  In this paper we analyzed the two peculiarities frequently observed in SQUID
magnetometer applications in TDEM based geophysical exploration such as the
decay of the ground response is frequency dependent and sign reversal or
zero-crossover effect at late decay times. A SQUID based TDEM system has been
developed, characterized with prototype transmitter and field survey has been
performed. Since the laboratory tests were done in a relatively noisy
environment, the frequency dependence and the sign reversal effects have not
been observed but the same have clearly been observed in the field survey. In
the field survey, the decay transients surprisingly reached negative values in
a terrain where a thin conducting layer is sandwiched between thick upper and
thin lower resistive layers and the decay transients were highly dependent on
base frequency of the transmitter waveforms. The major advantage of using the
SQUIDs in TDEM applications is its extremely high sensitivity maintained at low
frequencies. Therefore one can investigate the conductivity of the layers
buried in deep below the surface of the ground. Nevertheless, the decay
transients recorded with SQUID provided the information of the conducting layer
along with an unexpected zero cross-over at later decay times. In this article,
we describe the possible reasons for these peculiarities based on the
observations made by various researchers, understanding of the TDEM concepts,
behavior of the instruments and various experiments performed by us.
