4 (WFSD-4)

Borehole imaging is one of the most rapid and accurate methods for gathering in-situ geological feature information. The image logging data from WFSD-4 contain a mass of subsurface information, which plays a significant role in improving the understanding of the Presumed Slip Zone (PSZ) in the northern segment of the Yingxiu-Beichuan Fault. Using the electrical image logs with core data, the lithology and the PSZ characterisation were carried out. The results show abundant bedding and oblique fractures in the main lithologies. From the imaging dip interpretation, the bedding development of the 2012-2123-m section tends to be basically unchanged, but the dip angle varies frequently, and is mainly affected by the drag and pull of the fault. From 2123 m to the bottom of the well, the bedding is undeveloped, the formation is fractured seriously and the fracture breccia is obvious at 2190 m. There is a clear change in the strata: the dip angle alters from steep to gentle, and tends to change from north to south. The breakpoint position should be 2190 m, and the 2012-2190-m segment should be the hanging belt of the fault. Electrical images, conventional log as well as core analysis from WFSD-4, indicate that the slip may occur in fault breccia.