Metal detector for salt soils

A metal detector used for detecting a metallic target in a soil including:

• a. transmit electronics having a plurality of switches for generating a repeating transmit signal cycle, the repeating transmit signal cycle including at least a high voltage period followed by a low voltage period, the high voltage period including at least a duration of switched high voltage and the low voltage period including at least a duration of switched low voltage;
• b. a transmit coil connected to the transmit electronics for receiving the repeating transmit signal cycle and generating a transmitted magnetic field for transmission into the soil;
• c. a receive coil for receiving a received magnetic field from the soil and providing a received signal induced by the received magnetic field including a signal due to perpendicular components of a uniform conducting half-space in the soil, a signal due to components of substantially log-uniform viscous superparamagnetic soil and a signal due to a movement of the receive coil with respect to a static magnetic field;
• d. receive electronics connected to the receive coil for processing the received signal, the processing including synchronous demodulations of the received signal using predetermined synchronous demodulation functions after a predetermined time from the beginning of the low voltage period following the high voltage period,
• wherein the synchronous demodulations include a first synchronous demodulation using a first predetermined synchronous demodulation function, and an integral of first predetermined synchronous demodulation function is substantially zero within the repeating transmit signal cycle;
• and an averaging of post first synchronous demodulation signals for more than one repeating transmit signal cycle substantially cancels in a first output signal the signal due to perpendicular components of a uniform conducting half-space, the signal due to components of substantially log-uniform viscous superparamagnetic soil and the signal due to a movement of the receive coil with respect to a static magnetic field;
• the synchronous demodulations further include a second synchronous demodulation using a second predetermined synchronous demodulation function, and an integral of second predetermined synchronous demodulation function is substantially zero within the repeating transmit signal cycle;
• and an averaging of post second synchronous demodulation signals for more than one repeating transmit signal cycle substantially cancels in a second output signal the signal due to components of substantially log-uniform viscous superparamagnetic soil and the signal due to a movement of the receive coil with respect to a static magnetic field;
• and
• the receive electronics linearly combines all the output signals to produce an indicator output signal, the indicator output signal including a signal indicative of the presence of a metallic target in the soil.