Kansas Geological Survey, Open-file Report 2016-25
KGS Open File Report 2016-25
June 2016
This applied research project correlated measured shear-wave velocities with the condition of rock above dissolution voids, targeting the stress as related to shear wave velocity of the overburden. Shear-wave velocities were estimated using passive surface wave data acquired along a total of six profiles that intersected 26 abandoned brine production wells and in proximity to an additional five wells. Multi-channel 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 strength of the rock above possible salt jugs associated with the wells. Recommendations for further investigation are suggested where voids are suspected to have the potential for vertical migration.
Passive MASW profiles were acquired during four nights of data collection near the V&S railroad right-of-way in Hutchinson, Kansas, on March 16-19, 2015. Six lines and a 2-D grid of receivers were positioned over key wells. 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 3.5 Hz were recorded, with an average depth of investigation 55 meters (m), and in some locations exceeding 70 m, successfully sampling deep within bedrock.
With shear-wave velocity being a function of shear modulus and density, and the shear modulus as the ratio of stress over strain, it is possible to quantify 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.
Shear velocities in the majority of the study area represent a normal stress regime and natural geologic variation. Increased depths of investigation and maximum surface-wave wavelengths were observed near wells 14B and 41, and between wells 17 and 42. Velocities are consistent with the shear-wave velocity trends expected for bedrock at this site and likely represent natural variability. However, these anomalous depths indicate that there is something unique to these areas, which may suggest elevated velocity at depth or an off-line anomaly. Elevated shear-wave velocity above wells 8B, 15B, 46, and possibly well 92 suggests elevated stress and voids with under-supported roofs. The relative velocity and limited span of elevated stress implied by size of the anomalies suggest that the threat of failure is likely not imminent.
Comparison of shear-wave velocity profiles collected over time (time lapse) provides insight into void dynamics and overburden stability as wells as time lapse variability associated with seasonal/non-geologic processes. Future monitoring of the subsurface around wells 1B, 8B, 14B, 15B, 17, 41, 42, 46, and 92 is advisable in lieu of invasive investigations, providing direct measurements and the potential for remediation, if necessary. Considering that shear velocity observed in March 2015 may indicate elevated shear stress and, thus, elevated risk for vertical migration--particularly at wells 15B and 46--it would be advisable to perform repeat MASW surveys annually. 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 and proximity to surface structures, an annual monitoring program continuing through 2018 should be considered before considering invasively evaluating 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.
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Kansas Geological Survey
Placed online April 11, 2017
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