Wide Angle Reflection Study of Crustal Structure, Eastern Pennsylvania - Northern New Jersey

Abstract

A 12 channel, digitally recorded wide-angle reflection profile was conducted in the northeast trending Great Valley of eastern Pennsylvania-northern New Jersey. The wide-angle reflections were inverted to determine the crustal velocity structure of the region:, Specifically, the experiment was designed to determine if the Conrad discontinuity existed in eastern PA - northern NJ and to determine its depth and the thickness of the crust. Six quarry blasts, timed by a geophone located at the quarry, were recorded at three offsets of 73.8 km, 118.3 km, and 146.7 km. The common-receiver array was located at an abandoned railroad track parallel to Paulinskill Lake, Sussex, County, New Jersey. It consisted of 11 geophone groups spaced 213 meters apart. Since the reduced data contained a low S/N ratio for which the computer was unable to pick the P-arrivals, the data were treated subjectively for the sweep of the P-wave arrival across the array at the expected time. The travel time and normal moveout were then calculated for coherent phase signals across the traces. These data were plotted on a t(p) (intercept time-ray parameter) section and best-fit by hand with two elliptical curves. A cusp in the data suggests a 2 layer structure with a velocity discontinuity at 29 km. The upper layer has a near surface compressional wave velocity of 5.8 km. The lower layer is 17 km thick with a velocity of 7.7 km. This velocity transition suggests that the Conrad discontinuity has been observed in the study area.