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 OPTV 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 IGRF background, the profiles for the total
and vertical component, 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 B_A profile and a method based on the bipolar model. Theoretical modeling of B_A and B_(z,A) and the derivative profiles was then performed, using a 3-D 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.