The following is a semiannual progress report, preliminary in nature, covering work done in the period of December 1, 1958 to May 31, 1959.

Report discussing the progress made on a continuing geophysical program during the period from January to April, 1959. The purpose of the program was to determine the type and configuration of various rock units below the surface in areas of interest at the Nevada Test Site.

"This Progress report describes a part of a continuing geophysical program being conducted by the U.S. Geological Survey on behalf of the Albuquerque Operations Office, U.S. Atomic Energy Commission"

From introduction: The results presented in this preliminary report are principally concerned with the ground motion caused by seven underground nuclear explosions (table 1) in the bedded tuff of the Oak Spring formation in Area 12. Ground motions were recorded at stations as close as 5 kilometers and as far as 300 kilometers from the explosions (fig 1 and table 2). The results for the Bianca and especially the Evans explosion are meager because the first was accompanied by a Test Site-wide moznentary power failure and because the yield of the second was so far below the anticipated yield that uncertain results were obtained. Ground motions caused by most of the air explosions in Operations Plumbbob and Hadtack - Phase 2 were recorded and have been partly analyzed. Results are presented in this report only where they relate significantly to analysis of the underground explosions. The effects of most of the small underground explosions in Area 3 (center of Yucca Flat) that had significant yields were also recorded. The results have not yet been interpreted.