Chap. 10] 



ELECTRICAL METHODS 



773 



been applied for the normal field, the curves might have been improved 

 further. The maximum in the Y component and the trend of the Z curve 

 indicates that the syncline acts as a current concentration, probably 

 because the shales in the center of the syncline are of good conductivity. 

 The Haalck electromagnetic method (see page 765) was used for depth 

 determination of lignite beds in the Ville area by observing changes in the 

 tilt angle of the detection coil with changes in spacing of the primary elec- 

 trodes (Fig. 10-96). For the same area, resistivity-depth curves were 

 illustrated in Fig. 10-72. 



DthdiM 



250^ 100 



Fig. 10-96. Effect of electrode separation on tilt angle of electromagnetic field 

 in determining thickness of lignite beds at Ville, Germany (adapted from Haalck). 

 Left scale, electrode spacing; right scale, depth. 



B. Electromagnetic Methods with Inductive Power Supply 



The application of primary energy by insulated loops gives the electro- 

 magnetic-inductive methods a number of advantages over the electromag- 

 netic-galvanic methods. First, power can be transferred to the ground 

 without great loss, particularly in areas of poorly conductive surface beds. 

 While galvanic methods appear to be better adapted to massive geologic 

 bodies, inductive methods are more suited for sheet-like deposits. Good 

 conductors may be reached when covered by poor conductors; depth pene- 

 tration can be regulated by using different frequencies. 



