572 * EXPLORATION GEOPHYSICS 



The dips of the strata vary somewhat around the rim and the eleva- 

 tions of corresponding horizons also vary, mainly because of the effect 

 of the numerous near-radial faults. These faults are rather symmetrically 

 distributed around the crater and are very similar in character. Displace- 

 ments of the beds by these faults may amount to 85 feet in a vertical 

 direction. In addition to the faulting which now shows in the rim of the 

 crater, the rim rocks have suffered considerable fracturing. The main 

 geological features of the bottom of the crater are the extent and distri- 

 bution of quaternary fill material, which consists essentially of the talus 

 slopes extending from the crater floors up the inside slope of the rim, and 

 the very fine sandy material covering the location of the playa lake and the 

 lake beds below. The distribution of talus is also fairly uniform around 

 the crater rim, being less extensive on the northwest side of the crater 

 where, correspondingly, the fine fill material extends farther out toward 

 the rim. 



The chief purpose of the geophysical work was to determine the sub- 

 surface conditions in the crater and the possible occurrence of any large 

 body of meteoritic material. 



Electrical Survey. — The geoelectrical studies were conducted with a 

 low frequency alternating current resistivity apparatus. One current 

 electrode remained at a fixed position within the crater. The other current 

 electrode and the two intermediate potential electrodes moved prog- 

 ressively outward as the depth of investigation was increased, the lines 

 of measurement being radial with respect to the center of the crater. 



A center point selected within the crater was used as a hub or refer- 

 ence point. Resistivity studies were made at stations distributed circum- 

 ferentially around the crater as follows : ( 1 ) 1000 feet from the center 

 point and entirely within the crater proper; (2) 1500 feet from the 

 center on the inside slopes of the crater rim; (3) 2000 feet from the cen- 

 ter and on the crater rim; (4) additional studies at points within and 

 outside of the crater at several stations selected after the first section had 

 been completed and a general idea obtained as to depth of bedrock in the 

 crater. Spacing between stations was selected to give sufficient overlap 

 to allow proper mapping of the subsurface structure. The orientations of 

 the traverses were: (a) along lines radially from the center and (b) 

 along fines (chord lines) at right angles to (a). On both radial and 

 chord lines studies were made to effective depths of 1800 feet. 



Brief Summary of Results 

 Due to the symmetry of the crater, the resistivity curves all exhibit 

 similar characteristics. (Figure 354.) Final interpretation of the data 

 was based chiefly upon direct curve correlation. From the geophysical 

 results a subsurface structural map was prepared showing probable 

 thicknesses of fill material, bedrock contours, etc. This is shown by the 

 cross section of Figure 355. 



