a dual mode multiple - element resonant cavity piezoceramic borehole energy source is disclosed . thus , a source for generating detectable seismic waves in a wellbore is provided which is capable of transmitting useful signals over long distances between the source and the related receiver or receivers . a liquid - filled elongated column is provided which defines a longitudinal axis and has a resonant frequency . a resonator comprising at least one substantially cylindrical piezoelectric element having an axis approximately parallel to the axis of the borehole for inducing seismic waves in the borehole is furnished . the resonator is constructed and arranged for vibrating at the first resonant frequency , and is also constructed and arranged for vibrating at a second frequency in a hoop mode to generate seismic waves having the second frequency .