A new gravity interpretation: A case study from Pahute Mesa, Nevada test site

The separation of regional and residual components from observed Bouguer gr avity anomaly is crucial in gravity intepretation. A new approach is applie d in this study to compute the residual anomaly of the Pahute Mesa region l ying on the north-west corner of Nevada test site. The drilling in Pahute M esa revealed a structural depression, which in fact, is a caldera, known as Silent Canyon. This region is topographically very high (> 2000 m) and yet the lowest gravity values (similar to - 220 mGal) are observed here. This appears to be an exceptional case in the Nevada test site. Geologically, th e low-density Cenozoic sedimentary and volcanic rocks: in the Silent Canyon overlie the denser pre-Cenozoic basement. The thickness of the top sedimen tary and volcanic rocks needs to be redefined for the conduction of nuclear tests. The basement depth determination by a previous USGS study in the Pa hute Mesa region appears to be inconclusive. The basement thickness was pre dicted to be 7080 ft, whereas the drill hole UE 20f in the central region u p to a depth of 13,686 ft did not encounter the basement rocks. In sharp contrast, the thickness of the Cenozic sedimentary and volcanic se quences not too far from the drill hole UE 20f in the Silent Canyon, was es timated to be 17,000 ft by interpreting residual gravity anomaly obtained b y our new approach, based on the finite element concept. This depth estimat e seems to be geologically more realistic. The basement configuration of th e Pahute Mesa site has been constructed by interpreting a number of residua l gravity profiles.