574 



EXPLORATION GEOPHYSICS 



work two holes were drilled to a depth of 600 feet and both encountered 

 heavy zones of broken meteoritic iron from about 450 feet down, t 



Detailed Resistivity Mapping of the Wilmar Gravel Deposit/^' — 



The detail to which a gravel deposit can be mapped by fixed depth resistivity 

 traverses is shown by work at the Wilmar gravel deposit, near Wilmar, 

 Minnesota, reported by S. W. Wilcox.J The small abandoned sand pit 



SECTION ON L»CS ni2-B6 



Fig. 355. — Probable geologic sections, Meteor Crater. 



Qd— Debris 



Q^ — Landslides 



Ql — Lake beds 



T^jj — Moencopic sandstone 



(7k — Kaibab limestone 



^jj — Coconino sandstone 



^g — Supai formation 



(A.I.M.E. Geophysical Prospecting, Technical Paper, 1932.) 



located in the upper left of Figure 356 furnished the clue for the geo- 

 physical exploration of the locality. 



Four fixed depth traverses were run, spaced 100 feet apart, using a 

 20- foot constant electrode spacing to secure the data for drawing the equi- 

 resistivity contours to outline the deposit. Logs of the test holes put 

 down in the area investigated are included in Figure 356. Test hole No. 7 

 showed no sand or gravel and thus served to establish one limit of the 

 gravel body. It appears that the area within the 1000 ohm-foot contour 

 contains substantially all that remains of an ancient stream channel gravel 



t C. H. Wilson, "Drilling Proves Existence of Meteoric Mass," The Mining Journal, April 

 30, 1932. 



* Dart Wantland, personal communication. 



t S. W. Wilcox, "Sand and Gravel Prospecting by the Earth Resistivity Method," Geophysics, 

 Vol. IX, No. 1, January, 1944. 



