Borehole Magnetic Gradiometer

• Determine pile depth

The instrument detects variations in the earth’s magnetic field ferrous materials increase the intensity of the magnetic field. This allows the reinforcement within the pile to be detected. Best results are obtained when the borehole is within 0.5 m of the pile. Results become less precise the further away from the pipe (the response becomes lower in amplitude and becomes more dominated by the ambient field). Metallic contamination or the presence of metal fixtures close to the sensor may also affect the results. The middle of the sensor rod needs to ensure the bottom of the pile has been detected. If the material is loose, a plastic casing is needed to prevent the collapse of the hole. The diameter of the borehole needs to be approximately 70 mm.

The depth of pile is estimated based on the occurrence of a magnetic gradient inversion and decay.