The seismometer hole, Hole 1224D, was cored from 25.5 to 59 mbsf with 15.65 m of core recovered. Hole 1224F is <20 m southeast of Hole 1224D and was cored to a depth of 174.5 mbsf with 37.7 m of core recovered. Downhole logging measurements in Hole 1224F were made to a total depth (TD) of 174.5 mbsf. The water depth was 4967 m and the sediment thickness was ~28 m at the site. The base of the pipe in Hole 1224F was at 35 mbsf during logging.
Hole 1224F was logged using a nearly complete suite of wireline logging tools that measured nuclear, acoustic and shear, and electrical and magnetic properties. Borehole electrical image and temperature logs were also acquired. Because of the low core recovery (~20%), logging measurements in Hole 1224F were vital for inferring the structure of the upper oceanic crust at this site. Detailed descriptions of the logging tools and preliminary logging data analysis are referred to Stephen, Kasahara, Acton, et al. (2003).
The density (RHOB), neutron porosity (NPHI), compressional wave velocity (VP), and shear wave velocity (VS) acquired using the logging tools and used in this seismic study are shown in Figure F2. The five logging units shown in Figure F2 were classified using these and other logging data and based on core descriptions and downhole Formation MicroScanner (FMS) image analysis (Stephen, Kasahara, Acton, et al. 2003). 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 (63–103 mbsf) 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. Unit V (below 142 mbsf) is composed of dense basalts with a uniform density of ~2.9 g/cm3. In addition, a hydrothermal vein between 138 and 142 mbsf was detected by all the logging tools, particularly by elevated temperature and natural gamma radiation (Stephen, Kasahara, Acton, et al. 2003).
Shipboard moisture and density and compressional wave velocity measurements were compared with in situ downhole logging data. Descriptions of these shipboard measurements are referred to Stephen, Kasahara, Acton, et al. (2003). As shown in Figure F2, core measurements of porosity, density, and compressional wave velocity agree well with in situ downhole logging data where the rocks are dense, especially in Unit II. In other units, filled with breccia and pillows, core measurements approximate only the limits of the dense rock matrixes without sensing any existence of large-scale structures. Shore-based compressional and shear wave velocity measurements under elevated pressures were obtained on 11 samples without visible fractures from both Holes 1224D and 1224F. Good ultrasonic shear wave signals were obtained only on 7 samples. The compressional and shear wave velocity measurements at estimated in situ pressure are plotted in Figure F2 as triangles. In the dense Unit II, both shipboard and shore-based wave velocity measurements agree well with downhole logging data. Figure F3 shows typical velocity measurement results under pressure for a seawater-saturated core plug from Section 200-1224D-2R-3 from a depth of ~38 mbsf. The pressure effect on wave velocity under saturated condition is minimal for dense basaltic rocks, as expected, and the shear wave anisotropy is negligible.