Characters of Faults and Structures Revealed from Cores and Wire-line logs in Hole-A of the Taiwan Chelungpu-fault Drilling

In 2004-2005 two holes (hole-A and B) for Taiwan Chelungpu fault Drilling Project (TCDP) were drilled in Takeng, west-central Taiwan. Hole-A was drilled with continuously coring from 500 to 2003 m. Besides conventional wire-line downhole geophysical logs, Dipole Shear Sonic Imaging (DSI) and high- resolution micro-resistivity image (FMI/FMS) tools were run at the interval of 508 to 1865 m. Among all fault zones, several characters revealed from hole-A indicate that fault zone at depth 1111m is the best candidate for 1999 Chi-Chi earthquake. Here we present an integrated results from logs and cores including percentage of velocity anisotropy, differential cross energy, lithology, caliper, fast shear azimuth, fracture density and fracture distribution. We use on-site core description, wire-log data (natural gamma ray (GR)), and core images to determine lithology and correlate drilling and log depth. Fractures were picked from `unrolled' core images to obtain fracture density. Average fracture density from 500 to 2003 m is 1.91/m and reaches the highest around FZ1111 with a value of 14/m at 1107 m log depth. Percentage of velocity anisotropy from DSI tool has negative correlation to GR-derived lithology (covariance -0.49) in several sections, especially in sandstone. Mean azimuth of fast shear polarization (FSH) is 113° (95% confidence interval = ±2°) and is roughly parallel to the dip direction of bedding or the direction of regional maximum horizontal principal stress. Compared with overall data, the mean FSH azimuth in the interval of 1050-1200 m rotated toward south with increasing variance (mean resultant = 134°, 95% confidence interval = ±11°). Despite fracture density and distribution variance are high at depth 1050-1200 m, MSD data (mean-square dip analyzed from FMS) keeps the same orientation (103°) with respect to the mean dip direction of bedding and structure, indicating that FSH-Azimuth is less affected by fracture attitude. Besides, compiled data suggested that GR-derived lithology has some connection to maximum cross energy, energy anisotropy, and fast shear azimuth, which might reflect relationship of inherent and stress induced anisotropy with specific formations in the drilling site.