Preliminary Field Results on a New Transient Induction System Employing Pseudo-Noise Signals

An experimental system is described which utilizes pseudo-noise (PN) and cross-correlation techniques to characterize subsurface conducting bodies such as those pertaining to natural resources. The system is essentially an extension of the loop-loop, transient-electromagnetic, induction systems used in ore prospecting. In lieu of a low duty-cycle pulse, this system transmits a wide-bandwidth PN digital waveform. The mapping signature is obtained by cross-correlation of the received signal with a replica of the sounding PN waveform. The receiver also averages several signatures to improve the signal-to-noise ratio. This type of system has the potential advantages over previous transient systems of using smaller antennas and requiring less transmitter power for more portable operation. Preliminary field data is shown and discussed for a high conductivity dike of ilmenite magnetite and for the surrounding low conductivity anorthosite. Frequency response data is obtained digitally via the Discrete Fourier Transform operation on the time domain signatures. Interpretations are given for both time and frequency domain representations.