A method for investigating material surrounding a borehole, the method comprising: generating a first lower frequency acoustic wave within the borehole, wherein the first lower frequency acoustic wave induces a linear distortion and a nonlinear distortion in one or more inhomogeneity features in the material that are substantially perpendicular to a radius of the borehole; directing a first sequence of higher frequency pulses in a direction perpendicular with respect to the longitudinal axis of the borehole into the material contemporaneously with the first acoustic wave; and receiving at one or more receivers positionable in the borehole one or more second higher frequency pulses produced by an interaction between the first sequence of higher frequency pulses and the one or more features undergoing the linear distortion and the nonlinear distortion due to the first lower frequency acoustic wave, wherein: the first lower frequency acoustic wave has alternating compressional and dilatational phases; characterised in that the method comprises distinguishing between a crack present in the material and other said inhomogeneity features present in the material based on whether the one or more second higher frequency pulses are received dependent on the phase of the first lower frequency acoustic wave; and the second higher frequency pulses are received at the one or more receivers if the first sequence of higher frequency pulses encounters the one or more inhomogeneity features within the material that are oriented such that they direct energy back to the borehole while undergoing the distortion due to the first lower frequency acoustic wave such that the distortion alters a contrast in acoustic impedance during a cycle of the lower frequency acoustic wave. generating a first lower frequency acoustic wave within the borehole, wherein the first lower frequency acoustic wave induces a linear distortion and a nonlinear distortion in one or more inhomogeneity features in the material that are substantially perpendicular to a radius of the borehole; directing a first sequence of higher frequency pulses in a direction perpendicular with respect to the longitudinal axis of the borehole into the material contemporaneously with the first acoustic wave; and receiving at one or more receivers positionable in the borehole one or more second higher frequency pulses produced by an interaction between the first sequence of higher frequency pulses and the one or more features undergoing the linear distortion and the nonlinear distortion due to the first lower frequency acoustic wave, the first lower frequency acoustic wave has alternating compressional and dilatational phases; characterised in that the method comprises distinguishing between a crack present in the material and other said inhomogeneity features present in the material based on whether the one or more second higher frequency pulses are received dependent on the phase of the first lower frequency acoustic wave; and the second higher frequency pulses are received at the one or more receivers if the first sequence of higher frequency pulses encounters the one or more inhomogeneity features within the material that are oriented such that they direct energy back to the borehole while undergoing the distortion due to the first lower frequency acoustic wave such that the distortion alters a contrast in acoustic impedance during a cycle of the lower frequency acoustic wave.