Measuring Crack Porosity using Three-Dimensional Electrical Resistivity Tomography.

Desiccation cracks in Vertisols and other shrink-swell soils greatly alter the hydrological properties of the soil by intercepting runoff and increasing evaporation. Nondestructive monitoring and measuring of soil crack volume or porosity is difficult. Typically, crack porosity is predicted from measurements of soil subsidence or shrinkage; however, these measurements require installation of permanent equipment in the field. In this paper, we demonstrate the use of electrical resistivity tomography (ERT) to monitor soil crack porosity. Electrical resistivity tomography is a noninvasive geophysical technique that generates maps or images of the subsurface electrical resistivity of a soil. We conducted ERT surveys during the seasonal drying cycle of a Vertisol in Central Texas. Crack porosity near the end of a drying cycle was measured by filling cracks with cement and photographic analysis of horizontal cross-sections. Measured crack porosity was correlated to crack indicators on ERT images (r2 = 0.63). Total shrinkage porosity for each date of the study was predicted from soil water content loss. Measured crack porosity accounted for roughly one third of total shrinkage porosity. For dates where cracks were present, average total shrinkage porosity was well correlated with ERT indicators (r2 = 0.96). Our results suggest that ERT could be a viable, non-invasive tool for measuring crack porosity. ERT-based methods have several advantages over traditional shrinkage-based methods, mainly that ERT does not require installation of permanent field equipment and can provide data on the spatial distribution of cracks over pedon-scale areas. [ABSTRACT FROM AUTHOR]