366 MAGNETIC METHOD [Chap. 8 



D. Observatory Instruments 



Certain instruments now used in observatory practice are also applicable 

 in regional magnetic surveying. Some of these may, by suitable modifica- 

 tions, become of importance for magnetic prospecting. Observatory in- 

 struments are employed (1) for the absolute determination of magnetic 

 components, and (2) for recording magnetic variations. 



1 . Instruments for absolute determinations are sine galvanometers, mag- 

 netic theodolites, and earth inductors. These require no further discussion 

 here, since their principles have been described before. 



2. Instruments for the recording of magnetic variations fall into two groups 

 of different design, depending on whether they are intended for perma- 

 nent observatories or for temporary setups in connection with magnetic 

 exploration. In permanent observatories variation instruments are pro- 

 vided for declination, horizontal intensity, and vertical intensity. Some 

 observatories prefer to record north and east components of the hori- 

 zontal intensity. The variometers are usually set up in a straight line in 

 the magnetic prime vertical so that the declination variometer oscillates 

 in the magnetic meridian, the horizontal magnetometer in the prime ver- 

 tical, and the vertical intensity variometer in the magnetic meridian. ^^ 

 For convenience in scale value determinations, Helmholtz coils are perma- 

 nently installed on the instruments. Temporary observatories in magnetic 

 exploration generally utilize the regular vertical or horizontal intensity 

 magnetometers equipped with a mirror device. This device projects the 

 light reflected from the magnetic system on a recording drum and carries 

 a base-line mirror and a Bourdon tube for recording temperature. A 

 portable recording device using only the magnetic sj^stem of a balance 

 magnetometer is illustrated in Fig. 8-22. A method for remote recording 

 of magnetic variations by means of a balanced photoelectric cell arrange- 

 ment has recently been described by Graf.®^" 



IV. CORRECTIONS 



In magnetic prospecting it is necessary to eliminate or correct for tem- 

 perature, daily variation, planetary variation, base change, influence of 

 iron and steel objects, terrain anomalies, and regional anomalies. These 

 corrections will be discussed in the order indicated. 



A. Temperature 



1. Elimination. The action of temperature may be disregarded in 

 instruments which are well insulated, are compensated for temperature, 



'2 See Hazard, op. cit. (2nd ed.), Fig. 14, and pp. 108-112 for directions for operat- 

 ing a magnetic observatory. 



62" A. Graf, Beitr. angew. Geophys., 7(4), 357-365 (1939). 



