High Resolution Seismic Reflection Survey near SPR Surface Collapse Feature at Weeks Island, Louisiana
Richard D. Miller, Jianghai Xia, Joe M. Anderson, David R. Laflen, Jeffrey M. Erickson, Patricia M. Acker, Mary C. Brohammer,Don W. Steeples, Ross A. Black


Summary
Shallow, high resolution seismic reflection techniques detected the subsurface expression of a 40 ft wide and 30 ft deep sinkhole above DOE's Strategic Petroleum Reserve (SPR) storage cavities at Weeks Island, Louisiana. The underground cavities that presently hold 73 million barrels of crude oil are two levels of a former room and pillar salt mine. The mine is within a salt dome that at the sinkhole is approximately 180 ft below the ground surface and responsible for the more than 125 ft of topographic uplift that produced Weeks Island. Four nominal 180 shotpoint, 24-fold P-wave CDP lines acquired with 8 ft station spacing possess interpretable reflections with an average dominant frequency of approximately 80 Hz and apparent NMO velocities ranging from 1350 to 2100 ft/sec. The field recording parameters and quality control were based on the reflection interpreted during walkaway tests to be from a reflector about 150 ft deep. This coherent reflection event interpretable on all four lines is dramatically altered on the east/west line in the proximity of the sinkhole. An offset in the prominent reflection interpretable on all four lines traces a southwest/northeast trend across the study area. The disturbed reflection on the east/west line, presumed associated with the sinkhole, structurally resembles a chimney type feature or possibly a water table drawdown.
Introduction
This seismic reflection survey was designed to detect and delineate geologic or hydrologic features associated with the small sinkhole discovered on May 18, 1992, as well as identify areas potentially susceptible to subsidence above the SPR storage cavities. The two-level underground cavities, presently holding 73 million barrels of crude oil, are a former room and pillar salt mine. The old mineworks range in depth from about 500 ft to more than 700 ft below the ground surface. The top of the salt dome is approximately 180 ft below the ground surface at the sinkhole and is responsible for the more than 125 ft of topographic uplift that produced Weeks Island. The water table near the sinkhole is about 90 ft below the ground surface. The absence of a cap rock (at least in a classic sense) leaves the salt boundaries of this dome in direct contact with overlying unconsolidated marine sediments. At the time of this survey the sinkhole was approximately 35-40 ft in diameter, slightly more than 30 ft deep, and had surface expression within 50 ft of the south side of Morton Road (Figure 1). The proposed high-resolution seismic survey originally consisted of five lines, each approximately 1500 ft in length: four P-wave (compressional wave) lines and one S-wave (shear wave) line. The lines were laid out to maximize the potential of imaging the subsurface expression of the present sinkhole and to identify other areas that might be susceptible to subsidence. The primary target was the top of the salt dome. The secondary target was the water table, followed by structural or stratigraphic signatures within the salt dome. Dissolution features associated with evaporite beds have produced easily interpreted signatures on shallow seismic sections (Miller et al., 1993; Steeples and Miller, 1987; Steeples et al., 1986). Based on work at other salt domes (Black and Voigt, 1982; Leading Edge, August 1994 issue), a very irregular salt/sediment contact is likely at this site. Direct imaging of the salt-sediment contact has proved challenging on previous surveys at this salt dome (Kinsland and Rutter, 1994). Irregularities in the surface of the salt associated with either dissolution or joints and their expression in overlying sediments are of particular interest. The acquisition portion of the seismic reflection survey was conducted between March 2 and 4, 1994. The project consisted of several walkaway noise tests and four nominal 180 shotpoint, 24-fold P-wave CDP lines (Figure 1). The surface conditions varied from heavily wooded (hand-cleared 5 ft wide path) to manicured lawn to asphalt roads. Some secondary clearing was necessary along wooded paths previously cleared for elevation surveying. The asphalt and gravel roads were obstacles that not only resulted in minor reductions in fold but also provided a source for traffic noise. No shots were recorded while vehicles were close enough to active recording stations to produce more than 0.1 mV peak-to-peak of background noise. Underground utilities including a petroleum pipeline, a propane pipeline, high voltage power lines, a fiber optic communications link, standard telephone lines, and water lines inhibited continuous coverage across several sections of the four lines. The ground surface was damp, with several significant topographic and cultural obstacles including ditches, relatively steep terraces, partially buried foundations from previous surface structures, and about 55 ft of relative elevation change on lines 1, 2, and 3. The field recording parameters and quality control were based on the 150 ft deep reflector interpreted during walkaway tests on the north end of line 1. Future borehole logging, both geologic and geophysical, based on the surface seismic data should greatly enhance the quality and quantity of seismic interpretations.

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