Patent ID: 6534985
Filing Date: 2003-03-18
Classification: G01V

Abstract:
An electromagnetic sensing apparatus comprising:an electromagnetic transmitter for generating a transmitter field; at least one receiver device spaced from the transmitter for sensing a receiver local electromagnetic field proximate thereto and generating a receiver signal, wherein the receiver local electromagnetic field includes the transmitter field and a receiver secondary field induced by the transmitter field; at least one calibration device for sensing a calibration local electromagnetic field proximate thereto and generating a calibration signal, wherein the calibration local electromagnetic field includes the transmitter field and a calibration secondary field induced by the transmitter field, the calibration device being positioned such that the distance between the transmitter and the receiver device is greater than the distance between the transmitter and the calibration device; a switch connected between the at least one receiver device and the at least one calibration device for selecting one of the receiver signal and the calibration signal; and a processing means having a first input for receiving a signal from one of the receiver device and the calibration device and a second input for receiving a signal from the switch.