Central loop measurement has covered the full zone in the past 20 years. Nevertheless, articles dealing

with response features of the mediumdistal zone are very insufficient, which has caused perplexity among the

users, for example, the late delay response value is larger than the early delay response value (ε-overshoot)；the

results observed by two sorts of instruments are considerably different from each other at the same measuring

point. These problems have been analyzed in detail in this paper, and the results show that there basically exist

four basic response features in the mediumdistal zone, and the "ε-overshoot" is only one of them. The intensity

distortional response is caused by the receiver coil delay. Therefore, the coil character should be exactly known

and the transmitter loop length should be carefully selected before the performance of the survey in the field.