GRAVITATIONAL METHODS 409 



A valley next to a station acts in the same manner. It represents a deficit 

 in attractive force due to the absence of earth material in it. A gravimeter 

 would give a lower reading in this case than if the valley were filled (as 

 assumed in the Bouguer correction) instead of being occupied by air. The 

 meter reading has been reduced as if the ground were flat. The terrain 

 calculation shows the amount of over-correction and, by adding it to the 

 gravity value, rectifies the error. 



The above explains why the average height of a segment of terrain is 

 calculated without regard to sign. Although the statement appears contra- 

 dictory, it is true that hills and valleys act in a similar manner in their effect 

 on a gravimeter. 



Density Profiles. — The Bouguer correction as applied in gravity sur- 

 veys, as has been brought out, deals with the effect of all of the material 

 from the ground surface to the datum plane. For a given station the amount 

 of this correction in milligals equals the factor 0.0127 • density • the 

 distance in feet between the stations and the datum. In equation form, 

 correction = 0.0127 • d • distance in feet. 



It can be shown that for a distance of 100 feet from station to datum 

 an error in the assumed density of 0.1 would give an error of approximately 

 0.13 milligal. The value used for the density is considered to apply to the 

 entire slab of material from the station down to the datum. Unless density 

 determinations from drill cuttings for the geologic section to the datum 

 are available, surface sample density values plus a factor of judgment are 

 our only guides as to what the assumed density should be. 



However a method has been outlined by L. L. Nettletonf whereby the 

 effective, in place, density of a section of earth materials can be determined. 

 The procedure is not complicated. A special traverse of gravimeter stations 

 is run across a topographic feature, either a hill or a valley. The obser- 

 vations are corrected for latitude and for elevation in the usual manner. 

 The resulting free air gravity values are plotted on a cross-section showing 

 the elevations of the stations. These gravity observations (corrected for 

 latitude and elevation only) will show a certain agreement with the topo- 

 graphy across which they were taken. They will reflect the effects of the 

 hill or the valley. 



The next step is to complete the reduction of these free air gravity 

 values by applying the Bouguer correction, using a number of different 

 assumed values for the density factor. The Bouguer gravity values cor- 

 rected at the different densities are also plotted on the section. The graph 

 will show that the Bouguer gravities will still reflect the topographic feature 

 if the density chosen was too small. The graph will likewise show that where 

 the density selected is too great the observations will be over-corrected. It 

 will exhibit a decrease in the plotted graph across a hill. 



t L. L. Nettleton, "Determination of Density for Reduction of Gravimeter Observations," 

 Geophysics, Vol. 4, No. 3, July, 1939, pp. 176-183. 



L. L. Nettleton, "Geophysical Prospecting for Oil," McGraw-Hill, New York, 1940 



