Method for determining geology while drilling

A method for determining a geology at a geographical location while sonic drilling, which method comprises the steps of:

• performing sonic drilling into a ground surface at the geographical location with a sonic drill rig, wherein the sonic drill rig comprises a sonic drilling head and a drill pipe coupled to the sonic drilling head, and wherein the sonic drilling head has two counter-rotating weights for vibrating the drill pipe during sonic drilling;
• measuring the vibrations of the drill pipe during sonic drilling;
• converting the measured vibrations from the time domain to the frequency domain by applying a Fourier transformation;
• determining a soil composition at a momentary drill depth by comparing the frequency pattern from the Fourier transformation with predefined frequency patterns for different soil compositions, and
• adjusting sonic drilling parameters at the momentary drill depth based on the determined soil composition for the momentary drill depth, wherein the sonic drilling parameters comprise drill force, drill speed, and the frequency of the vibrations with which the drill pipe is vibrated,
• wherein the predefined frequency patterns are defined by repeating a plurality of times the following steps at different geographic locations:
• performing sonic sample drilling with a hollow drill pipe, a sonic drilling head, wherein the drill pipe is coupled to the sonic drilling head and wherein the sonic drilling has vibration means two counter-rotating weights for vibrating the drill pipe;
• measuring the vibrations of the drill pipe during sonic sample drilling;
• converting the measured vibrations from the time domain to the frequency domain by applying a Fourier transformation;
• storing the frequency pattern from the Fourier transformation for the specific drilling depth;
• after reaching an end depth, retracting the hollow drill pipe;
• examining a sample contained in the hollow drill pipe for the soil composition along the length of the sample;
• matching the soil composition at a position along the length of the sample with the stored frequency pattern for a specific drilling depth to compose a predefined frequency pattern for the soil composition.