Extending the range of time constants recorded by the SPECTREM AEM system

In order to get the correct amplitudes of the secondary field measurements, the correct value for the primary signal must be subtracted from the input signal where both secondary and primary are recorded together. This is due to the secondary signals being measured while the transmitter is on. The onboard data system approximates the transmitter signal by the total signal value in the last window, it being assumed that the secondary signal has decayed to a negligible value at that delay time.

The result of subtracting too much from the secondary signal and adding this excess to the primary signal is that the secondary signals are no longer sums of simple exponential decay functions. A procedure is described to estimate the decay functions as well as the offset to be subtracted from the initial estimate of the primary.

The Spectrem AEM transmitter has no off-time; secondary signals are recorded at the same time as the transmitter primary. By assuming the secondaries have decayed away by the last window, the signal value is used as an estimate of the primary. The result is underestimation of the secondary amplitudes if the target is highly conductive. This paper describes a method to compute a better estimate of the primary amplitude.