CRACK CLOSURE PRESSURES INFERRED FROM ULTRASONIC DRILL-CORE MEASUREMENTS TO 8 KM DEPTH IN THE KTB WELLS

Ultrasonic P-wave traveltimes were examined in 73 directions of cylind rical core specimens, measuring 30 mm in length and in diameter, at ro om temperature in a 400 MPa pressure vessel. The wave-velocity analysi s was carried out on 66 crystalline drill cores taken at depths of bet ween 127 and 3888 m in the KTB-VB well and on 18 drill cores taken at depths of between 4195 and 8080 m in the KTB-HB well. Based on omnidir ectional wave velocities versus pressure, a method is developed to sep arate crack-caused and textural velocity anisotropies. The crack-cause d part of the velocity anisotropy is used to infer crack closure press ures. The ratio of horizontal to vertical differential pressure obtain ed from textural-reduced crack closure curves decreased from four near the surface to one at depths greater than 5 km. The difference in hor izontal crack closure pressures was higher than the overburden pressur e only for specimens from shallow depths (<1 km); it reached a maximum value of 40 MPa at 2 km depth and decreased to 15 MPa in the depth ra nge from 4 to 6 km. For 45 cores we observed a rotation in velocity az imuth under pressure. Assuming the velocity azimuth of nine cores with weak fabric and the crack velocity azimuth of 23 anisotropic cores wi th rotating azimuths to be parallel to the S-H-direction, our data imp ly a significant rotation of the stress field in the depth range from 1 to 4 km in the KTB pilot hole. Our core results support the previous ly determined average S-H-direction of N162 degrees E at the drill sit e only in the suite of metabasites at depths ranging from 3629 to 3746 m.