GRAVITATIONAL METHODS 429 



presence of the structure with sufficient accuracy for reconnaissance 

 purposes. 



The Mykawa field is on an uplift supposedly produced by a deeply 

 buried salt mass. If this supposition is correct, the salt mass must lie at a 

 depth greater than that of the deepest well which was drilled to 7,350 feet 

 without hitting the salt. The Mykawa production comes from sands at 

 depths of 4100 to 4900 feet. 



The gravity minimum extends over a considerable area. In fact, it ex- 

 tends considerably beyond the limits of this survey in a southeasterly 

 direction. The field is somewhat displaced from the center of the minimum, 

 but such displacement could have been predicted by noting that the value of 

 gravity drops toward the minimum much more rapidly on the northwest 

 than on the southeast. This condition is produced by a salt mass of such ver- 

 tical section that the center of gravity of the salt is not directly below its 

 highest point. The center of the minimum is of course very nearly above the 

 center of gravity, while the top of the uplifted beds is above the highest 

 point. From a study of the exact shape of such a minimum it is possible 

 to make a good estimate of the location most favorable for finding pro- 

 duction. 



Gravimeter Survey over Sedimentary Structures. — An interesting 

 example of the way in which a gravimeter survey reveals subsurface geo- 

 logic structure as related to the search for petroleum and of some of the 

 geologic factors which enter the interpretation has been given by L. H. 

 Boyd.f This example illustrates how in three cases gravity maxima were 

 associated with structural highs, while in a fourth case a gravity minimum 

 reflected a structural high, all in the same general area. 



Boyd presents the results of gravimetric work along the Kettleman 

 Hills-Lost Hills trend on the west side of the San Joaquin Valley in Cali- 

 fornia. Figure 260 shows the gravity contours for the area, at an interval 

 of 0.50 milligals, based on station values reduced to datum by correcting 

 for geodetic position, for elevation and for the Bouguer efifect. For the 

 latter two, the constant 0.069 milligals per foot was applied, which assumes 

 a density of 2.0 for the surface sediments. For the first, or latitude correc- 

 tion, the international formula for gravity of the U. S. Coast and Geodetic 

 Survey was employed. 



In the northwest part of the gravity map (Figure 260) there is an 

 elongated closed gravity maximum, the axis of which runs in a northwest- 

 southeast direction through T. 22 S. R. 18 E. This prominent gravity high 

 correlates with the structural closure on Kettleman North Dome, as 

 expressed by subsurface contours given in Figure 261. The agreement is 

 quite striking in respect to width and alignment between gravity and 

 structural features. 



t L. H. Boyd, "Gravity Meter Survey of the Kettleman Hills-Lost Hills Trend, California,' 

 Geophysics, Vol. XI, No. 2, April, 1946, pp. 121-128. 



