CLASSIFICATION OF CRYSTALLINE DRILL CORES FROM THE KTB DEEP-WELL BASED ON STRAIN, VELOCITY AND FRACTURE MEASUREMENTS

Anelastic strain recovery, ultrasonic wave velocity and Brazilian tens ile strength measurements were made on 52 crystalline core samples tak en at depths between 127 and 3888 m in the KTB pilot hole. The purpose was to quantify the alterations of physical properties of rock caused by time-dependent, stress-relief microcracking within the drill cores . Asymptotic values of principal recovery strains decrease exponential ly with increasing depth irrespective of lithologic units investigated . From the pressure-dependent wave velocity data it was found that gne isses have both the highest percentage of crack-induced (19%) and text ural (11%) anisotropy among all rock types analyzed. Amphibolites, lam prophyres and metagabbros show a total velocity anisotropy of about 10 %, from which one-half in the former two cases, and one-third in the l atter case is crack -induced. The azimuth of minimum P-wave velocity r otates from N30-degrees-E at a depth of 2 km to N160-degrees-E at 4 km . The dip of minimum P-wave velocity is very steep (greater-than-or-eq ual-to 80-degrees) throughout the pilot hole, and only decreases in th e depth interval from 2.8 to 3.2 km. Based on a double-normalized velo city plot one can distinguish between gneisses (parallel crack pattern ) and metabasites (randomly-oriented cracks). The average tensile stre ngth of drill cores ranged from 11 to 25 MPa depending upon rock type and loading direction. The average strength anisotropy found was 25, 1 8, 13, 10 and 5% for gneiss, lamprophyre, amphibolite, marble and meta gabbro, respectively. Lamprophyres and metabasites are characterized b y a pre-dominate viscoelastic component of recovery strain, whereas th e expansion of gneissic core samples can mostly be explained by dilata nt, stress-relief microcracking.