A rod with a proton magnetometer at each end, which may be stood on the ground in a vertical position to measure local variations in the strength of the earth's magnetic field at ground level (such variations being due to features just below the surface).
Origin
1960s.
Definition of proton gradiometer in US English:
proton gradiometer
noun
A rod with a proton magnetometer at each end, which may be stood on the ground in a vertical position to measure local variations in the strength of the earth's magnetic field at ground level (such variations being due to features just below the surface).