CHARACTERIZING KARST HAZARDS AND DEPTH-TO-BEDROCK IN DRIFTLESS AREA AGRICULTURE SETTINGS IN SOUTHEASTERN MINNESOTA

The karst of the Driftless Area of southeastern Minnesota is mantled with up to 15 m of soil and unconsolidated sediments. The mantle material frequently has enough cohesiveness to form a “roof” over underlying processes of sinkhole development until the undermined roof collapses, often catastrophically. Dairy farming has traditionally been a widespread agricultural practice in the region, but economic and regulatory forces have conspired in recent years to require consolidation and expansion of operations in order to maintain their financial and environmental sustainability. One of the sources of controversy surrounding the consideration of proposed expansions is the possibility of karst hazards undermining manure storage facilities.

Over the past eight years, Electrical Resistivity Imaging (ERI) techniques have been employed to screen for karst hazards during the planning phase of feedlot expansions and to more accurately characterize the nature of karst hazards—when they are present. In addition, ERI has proven to be very useful in imaging variations in depth-to-bedrock, which is highly variable in the karst terrain of southeastern Minnesota. ERI has been performed using a 56-channel AGI Supersting^TM system with post-processing of the data in EarthImager^TM software. Dipole-Dipole and Wenner electrical resistivity arrays have been the most useful for identifying karst hazards and depth-to-bedrock. Electrode spacings of 3 to 5 m have yielded a good balance between depth-of-image and the spatial resolution. Soil boring data have provided complementary data to the ERI and have proven critical to calibrating the interpretation of ERI data.

With some site-to-site variation, most southeastern Minnesota Earth materials have resistivities that fall within typical ranges: 20-80 ohm-m for soils, 80-100 ohm-m for epikarst and weathered residuum, and >100 ohm-m for bedrock. Subsurface voids typically have resistivities greater than 1000 ohm-m, providing good contrast with the resistivities of the surrounding bedrock. ERI has been an effective tool in identifying karst hazards in southeastern Minnesota agricultural settings. In addition to improving site assessments, ERI has reduces controversy surrounding the karst hazards of proposed projects by providing more certainty about the underlying geology.