Shear‐Wave Velocity as an Indicator of Increased Stress and Failure Potential Associated with Dissolution‐Mining Voids

Abstract

The use of dissolution wells for mining salt has been common practice for over a century, leaving behind brine-filled "salt jugs" or voids in the subsurface which, over time, can migrate through overlying rock formations, potentially leading to sinkhole formation and public safety hazards. In an effort to determine the relative range of stress on the roof rock above these jugs at various stages of failure, evaluate the extent of void migration, and aid in remediation planning, shear-wave reflection surveys were conducted at a well field near Hutchinson, Kansas, where drill-confirmed dissolution features exist in the 125-m deep Hutchinson Salt Member. Since shear-wave velocity (V s) is directly related to stress through the shear modulus, V s profiles were analyzed at known well locations to determine if measurable variations in velocity were consistent with zones likely experiencing elevated relative stress and therefore, increased failure potential. Changes in V s upwards of 20% were observed along lines where voids were known to be present and had experienced significant migration into the shale cap rock. Localized velocity changes were coincident with the findings of drilling and associated borehole investigations. Alluvium velocities for the same lines remained relatively constant, suggesting that V s changes are constrained to consolidated rock above mapped subsurface dissolution voids and are not related to depositional changes in the unconsolidated overburden.