Methods and apparatus are provided for determining an indication of a nonlinear property of the formation. A sonic tool is used in a borehole to generate a signal having multiple cycles of a wave of a predetermined frequency. The amplitude of a tube wave resulting from the signal is measured. The measured amplitude is then used to determine an indication of the nonlinearity of the formation according to a relationship which relates the measured amplitude to the amplitude of a second harmonic tube wave which is generated in the borehole by the tube wave of the predetermined frequency due to the nonlinear property of the formation. The indication of nonlinearity may then be utilized to give an indication of the relative consolidation of the formation surrounding the borehole.