Knowledge of in-situ principal stress directions is vital for planning optimum orientations of inclined and horizontal wells, to minimise borehole instability and formation break-out.
This information can be obtained by measuring the orientations of natural and induced fractures by goniometry or magnetic methods and/or from sonic velocity anisotropy determinations.

When a core is brought to the surface, relief from the ambient stress commonly causes differential expansion, through the development of numerous microfractures. This expansion is greatest in the direction of maximum horizontal stress (Sigma 2) and least in the direction of minimum horizontal stress (Sigma 3). Ambient Sigma 1 is usually vertical, due to overburden.

The orientation of the microfracture network can be measured accurately in 3-D using magnetic methods. Alternatively, it can be determined from acoustic anisotropy, since acoustic velocity measured across the core parallel with the microfractures is greater than that measured perpendicular to them. In both cases, microfracture orientations are referred to North by paleomagnetic core re-orientation.