3D Seismic Reflection and Tomography Experiment

Hill Air Force Base, Utah
July-August 2000

Experiment Description |  Photographs and Figures

RICE 3-D Reflection and Tomography Experiment: Hill AFB, OU-2, Utah
 July-August 2000

Alan Levander and Colin Zelt
Department of Earth Science
Rice University, Houston, TX 77005

Sponsor: Department of Energy, Environmental Management and Science program

ABSTRACT

 In July and August 2000 we conducted 3-D reflection and 3-D tomography experiments, 2 vertical seismic profiles, and 6 check shot surveys at Hill Air Force Base Operable Unit 2 (OU-2). The surveys were designed for environmental characterization of a shallow (< 20 m) aquifer contaminated with trichloro-ethene, a dense, nonaqueous phase liquid (DNAPL). The DNAPL ponds in low points of a paleochannel incised in a Quaternary clay that has been covered with 5-20m of Quaternary sands, gravels, and clays. The goal of the seismic experiments is to characterize the base of the paleochannel with resolution of 20-40 centimeters to aid placement of wells to be used in further remediation efforts.

The seismic experiments made use of 624 Texan portable seismographs, and 2 Geometrix multi-channel seismographs. For sources we used 223 caliber single shot rifles, fired in 6 cm drillholes. The Texan seismographs were deployed in 6 parallel lines with cross-line separations of 2.1m and with inline geophone spacing of 35 cm. Shots were fired in a rotated staggered brick pattern, with ~120 shots/line. Forty-six seismic lines were occupied, producing a survey area of 94.5 by 36.05 m. The same area was covered by a 3-D tomography experiment in which 600 seismic receivers were placed at 2.8 m increments inline and 2.1 m increments cross line on a staggered grid. Shots were fired at every receiver position.

Lastly we acquired six check shot surveys and two vertical seismic profiles using the University of Texas El Paso 3-component downhole instruments. For the checkshot survey, surface shots were recorded by receivers spaced at 0.5m increments in 6 different boreholes. For the VSPs the same receiver increment was used in the boreholes, and surface sources were fired every 0.7 m to offsets of 21m.

Overall the data quality is excellent. The 223 rifle produces a broad bandwidth (50-350Hz) signal of large amplitude. In all we acquired approximately 4500 shot records, totalling ~40 Gbyte of data. The project was funded by the Department of Energy Environmental Management and Science Program. We received considerable support fromÊ from IRIS/PASSCAL, the University of Texas at El Paso, Duke Engineering, URS, and the U.S. Air Force. The field crew consisted of 20 people from a large number of U.S. and international universities, as well as from IRIS, UTEP, and Rice.

Photographs & Figures

Seismic Targets | Survey Geometry | Sunrise | Seismic Sources

3-D Reflection Experiment | VSP Survey | Sunset

Target

The goal of the 3-D study is to image the paleochannel incised in the top of a clay aquitard that serves as a structural trap for DNAPL. The map below shows clay depth estimates from the interpretation of well data (triangles). Despite the exceptional well control, the paleochannel is not adequately characterized for full environmental remediation. The 3 seismic lines shot in the 1998 pilot study are shown, as is the main treatment pipe.

Survey Geometry

Every 5th geophone location is shown as a blue circle, every sixth shot is shown as a red circle. South is at the bottom of the figure

Sunrise over the Porta-Potties

Seismic Sources

The 223 source with the geomtrics trigger and a Texan

Karin with the 223 source
Jojanneke holds the bullets

Sharleen fires the source

3-D Reflection Experiment

Pavel prepares for the day's shooting.

3D reflection spread looking east toward the Wasatch front. The green Texan seismographs and orange geophones are seen on the 6 receiver lines. The shotlines are outlined by the white plastic shothole liners.

Shooters are in the center of the spread. Timing of the shooting is controlled from the sunshade. Temperatures reached 105F.

The Reftek Texans with 40Hz geophones

VSP and Surface Profiling

The 2 UTEP downhole tools: each has a 3-component geophone

The surface seismic spread runs between 2 wells with downhole tools in them.

Shooting at the well.

Sunset