Relating geophysical and geochemical signatures to bulk electrical resistivity

Abstract:

Novel geophysical techniques such as electrical resistivity (ER), are becoming increasingly popular for studying the critical zone. Some of the primary benefits of geophysical approaches include the ability to survey large areas quickly and with minimal disturbance, however uncertainty in the petrophysical relationship between ER and water content can limit the utility of such approaches for quantifying subsurface features. In the Lower Peninsula of the Michigan Basin, this relationship is further complicated by the unique geologic history that includes deposition by ancient seas and more recent repeated glacial advances. This geologic history heavily influences the geophysical and geochemical properties of the critical zone, which as a result is dominated by deep carbonate-rich sands and gravels.

To study the influence of these properties on the bulk resistivity of the shallow subsurface, we collected two 3.6 m deep soil cores from a former agricultural field in Michigan, USA which is undergoing the process of forest succession. The first core is located in a second-growth forest, and the second in adjacent open grassland to compare biogeochemical influences. Grain size, volumetric water content, organic matter content, and effervescence were sampled in conjunction with soil box resistivity tests at 8 cm intervals to build relationships between resistivity, water content, and the geochemical signatures of carbonate dissolution processes. The results from this work highlights the importance of understanding geochemical processes when interpreting ER data and the challenges of field-scale heterogeneities.