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VSP Recording

Seismic Recording

The goal of the seismic reflection research was to determine whether high concentrations of DNAPL (CCl₄) in the subsurface at 200 West Area could give rise to seismic amplitude anomalies. If such amplitude anomalies exist, the distribution can be mapped and calibrated versus borehole measurements. A secondary goal was to map the subsurface geology and delineate probable preferential pathways for DNAPL transport.

The research was conducted in three phases: (I) site evaluation and seismic modeling studies to determine the likelihood that an amplitude anomaly exists and its probable manifestation, (II) seismic data acquisition to image areas of probable high concentration of DNAPL, (III) data processing and interpretation of the seismic data.

Evaluation of existing geological, chemical, and hydrological data at 200 West Area indicate that the highest concentrations of DNAPL occur at the contacts of the Handford Fine unit/Pilo-Pleistocene unit and the Plio-Pleistocene unit/caliche layer. Because no subsurface velocity information was available, P and S-wave vertical seismic profiles were recorded in three wells. Density information was used from previously published measurements. No cores were available to do velocity/DNAPL saturation measurements; thus, mathematical models based on Gassmann's theory of fluid saturated media were input in to the seismic model.

Seismic amplitude versus offset (AVO) models were constructed using Shuey's approximation to the Zoeppritz equations to study the reflection response from these contacts with and without the presence of DNAPL. The models indicate that the reflection coefficient between the Hanford Fine and the Plio-Pleistocene should be strongly positive and change to weakly negative in the presence of DNAPL in the Hanford Fine. In contrast, the reflection coefficient between the Plio-Pleistocene and the caliche layer is moderately positive and becomes strongly positive if CCl₄ is saturating the Plio-Pleistocene.

The results of the model study strongly suggested that the presence of high concentrations of DNAPL at the target horizons could cause detectable seismic amplitude anomalies. Furthermore, the models indicated that the seismic frequencies greater than 100 Hz would be need to resolve the interfaces.

Phase II - Seismic Acquisition

In March 1999 four seismic lines were collected by ESRI around the 216 Z-9 crib. Because the caliche layer is a strong acoustic boundary near the DNAPL contaminated layers, the recording parameters were optimized to image this reflector. Other considerations included having adequate temporal and spatial sampling necessary for high-resolution recording and the need to attenuate acoustic noise. The station spacing was set to 1 meter and 120 channels were recorded in an asymmetric split spread geometry. A single 12,000 lb. peak force vibrator sweeping 30-300 Hz was used with shot spacing of 1 meter to yield 60 fold data.

(Line Z-9-1 seismic recording)

During seismic acquisition data were stacked daily for QA/QC purposes. These stacks were essential to ensure that survey objectives were being met and that the data quality was consistent throughout the survey. Ground roll was found to be quite severe to the extent that raw field records showed no discernible reflected energy. Extensive detailed data processing was required to obtain useful images. The final stack sections present a detailed high-frequency (250 Hz) image from the near surface to the middle Ringold Formation (100+ m). No reflection could be positively identified as the top of the basalt.

Structural contour maps were prepared from the seismic data for the top of Plio-Pleistocene and top of caliche layers. Seismic amplitude maps were prepared for these intervals and compared to the corresponding CCl₄ isoconcentration map. These comparisons show a correlation consistent with the seismic model results. That is, low and negative amplitudes of the Hanford Fine/Plio-Pleistocene event correspond to high concentration of CCl₄. High amplitude values along the caliche layer correspond to high concentrations of carbon tetrachloride.

The results appear promising that high DNAPL concentrations can be imaged in this area. However, validation of these assertions and the model requires test boreholes. Analysis of these data is ongoing (3/2000). 

Line Z-9-2 enhanced amplitude display. TC = Top Caliche. TPP = Top Plio-Pleistocene