Method for enhancing low frequency output of impulsive type seismic energy sources and its application to a seismic energy source for use while drilling

A system for generating seismic energy in a wellbore, comprising:

• a spark gap disposed in a drill string;
• a power supply disposed in the drill string and configured to discharge through the spark gap;
• a control circuit disposed in the drill string and operable to cause the power supply to discharge in firing sequences having at least two discharges separated by a time selectable to result in a determinable acoustic energy frequency output from the discharges;
• and
• an acoustic reflector disposed in the drill string proximate the spark gap and configured to reflect acoustic energy from the spark gap into the wellbore, the reflector including a positioning device configured to move a wall of the reflector to a selectable distance from a wall of the wellbore.