Abstract: A pack-off locking means for use in downhole geological seismic work. An upper and lower endcap rigidly connected together by a inelastic vertical member which is adapted to hold a seismic energy source. The endcaps have sealing means attached thereto for isolating a cylindrical section of the well borehole, thus, forming a sealed chamber wherein the seismic energy source contained therein transmits acoustical energy in a mostly horizontal and omni-directional plane into geological formations of interest. Minimal acoustical energy is transmitted into the borehole vertical fluid column because the seal-off means prevents communication thereto. Means for pressurizing the chamber formed by the pack-off means and for measuring the pressure thereof.