ELECTRICAL METHODS 617 



the tuned vertical coil. (A reversing switch is provided for obtaining 

 the opposed phase relationship.) The number of turns in the horizontal 

 coil is a measure of the field threading the vertical coil. Thus, when the 

 plane of the ellipse is parallel to that of the vertical coil, the number 

 of turns in the secondary coil is proportional to the ratio of the minor to 

 the major axis of the ellipse. 



In practice, two separate readings are taken at each station, instead 

 of spending the necessary time required in determining the plane of the 

 ellipse. That is, after a balance has been obtained for one orientation, the 

 apparatus is rotated through 90° about the vertical axis and another 

 balance reading is made. The two readings (number of turns) are then 

 compounded vectorially to obtain a resultant whose magnitude is pro- 

 portional to the ratio of the minor to the major axis of the ellipse. 



Electrical Characteristics of Apparatus. — The amplifier and coupling trans- 

 former should be well shielded electromagnetically and electrostatically. Undesirable 

 coupling between circuits can be minimized most successfully by feeding the output of 

 the horizontal and the vertical coils into separate peak-tuned input windings of the 

 transformer. These windings are connected so their fluxes oppose and, when balanced, 

 give zero induced potential in the secondary coil. An electrostatic shield should be 

 provided between the two primary windings and the secondary winding. 



The alternator supplying power for energizing the loop should be of good design 

 to produce as pure a sine wave as possible in order to minimize harmonics. Also, an 

 input filter should be provided in the amplifier to remove harmonics, because in circuits 

 of this type proper null point balance can not be obtained unless the higher harmonics 

 are well suppressed. 



Field Procedure, — The initial step in the survey is the layout of the large hori- 

 zontal loop for energizing the area under investigation. The loop should extend well 

 beyond the limits of the area in order to minimize the effects of a non-uniform field 

 near the loop itself. If terrain conditions permit, a square or rectangular loop is 

 usually the most desirable. Inside the loop, a regular gridwork or pattern of obser- 

 vation points where the readings are to be made is surveyed. If a square loop 

 is used, the observation points are preferably laid out by intersecting lines parallel to 

 the sides of the loop. These points of observation are to be plotted on the map. 



If the energizing loop is laid in flat country, the magnetic field within the loop 

 (primary field) is substantially vertical at all of the observation points, provided these 

 are not too close to the cable. The magnetic field due to induced currents, however, 

 will usually have a horizontal component at the surface. Hence, the resultant magnetic 

 field at the surface is usually elliptically polarized, and the ratio of the major to the 

 minor axis at a series of stations will give a measure of the magnitude, direction, 

 and relative phase of the secondary field. For example, if measurements are taken 

 near a large conducting body, a large out-of-phase secondary field will occur and this 

 will give rise to a relatively large minor axis in the representative ellipse, t On the 

 other hand, if the secondary field is zero or if it lags or leads the primary field by 

 180°, the minor axis of the ellipse vanishes. 



Double Coil Method for Measuring Phase and Amplitude 



Hedstrom $ has developed the apparatus shown in Figure 385. The 

 apparatus comprises two coils, each having approximately 1200 turns on a 



t A. B. Broughton Edge and T. H. Laby, loc. cit., p. 60, 



t H. Hedstrom, "Phase Measurements in Electrical Prospecting," A.I.M.E. Geophysical Pros- 

 pecting, Tech. Pub. 827, 1937, 



