Sinkhole progression at the Weeks Island, Louisiana, Strategic Petroleum Reserve (SPR) site Page: 7 of 20
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were studied analytically to complement the field data. A variety of geophysical methods were
employed to gain diagnostic information but met with limited usefulness.
Seismic reflection profiling identified an apparent deflection in the reflector near the sink-
hole center that at first was thought to be a hydrologic cone of depression (Neal and Myers,
1995). Later detailed study showed this reflector was more apt to represent a stuctural or mate-
rial discontinuity. But the perceived anomaly led the way to obtain detailed data that showed a
very flat piezometric surface near sea level within highly permeable sediments of the ancestral
Mississippi River delta.
Crosswell seismic tomography was conducted across the throat of the sinkhole through
four wells constructed in opposite quadrants outside the sinkhole. The velocity tomograms
showed a distinct low-velocity zone typical of saturated sediments below the surface sinkhole but
failed to reveal detailed throat geometry. The borehole locations in competent high-velocity salt
confirmed a strongly vertical structure at depth (Harding, 1994).
Self potential (SP) surveys that showed hydrologic streaming potential at another mine
sinkhole locality were attempted at Weeks Island. Although apparent anomalies were measured
near the sinkhole, their interpretation was uncertain, but thought to show downward hydrologic
flow along a planar sheet.
Gas mapping of trace hydrogen and methane was conducted to test connectivity with the
SPR mine or with anomalies within the salt. Although some anomalous areas around the sinkhole
were observed, they did not reveal definitive diagnostic information (LSU, 1994). Further.
evaluation of gas mapping methods are continuing, as later tests showed evidence of anomalies
along mine edges, and near anomalous zones (Carney et al., 1995).
Slanthole drilling directly into and below the sinkhole provided the most direct confirma-
tion of dissolution geometry as evidenced by the drilling of boreholes BH-7A and BH-9 (Figure
2). Slanthole BH-9, adjacent to the sinkhole, was drilled at a high-angle approach directly over
the top of the subsurface extension of the surface sinkhole expression. It extended below the top-
of-salt elevation encountered in the tomography holes. This wellbore provided the opportunity in
July 1994 for injection of rhodamine dye directly into the throat of the sinkhole. The dye, if de-
tected in the fill hole sump, would provide unequivocal evidence of hydrologic connection with
the mine. However, dye has yet to be detected in the mine, even after 30 months of monitoring.
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Neal, J.T.; Bauer, S.J. & Ehgartner, B.L. Sinkhole progression at the Weeks Island, Louisiana, Strategic Petroleum Reserve (SPR) site, article, November 1, 1995; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc620914/m1/7/?rotate=90: accessed May 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.