The author has developed a small and powerful high frequency cam-type vibrator using roller thrust bearings with uneven races. The diameter of the vibrator is nearly the same as the outer diameter of the thrust bearing, and it is very small compared with other vibrators. This vibrator can be installed at the bottom end of a drill string as a high frequency vibro-hammering gear.
Although the amplitude of the axial displacement is fixed, combination of two vibrators of this type can make the amplitude variable by superimposition. This variable superimposition can also be used as an On-Off switch of the vibration.
The underwater sound used for seismic survey is usually a short pulse with a duration of several to several tens of milliseconds. The vibrator described in this paper is capable of generating such short sound pulses owing to the variable superimposition.
Present sound generators for seismic survey are usually big and heavy and generate sound pulses with a wide frequency spectrum, centered around 100Hz. The cam-type vibrator described in this research is much smaller and lighter than present systems, making the deployment near the seafloor easy for even at great depths, which in return leads to more detailed results in stratum surveys. The emitted sound is a pure tone whose frequency can be anywhere between 100 and 1000Hz.
The author has tried to develop this type of sound generator under the support of JOGMEC (Japan Oil, Gas and Metals National Corporation).
In this paper, the principle of the vibration, the design of the sound generator and the result of the experiment shall be discussed.