It is evident, as shown in Figures F6, F7, F8, and F9, that conventional seismic data with dominant frequency <100 Hz could hardly discern the detailed internal structure of the upper oceanic crust of <140 m thick revealed by logging and core data. In comparison with the original seismic section in Figure F4 and the BP-filtered seismic section in Figure F12, the quality of seismic data was drastically improved after Highres processing and resolution enhancement as shown in Figure F13. Many features, including clear-cut faults, can now be interpreted.
The seismic reflections from the boundaries of the five logging units can be easily identified on the Highres-processed SCS data as shown in Figure F11, in comparison with the synthetic seismogram generated at the drill site. Because SP 16002 is in the close vicinity of the drill site and the logging data are overall of good quality, the correlation between the synthetic seismogram and the Highres SCS data is quite good. Once the seafloor reflection or the water/sediment interface reflection in the Highres seismic section is tied up with that on the synthetic seismogram, the rest of the boundary reflections from the interface between the sediment and Unit I and those between the boundaries of the five logging units are well-aligned, as indicated in Figure F11.
The reflections from the hydrothermal vein on both synthetic seismogram and Highres SCS data show reversed polarity compared to the seafloor reflection as shown in Figure F11. This is because of the low acoustic impedance contrast between the fluid-flowing hydrothermal vein and the adjacent basement interfaces as shown in Figure F6 and F7. Also in Figure F11, the reflection caused by the hydrothermal vein on the synthetic seismogram appears much stronger than that revealed on the Highres SCS data as well as the reflection from the interface between the sediment and basement Unit I. These are attributed to washout during drilling (Stephen, Kasahara, Acton, et al., 2003). Compared to the large impedance contrast caused by the presence of the hydrothermal vein, the impedance contrast, as well as the reflection and seismic amplitude of the boundary between Units IV and V, is relatively weak (Fig. F7). The boundary reflection between Units IV and V is thus masked by the presence of the nearby hydrothermal vein as shown in Figure F11; however it can still be clearly seen on the Highres SCS data as indicated in Figure F13.
Seismic imaging of the fault systems in the vicinity of Site 1224 was drastically improved as can be seen by a comparison of the unprocessed SCS section in Figure F4 and the Highres-processed SCS section in Figure F13. A fault cutting the structures at SP 16087 that is hardly visible in Figure F4 is clearly seen on the Highres SCS section in Figure F13. Many faults clearly imaged on the Highres seismic section could have been overlooked using the unprocessed data as shown in Figure F4. More importantly, the Highres seismic data in Figure F13 indicate that the hydrothermal vein identified using both logging and seismic data above the boundary between Units IV and V is connected to the fault cutting the sediment and basement structures at SP 16040. It is evident that this hydrothermal vein remains open to seawater through its link to the fault systems in the southwestward uplift. This finding could provide geophysical evidence of possible hydrothermal circulation in this thin upper oceanic crust that may host the observed microbial activity at the drill site (Stephen, Kasahara, Acton, et al., 2003).