The following are patents on multi-frequency VLF-type metal detectors. Multi-period PI patents are not included here.
Download	Patent #	Title	Description
322K	US3012190	Multiple Frequency Alternating Current Network	1961 (filed 1946!) First multifrequency patent?
1061K	US3686564	Multiple Frequency Magnetic Field Technique for Differentiating Between Classes of Metal Objects	Multi-frequency detector from 1972.
1275K	US4942360	A Method and Apparatus of Discrimination Detection Using Multiple Frequencies to Determine a Recognizable Profile of an Undesirable Substance	Minelab, multiple frequency.
325K	US4975646	Detector System for Recognizing a Magnetic Material	Multi-frequency
2369K	US5537041	Discriminating Time Domain Conducting Metal Detector Utilizing Multi-Period Rectangular Transmitted Pulses	Minelab multifrequency (BBS).
2199K	US5642050	Plural Frequency Method and System for Identifying Metal Objects in a Background Environment Using a Target Model	White's multi-frequency detector (see 5654638).
2306K	US5654638	Plural Frequency Method and System for Identifying Metal Objects in a Background Environment	White's multi-frequency detector (see 5642050).
132K	US6879161	Method and Apparatus for Distinguishing Metal Objects and Employing Multiple Frequency Interrogation	White's
168K	US6911823	Metal Detector Employing Static Discrimination	White's
44K	US7579839	Metal Detector	Minelab, multi-frequency demod technique
130K	US8063777	Real-Time Rectangular-Wave Transmitting Metal Detector Platform with User Selectable Transmissions and Reception Properties	Minelab, IB/MF
176K	US8159225	Multi-Frequency Transmitter for a Metal Detector	Minelab, MF TX method
128K	US8237560	Real-Time Rectangular-Wave Transmitting Metal Detector Platform with User Selectable Transmissions and Reception Properties	Minelab, IB/MF