A method of increasing the useful seismic energy in a generated primary pulse and at least one article for carrying out the method is disclosed. The article comprises a pneumatic sound generator, as an air gun for generating a primary seismic pulse and an openended elongated cylinder surrounding the air gun and having an end opening are nearest the air gun exhaust ports greater than the area of said ports for shaping the primary pulse so that its dominate frequency will be lowered, placing more of the energy in the useful seismic frequency range, resulting in greater penetration of the seismic signal within the sedimentary section.
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