Signal processing for through-the-earth electromagnetic systems

The deep-mine application of electromagnetic location systems requires the use of extremely low frequency transmissions. The associated higher noise levels and lower signal levels necessitate the incorporation of nonlinear processing and adaptive noise cancellation. A demodulator with nonlinear processing is implemented by placing an adaptive clipper inside a signal-cancelling feedback loop. Adaptive noise cancellation is applied to the narrowband demodulator outputs. Its weighting matrix is determined by the direct-matrix inversion algorithm with iterative estimation of the signal vector and cross-correlation matrix. The signal processing system is tested with both simulated and recorded extremely low frequency noise