High-Resolution Seismic Reflection Investigation
Of the Subsidence Feature on U.S. Highway 50 at Victory Road
Near Hutchinson, Kansas
Richard D. Miller
High-resolution seismic reflections were used to map the upper 800 ft of sediments around and below an actively subsiding sinkhole currently affecting the stability of U.S. 50highway in Reno County, Kansas. Primary objectives of this study were to delineate the subsurface expression of this growing salt dissolution induced sinkhole and appraise the sinkhole’s threat to both highway stability and the highway’s characteristically heavy traffic load. The high signal-to-noise ratio and resolution of these seismic reflection data allowed detection, delineation, and evaluation of rock failure and associated episodes of material collapse into voids left after periodic and localized leaching of the 400 ft deep, 125 ft thick Permian Hutchinson Salt member. Mechanisms and gross chronology of structural failures as interpretable from stacked seismic sections suggest initial subsidence and associated bed offset occurred as accumulated stress was rapidly released within a tensional dome defined by reverse fault planes. As the downward movement (settling, relaxation) of sediments slowed with little or no incremental build up of stress, gradual subsidence continued in the subsurface, advancing as an ever-expanding bowl, geometrically defined by normal fault planes. Several episodes of subsidence are evident in dissolution related features (current and paleo) imaged on these two ¾-mile long seismic profiles. The rate of destabilization and failure as well as the load bearing potential of the rock layers above zones of dissolution are strongly 2 influenced by both the original subsidence geometries and dimensions as well as the subsequent reactivation of subsidence along the profiles. The sinkhole at the intersection of U.S. 50 and Victory Road is probably related to the reactivation of natural salt dissolution processes that produced the seismically imaged, 1500 ft wide paleosubsidence feature interpreted to have been active during Tertiary and/or Quarternary. Alternately, this current sinkhole could be from delayed failure of overlying Permian rock layers bridging voids or rubble areas in the salt for hundreds of thousands of years following the initial Tertiary to Quaternary subsidence.Interpretation of reflections from key stratigraphic horizons suggests initial plastic deformationof rock layers over dissolution voids was followed by roof rock failure along reverse fault failure planes within an earth volume known as the tension dome. The original tensional dome was centered on the dissolution volume and extended from the base of the salt interval to nearthe ground surface. A long period (a significant portion of late Tertiary and early Quaternary) of relaxation of stress associated with layers outside the tensional dome remaining after failure occurred along normal fault planes. A much smaller tensional dome located at the western extreme of the original tensional dome controls recent and current subsidence. Subsidence associated with failure defined by this most recent dome has followed a somewhat asymmetric path from the salt to surface. With a subsidence history at this site potentially extending as far back as mid-Tertiary, it is unlikely subsidence will end within the next millennium. Until the highway started sinking at this location during or just prior to 1998, little if any subsidence seems to have been associated with this buried sinkhole throughout late Quaternary. This long period of inactivity followed by the localized, rapid subsidence currently observed at this site makes it reasonable to expect other small sinkholes will form without warning above this or other similar paleofeatures in the area. Considering the interpreted bed geometries, slumping of the ground surface at the intersection of Victory Road and U.S. 50 will likely continue gradually along the northern and eastern edges of the seismically defined paleostructure, elongating the sinkhole in those directions. Besides the obvious disruption to the road system, unfortunately this subsidence feature provides a pathway for fresh waters of the Equus Beds to come in contact with the more brackish waters of the Permian, or vice versa. Sufficient bridging and undercompacted rock layers still exist beneath this sinkhole to sustain the current gradual subsidence rate of around 1 ft/yr for several years to come. If salt dissolution has begun again at this site—anthropogenic or natural—it is not possible with these data alone to confidently identify a fluid source or pathway. However, with the superimposition or juxtaposition of this modern sinkhole and the mid-Tertiary to early Quaternary subsidence feature, and considering the nearest disposal well with a history of fluid containment problems is more than 1.5 miles away, the sinkhole is likely natural in origin. Unfortunately, considering the long history of oil field disposal well-induced dissolution in this area and the proximity of this particular site to the natural dissolution front, neither of these catalysts or fluid sources can be completely ruled out. Apparent undulations in the surface of the Hutchinson Salt layer could be indicative of dissolution features bridged by undisturbed rock layers with a span not yet sufficient for the accumulated load to instigate roof failure or creep. Roof rock failure above voids associated with excessive leaching of salt can proceed at varying rates and affect different portions of the over- 3 lying rock column. These undulations could also be indicative of changes in water chemistry (salinity) during or near the conclusion of salt deposition in this area. An apparent halt in the upward movement of a dissolution feature (downward movement of sediments) at the boundary between the Ninnescah Shale and the Upper Wellington Shale provides insight into the effective unsupported span of roof rock these shallow shale layers can support before failure, and, therefore, the size of the tensional dome to be expected along the dissolution front. This study also evaluated the effectiveness of using high-resolution vibroseis on the shoulder of U.S. 50 when traffic was slowed but not stopped. Previous data collected in this area was acquired along a quiet, east/west county road 1 mile south of U.S. 50 using a small recording channel seismograph and an invasive, low energy, impulsive source survey. Equivalent dominant frequencies were recorded on both surveys, with the more recent efforts resulting in significantly greater energy penetration and a signal-to-noise ratio sufficient that usable data were recorded regardless of background noise levels. The bed resolution, coherency of bedding within subsidence features, and overall signal-to-noise ratio were greatly improved using minivibroseis survey techniques.
This report also includes three appendices which include field photographs and printouts of PowerPoint presentations of this study and seismic reflection profiling in general.
SAGEEP 2002 Poster Presenation 13.5
Full Paper 2002-17 US50Text.PDF 8.57 MB
Appendix A 2002-17 US50 ApdxAPDF 1.7 MB
Appendix B 2002-17 US50 ApdxBPDF 13.6 MB
Appendix C 2002-17 US50 ApdxCPDF 886 KB