We evaluated the capabilities of vertical seismic profiling (VSP) for imaging the complex heterogeneous unconsolidated sedimentary structures at a shallow site. We deployed a 24-level hydrophone array with 0.5-m level spacing down a preexisting poly vinyl chloride (PVC) cased well. Data acquisition time was quick. Only 15 multioffset shot points using a hammer-on-plate source were needed to acquire reflection data between the water table at 3 m and the bedrock at 35 m to produce a depth section image. This image extended 9 m from the receiver well, yielding resolutions between fresh-waterbearing sands and impermeable muds and clays of better than 1 m. Depth accuracy of the image was confirmed by good correlation with cone penetrometer logs. We used conventional wavefield separation and VSP-CDP mapping techniques to image the data. Tube waves, created by seismic arrivals at crosssectional area changes in the borehole fluid column, were the primary source of coherent noise in the data. The tube-wave arrival structure was complicated by the hydrophone array, which generated and scattered tube waves at each hydrophone pod. To combat the tube wave interference, we inserted closed-cell-foam baffles between elements. The baffles attenuated and slowed the tube waves, and reduced generation and scattering. A comparison between unbaffled and baffled VSP data showed that baffling increased the maximum useful frequency from 300 Hz to over 900 Hz. By contrast, surface shot data recorded at the same site, using buried 40-Hz vertical geophones, exhibited useful frequencies of less than 250 Hz. In addition, coherent noise in surface shot records caused by air waves and first arrivals made it very difficult to identify shallow reflections above 25 m. Reflections from depths as shallow as 10 m were easy to identify in the baffled VSP data.

A vertical seismic profiling (VSP) method was developed to produce compressional (p-wave) velocity sections in a 3-D survey volume, down to a maximum depth of 57 m. The VSP method consisted of a multilevel hydrophone tool deployed down several wells. Multiple offset surface shotpoints were recorded at multiple azimuths around each well head, using an impact source.The VSP data were processed and inverted to produce p-wave interval velocities. These velocity sections were then projected into a model of the survey volume, together with borehole log information and other geologic information, to produce a 3-D visualization which aided the location and interpretation of aquifer boundaries below the water table. The final image displayed several low velocity zones, and these were attributed to partially saturated pore spaces, possibly both natural, or pump induced by air suction. Low velocity layers appeared above several actively pumped aquifers, and it was concluded that the multi-offset, multi-azimuth, VSP method may be suitable for mapping both aquifer channels, and zones of pumping influence.

No abstract

This Site Safety Plan for the Environmental Restoration Division's environmental investigations has been prepared specifically for the Lawrence Livermore Nationai Laboratory (LLNL) Site in Livermore, California, on the basis of available data. This Health and Safety Plan was written to comply with applicable requirements of Occupational Safety and Health Administration regulations, including 29 CFR 1910.120, Hazardous Waste Operations and Emergency Response. This document is intended as a gen',ral health and safety plan for LLNL and will be supplemented by Operational Safety Procedures for activities which are not sufficiently covered by this plan. As additional facts, monitoring data, or analytical data on hazards become available, this Plan will be modified. It is the responsibility of the co-Site Safety Officers (SSOs) to evaluate data that may impact health and safety dunng these act-vities and to modify the Plan as appropriate. Either of the co-SSO shall have the authority to institute any change that provides greater health and safety protection for workers at the LLNL Site. II Each time the term SSO or co-SSO is used, either the co-SSO or a designee should take responsibility or complete the action. Employees are encouraged to bring to the attention of their supervisor any unsafe or hazardous condition that they observe as they carry out their responsibilities. The supervisor shall then inform the SSO promptly so that the situation can be corrected and personnel can be advised of an improved procedure, lt is clearly impossible to anticipate 'ali specific safety and health hazards beforehand; therefore, ali personnel must exercise common sense and good judgment in their approach to a given situation. The Health and Safety Training described in Section 9 of this document will assist in preparing indi_ 2duals to recognize hazards. This document incorporates appropriate rules, guidelines, and recommended work practices contained in previously published material. Ali personnel potentially exposed to physical or chemical hazards as described in this Plan shall follow the safety and health procedures set forth herein. Any modifications to this Plan will be issued as attachments to this document.