Mapping desiccation fissures using 3-D electrical resistivity tomography

Discontinuities which may form in a soil mass due to desiccation have been shown to be a source of engineering concern in situations where low permeability barriers are required. Current methods for assessing the condition of such geo-barriers, are mainly based on visual inspections, which are inadequate when the soil surface is obscured by vegetation. Electrical Resistivity Tomography (ERT) has been shown to be sensitive to the presence of fissures in 2-D with inversion models mapping the fissure's positions and enabling their growth to be monitored. This paper presents a method for mapping fissured networks in 3-D using miniature arrays under laboratory conditions. The results of two experiments using compacted clay from two separate embankments show a good comparison between the visible surface fissures and the surface model obtained from a 3-D inversion, as well as showing good agreement between the vertical sections of the model and the exposed model interior following dissection. Additionally the results confirm that the 3-D method is suitable for monitoring the onset of fissuring. A comparison of Schlumberger, Dipole–Dipole and combined arrays for visualising the fissures is also presented, indicating that the combined method produces the most accurate image of the subsurface, while the Schlumberger array provides a greater resolution than the Dipole–Dipole array, alongside the added benefit of shorter survey times.

► ERT monitoring of desiccation fissuring in 3D using miniature array.
► Comparison of resistivity model with dissected physical model.
► Comparison of Schlumberger, Dipole–Dipole array methods.
► Proposed contour values defined for fissures in clay and verified with resistivity data.