Near-surface void characterization and sensitivity analysis using enhanced processing procedures on passive Multi-Channel Analysis of Surface Waves (MASW) data

Abstract

Enhanced processing procedures on passive multichannel analysis of surface-waves (MASW) data were utilized to identify velocity anomalies above known salt solution voids in Hutchinson, Kansas, likely caused by the changing stress field due to the migration and/or expansion of the void. Previous geophysical studies within the study area provided information about the origin of the dominant passive surface-wave energy, allowing for an optimal spread orientation consisting of both 1D survey lines and a 2D grid. Occasional passing trains throughout the night generated surface-wave energy ranging from ~4 Hz to ~20 Hz for most recorded events. The use of the 2D grid allowed for identification of the orientation of wave propagation to correct the high apparent velocities caused by the oblique source orientation. Following acquisition, enhanced processing procedures such as time window stacking, percent keep, and source stacking, generated an overtone image with a higher signal-to-noise ratio and more pronounced fundamental mode energy. This visual improvement facilitated the extraction of fundamental mode energy, ultimately increasing the accuracy of the final shear-wave velocity profile. Velocity anomalies within the velocity profiles could likely be attributed to the changing of the stress field during the movement of the void. As the void expands laterally, the roof span increases. The increasing roof span likely increases the stress and shear velocity in the overburden load causing high-velocity haloes in the velocity profile. When the roof span becomes too large to support the overburden load, the roof rock will collapse, causing the migration of the void. Upon collapse, the non-collapsed overburden should accumulate stress due to the lack of underlying support, while the collapsed rock (rubble) should decrease in shear-wave velocity.