Seismic imaging of a salt-dissolution void in Central Kansas

Abstract A subsurface anomaly in McPherson County, Kansas, was caused by salt dissolution following a casing breach in a brine wastewater disposal well during the1980s. The breach led to the formation of a void and possible subsidence within and above the 150-meter-deep Permian salt layer. While borehole camera images confirm the presence of the air-filled void, limited camera illumination restricts conclusive measurements of its horizontal extent. High-frequency seismic reflection data were acquired to delineate the extent of the void and assess potential surface subsidence. To achieve a more accurate mapping of the void, a 2D elastic multi-scale Full-Waveform Inversion (FWI) technique was employed, which involved analyzing seismic body waves with frequencies ranging from 20 Hz to 50 Hz and a maximum source offset of 1050 meters. First-arrival travel time tomography refined the initial velocity model above the salt layer, while automated first arrival picking from seismic shot gathers was achieved through a custom-trained Convolutional Neural Network. The FWI velocity updates accurately locate the salt cavity, estimating it to be around 100 meters wide and over 50 meters tall, consistent with historical invasive measurements. Cavities of this nature can pose significant environmental or public safety risks, since potential subsidence or collapse could lead to ground instability and negatively impacting groundwater or infrastructure. Consequently, FWI and high-resolution reflection imaging serve as complementary tools for delineating and monitoring these salt-related anomalies. This specific salt interval is heavily used in this area for seasonal storage of liquid petroleum, solution and room and pillar mined for salt consumption, and highway applications, but is unsuitable (uneconomical) for energy or CO2 storage due to its shallow depth and lack of potential for high pressure fluid containment. However, this study helps establish a dedicated workflow for effective monitoring of deeper, reservoirs more suitable for high pressure storage.