It can be said that quadrature systems failed the competition with powerful time-domain and towed rigid-boom frequency-domain systems. It seems that there is no place for them but they continued to be in use for a long time and the reasons are: first, in contrast to time-domain systems quadrature systems could be applied in weak conductive environment. But one of them, namely, EM4H is still widely used for survey – about 100 000 line kilometers in a year. Experience has shown that new advantages of EM4H have improved essentially survey results and noticeably risen the potential of frequency-domain systems with large receiver-transmitter separation. In the paper basic features of the EM4H system are described and illustrated by survey results. Particular attention is paid to estimation of effectiveness for searching conductive target depending on distance.