Electrical Resistivity for Seeing Deep Foundations: A New Framework for Identifying Long, Slender Elements

The use of applied geophysics in geotechnical engineering is steadily increasing. Electrical resistivity is a common near-surface geophysical method that has traditionally been used for site characterization in large projects. More recently, there has been a rise in its potential use for locating and/or examining infrastructure. One potential use for electrical resistivity is to “see” deep foundations; specifically, to identify defects during construction, evaluate existing structures for re-use, or determine the dimensions of unknown foundations. Electrical resistivity has been successful with large foundations (>1 m diameter) but the method fails at long, slender elements like singular driven piles. The limitations of imaging the foundations are in the data post processing. Most geophysical inversion schemes solve the forward problem using the finite element method (FEM), which cannot capture small heterogeneities without over discretization of the element mesh. Additionally, the FEM inversion schemes have the propensity to smooth inverted ERT images and create near surface mathematical artefacts, making the final images inaccurate representations of the true electrical resistivity distribution of the subsurface. This paper presents an alternative solution using the analytic element method (AEM). An advantage of AEM over FEM is its ability to directly solve electrostatic interface conditions across discontinuities (such as a concrete pile) without needing a finely discretized mesh at these interfaces. This paper presents two examples comparing the FEM with the AEM using electrical resistivity: a steel H-pile and a reinforced concrete pile driven in sand. The FEM solutions show that the foundation cannot be discerned from the background soil. The pile elements are more accurately represented in the AEM models where sharp interfaces are easily seen. The next step in this research will be to incorporate the AEM in an electrical resistivity inversion to fully realize the capabilities of the AEM and capture a true representation of the subsurface electrical resistivity distribution. This has the potential to expand the use of electrical resistivity for examining deep foundations.