✓ non destructive method
✓ fast and accurate
Key notes
- The combined use of GPR and FDEM adds value to your survey
- Streamlines the process and saves time.
- Removes ambiguities that may exist when using only one method
The use of non-destructive geophysical methods has become an essential component in technical projects, applied both during the initial stages of design and throughout the lifecycle of structures. These methods are invaluable not only for monitoring new constructions but also for investigating older structures. In the latter case, geophysical surveys can reveal critical information, such as the location of foundational elements and subsequent interventions.
Electromagnetic (EM) methods with Ground-Penetrating Radar (GPR) enhances geophysical investigations by leveraging the complementary strengths of both techniques. EM methods, such as Frequency-Domain and Time-Domain Electromagnetics (FDEM and TDEM), are highly effective for detecting variations in subsurface conductivity, making them ideal for mapping groundwater, contamination plumes, and lithological changes.
GPR, on the other hand, provides high-resolution imaging of shallow subsurface features based on dielectric contrasts. When used together, EM surveys can guide GPR data collection by identifying areas of interest, while GPR refines the interpretation by providing detailed structural information. This combined approach improves target detection, minimizes ambiguity, and is particularly valuable in environmental studies, engineering site assessments, and archaeological investigations.
The objective of this geophysical survey presented was to detect anthropogenic structures beneath the floor of a historic airplane silo constructed in 1938.
GPR Survey
The GPR survey was designed to provide both 2D and 3D interpretations of subsurface features. While 2D sections allow for an initial assessment, a 3D interpretation offers a comprehensive visualization of the detected structures. streamlining the process of building. However, the process can be challenging in environments with high subsurface heterogeneity, particularly beneath areas with extensive preparatory construction work.
The depth axis on each GPR section was calculated using an electromagnetic propagation velocity of 0.1 m/ns. It is important to note that this velocity may vary, with looser formations potentially resulting in reduced values and shallower depth estimates. This velocity was determined through the analysis of diffractions observed in the GPR data.
A notable observation in all GPR sections is the presence of a foundational structure at the western end of the silo, near the sliding doors. This structure is visible at depth and likely represents iron-cemented foundation walls connecting columns on opposite sides.
When presented in 3D mode, the GPR data highlights the alignment of metallic features previously detected in 2D sections. For instance, continuous reflections interpreted as metallic tubes are aligned in certain areas, corroborating the findings of FDEM maps. At the foundation columns near the sliding doors, significant reflectivity was observed in the perpendicular GPR sections, further indicating the presence of reinforced foundation walls.
Two important conclusions:
- By slicing the 3D GPR data at 1, 1.5 and 3 meters depth, additional features were revealed, alongside areas filled with coarse material.
- This horizontal slice consolidates the previously detected features and provides new insights into subsurface.
FDEM Measurements
The FDEM (Frequency Domain Electromagnetic) method is a non-invasive technique used to map subsurface features by measuring variations in electrical conductivity and magnetic susceptibility.
This method involves transmitting electromagnetic fields into the ground and recording the resulting secondary fields generated by the subsurface materials. FEM is particularly effective in detecting metallic structures, such as pipes, reinforcements, or buried utilities, and identifying contrasts in geological formations. By analyzing the in-phase and quadrature components of the signal, FDEM can provide detailed maps of subsurface anomalies, aiding in the interpretation of anthropogenic and natural features.
The FEM survey utilized conductivity meters operating in VCP mode, with effective depths of 1.5 meters and 3 meters. The resulting in-phase component maps, which are sensitive to the presence of anthropogenic metallic structures, reveal elongated features with high values.
These features correspond to metallic caps and linear elements aligned perpendicular to the silo’s longest side and are associated with reinforced walls or large metallic pipes, adding to the findings of the GPR survey.