Patent US 4,263,553

A discriminating metal detector that can compensate for ground minerals includes an oscillator circuit having at least two attenuation coils physically located in a probe of the metal detector for ground control. The probe also includes a resonant LC circuit having an inductive coil on the same plane as the attenuation coils but on a different axis than the attenuation coils. The attenuation coils of the oscillator circuit have no magnetic effect on the inductive coil or on the natural response of the inductive coil to metal objects. The attenuation coils of the oscillator operate at a substantially higher frequency than the inductive coil of the resonant LC circuit. As the resonant frequency of the discriminating LC circuit varies due to mineral changes in the ground the oscillator frequency shifts likewise proportionally in order to maintain a constant separation between the two frequencies. This desirable effect is provided by incorporating the oscillator coils inside the probe with the inductive coil thereby assuring that both the inductive and attenuation coils are exposed to the ground minerals and effected at the same rate. The discriminating metal detector is capable of detecting gold, silver, coins, rings, and other valuables while rejecting trash like iron, pull-tabs, foil, bottle caps in addition to cancelling the effect of ground minerals in high mineral areas.