400 



EXPLORATION GEOPHYSICS 



Fig. 243. — Drift curve for consecutive reading 

 of stations. (L. M. and F. W. Mott-Smith, The 

 Petroleum Engineer.) 



day's work and builds up the gravity anomaly map. The usual process of 

 computation is as follows : (a) The instrument scale readings are first 

 converted into milligals by using a calibration curve; (b) the drift 



curve is then obtained by plotting 

 the value of gravity against the 

 time at which the reading was 

 taken. Figure 242 shows a drift 

 curve representing the work of an 

 average day. Analysis of the drift 

 curves gives considerable informa- 

 tion on the progress of the work 

 during day. It also serves as a par- 

 tial check on instrument operation 

 and is used to compute the dif- 

 ferences in the gravity between the 

 several stations. 



On this particular day the operator first occupied base number 4 at 

 8 :00 A.M. and made an observation. He then occupied stations 16 to 22 and 

 returned to the initial base number 4 for a check reading at 10:20 A. M. 

 If the instrument were perfect, he would have obtained exactly the same 

 reading. Actually, due to slight instrumental inaccuracy or drift in the 

 two hour and twenty minute interval, a reading was obtained which was 

 0.05 milligal less than the first reading. The instrument was then returned 

 to station number 22, thus securing a second observation at this station and 

 estabhshing it as a new base. Four new stations were then observed and a 

 check reading at the new base taken. Station 26 was then established as a 

 base and finally seven new stations were observed, with two check readings 

 at base 26. It will be noted that on this particular day the instrument was 

 set up 25 times, 18 new stations were determined, and two new bases 

 established. 



The difiference in the gravity between the various bases and stations is 

 taken directly from the drift curve. Small discrepancies between the various 

 observations at a base are allowed for by drawing a mean straight line 

 through the points. This procedure involves the assumption that the in- 

 strument drifted at a constant rate between observations. In usual field 

 work the drift is fairly constant, although erratic results occur occasionally 

 due to peculiarities of gravity instruments when subjected to rough hand- 

 ling, etc. However, experience has shown that this method is sufficiently 

 accurate for practical purposes. The gravity difference in milligals between 

 each station and the base to which it is referred is taken from the plot by 

 measuring vertically between the drift lines. 



Knowing the difference in gravity between each station and one of the 

 bases, and the differences between bases, the gravity difference between 

 all stations and bases and the main base can at once be obtained by simple 

 arithmetic. At the start of the survey the main base in each area is given an 



