Depth to clay layer map of OU2 based on the logged depth to clay from each of the 278 monitoring wells and 115 cone penetrometer readings. The data points are shown as are the locations of the three seismic profiles acquired during the 1998 pilot survey. The pipe is part of the remediation project. The white box approximately outlines the coverage of the 3-D survey acquired in August 2000.

Abstract

In July and August 2000, we conducted 3-D reflection, tomography, and downhole seismic studies at Operable Unit 2 (OU2) at Hill Air Force Base in Ogden, Utah. OU2 has been the subject of ongoing remediation efforts to remove dense non-aqueous phase liquids (DNAPLs) that contaminate a shallow (less than 20 m) aquifer.

The survey target is a paleochannel buried beneath Quaternary sands, gravels and clays that acts as a local trap for contaminants. The highly irregular, steep walled paleochannel was imaged by a pilot 2-D survey conducted in 1998, demonstrating the viability of seismic methods for investigating the upper 20m at this site.

The four week experiment included 3-D seismic reflection, 3-D tomography, six check shot surveys in 15m boreholes for velocity estimation, and two vertical seismic profiles. A 223 rifle was used as the seismic source in all experiments, producing frequencies from 40Hz to greater than 300Hz. Approximately 6000 shot records were taken in the various experiments; the experiment at times generated 9Gbytes of data per day.

The 3-D reflection and tomography experiments both occupied an area of 95m by 36m, and utilized over 600 RefTek Texan instruments. For the reflection experiment the instruments were deployed in swaths of six receiver lines 2.1m apart, with geophone intervals in the inline direction of 0.35m. The shots were fired on a rotated brick pattern with 0.35m shot intervals in the inline and crossline directions, producing a nominal 52 fold survey. Data quality appears to be excellent, despite a high level of cultural noise. The shot gathers show strong reflections with conflicting dips, characteristic of steeply dipping features.

The field effort involved approximately 20 people. A high level of field support was provided by IRIS/PASSCAL and UTEP.

This figure is a depth migrated image of the 1998 Line 2 rifle data. The velocities used for the migration were smoothed and depth converted RMS velocities used to create the initial stack. The interpreted top of the targeted clay layer is indicated by the green line. The depth to clay from interolated well data is shown by the brown line. The depth to clay from an adjacent well (U2-050) is indicated by the pink line (approx. 13m) as is the top of the groundwater table (9 m) (dashed blue line) for several wells in the vicinity.