MAGNETIC METHODS 149 



(7) Clamp the magnet system, remove instrument from the tripod 

 and place in the carrying-box, and proceed to the next station. 



Stray Magnetic Material 



During a magnetic survey, the operator must always be on the alert to 

 note the presence of any "tramp" iron which may cause erroneous anom- 

 alies. Iron or steel structures, pipe lines, culverts, railroads, direct current 

 power lines (including street car lines with ground return circuits), junk 

 piles, etc., are possible sources of error. A careful record of observed 

 surface conditions should be kept, in order that the readings made in the 

 neighborhood of such disturbances may be properly evaluated in the final 

 interpretation of the magnetic data. 



The operator should always remove personal articles which may be 

 magnetic. Errors are oftentimes caused by wrist watches, notebooks 

 with steel rings or backs, steel belt buckles (usually plated with the more 

 expensive metals), iron arch supports in shoes or boots, steel-plated eye- 

 glass frames, pocket-knives, Brunton compasses, fine wire in hat bands, 

 zippers, cameras, metal pencils, buckles on shoes, metal buttons on clothing, 

 etc. Horn-rimmed glasses can be obtained that are essentially non-magnetic. 



For the best field results it is desirable for a magnetometer operator 

 to develop his own standard routine practices in setting up and reading 

 the instrument. Such practices include : remaining on the same side of the 

 instrument when taking readings in both direct and reversed orientations ; 

 not walking around the tripod when it is set up more than is necessary; 

 keeping a wrist watch the maximum distance from the instrument by 

 placing the hands behind the back when taking readings ; and keeping other 

 persons at a distance while the instrument system is released. 



Preferred Field Technique and Calculation of Field Notes 



The following calculation of a sample set of field notes taken with a 

 vertical magnetometer will illustrate one satisfactory procedure for field 

 work. The form illustrated in Figure 64 was designed by Mr. John H. 

 Wilson. Bound field notebooks of this form are available.* 



The magnetometer is first set up at the base station with the tripod head 

 properly oriented. The first reading is taken with the N end of the instru- 

 ment in the magnetic east. Usually two readings are made (or more, if 

 necessary) to obtain readings that check within 0.1 to 0.2 scale divisions. 

 The two readings are entered in column 2 of the form under N. to E. 



The instrument is then reversed 180°, using an index mark on the tripod, 

 and a second set of readings is taken and recorded. Temperature and time 

 are also recorded, as shown. The position of the base and subsequent 

 stations must be known as to their location in a section, or on a control 



* The form is used here through the courtesy of the Kendrick-Bellamy Co., Denver, 

 Colo. 



