Prior knowledge assumes a homogeneous and uniform basement at Site 1224 that is typical of <100 Ma oceanic crust on the Pacific plate. The core and logging data, however, revealed five distinct units with varying petrophysical properties. Unit I (above 45 mbsf) contains high-porosity, low-density, and low-velocity rocks filled with basaltic fragments. Unit II (45–63 mbsf) is characterized by uniformly high velocity and low porosity massive basalt flows. Unit III (63–103mbsf) consists of relatively uniform and high-velocity breccia and sheeted lava flows. Unit IV (103–142 mbsf) contains pillow lavas as seen from the FMS log, and Unit V (below 142 mbsf) is composed of dense basalts with a uniform density of ~2.9 g/cm3. In addition, there is a hydrothermal vein at the base of the upper basement at the site based on FMS image analysis and interpretation of temperature and gamma ray logs.
This study further shows the seismic evidence of this five-layer structure of the upper oceanic crust in the vicinity of the drilled site. Conventional seismic data with dominant frequency <100 Hz cannot resolve the detailed internal structure of the <140-m-thick upper oceanic crust as revealed by logging and core data. Only through advanced seismic resolution enhancement can the boundaries of these logging units be clearly identified on the processed seismic section and extended laterally away from the drill hole. The fault systems at and near the site can also be imaged clearly. The hydrothermal vein can be uniquely identified by a strong reversed polarity seismic signature compared to the seafloor reflection. The hydrothermal vein is open to seawater through its link to the fault systems in the southwestward uplift. This finding may indicate geophysical evidence of possible hydrothermal circulation in the upper oceanic crust, which could help explain the observed microbial activity at the drill site.
During the course of this study, laboratory core measurements of porosity, density, compressional wave velocity, and shear wave velocity under pressure were also performed. These measurements agree well with in situ downhole logging data where the rocks are dense. In rocks filled with breccia and pillows, core measurements approximate only the limits of the dense rock matrixes, neglecting any existence of large-scale structures. This is the ever-challenging upscaling problem from the centimeter scale of the core plug to the meter scale of the logging measurements. It is evident that in situ downhole logging and high-resolution seismic data are essential to reveal meter-scale shallow oceanic crustal structures. This can become important and essential as the need increases to better map the internal structures of the upper oceanic crust and therefore understand the hydrothermal circulation systems and their associated microbial communities.