228 



EXPLORATION GEOPHYSICS 



lying rocks. Thus, if the magnetic beds overlie a synclinal or old erosional 

 low feature, a magnetic high will be produced, as illustrated in Figure 111 A, 

 due to the greater thickness of the magnetic material over the deeper con- 

 tact. When the upper magnetic beds overlie a buried hill or other feature 

 of relatively low permeability, as in Figure 11 IB, a magnetic low will 

 result due to the lesser thickness of magnetic material over the basement 

 high. (A supplemental geophysical method (such as electrical or seismic) 



Travtrv* V jfanc^ 



Trayfr^f D jM/JCr 



Maqntfic P*rmtabilihf: a >h 



Fig. Ill — A, magnetic high associated with increased thickness 

 of shallow magnetic beds, and B, magnetic low associated with 

 decreased thickness of shallow magnetic beds. 



capable of indicating variations of thickness in the upper layer may be 

 employed in problems of this type. Interpretation would then be based 

 upon both types of data.) 



Placer Deposits. — In a majority of placers the greatest concentra- 

 tion will be on the bedrock. Occasionally, the best values occur on a false 

 bedrock ("hardpan" or conglomerate) which overlies the bedrock. The 

 placer material is composed chiefly of the magnetic black sands and the 

 other heavier materials, including the gold values. The magnetometer may 

 be employed to locate the areas of magnetic high, which will be the zones 

 of concentration of the placer material. Because the gold is found adjacent 

 to the bedrock or false bedrock, it is important to know the depths to the 

 bedrock and the thickness of the fill material. This may be accomplished 

 by electrical or shallow refraction seismic methods. 



Figure 112 shows a typical cross-section of a placer from a geophysical survey 

 conducted in Trinity County, California. The largest concentration of placer material 

 was indicated by the magnetic high, mapped with a vertical component Askania mag- 

 netometer. With the magnetometric method the complete outline of the old stream bed 

 was obtained, together with the thickness of the lower gravels and the overlying clay 

 and sand fill. With this information the necessary depth of test pits and drill holes 

 was easily determined, and test locations recommended. 



Another simple magnetic survey is illustrated by the contour results of a survey 

 of a placer property in Pinal County, Arizona.f (Figure 113.) The general regional 



t J. J. Jakosky and C. H. Wilson, "Geophysical Studies in Placer and Water-Supply Problems," 



A.I.M.E. Geophysical Prospecting, Tech. Pub. 515, 1933. 



