Abstract

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 α-to-β apparent resistivity for the square array and is determined for each coplanar set of four electrodesusing 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.

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