Chap. 10] ELECTRICAL METHODS 765 



lated and corrected for. Corrections need not be applied when com- 

 pensators are used with inductive field excitation, since the loop field 

 appears in the in-phase component, whereas the field induced in an ore 

 body will generally affect most strongly the out-of-phase component. 



2. Receiving equipment used in electromagnetic methods is virtually 

 identical with that in inductive methods and is discussed in that connection. 

 A piece of equipment not described there is the Darley pipe locator, since 

 it is a direct application of the electromagnetic method with galvanic 

 current supply. One pole of a small A.C. generator (buzzer) is connected 

 to an accessible point of a system of pipes; the other pole is grounded. 

 The receiving device is a small rectangular coil (carried in horizontal 

 position over the ground), connected to an amplifier with phones. As 

 such a coil will respond to the vertical component, which is zero directly 

 over a conductor (see Fig. 10-92o), the point of minimum or zero signal 

 will indicate the position of the pipe. In other types of pipe locators where 

 the coil is carried in a vertical position, the horizontal component is re- 

 ceived and therefore the pipe is indicated by a maximum. 



A method not strictly classifiable under the groups tabulated on page 764 

 has been proposed by Haalck.*° It is based on a comparison of the hori- 

 zontal field of the ground currents with the field produced by the electrode 

 leads. The latter are predominantly vertical; the former predominantly 

 horizontal, at least on the electrode base. The receiving arrangement is 

 set up in the center of the electrode base and the cable is carried around 

 this location in a half rectangle. If the current flows in the cable from 

 front to back, the cable field Zo is upward and that of the gro\md return 

 current is to the left. The two fields are approximately in-phase. If the 

 reception frame is set up with its horizontal axis of rotation parallel with 

 the electrode base, a minimum of sound will be observed in the head- 

 phones when its plane is in the resultant direction of the two fields, that is, 

 if tan (f = Zo/H. 



The sensitivity may be increased by using two frames, one stationary 

 and the other rotatable (Fig. 10-96). In investigating stratified ground 

 with this method, the electrode separation is increased in steps and the 

 change in tilt angle is observed. If the ground is homogeneous, the hori- 

 zontal field will decrease with an increased electrode separation due to 

 greater depth penetration of the current. Hence, the tilt of the frame 

 will increase. If bodies or strata of different conductivity are present, 

 the depths of the effective current concentration will change; therefore, 

 changes in the regular trend of tilt variation will occur. There is a definite 

 relation between electrode separation and depth, the factor being 2^ to 3, 

 depending on conductivity contrasts involved. 



8" H. Haalck, and A. Ebert, Zeit. Geophys., 8(8), 409-419 (1932). 



