GRAVITATIONAL METHODS 



427 



ness of some 3 to 4 m. The uncorrected (primary) curve shows an effect 

 of the contact with the schist. The reduced curve, which inckides a correc- 

 tion for the effect of the contact on the assumption that the difference in 

 density between the schist and leptite is 0.2, shows indications of two ore 

 zones.* 



Fig. 258. — Gravity anomaly over a pyrite deposit located under a lake 

 in the Skellefte district, Sweden. (Hedstrom, A.I.M.E. Geophysical 

 Prospecting, Tech. Pub. 953.) 



Petroleum Surveys t 



Gravimeter Surveys over Salt Domes. — Figure 259 shows an exam- 

 ple of a survey conducted with a gravimeter in the vicinity of Houston, 

 Texas. The survey was made in nine working days, included 169 stations, 

 and covered an area of approximately 100 square miles. 



The area covered during the survey includes the Pierce Junction and 

 the Mykawa oil fields. The Pierce Junction field is a piercement type dome 

 having a substantially vertical salt plug approximately one mile in diameter. 

 The cap rock overlying the plug is about 250 feet in thickness, and this in 

 turn is covered with about 700 feet of recent material. The top of the salt 

 is approximately 950 feet below the surface. The salt plug has a lower 

 density than the surrounding rock, and its presence causes a gravity low 

 or minimum. The gravity picture is complicated by the presence of a re- 

 gional change of gravity, with the values increasing toward the south against 

 the latitude gradient. l*\irther complications are introduced by the neigh- 

 boring gravity minimum due to the Mykawa field toward the southeast. 

 The diagnostic gravity feature is a large so-called minimum nose, clearly 

 indicated by several contours. This feature results when the gravity mini- 

 mum caused by the salt plug is superposed on the regional or lateral 

 change of gravity. A study of the contours shows that the decrease in 

 gravity due to the dome is about two milligals and its effect is noticeable 



* Note that the gravimeter did not "resolve" or differentiate between the two 

 southern veins. 



t L. M. and F. W. Mott-Smith, personal communication; also, see "Advancements in the Use 

 of the Gravimeter, in Oil Exploration," The Petroleum Engineer, July 1939, pp. 85-97. 



