(tur’ am) Electromagnetic-survey method employing an energizing source consisting of a long insulated cable grounded at both ends, or a large horizontal loop. The cable is often several kilometers long and energized at 100 to 800 Hz. Measurements are made of the field-strength ratio and phase difference of the voltages induced in two receiving coils about 100 ft apart. Usually the plane of the two loops is horizontal. Many profiles may be made using the same source location. See Bosschart (1964) and Figure E-7.
The method was invented by Erik Lars Helmer Hedstrom in the mid 1930s
- Hedstrom, E. H. (1937) Phase Measurements in Electrical Prospecting. AIME Tech. Publ. 827.