RESIDUAL STRAIN, WAVE SPEED AND CRACK ANALYSIS OF CRYSTALLINE CORES FROM THE KTB-VB WELL

We investigated eight drill cores from the German KTB pilot hole in th e depth range from 935 to 3747 m using three experimental methods. Res idual strains were measured by secondary overcoring (SOC), core P-wave anisotropy by wave velocity analysis (WVA), and the distribution of m icrocrack orientation by chemical contact imagery (CCI). The orientati on of cracks and the direction of maximum wave speed coincided for all cores investigated. The residual strain azimuths differed from the or ientation of cracks and maximum wave speed. The elastic residual strai ns measured shortly (minutes) after SOC showed a nearly constant azimu th versus depth at the drill site. The average value of calculated min imum horizontal strain azimuth was N117 degrees E. The final strain az imuth (elastic plus anelastic deformation) recorded days after SOC sho wed scattered values versus depth. Magnitudes of overcored strain gaug es (with outer free surface) showed lower values compared with magnitu des of undercored gauges (with inner free surface) for cores from shal low depths below 2 km. The situation was reversed for samples from gre ater depth. While crack opening was assumed to be responsible for the strain pattern after the Ist coring in situ, partial crack closure cou ld explain the observed residual strains after 2nd coring. Since the e lastic part of residual strains obtained from 2nd coring was not corre lated to rock texture, this part of strain might store information on the stress field at depth. Our minimum residual strain azimuth is in a greement with the regional paleostress direction of S-H and deviates b y about 30 degrees from the recent in situ S-H-direction at KTB.