Borehole breakout provides another method to assess biaxial stress distribution around a hole. The method of measurement here is the failure of the borehole and this is measured by an acoustic scanner. If the wall stresses are insufficient to induce compressive failure of the borehole wall then no indication of the stress field may be made. Alone, the acoustic measurement of breakout only permits the direction of major stress perpendicular to a borehole to be estimated.
If the angular width of the breakout is measured, there is a basis for knowing the minimum stress and in addition if there is a good knowledge of the material strength where the breakout has occurred, then the maximum stress perpendicular to the hole may be calculated. Thus if core has been taken from the hole at the location of breakout for uniaxial compressive strength (UCS) testing and the closure pressure from hydrofracture is also known, then there is a basis for calculating the maximum stress perpendicular to the hole.
Doing this requires good information on the rock strength. For example, trying to establish a UCS by correlation from a sonic log would usually not provide an adequately accurate value for use in calculating stress from breakout. What is really required is a UCS measurement transverse to the hole.
Breakout due to wall failure perpendicular to the major stress
Acoustic scan image of borehole breakout