Borehole Magnetic Gradiometer

Applications
  • Determine pile depth
Method

The instrument detects variations in the earth’s magnetic field. Ferrous materials increase the intensity 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 pile (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 extend beyond the end of the pile by approximately 2 m in order to ensure the bottom of the pile has been detected. If the material is loose, a plastic casing is needed to prevent collapse of the hole. The diameter of the borehole needs to be approximately 70 mm.

Data analysis and presentation

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

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