Currently, sites well studied by geological and geophysical methods are used as natural reference models for testing measurement systems used in the transient electromagnetic (TEM) method. However, the electrical properties of these models are rarely known in such detail that the difference between the measured and modeled transient responses can be safely attributed to the measuring system's intrinsic response, rather than the properties of a reference model. In this article, we present and discuss the results of testing TEM systems on Lake Baikal. The measurements were taken in early spring when the lake was covered with ice. More than 1000 m deep, homogeneous and isotropic water column of the lake is a unique natural model with precisely known properties. Testing works on Lake Baikal answered the question why, when using small central loop arrays in resistive environment, an anomalously slow decrease in transient EMF (long tail) is observed at late times. As it turned out, long tails are a slowly decreasing component of the input amplifier's intrinsic response to a voltage pulse induced in the receiving loop during transmitter current turn-off. As for measurements at early times, it was shown how delay effects in turning off the transmitter current manifest themselves in the early-time loop-loop TEM data.
