Abstract: An electromagnetic gradiometer generates a time varying primary electromagnetic field, such as by use of a transmitter coil attached to a waveform generator. The primary electromagnetic field induces a responsive, relatively weak, time varying secondary electromagnetic field about a hidden object. The combined primary and secondary electromagnetic field is measured simultaneously at two spaced apart detectors, such as at two distinct receiver coils. The detectors are configured such that they detect the primary field in equal magnitudes but in opposite sense and connected in series such that the primary electromagnetic field is automatically nulled. The difference in the secondary electromagnetic field measured at the two points is divided by the distance between the detectors to determine the electromagnetic field gradient therebetween.

Abstract: A spectrogram of secondary magnetic field strength as a function of frequency and spatial relationship is used to identify hidden objects. A time-varying multi-frequency primary electromagnetic field is generated, preferably over the range 100 Hz to 300 kHz, which induces a time-varying multi-frequency secondary electromagnetic field about the hidden object. The strength of the secondary field, typically inphase and quadrature, is plotted as a spectrogram over a low frequency broadband spectrum as a function of frequency and spatial relationship between the hidden object and the secondary field strength detector. From this spectrogram, indications may be had of the hidden object's characteristics such as location, size and shape, and material composition. Preferably, the measured spectrogram is compared against a library of reference spectrograms by a computer to identify the hidden object.

Abstract: The present invention provides an apparatus and method for creating a magnetic cavity in a region about the centerpoint of two concentric, magnetic field-generating electrical wire coils. The outer of the two coils generates a strong primary magnetic field that may induce a relatively weak magnetic field in a remote conductive material, such as subterranean mineral deposits. The inner of the two coils generates a secondary magnetic field having a smaller amplitude and an opposite polarity from the primary field. Various parameters of the apparatus are calculated so that the two oppositely polarized magnetic fields mutually cancel each other in a specified region inward of the two coils about their centerpoint, creating the magnetic cavity. A magnetic sensor can then be isolated within the magnetic cavity for detecting the weak induced magnetic field in the remote conductive material without interference from the nearby primary and secondary magnetic fields.