The effective area of a circular wire loop to a uniform field is reduced considerably if it is placed close to a large superconducting disc. The effective area to the field of a nearby magnetic dipole is reduced much less. Such a selective pickup loop is used to construct a small first order gradiometer, with a baseline of 3 cm. Its sensitivity to fields of nearby sources is comparable to that of a much larger symetric second order gradiometer wheras its sensitivity to inhomogeneous magnetic noise is much less. Another advantage of the unsymmetric configuration is, that it has a smaller selfinductance than a conventional gradiometer, so that a better flux transfer to the SQUID is possible.
Overweg, J. A., & Peters, M. J. (1978). A miniature squid magnetometer for biomagnetic measurements with improved selectivity. Journal de Physique. Colloque, 39(C6), 1226-1227. https://doi.org/10.1051/jphyscol:19786542