As introduced on the White's pulse induction detectors beginning in 2008. https://patents.google.com/patent/US7994789 Abstract A new dual field search coil for pulse induction metal detectors has multiple coplanar wire coils of different diameters connected in series with the output of a pulse source, overcoming the loss of target size resolution associated with a single coil search loop. Small objects are sensed by an internal small coil and larger objects are sensed by the large