Detection of subsurface soil cracks by vertical anisotropy profiles of apparent electrical resistivity

To date, an understanding of crack dynamics has been fundamentally hampered by the lack of available techniques to observe or monitor crack dynamics below the soil surface. A new technique relates the growth of soil cracks to a progressive increase in the electrical anisotropy of the soil. Although a single measurement of anisotropy is possible using a surface array of electrodes, the use of four strings of electrodes installed vertically at the corners of a square provides a valuable picture of the crack pattern at depth. In addition, time-lapse electrical surveys allow the growth of cracks to be clearly monitored. The electrical anisotropy is defined as the ratio of the alpha-to-beta apparent resistivity for the square array and is determined for each coplanar set of four electrodes using one electrode from each of the four vertical strings. In a laboratory, we measured the electrical anisotropy in a sand-filled lysimeter with a plastic sheet, introduced to represent an electrically insulating crack. Measurements were then repeated in a cracking-soil-filled lysimeter. Finally, measurements were made in a field where a flood-irrigated sorghum crop was grown on cracking soil. Measurements under all three conditions demonstrate that the lateral and vertical extents of cracking in a soil profile strongly influence the electrical anisotropy. The larger the cumulative cracking volume, the higher the electrical anisotropy. Soil-moisture changes after crack closure have a minor influence on the measured anisotropy, as have sorghum roots. These experiments demonstrate that electrical-anisotropy profiles are a valuable tool for monitoring crack dynamics within a soil profile.