High-frequency reflections in granite? Delineation of the weathering frontin granodiorite at Pinon Flat, California

We analyze data from two orthogonal seismic Lines 336 m in length collected at Pinon Flat, California, over weathered granodiorite bedrock. Each Line was made up of 10 reversed segments 84 m in length. We analyzed the first a rrivals from these data and found dramatic variations in velocity along the profiles. An upper layer (approximately 2-m thick) known from trenching ne to be composed of soil and sandy grus had measured velocities ranging from 400 to 700 mis. Velocities inferred from refraction analysis of first arri vals of the reversed lines revealed a heterogeneous lower layer below the s oil with measured velocities of 1600-2700 mis by a depth of 15 m. We interp ret these data to be measuring velocities of a deeply weathered unit charac terized by granodiorite corestones embedded in a matrix of saprolite. The m ost remarkable feature of these data emerged from attempting to process the same data as reflection data. Simple bandpass filtering in the 250-400 Hz band revealed a bright, impulsive arrival with three characteristic propert ies: (1) irregular velocity moveout that is inconsistent with that expected from a layered earth model, (2) the arrival is at a nearly constant time-d epth on all data, and (3) the arrival tends to be followed by a ringing cod a whose frequency varies from trace to trace. This arrival ties exactly wit h a velocity discontinuity measured in a borehole located on one of the pro files that we interpret as the base of the weathered layer. We suggest this arrival is a specular reflection from a weathering front that occurs along horizontal sheeting joints at a fixed depth below the surface. This surfac e acts as an effective mirror for high-frequency seismicwaves which are the n channeled upward through an intact, high-e path of unaltered blocks of gr anodiorite to define the observed signals at the surface.