1146 Subsurface Geologic Methods 



zone. By this means an accurate relationship between the fault zone and 

 the structure foundation was shown. 



Keyhole Dam Site 



The Keyhole dam site is located on the Belle Fourche river near 

 Moorcroft, Wyoming. The river at an earlier stage aggraded a deep 

 channel through sediments of the Lakota formation. This channel has 

 subsequently been refilled with gravel, sand, and clay. One of the pro- 

 posed types of structure for the site was a slab-and-buttress-type dam. 

 It became necessary to know if the river-fill materials were of sufficient 

 density to provide adequate support for the buttress foundations. Normal 

 means of exploration were not deemed advisable. The alluvium contained 

 large-size gravel and cobbles, which precluded the use of normal drilling 

 and sampling methods. The water table was near the surface, making the 

 dewatering of a test pit some 50 feet in depth a costly operation. 



It, was decided to explore the area by means of a gravimetric survey. 

 The sediments are essentially horizontal, and sufficient drilling had been 

 done to outline the river channel in the area to be studied. A gravi- 

 metric survey, as applied to the regional exploratory problems, makes 

 use of the following two corrections to bring the data to a usable base 

 level: (1) a correction for elevation differences or a free-air correction 

 and (2) a correction for the density of the near-surface alluvia by means 

 of the Bouguer reduction. The first of these can be readily obtained from 

 the surface elevations. The second, namely, the Bouguer correction, is 

 the one of interest to this problem since this correction is made by assum- 

 ing various densities for the material between an arbitrary datum and 

 the ground surface. The correct average density will give a straight 

 line through the corrected points, which may dip in a direction of the 

 regional dip and may reflect the dip of the strata ; or, in the case of hori- 

 zontal strata, it may be horizontal. 



The theory behind this correction has been discussed by Nettleton,^ 

 who makes an interesting comment: 



The analysis of a density traverse consists primarily of plotting profiles 

 of the elevation and of the gravity values with the usual reduction for latitude 

 and free-air corrections and with different curves for the Bouguer correction 

 made with different densities. Under favorable conditions a quite definite selec- 

 tion can be made of the density which comes closest to giving reduced gravity 

 values on a straight line across the topographic feature. Frequently there are 

 some stations which do not fit into a smooth curve for any density having de- 

 partures of a few tenths of a milligal (milligal numerically equal to a milli- 

 dyne). Our experience indicates tliat these irregularities are caused by real 

 inhomogeneities in the material, as checked observations have confirmed the 

 gravity differences. 



The foregoing clearly sets forth the idea on which the field test 



' Nettleton, L. L., Determination of Density for Reduction of Gravimeter Observations: Geophysics, 

 vol. 4, no. 3, pp. 180 ff., July 1939. 



