Voids detection in Marine docks by Ground Penetrating Radar (GPR)
The GPR method uses radar propagation and reflection of electromagnetic waves of different frequencies.
A microwave antenna transmits pulses of very short duration in the soil or the structure, generally at frequencies ranging from 16MHz to 2.6 GHz (according to depth and resolution objectives). When the waves encounter contact between two different dielectric permittivities environments, part of their energy is reflected, while the other goes deeper.
The acquisition system measures the travel time of the radar wave between its transmission and its reception. The depth is calculated using the travel time and the relative dielectric permittivity εr of the soil (v = c /√εr where c: speed of light and v: soil velocity).
However, this depth may be lower or even 0 if there is the presence of natural or artificial screens (clay soils, rebar mesh, metal plate, etc.).
During this GPR campaign, we used a 270MHz antenna that allows auscultation close to 6m deep.
The above radargram highlights anomalies, between 32 and 48 abscise, linked to voids extended between 3.5m and beyond 5.6m deep. GPR measures detect also geometric targets regularly spaced that appear to be related to wooden beams.