A novel test ground for the equipment qualification of magnetic gradient sensors used for unexploded bomb detection

• We built up a test ground for sensors used for unexploded bomb detection.
• We positioned movable test coils in tubes underneath of our test ground.
• By using split coils flux gradients of unexploded bombs can be simulated.
• Tests of the signal resolution and of the relative spatial resolution are feasible.
• Traceability of detectors to national metrological standards is possible.

Scanning of the ground level by magnetic gradient sensors (fluxgate sensors) is the primary detection technique for unexploded bombs (UXBs). In order to allow a classification of the test equipment (magnetic sensors and associated evaluation software) as well as training and examination of the skills of sensor operating teams we built up a test facility. In the first step to generate the stray magnetic fields of UXBs, we positioned solenoids of the same dimension as the simulated bombs under a test ground using the principle of the equivalent current shell. From numerical investigations it has been found, that for depths exceeding 1.2 m, the gradient field profiles of these solenoids and the gradient field profiles of small multi-layer split coils agree very well (far field regime). This was verified later experimentally: By positioning these movable small multi-layer split coils in tubes running diagonally underneath the test ground and controlling the current flowing through these coils, we were able to find a good agreement between calculated and experimental data of the gradiometer signal scans on the measurement plane for (i) tests of the signal resolution and (ii) tests of the relative spatial resolution of the gradient sensors.