SOME MODEL EXPERIMENTS RELATING TO ELECTROMAGNETIC PROSPECTING WITH SPECIAL REFERENCE TO AIRBORNE WORK*

After a discussion of the underlying principles, some electromagnetic model experiments made primarily in connection with the development of the ABEM airborne method are described. The experiments were made on thin vertical and horizontal conductors of “infinite” extent with coil arrangements involving one transmitter and one receiver unit. The in‐phase and out‐of‐phase components of the field picked up by the receiver were measured in percent of the amplitude of the normal field at the receiver. In this case 1° phase‐shift corresponds to 1.75 % of the normal field amplitude. The experiments were made on a scale 600–2000 times smaller than the natural scale. The ores and overburdens normally encountered in the field were simulated by sheets of Cu, Al, Zn and Pb of varying thicknesses. The frequencies used were 500, 880 and 1500 c/s. The variation of the secondary fields with the thickness, resistivity and depths of the conductors causing them is discussed in some detail. The bearing of the laboratory work on ground and especially airborne electromagnetic methods is indicated. A few miscellaneous experiments are also described. The results of an airborne survey over a known ore body and those of the corresponding model experiments are given. The ABEM airborne electromagnetic method and the so‐called “Canadian” method are briefly compared in the light of the model experiments described.