ANALYSIS AND IMPROVEMENT OF THE PRIMARY FIELD WAVEFORM OF THE GROUND TEM METHOD*

Many ground TEM systems use an on-off type of primary field waveform and measure the secondary field only in the transmitter off-time, so as to realize the pure secondary field measurement. The width of the trailing edge of the primary pulses and the duration of the transmitter on-time form a time-constant (T_c) window. Most effective excitation is achieved if the target conductor has a T_c falling into this window. To be able to measure in the transmitter on-time, two primary field waveforms are proposed. The first is a series of unipolar pulses with identical steep ramps at both leading and trailing edges. Measurements made in the on-time after the leading edges simply reduce the stacking time needed and power consumption by half. Also, long- T_c conductors are better excited. The second is a series of bipolar pulses with long ramps as the leading edges and short ramps as the trailing edges. This waveform presents two different T_c-windows at the same time and helps to improve the detection of long-T_c conductors in the presence of short- T_c ones.