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

Abstract

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