Sonic drilling is a soil penetration technique that fluidizes porous materials. Using a sonic head, drill casing and rods are brought to a vibration frequency of 100-200 Hz, which is within the range of human hearing. These waves are transmitted through the drill string to the end of the casing and reflected, causing the casing to stretch and thin, and to shorten and thicken 100-200 times per second. This intense vibration causes a very thin layer of soil directly around the drill rods to fluidize. The fluidized soil zone extends a maximum of 5 millimeters from the rod. The soil in the influenced area behaves like a fluid, which dramatically reduces the friction between the drill rod and the surrounding soil, allowing very rapid penetration. The same friction reduction applies to the inner surface of the soil sampling tool, and enables collection of very long sample cores (up to four meters). In addition to the vertical vibration, compact sonic drill heads can rotate to easily make up and break drill rods, and to cut concrete or asphalt. Sonic drill heads vibrate and rotate simultaneously, providing cutting at the bit face in addition to the sonic vibration, to penetrate competent material.
The sonic drill head contains two or more synchronized eccentric weights that are driven by high-speed hydraulic motors, (6-12,000 rpm). The drill head is mounted in oversized isolation blocks to concentrate energy on the drill string. The movement of the eccentric weights brings the casing into a purely vertical vibration. With low pull-down or pull-up force, the casing can be driven and retrieved.