Use Of Vsp Techniques To View, In 3D, Partially Saturated Layers In The Shallow Subsurface At Lawrence Livermore National Laboratory, California

A multi-offset VSP (vertical seismic profiling) method was developed to help define aquifer boundaries and below<br>water table partially saturated zones at the Lawrence Livermore National Laboratory, Livermore Site, California.<br>The VSP method consisted of using a multilevel hydrophone array giving 48 channels of recorded seismic data per<br>shot. Hydrophone spacing was 0.5 m, with anti-tubewave baffling devices inserted between each phone, and at each<br>end of the array to slow down and attenuate tube wave noise contaminating the desired data signals. The seismic<br>source consisted of an impact weight-drop machine.<br>The VSP data was processed, and inverted to produce 2D p-wave interval velocity sections with up to 0.5 m of vertical<br>resolution in the unconsolidated sediments below the water table. Multiple 2D velocity sections were created,<br>corresponding to radial arms of walk-away shot point records from multiple wells. These 2D velocity sections were<br>then visually projected into the 3D survey volume, together with well locations, predefined hydrostratigraphic unit<br>(HSU) layer boundaries, and the interpreted isopach map of a major HSU of interest. The interactive graphical display<br>of all these data in 3D allowed correlations to be made between the VSP velocity images and the pre-existing data.<br>The discovery of several low velocity zones below the water table was attributed to partially saturated pore spaces.<br>Two thin (less than 1 m thick) layers with p-wave velocities as low as 500 m/s appeared to be perched on top of major<br>aquifers undergoing pump extraction and treatment. Gas analysis from water samples taken from one of the aquifers<br>indicated normal atmospheric air as a probable gas source. High pumping flow rates (20 to 30 gal/min) may<br>help to explain how venturi action could be responsible for the intake of air from the vadose zone into porous but<br>less permeable silty-sand units overlying sand-gravel aquifers. Possible uses for this VSP technique would be to delineate<br>the zone of pumping influence, and help in the choice of suitable new well locations for either pumping or<br>monitoring.