Void Hunting: Ambient noise tomography for spatio-temporal subsurface imaging and monitoring in karst environments

Abstract:

Karst environments are characterized by voids, i.e. sinkholes and conduits of varying size that arise from the active dissolution of carbonate rock by acidic groundwater. These voids, whether air-, water-, or soil-filled, can be difficult to image using near-surface geophysical methods due to the limited investigation depths of most active-source methods. In addition, due to the significant effort it takes to collect active-source data, investigators are often unable to monitor spatio-temporal variations in the subsurface. The ability to detect, image, and monitor subsurface voids improves our understanding of the processes that create and transform voids, a vitally important insight across a variety of scientific disciplines and engineering applications, including hydrogeology, geotechnical engineering, planetary science and even issues of national security. Using a 54-element nodal array (1C and 3C sensors), we image the subsurface of the USF GeoPark with ambient noise surface wave tomography. We also collected complementary active-source geophysical datasets (e.g. 2D ERT) that are used to constrain and/or validate the tomography results. In addition to the ability to use seismic temporal monitoring, ambient noise provides lower frequencies than what are achievable with active-source seismic methods, thus enabling us to image deeper at this site than any previous seismic study. We are working to address three research questions with this study: (1) Can ambient noise tomography resolve voids in the karst environment? (2) Can ambient noise methods be used to monitor spatio-temporal changes in sinkhole and conduit formation? (3) How do ambient seismic methods complement active-source near-surface methods? In this presentation, we will discuss our current answers to these questions and present the current state of surface wave methods in the karst environment.