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CONTENTS:ABSTRACTTwo three-dimensional (3-D), high-resolution seismic reflection pilot studies were conducted in California at two sites, where the primary contaminants of concern are solvents (volatile organic compounds). The objectives of the surveys were to:
The second site, in Southern California, is located in a broad, synclinal depression in the Transverse Range. Shallow alluvium overlies a marine turbidite sequence that crops out as massive sandstone beds. Field work for both surveys took place in August 1992. A Bison Model 90120-A, 120-channel (DIFP) seismograph was used to record the data. Thirty-hertz, natural-frequency geophones were used to receive the data, and an Elastic Wave Generator (EWG) was used as the seismic source. The use of a signal-stacking, noninvasive source was found to be an effective method of overriding background noise at the sites. Prior to the commencement of the 3-D pilot studies, a two-dimensional (2-D) profile was recorded to test the acquisition parameters, which included the geometry of the survey, digital sample rate, and analog filter settings. The data were monitored in the field with a Bison 486 Explorer outdoor computer. The 2-D data were processed and displayed in the field. Both sites displayed coherent seismic reflections from the depths of interest on the field-stacked sections. The seismic data were transferred to a nine-track tape. Each survey required about 2 days to perform. The first survey covered an area of about 300 x 300 ft and consisted of 26,000 waveforms. The second survey was approximately the same size and used 7,200 waveforms. The surveys were processed and displayed using Mercury International Technologies (MIT) Software and a Perkin-Elmer hardware platform so that the structural and stratigraphic features, which may act as conduits for the contaminants, could be viewed from all perspectives. The more complete picture which these surveys yielded could not be duplicated with traditional 2-D seismic methods combined with well log correlation. Future plans include using this data to more precisely locate remedial systems and to extend the use of the techniques to other areas.
INTRODUCTIONIn designing a remediation system, especially if pump-and-treat technology has been chosen for ground water cleanup, it is vitally important to understand the subsurface geology. Often, the geology is heterogeneous and the success of the remediation is dependent upon placing the recovery wells in optimum locations of preferential contaminant transport. Recently, through the use of two-dimensional (2-D) and three-dimensional (3-D) high-resolution seismic techniques, it has become possible to image the subsurface at shallow depths that are meaningful for environmental cleanup. In 1988, seismic imagery techniques were used to assist in designing a pump-and-treat remedial system in fractured bedrock (Adams et al. [1]). In 1992, a 3-D high-resolution seismic reflection survey was performed for the U.S. Geological Survey/Environmental Protection Agency (EPA) at Haddam Meadows near Hartford, Connecticut. Data are still being processed from this study that showed a meandering paleochannel above bedrock. Late in 1992, 3-D pilot surveys were performed at two sites in California (Figure 1). The surveys were performed to determine the feasibility of using these techniques at geologically complicated sites to:
SITE HISTORY AND GEOLOGYLawrence Livermore National LaboratoryLawrence Livermore National Laboratory (LLNL) is a research and development facility owned by the U.S. Department of Energy and operated by the University of California. Initial releases of hazardous materials to the environment occurred in the mid- to late-1940s when the site was used by the U.S. Navy as an air training base. Since 1950, additional releases have occurred due to localized spills, landfills, surface impoundments, and leaking tanks. In 1983, organic solvents were discovered in ground water onsite and offsite by LLNL. In 1987, LLNL was added to the EPA National Priorities (Superfund) List.The LLNL site and surrounding vicinity are underlain by hundreds of feet of unconsolidated Tertiary and Quaternary alluvial sediments overlying Mesozic basement rocks of the Franciscan Assemblage. The younger sediments are on the order of tens to hundreds of feet and include lacustrine, alluvial fan, and channel deposits. Rapid lateral and vertical facies changes are common, with most of the contaminants transported in the coarse-grained buried stream channels at depth. The site is bounded by the Greenville Fault to the east and the Las Positas Fault to the south, both of which are considered seismically active. Although older ancestral faults have been postulated to underlie the site (Carpenter et al., 1984 [2]), no active faults are present at the site. Depth to ground water varies from over 110 ft in the southeast corner of LLNL to about 30 ft in the northwestern corner. Ground water generally flows to the west, but locally to the south and southwest. Substantial vertical hydraulic gradients exist in some portions of the site. Pumping tests indicate horizontal hydraulic communication in the more permeable sediments on the order of hundreds to thousands of feet, and vertical communication on the order of tens of feet. Hydraulic conductivities of the subsurface sediments vary over seven orders of magnitude. Contaminants are generally confined to the upper 200 ft, the primary contaminant of concern being tetrachloroethylene.
Site North of Los AngelesSolvent contamination (primarily trichloroethylene) was discovered at this site in 1984. The plume of contamination was found to underlie much of the site's 2,700 acres. Nearly 200 wells were installed to monitor the contaminant plume. By 1991, a treatment system had been installed to treat ground water from the aquifer so that the contaminants could be contained onsite. However, the pumping of an offsite well east of the facility may have drawn the plume offsite. The pilot 3-D seismic survey was performed to correlate the geology between areas of known contamination and where it may be spreading to the east.The site is located in the northern part of the Transverse Range in a broad synclinal depression. The formation beneath the site is composed of a Cretaceous marine turbidite sequence of sandstone interbedded with siltstone/mudstone and conglomerate lenses (Colburn et al., 1981 [3]) called the Chatsworth Formation. At the surface, the site is characterized by massive sandstone beds overlain in some areas by shallow alluvium up to depths of about 30 ft. The arkosic sandstone is interbedded with siltstone and claystone and cemented with carbonate. Zones of weakness, some of which have eroded to canyons infilled with alluvium, are formed by fractures or faults or siltstone/claystone outcrops. The beds dip to the northeast at 20 to 30 degrees. Fault and fracture zones are prominent at the site and are often expressed as surface lineaments that extend a mile or more. Three main strike directions are present for the lineaments: northwest to southeast, northeast to southwest, and east to west. A shear zone trending northeast to southwest borders the area to the north where the pilot seismic study was performed. Ground water is present in a shallow, unconfined zone represented by alluvium, weathered bedrock, and faulted rock, and in a deeper regional system in the fractured Chatsworth Formation. The two zones are hydraulically interconnected. The shallow ground water is found mostly at depths of 30 ft. The zone is discontinuous and may be only seasonally wet. Static water levels in the deeper aquifer vary from 2 to 370 ft over the site. The formation is nearly impermeable except where fractures are located. No wells exist in the area where the seismic survey was performed; however, a contaminated well about 1,000 ft south of the survey shows a static water level of 42 ft. This well was drilled to a depth of 120 ft in fractured rock with 4 ft of alluvium near the surface.
Field WorkField work commenced in August 1992 at LLNL in a cooperative effort between LLNL, Resolution Resources, and Rutter & Willbanks. A Bison Model 9120, 120-channel (DIFP) seismograph was used to record the data. Thirty-hertz, natural-frequency geophones were used to receive the data, and an Elastic Wave Generator (EWG) was used as the seismic source for both surveys. Since both sites were acoustically noisy, the use of a signal-stacking, noninvasive source was very important. The use of the EWG permitted the noise at the sites to be canceled as the signal-to-noise ratio was enhanced. The use of a noninvasive source allowed the survey to proceed in a more timely fashion, with the source cycling approximately every 6 seconds. On an average, eight impacts were used at every station. Also, the source provided for increased safety with no exposure to the contaminants present.Prior to designing the 3-D survey, a 2-D line was recorded at each site with tight geophone spacing at 4-ft intervals. A noise test was conducted to determine the acquisition parameters, which included the geometry of the survey, digital sample rate, and analog filter settings. The data were monitored in the field with a Bison 486 Explorer Outdoor Personal Computer (PC). The 2-D data were processed and viewed in the field. Coherent seismic reflections were displayed in the stacked sections for each site. Once 2-D stacked sections (Figures 2 and 3) were processed for each site, the 3-D survey was implemented. Each survey required 2 days to perform. A grid of geophones was laid out with cables connected directly to the 120-channel seismograph. This enabled an area approximately 300 by 300 ft to be surveyed. At LLNL, 26,000 waveforms were used. At the Los Angeles site, 7,200 waveforms were used. The LLNL site was characterized by a grid with a 55-ft shot point spacing and 27.5-ft receiver spacing (Figure 4). At the Los Angeles site, a 50-ft shotpoint interval with a 25-ft receiver spacing was utilized. The geometry of the grid differed to conform to conditions unique to the site. The source impacted the ground along a grid whose nodes were centered halfway between geophone and halfway between where the lines were positioned. The source was employed outside the grid so that greater fold (volume of data) could be developed near the edge of the grid.
Processing the DataThe 2-D data sets were processed in the field using Seistrix 2-86 by Interpex, Golden, Colorado, and displayed using the Bison Explorer PC.The 3-D field data were stored in the seismograph on two separate rugged hard drives. At the end of each day, the information was transferred to the Bison PC. The PC was used to convert the data from a proprietary Bison format to standard, oil industry format, Society of Exploration Geophysicists format Y (SEG-Y). The SEG-Y data were transported to a nine-track tape via the small computer system interface (scsi) port on the Explorer. The tapes were then shipped to Trend Technologies in Midland, Texas, where Mercury International Technology (MIT) IXL software was used to sort the field traces into bins. Bins are files that contain field traces with common reflecting points. The binned data were analyzed and corrected for statics: the velocity was analyzed; the digital data were bandpass filtered; the normal move out was corrected, stacked, migrated, and output to a 3-D SEG-Y file. This 3-D SEG-Y file was stacked, and traces were arranged as a series of in-lines (parallel to the X axis) and cross lines (parallel to the Y axis). The time series of the waveform (two-way reflection times) correlates to the depth (Z axis). This volume of data was displayed and analyzed by extracting certain in-lines, cross lines, and/or arbitrary lines, as well as time slices (depth slices) to trace amplitudes from coherent reflections. The data were then used to create 3-D perspectives. While the data at LLNL have been preliminarily processed, considerable additional processing still can be performed. At the second site, only time slices have been analyzed, and the data are still being processed.
Discussion of ResultsAt LLNL, the presence of buried stream channels, confirmed by drilling, were identified by the 2-D seismic reflection profile. The shape, orientation, and configuration of the channels were then delineated by the 3-D acquisition and processing.Three correlatable seismic features were identified, one of which is believed to represent a shallow unsaturated channel, one which represents the uppermost saturated channel, and a deeper horizon which is believed to represent the base of a known, broad, deeper channel sequence. Both saturated channels are primarily contaminated with low concentrations of tetrachloroethylene, with the greatest mass carried in the center, more permeable portions of the channels. Time-structure contour maps were constructed for each channel, and isochron (thickness) contour maps were constructed between the base of each channel sequence. In addition, 3-D plots of the subsurface expression of each horizon were also constructed (Figure 5). Although the 3-D processing is only preliminary at this time, the results are promising. Complications from possible ancestral faulting in the area, a relatively long shot to receiver distance, and extremely limited velocity control may limit the resolution of additional processing. The 2-D data at the site north of Los Angeles show massively bedded strata dipping to the northeast with a shallow cover of alluvium that deepens from about 60 ft near the eroded shear zone. The 2-D section (Figure 3) shows at least three fractures are present over a 480-foot section. By time slicing vertically through the site, using 3-D, it is possible to see what may be faulted or eroded zones, trending predominantly northwest to southeast and east to west, but also northeast to southwest. This correlates to information in the literature. The east to west zones become more prominent at a depth of about 700 ft. While these data are very preliminary and may require additional studies on more closely spaced grids, they suggest trends that change with depth throughout the study area as might be expected from the known geology. Since ground water is found only in eroded or fault zones in the bedrock, the knowledge of the location of these permeable zones, which act as contaminant pathways, may prove invaluable in understanding the hydraulic interconnections. Previously, it was neither feasible nor economically possible to characterize a bedrock aquifer in this detail.
CONCLUSIONSThe shallow 3-D high resolution seismic reflection pilot study recorded in 1992 indicated that this technology could be successful in assisting in the optimization of remediation at LLNL. Although recording parameters were not optimum due to time and financial constraints, the 3-D high resolution seismic modeling delineated the spatial distribution of seismic features observed on a 2-D seismic profile recorded within the study area that correlated with known buried stream channels, previously identified from extensive drilling.
While both 3-D surveys were pilot studies with only preliminary interpretations of data completed to date, they provided what appears to be valuable information on the heterogeneity of the subsurface geology. This information, once verified, can be used to determine likely migration pathways for contaminants. The data provide images without penetrating into known contaminated areas, give guidance for the optimum placement of extraction wells to minimize the spread of contamination into fine-grained sediments, and also render information from existing boreholes more valuable. The use of seismic techniques allows for the optimal design and placement of recovery systems.
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