ELECTRICAL METHODS 449 



first step consists in surveying two base lines at right angles to each other 

 (lines OF and 04 in Figure 266) and then surveying two series of regu- 

 larly spaced traverse lines, each series being parallel to one of the base 

 lines. Their intersections form a grid-work of stations. A satisfactory 

 field procedure consists in setting up the potentiometer at a station on one 

 of the base lines and measuring the difference in potential between that 

 reference point and other points along a traverse line which passes through 

 the station and is parallel to the other base line. (To connect the potentiom- 

 eter with the distant base line, it is convenient to use a reel containing 

 about 1000 feet of insulated wire.) 



Upon completion of the traverse line the wire is disconnected from the 

 instrument and anchor stake and is wound up by the reelman as he returns 

 to the instrument. The instrument is then carried to the next station, set 

 up, and the potential measured between the new base station and the pre- 

 vious station. The reel is now moved along the new traverse and the pro- 

 cedure repeated. Should the work extend over a distance greater than 

 1000 feet, the potentials between the various base stations should be checked 

 once or twice daily, so that corrections can be made for marked potential 

 fluctuations. 



An average lineman, who handles the reel and digs holes (to a depth 

 of 3" to 6''') for the non-polarizing electrode, usually can contact from 

 10 to 15 stations, at one hundred foot intervals, per hour. Slightly greater 

 speed is obtained by having an assistant dig the electrode holes. 



Sources of Error 



Considerable potential variations will oftentimes occur in an area due 

 to natural earth currents, rains, changes in temperature (especially in 

 regions where the nights are cool and the mid-days hot), and freezing and 

 thawing weather. Additional potential variations caused by industrial or 

 mining operations will be found in many localities, and these are usually 

 the most common source of error in obtaining accurate field data. 



Earth Currents. — The phenomenon designated as "earth currents" 

 is due to a great variety of causes. The potential difference between two 

 grounded electrodes consists of several components ; some change with time 

 relatively slowly, and others fluctuate rapidly and irregularly. The earth 

 potential components of preponderantly direct-current character include : 

 the electrode potentials already described ;* potentials due to oxidizing ore 

 bodies ; a regional gradient in the area which amounts, according to 

 place and time of year, to some 10 to 100 millivolts per kilometer. 



* The electrode potential variations are, of course, minimized when non-polarizing 

 electrodes are used. 



