-in drill bit to 70 mbsf.
The cased reentry hole (Hole 1224D), which is suitable for a broadband borehole seismometer installation, is located at 27°2.916´N, 141°9.504´W in 4979 m water depth, 1.48 km northeast of the H2O junction box. Until the downhole seismometer is properly installed, casing will act to stabilize the reentry cone. A reentry cone with 24 m of 20-in casing was installed by jetting-in. Suspended below the reentry cone is 58.5 m of 10.75-in casing, which was cemented into a 30-m-thick, well-consolidated, massive basalt flow underlying 28 m of soft, red clay.
Prior to installation of the 10.75-in casing, Hole 1224D was cored using the rotary core barrel (RCB) through the sediment/basement interface at ~28 mbsf and down through basaltic basement to a depth of 59 mbsf. We cored ~31 m of aphyric basalt with 15.7 m recovery. Basalts consist of several flows. In the upper part of the red clay layer, radiolarians are abundant. After RCB coring, we expanded the hole using a 14-in drill bit to 70 mbsf.
After preparing Hole 1224D for the borehole seismometer, we drilled a single-bit hole (1224F), to 174.5 mbsf to acquire sediment and basalt samples for shipboard and shore-based analysis as well as to run a logging program. Hole 1224F is <20 m southeast of Hole 1224D, and measurements in Hole 1224F can be used to infer the structure surrounding both holes. Since Leg 200, another cased reentry hole (Hole 1243A) suitable for a borehole seismometer was drilled during Leg 203 at 5°18.0541´N, 110°4.5798´W in the eastern Pacific (Orcutt, Schultz, Davies, et al., 2003).
Amplitude-spectral characteristics of seismic signals could be affected by wind speed, sea state, shear resonance effects in the sediments, whales, air and/or water gun shooting, earthquakes, passing ships, and drilling-related activities such as bit noise and running pipe. Drilling-related noise from the JOIDES Resolution was observed in data collected by the shallowly buried seismometer (frequency band 0.01–80 Hz) being operated at the site by the University of Hawaii (Duennebier et al., 2002) (www.soest.Hawaii.edu/H2O/). Seismic data from the H2O observatory were acquired continuously from October 1999 until May 2003, and the data are available to scientists world-wide through the Incorporated Research Institutions for Seismology (IRIS) Data Management Center in Seattle. Drilling activities and sea bottom noise were well recorded by the H2O seismometers. During the operations, weather and sea-state data were recorded. The comparison of seismic signals with local and distant weather and sea-state data has enabled us to better understand the behavior of double-frequency microseisms in the open ocean (Stephen et al., this volume; Bromirski et al., 2005).
Figure F4 shows spectrograms acquired during this period (Stephen et al., this volume). The spectrograms show very large noise levels from 0.1 to 1 Hz with a peak at ~0.2 Hz, especially on the horizontal components. Large noise levels also appear at frequencies <0.02 Hz and higher than ~30 Hz. In the frequency band between 0.02 and 0.1 Hz, noise levels are very small. In the open ocean the double-frequency microseism peak split into two peaks. Comparison of the ship weather log with seismic data from ~0.2 to 0.5 Hz shows a strong correlation between seismic amplitude and local wind speed, suggesting the seismic noise at the ocean floor in this band is generated by local weather at the sea surface (Bromirski et al., 2005). On the other hand, in the band 0.1–0.2 Hz, the noise correlates poorly with local weather, suggesting a long-distance source. Noise in this band seems to correlate with high sea states impinging on distant coastlines. Large quasi-periodic noise was also identified (Stephen, Kasahara, Acton, et al., 2003) and was surmised to be whales singing. Other noise sources were identified as drilling noise by coincidence with drilling activity (Stephen, Kasahara, Acton, et al., 2003). Overall noise spectra are shown in Figure F5.
A long-standing problem in the red clay province of the eastern Pacific is to adequately resolve chert layers and basement in the presence of sediments <50 m thick. During Leg 200 we lowered a battery-powered, free-running 3.5-kHz pinger to the seafloor on the vibration iosolated television sled and recorded the pulse on the ship's 3.5-kHz acquisition system. This increased the sound level incident on the seafloor and improved penetration into the subbottom reduced the footprint of the sound on the seafloor and increased the received signal levels. Two prominent reflections were observed at ~10 mbsf (= 13 ms two-way traveltime [TWT]) and 28 mbsf (= 38 ms TWT), but the continuity of these reflectors varies with time throughout the survey, even though the ship moved only a few meters (Fig. F6) (Bolmer et al., this volume). The origin of the first subseafloor reflection at 10 mbsf could not be clearly identified in the core. VP obtained by physical property measurements on the JOIDES Resolution shows gradually increasing velocities from 1.48 km/s at the ocean bottom to 1.50 km/s at 7 mbsf (Stephen, Kasahara, Acton, et al., 2003). The population of radiolarians increases toward 7 mbsf. This suggests that the 13-ms reflector may be a radiolarian abundant layer similar to one observed at the Barbados décollement (Moore et al., 1998; Moore, 2000). In Barbados, the décollement layer comprises a radiolarian layer with an abundance of fluid methane lodged in large cavities of the radiolarians. The depth of the second subseafloor reflector at 28 mbsf corresponds to basaltic basement.
