428 



EXPLORATION GEOPHYSICS 



at a distance of about three miles from the center. There is also a small 

 positive gravity effect superposed on the large minimum, appearing 

 as a maximum of four-tenths of a milHgal near the center of the dome. This 

 feature, no doubt, is caused by the shallow denser cap rock. The gravity 



GRAVIMETER SURVEY 



PIERCE JUNCTION -MYKAWA AREA 



HARRIS COUNTY. TEXAS 



GRAVIMETER PROFILE A- A 



GEOLOGICAL CROSS SECTION A -A 



Fig. 259. — Pierce-Mykawa gravimeter survey. (L. M. and F. W. Mott-Smith, Personal 



Communication.) 



maximum is somewhat displaced from the actual position of the cap rock 

 as outlined by the producing area at the periphery of the dome. The dis- 

 placement is principally caused by the regional gravity change and can be 

 corrected for by applying corrections for regional change.* It can be seen, 

 however, that even without correction, the gravity contours indicate the 



*In using gravity work for detailing such structures, corrections for regional 

 change are made whenever it is large enough to be important. 



