Effects of Anisotropy on Borehole Breakout Direction: A Physical Modeling Approach

Stress measurements from the Earth’s crust are useful for better understanding plate tectonic processes, the strength of major lithospheric faults, and seismic hazards. One common data set for characterizing in situ crustal stress comes from borehole breakouts. To link borehole breakouts to regional stress patterns, structural geologists assume that the crust is homogeneous and isotropic. However, recent numerical, experimental, and field studies indicate that stress directions inferred from breakouts may be misleading. I take a physical modeling approach to study the effects of anisotropy on the direction of borehole breakouts. The model material is gelatin, which is incompressible and linearly elastic. The gelatin blocks are prepared in three different manners: un- layered; horizontally layered; and obliquely layered. Dip angle and dip direction of layers in the obliquely layered blocks are varied across trials. Each block of gelatin is subjected to an external uniaxial compression, after which a borehole is made and breakouts form. I found that neither dip angle nor dip direction had a significant effect on the direction of borehole breakouts (p=0.99, p=0.06). However, the model set up had a major flaw that imparted bias to the results. Correcting for that bias, the effects of dip direction may become significant. My results highlight the importance of future research investigating how anisotropy effects borehole breakouts and bring into question the validity of interpreting regional stress directions from borehole breakouts gathered in anisotropic rocks