Kansas Geological Survey, Open-file Report 2016-26
KGS Open File Report 2016-26
June 2016
This applied research project correlated measured shear-wave velocities with the condition of rock above dissolution voids, targeting the stress as approximated from the shear-wave velocity of the overburden. Shear-wave velocities were estimated during this study using passive surface wave methods with data acquired along two profiles located on or near two key abandoned brine production wells. Multichannel analysis of surface waves (MASW) was used to estimate the shear-wave velocity, loosely map stratigraphic contacts above the top of the "three finger" dolomite, and evaluate the relative difference in the rock above possible salt jugs associated with the wells compared to rocks above undisturbed salt. Comparison of shear-wave velocity profiles over time (time lapse) was used to provide insight into void dynamics and overburden stability.
Passive MASW profiles were acquired near the Irsik & Doll grain elevator, along William Street, and south of the BNSF Railroad in Hutchinson, Kansas, in 2015. In total, five lines and a 2-D grid of receivers were positioned over wells 2A, 4A, 4B, 6B, 7A, 7B, 52, 53, 59, and 60. Continuous sampling was utilized to record and allow for evaluation of all available sources of passive source energy, ensuring optimal source orientation and surface wave characteristics for each line. Surface waves with frequencies as low as 4 hertz (Hz) were recorded with an average depth of investigation 55 meters (m), and in some places exceeding 80 m, successfully sampling deep beneath the bedrock surface at a depth of around 20 m.
With shear-wave velocity being a function of shear modulus and density, and shear modulus the ratio of stress over strain, it is possible to estimate relative stress of overburden rocks (shear modulus) by shear velocity values. Local increases in shear velocity without changes in lithology can be equated to increased stress associated with overburden roof load over dissolution jugs. Relative shear velocity lows may be associated with collapse features whose vertical movement has been arrested by bulking, reduced stress to within roof rock strength, or changes in strength due to geologic features related to natural variation in deposition or erosion.
Reasonable consistency in the surface-wave dispersion patterns over well 2A between November 2014 and May 2015 surveys suggests the obvious change observed in key elastic physical properties of the overburden between the March 2013 and November 2014 is real. The disturbed volume from approximately 25 m below ground surface, down to the maximum sampling depth of these studies, appears to have stabilized since the November 2014 survey. An incremental change appears to have resulted in reduced strength and/or redistributed stress in the overburden above the well 2A void between the November 2014 and May 2015 surveys. This change in strength or stress appears to be limited to depths greater than 25 m (~10 m beneath the top of bedrock). A marked reduction in apparent velocity between the November 2014 and May 2015 surveys is prominent in bedrock between wells 2A and 4B. The velocity trend south of well 4B is consistent with the March 2013 baseline survey, suggesting a normal/stable stress regime in this area. In this transition zone between wells 2A and 4B, the reduced velocity may be related to interference of multiple surface-wave modes as the wavefield transitions from low velocity at well 2A to normal velocity south of well 4B.
Changes in dispersion patterns representative of subtle drops in sub-bedrock shear velocity at well 52 along the William Street line between October 2012 and June 2015 suggest some kind of incremental change has occurred above the old dissolution jug that may have reduced strength and/or redistributed stress in the overburden, such as a failure related to a vertically migrating void. This change in strength or stress appears to currently be confined to depths greater than 28 m below ground surface--approximately 18 m beneath the top of bedrock--and may have a lateral extent of up to 50 m, although this is uncertain and likely an over-estimate. Shear velocity of bedrock at well 53, although elevated in the June 2015 survey, is consistent with the October 2012 survey. This suggests that the void at this well was stable during the time between surveys.
No significant increases or decreases in shear velocity were observed along the BNSF railroad line that intersects wells 7A and 4A or any wells sampled along William Street, with the exception of well 52 as noted previously. A decrease in surface-wave penetration depths coincident with well 7A (BNSF railroad line) may be related to a localized zone of reduced shear velocity at depths of 45 m or greater. However, this interpretation is relatively low-confidence and reduced surface-wave penetration likely represents natural geologic variation. Overall, there appears to be no change above wells 6B and 7B previously surveyed in the vicinity of the Irsik & Doll elevator and the first-time surveys over BNSF railroad proximity wells have no indications of elevated shear velocity relative to surrounding rock layers that exceed reasonable variations for this area.
Future monitoring of the subsurface around wells 2A and 4B near the Irsik & Doll elevator and wells 52 and 53 along William Street is advisable in lieu of invasive investigations, providing direct measurements and the potential for remediation, if necessary. We propose a new field layout designed to incorporate all designated wells near the Irsik & Doll grain elevator, William Street, and south of the BNSF Railroad using optimal acquisition parameters (i.e., line lengths) determined during previous surveys. Considering the changes in shear velocity observed between surveys, it would be advisable to perform repeat MASW surveys on an annual basis. If change is observed and appears to be accelerating or migrating vertically, then shorter lags between surveys would be advisable. In light of the history of the jugs in question, proximity to surface structures near wells 2A and 4B, and known or suspected migration into the shale overburden near wells 52 and 53, an annual monitoring program continuing through 2018 should be considered before reevaluating subsurface condition. Future evaluations should focus on consistency between annual surveys and relative shear velocity over the jugs compared to native areas around the site.
To view this map, you will need the Acrobat PDF Reader, available free from Adobe.
Kansas Geological Survey
Placed online April 11, 2017
Comments to webadmin@kgs.ku.edu
The URL for this page is http://www.kgs.ku.edu/Publications/OFR/2016/OFR16_26/index.html