Borehole Magnetometry Test for Evaluating a Caisson’s Reinforced Depth: Experimental Results and Theoretical Modeling

Abstract

Abstract The borehole magnetometry (BM) test was performed to evaluate the foundation reinforcement depth at a site where a telecommunication tower is supported by a single 2.3-m-diameter caisson foundation of known as-built design. An optical televiewer probe containing a three-axial fluxgate magnetometer was lowered into a vertical borehole 2.35 m distant from the center of the caisson and the profiles for the total magnetic field flux density, and its vertical component were acquired and used to generate, by subtracting the International Geomagnetic Reference Field background, the profiles for the total, BA, and vertical component, Bz,A, anomalous magnetic field. Four distinct graphical methods were used to evaluate the reinforced depth from the anomalous profiles and their first- and second-order derivatives. Two of the methods, including the one proposed in this study, based on locating the inflection points in the derivative profiles, evaluated the reinforced depth very close to the as-built depth (8.0 m). The reinforcement intensity of magnetization was then evaluated using the BA profile and a method based on the bipolar model. Theoretical modeling of BA and Bz,A and the derivative profiles was then performed using a three-dimensional prismatic model. By comparing modeled and experimental results, the induced magnetization was found to be an unsuited modeling assumption, with remanent magnetization being a better representation of the magnetic field around the caisson’s steel reinforcement, in agreement with the theory, given the high Koenigsberger ratio for steel. Also, the modeling revealed the need for a more complex representation of the magnetic sources, with added prisms to represent the effects of a magnetically-noisy environment and above-ground structures, as well as the presence of inhomogeneity and polarization changes along the reinforcement length.