A method for determining the geology at a geographical location while drilling is disclosed, which method includes the steps of: performing sonic drilling with a drill pipe, a sonic drilling head, wherein the drill pipe is coupled to the sonic drilling head and wherein the sonic drilling head has vibration means for vibrating the drill pipe; measuring the vibrations of the drill pipe during drilling; converting the measured vibrations from the time domain to the frequency domain by applying a Fourier transformation; and determining the soil composition at the momentary drill depth by comparing the frequency pattern from the Fourier transformation with predefined frequency patterns for different soil compositions.