Borehole magnetometry to determine the depth of reinforced foundations in telecommunication towers

Abstract

Technical information about deep foundations of built structures may in some cases be unavailable or incomplete due to the lack of construction documentation and electronic media in which the reports are stored. This problem is particularly relevant for telecommunication towers constructed two-to-three decades ago but still under operation and subject to new loading demands from expanding networks. Nondestructive evaluation with geophysical methods has been applied to determine the depth to the bottom of deep foundations, a key parameter to evaluate their bearing capacity and settlements. This is done by lowering a borehole magnetometry (BM) sensor down a drillhole installed close to the unknown structure, thereby measuring the magnetic field along the testing drillhole. Determining the bottom depth of the structure using BM data configures an edge detection problem: a classical problem in magnetic data interpretation, but in this case with poor data coverage (only a profile close to the structure) to characterize a complex magnetization distribution. Common practices to locate pile bottom from characteristic peak or inflection points in the observed magnetic anomaly may work at some sites but usually fail to account for the complex anomaly patterns from built structures. We summarize a set of numerical and field-work applications showing that no single characteristic attribute of the observed field can undoubtedly be used to determine the foundation depth. We verify that most of the measured field along the testing drillhole is disturbed by fields from facilities at the ground surface and develop a data inversion procedure enforcing data fitting to measurements at the lower portions of the drillhole. The efficacy of this technique is illustrated with field tests in different geologic scenarios with results compared with as-built documentation and/or independent evaluation with other methods.