Simple evaluation of the fold axis, axial plane, and interlimb angle from a borehole image log

Abstract

Folds and fractures are important structures that preserve information on the past stress evolution; however, folds remain largely unexplored. Studying folds remains challenging, as no simple and unified method can be used to evaluate fold parameters, which include the fold axis, axial plane, and interlimb angle with depth. In this study, we propose a method to calculate the fold parameters of cylindrical concentric folds by considering the point at which the bedding trend changes as an inflexion point of the fold. The inflexion point is identified from the analysis of bedding orientation, which can be obtained by borehole image log. The orientation of the fold axis and the axial plane were geometrically calculated based on the inflexion surfaces at both ends of the folds. The application of this method is illustrated using a simulated fold model. It is shown that these fold parameters are calculated using the depth of the fold and are reliable to a certain extent, despite the uncertainty of the inflexion points. Although the extraction method assumes cylindrical concentric folds, it can be applied to symmetric folds to estimate the orientation of the fold axis and axial planes. The method developed in this study is expected to have a wide range of applications in structural geology as it can estimate the fold parameters of each fold traversed by a borehole.