Acquisition and preliminary analysis of oriented multi-component multioffset VSP data: DOSECC Cajon Pass Deep Scientific Drillhole

In October 1987 a four‐offset VSP (Vertical Seismic Profile) was acquired at the Cajon Pass Deep Scientific Drillhole. Preliminary velocity analysis of the P‐wave direct arrival has identified zones in the upper crystalline section of the borehole where interval velocities derived from a long‐spaced sonic log differ greatly from the VSP interval velocities. Large velocity differences were observed between the three near‐offset VSPs in the upper crystalline section of the borehole. These differences are interpreted as being associated with the complex geologic structure near the Cajon Pass borehole. Two source positions, one 35 m from the borehole and one 346 m from the borehole were used to analyze the horizontal component holograms of the P‐wave direct‐arrival. The near‐offset hodograms exhibited particle motion directions which were not coincident with the VSP source direction when the downhole receiver was in crystalline rock. The far‐offset hodograms exhibited particle motion directions which were generally consistent with the source direction.