During Ocean Drilling Program Leg 200, 16 December 2001–27 January 2002, Hole 1224D (27°53.3699´N, 141°58.7525´W) was drilled in the eastern Pacific (Fig. F1A) for later downhole installation of a broadband seismometer as a part of the global Oceanic Seismic Network (Stephen, Kasahara, Acton, et al., 2003). Within 20 m of this seismometer hole, Hole 1224F (27°53.3634´N, 141°58.7567´W) was drilled to a total depth of 174.5 meters below seafloor (mbsf) for logging to characterize the physical and chemical properties of the basement at the site.
The oceanic crust at Site 1224 has an age of ~45–50 Ma with a spreading rate of ~140 mm/yr (full rate). Prior studies (e.g., Raitt, 1963; Menard, 1964) concluded that the upper oceanic crust on the Pacific plate with age <100 Ma exhibits no internal structure other than being simple and uniform. The core and logging data acquired at this site, however, revealed five distinct logging units differentiated by petrophysical property variations within the <150-m-thick upper basement (Stephen, Kasahara, Acton, et al., 2003). A hydrothermal vein was identified near the bottom of the drill hole from logging data analysis. Microbial activity and fungal fossils were also observed at this site (Stephen, Kasahara, Acton, et al., 2003). It is quite evident that the physical and structural properties of the upper oceanic crust at this site are more complicated than previously inferred using seismic technology in the 1960s.
In this paper, we provide for the first time seismic evidence of the five-layer structure of the upper oceanic crust at the drilled site correlated with the logging units. Using advanced methods of seismic resolution enhancement, the boundaries of these logging units as well as the fault systems can be clearly imaged. The hydrothermal vein has a strong seismic signature with a negative polarity compared with the seafloor reflection. Seismic data further show that there are faults linking the hydrothermal vein to the seafloor. The detailed methodology of core-log-seismic integration is described in Sun (2000, 2004) and Sun and Goldberg (2000). Details of the seismic resolution enhancement method are given by Sun (2004).