TO DETERMINE WHETHER USEFUL INFORMATION RELATING TO THE IN-SITU STRESS FIELD CAN BE DERIVED FROM INFORMATION ON BOREHOLE BREAKOUTS.
Stresses on the Earth's crust are speculated to arise from many sources: plate driving forces, temperature changes (cooling and heating), gravitational loading by volcanic and sedimentary processes, and from poorly understood processes of midplate tectonics. The magnitude and direction of the principal stresses in the brittle crust influence and propagation of faults, the opening and closing of fractures, and pore water circulation.
Until recently very little in situ stress data has been available from the United Kingdom crust. Literature reviews list only a limited number of measurements which include information on the direction of maximum horizontal compression. Unequal horizontal crustal stresses imposed on vertical boreholes will often cause localized rock spalling along the direction of least compressive stress. The resultant elongations of the borehole cross section, called breakouts, can be identified from 4 arm caliper records collected routinely with conventional geophysical dipmeter logs. 81 boreholes in the United Kingdom have been analysed for breakouts using criteria for identifiaction of breakouts unique to this research. The majority of borehole breakouts in southern Britain are aligned northeast southwest, indicating a maximum horizontal stress in a northwest southeast direction. There are insufficient data to confidently assess a dominant borehole breakout direction in northern Britain, although the 4 boreholes analysed indicate possible variations in the crustal stress field.
THE CONTRACTOR HOLDS GEOPHYSICAL DATA FOR THE GREAT MAJORITY OF BOREHOLES DRILLED ON-LAND IN THE UK, AND WILL ANALYSE CALLIPER DATA FROM A NUMBER OF THEM TO SEE WHETHER THE PHENOMENON OF "BOREHOLE BREAKOUT", A SPALLING OF THE BOREHOLE WALL RESULTING IN AN ELLIPTICAL CROSS-SECTION, CAN GIVE USEFUL INFORMATION ABOUT THE LOCAL STRESS FIELD. IT SEEMS CLEAR ALREADY FROM WORK ELSEWHERE THAT BREAKOUTS PROVIDE A USEFUL POINTER TO THE DIRECTION OF MAXIMUM HORIZONTAL STRESS; SUBSEQUENT WORK WILL INVESTIGATE WHETHER SEMI-QUANTITATIVE INFORMATION ABOUT THE STRESS FIELD CAN BE OBTAINED AND WHAT ROCK CONDITIONS ARE NECESSARY FOR USEFUL INTERPRETATION OF THE DATA.