Detection of subsurface cracking depth through electrical resistivity anisotropy

Abstract

To date, an understanding of crack dynamics has been hampered by the lack of techniques to observe or monitor crack dynamics below the soil surface. This study introduces a new technique for the detection of subsurface cracks that relates the development of soil cracks to changes in the electrical anisotropy of the soil. Here electrical anisotropy is defined as the ratio of the apparent resistivity measured with the alpha and the beta square array. While measurements of electrical anisotropy have been carried out at the soil surface, here an electrode array is proposed that allows anisotropy measurements at regular depth intervals throughout a soil profile. Four strings of electrodes are installed vertically at the corners of a square so that square array measurements can be made on a series of coplanar electrode squares using one electrode from each of the four vertical strings. Anisotropy responses to different crack scenarios were simulated numerically before measurements were carried out in the laboratory and in the field. In the laboratory anisotropy measurements were carried out 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 at two field sites, where flood irrigated sorghum and cotton were grown on cracking soil. For the field measurements, a 3D electrical resistivity tomography routine was designed that could be run with the same electrode arrangement used for the anisotropy measurements. Time-lapse measurements of 3D electrical resistivity allowed tracking of water infiltration and gave information about the extent of soil cracking. This was then compared to the anisotropy results. The numerical simulations, the laboratory and field measurements all demonstrated that the lateral and vertical extent of cracking in a soil profile strongly influences the electrical anisotropy and that a well developed anisotropy profile allows determination of a cracking depth range. Soil moisture changes after crack closure only have a minor influence on the anisotropy as has the presence of sorghum and cotton roots. It is concluded that electrical anisotropy profiles are a valuable tool for monitoring crack dynamics within a soil profile.