GRAVITATIONAL METHODS 399 



car. The instrument is carried in a shock-resistant cradle. When a reading 

 is to be made, a small instrument tripod is placed upon the ground. The 

 table of the tripod is leveled with a level bubble. The gravity meter is 

 then lifted from its cradle, placed upon the tripod, and carefully leveled 

 by means of the level bubbles on the meter. 



The general field procedure* with direct gravity instruments of the pen- 

 dulum or gravimeter type is similar to that with a magnetometer. The 

 initial step for a field survey is the lay-out of a set of station locations in the 

 area to be explored. The location of stations is done by a field surveying 

 party. Each station is marked with a flag or stake for easy discovery and 

 preferably located along roads or trails. The stations may be arranged in 

 checkerboard fashion, or placed along traverse lines, etc., depending upon 

 the nature of the problem. Each station is plotted on a field map or re- 

 corded by proper survey notes. For most reconnaissance work one sta- 

 tion per mile is sufficient. In areas where more detail is needed as many 

 as ten stations to the mile may be required. Occasionally when making a 

 detailed survey in areas where abrupt changes are present, a closer spacing 

 may be advisable. The optimum spacing of stations is governed by local 

 conditions. The surveyors run relative elevations on all of the stations, 

 and if necessary plot topographic contours. The survey work proceeds 

 simultaneously with the gravity survey, with the surveyors working suffi- 

 ciently ahead of the instrument party to prevent delays in finding stations. 



The instrument is first set up at a chosen base station and adjusted for 

 the area. A measurement is then taken, followed after a short interval by 

 a second observation to guard against mistakes and to allow averaging of 

 results to increase the accuracy. This procedure is then repeated at succes- 

 sive stations. After five to ten stations have been occupied the instrument 

 is returned to the base station for a check observation. Usually the next 

 step is to make a subsidiary base station of one of the new stations con- 

 veniently located for further expansion of the survey. This is accomplished 

 by taking a second or repeat observation at the desired new base station. 

 Each base station is simply a field station whose gravity value relative to 

 another base has been made more certain by being occupied or measured at 

 least twice. The survey then proceeds in the same manner from the new 

 base, check readings now being taken at this base. In this way the area 

 is gradually covered with a series of bases and ordinary stations. The use 

 of subsidiary base stations avoids long trips back to the main base for check 

 readings. 



During the field work, a computer at the field office calculates the prior 



* Personal communication from L. M. and F. W. Mott-Smith. Compare L. M. and 

 F. W. Mott-Smith, The Petroleum Engineer, July, 1939. A major portion of this sec- 

 tion is essentially a description of the general technique employed by the Mott-Smith 

 Corporation and summarizes the important aspects of tested field procedure, 



