Apparatus for imparting seismic signals into the earth

Seismic signal generating apparatus (10) of the impact type that includes a signal generator (24) having an impact mass (26) therein, a base plate (14) or coupling member that is engaged by the impact mass (26) to generate the signal, a housing (28) attached to the generator (24) that encircles the impact mass (26) and extends toward the base plate (14), and means for excluding water from within the housing so that the impact mass (26) travels only through gas prior to engaging the anvil (66). The apparatus (10) also includes control means for positioning the generator angularly and azimuthally.