Collaborative Research: hydrogeological-geophysical methods for subsurface site characterization. 1998 annual progress report
- Stanford Univ., CA (US)
- Univ. of California, Berkeley, CA (US)
'The general purpose is the subsurface characterization of LLNL superfund site. The goal is to get the most accurate map of the hydrogeological parameters, necessary for modeling and designing the cleanup efforts at the site, using well log data and remote sensing geophysical techniques. In the second year of the project progress has been made in several areas: gathering and interpreting Vertical Seismic Profile (VSP) and Electromagnetic (EM) surveys; investigating the impact of various seismic measurements on upscaling of rock physics relations between sediment properties; and developing a new approach to integrate geophysical and hydrological data using state of the art methods to characterize the subsurface lithology. Vertical Seismic Profile data has been gathered from selected wells at the Treatment Facility D (TFD) during April 1996 and April 1998. The most striking finding here is the detection of anomalies related to saturation conditions. Preliminary results have revealed three anomalously low acoustic velocity zones with velocities below 1,000 m/s; this is lower than the natural acoustic velocity in saturated media by pure water (1,500 m/s). These three zones appear to be associated with HSUs 3a, 3b and 5. Velocities below 600 m/s have been revealed in the 3a and 3b HSUs (http://www.ce.Berkeley.edu/{approximately}ezzedine/DOE/paul.html). The authors believe that these anomalies are indicative of partial saturation. This explanation is supported by the water samples taken from pumping stations near the VSP well sites which appears to contain air bubbles. A gas analysis of water samples has not yet been performed. The authors hypothesize that this gas can be either air being sucked-in from the vadose zone above the water table, or from some chemical reaction. As a matter of fact, the natural water table level at this site was around 20 m below ground surface before any large scale pumping began, and had dropped to 25.5 m, in April 98. Furthermore, some of these low velocity zones are occurring not only in the major free-flowing sand or gravel parts of the HSU, but in boundary layers of silty sand either above or below the main HSU conductor. An electromagnetic survey was conducted at the site during June 4--25, 1997, and they worked on its interpretation. Seven cross well EM data sets were collected. Both 1D and 2D simulations, approximating the actual site and survey setup, were conducted. The 1D simulations were conducted using the code EM1D for one data set. Newman and Alumbaugh''s 3D forward code was used to simulate the response of both a resistive layer (representing HSU 4) and a conductive layer at the same location. Three separate inversion algorithms were applied to the data: Newman and Alumbaugh''s 2.5D finite difference and integral solution algorithm, Alumbaugh''s iterative Born approximation with a cylindrical symmetry, and Tseng''s 3D extended born approximation. The field EM data are still being analyzed.'
- Research Organization:
- Univ. of California, Berkeley, CA (US)
- Sponsoring Organization:
- USDOE Office of Environmental Management (EM), Office of Science and Risk Policy
- OSTI ID:
- 13555
- Report Number(s):
- EMSP-54655-98; ON: DE00013555
- Country of Publication:
- United States
- Language:
- English
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