Residual strain, wave speed and crack analysis of crystalline cores from the KTB-VB well

• Published in: Tectonophysics Vol. 263; no. 1; pp. 219 - 234
• Main Authors: Zang, A., Lienert, M., Zinke, J., Berckhemer, H.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15-10-1996
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/S0040-1951(96)00033-9

We investigated eight drill cores from the German KTB pilot hole in the depth range from 935 to 3747 m using three experimental methods. Residual strains were measured by secondary overcoring (SOC), core P-wave anisotropy by wave velocity analysis (WVA), and the distribution of microcrack orientation by chemical contact imagery (CCI). The orientation of cracks and the direction of maximum wave speed coincided for all cores investigated. The residual strain azimuths differed from the orientation of cracks and maximum wave speed. The elastic residual strains measured shortly (minutes) after SOC showed a nearly constant azimuth versus depth at the drill site. The average value of calculated minimum horizontal strain azimuth was N117°E. The final strain azimuth (elastic plus anelastic deformation) recorded days after SOC showed scattered values versus depth. Magnitudes of overcored strain gauges (with outer free surface) showed lower values compared with magnitudes of undercored gauges (with inner free surface) for cores from shallow depths below 2 km. The situation was reversed for samples from greater depth. While crack opening was assumed to be responsible for the strain pattern after the 1st coring in situ, partial crack closure could explain the observed residual strains after 2nd coring. Since the elastic part of residual strains obtained from 2nd coring was not correlated to rock texture, this part of strain might store information on the stress field at depth. Our minimum residual strain azimuth is in agreement with the regional paleostress direction of
S
H and deviates by about 30° from the recent in situ
S
H-direction at KTB.