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Expedition 311 Scientists2
Integrated Ocean Drilling Program (IODP) Site U1327 (proposed Site CAS-01B; Collett et al., 2005) is near Ocean Drilling Program (ODP) Leg 146 Site 889 (375 m southeast from Hole 889C). During Leg 146, three holes were drilled in the northern Cascadia margin (Westbrook, Carson, Musgrave, et al., 1994): a reference or background site for no gas hydrate or free gas in the Cascadia Basin (Site 888) and Sites 889 and 890 approximately at the mid-slope of the accretionary prism over a clearly defined bottom-simulating reflector (BSR).
The area of Sites 889 and 890 has been the focus of many interdisciplinary studies, including
The bathymetry at Site U1327 is dominated by two topographic highs, which rise ~200 m above the surrounding seafloor (Fig. F1). The ridges, almost completely composed of seismically inferred accreted sediments that lack any coherent seismic reflectivity, are associated with underlying thrust faults that resulted in the overall uplift of the area around the two ridges (Westbrook, Carson, Musgrave et al., 1994; Riedel, 2001).
Using 3-D seismic data, a map of the seafloor reflection coefficient was generated around Sites U1327 and 889 (Fig. F2) by calibrating the acoustic seafloor with the physical property data from piston cores, as well as employing Warner's method (Warner, 1990) of using the ratio of primary and multiple seafloor reflection strength (Riedel, 2001). The map in Figure F2 shows an area of high seafloor reflectivity northwest of Sites 889 and U1327, which was found to be the product of a widespread sand layer as determined from piston coring. Reflection coefficients for the rest of the survey area are relatively constant. High-resolution 3.5 kHz imaging also shows that the seafloor near Site U1327 is not covered with the typical transparent Holocene layer seen in the region and as observed at Site U1325 (Fig. F3). However, farther to the northwest, this transparent layer is again observed (Shotpoints 100–400 along seismic Inline 38).
The core and downhole logs from Site 889 provided the primary data needed to calibrate the seismic data for this area. Seismic Inline 38 from the 3-D MCS data in close proximity to Hole 889A is shown in Figure F4 and compared to the lithostratigraphic interpretation from Hole 889A. The distinction between incoherent accreted and bedded slope basin sediments is very clear in this data set. The core and log analyses showed three major lithostratigraphic units within the uppermost 400 meters below seafloor (mbsf). Lithostratigraphic Unit I was divided into Subunits IA and IB. Subunit IA, from the seafloor to 87 mbsf, comprises mostly clayey silts and silty clays with interbedded thin sand layers. This unit is characterized by subhorizontal to shallowly dipping beds with little deformation and is interpreted to represent slightly deformed slope basin turbidites and pelagic sediments. Subunit IB is also mainly silty clay but with less abundant sand layers. It was interpreted that the sediments in Subunit IB represent a series of sediment gravity flow deposits caused by tectonic uplift of the deformation front; therefore, Subunit IB may represent a transition between the abyssal plain sedimentation of lithostratigraphic Units II and III to the subsequent deposition of Subunit IA, which represents slope basin–type sedimentation. Units II and III consist of mainly clayey silt with a low abundance of sand layers. Lithostratigraphic Units II and III are distinguished on the basis of a significant increase in glauconite in Unit III, but no other compositional or structural differences were observed. These units were interpreted as typical abyssal plain sediments that were heavily deformed and fractured during the accretion process. Seismic profiles show that Subunit IB is seismically incoherent. The boundary between accreted and slope sediments was placed at ~90 mbsf (two-way traveltime of 1.88 s). The sediments of Subunit IB have a different history and slightly different composition compared to the accreted sediments; however, seismically there is no difference between the Subunit IB sediments and the deeper accreted sediments.
A strong BSR is observed in seismic data at this site. On MCS Line 89-08 (from the Leg 146 presite survey) the BSR is the dominant seismic reflection (Fig. F5), whereas in the higher frequency seismic data from the 3-D MCS survey, the BSR reflection strength is much weaker as seen on Inline 38 and Xline 3 (Figs. F6, F7). The BSR occurs within the accreted sediments and can be easily identified between common depth points 200 and 800 of Inline 38. In the deeper slope basin sediments, however, the BSR merges with regular, seafloor-parallel reflections and can no longer be isolated. The 3-D seismic data were used to generate a map of the BSR reflection coefficient in this area (Fig. F8) that shows values generally smaller than –0.1, with an exception of a bright spot associated with the cold vent at Site U1328 (see the "Site U1328" chapter).
The frequency-dependent reflection coefficient of the BSR was attributed to a layer of finite thickness (several meters to a maximum of 10 m) in which the seismic velocity decreases from the gas hydrate–bearing section to the gas hydrate–free and potentially free gas–rich sediments below (Chapman et al., 2002). Using the vertical seismic profile (VSP) data from Site 889 (MacKay et al., 1994), the BSR was estimated to occur at a depth of ~225 mbsf. VSP velocities were averaged to give a single interval velocity of 1636 m/s, which represents the entire section between the seafloor and the BSR. This average velocity was later used to calculate BSR depths at all other sites along the transect.
Special seismic techniques were used to describe the BSR including amplitude versus offset (AVO) and full waveform inversion (FWI) (Yuan et al., 1996, 1999). AVO can be used to define sediment properties above and below an interface of interest. In the case of the BSR, the question is how much free gas is present below and/or how much hydrate is present above the interface. Yuan et al. (1996) showed that no unique combination of free gas and gas hydrate content could be found to explain observed AVO trends in the MCS 89-08 data. FWI of the same data set yielded a fine-scale velocity model through the BSR and confirmed lower velocities from the VSP study of MacKay et al. (1994). The observed velocity drop below the BSR to values as low as 1480 m/s was converted to free gas concentrations using the Biot-Gassmann theory and yielded values of <1% of free gas (Desmons, 1996).
Leg 146 sonic and electrical resistivity logging data as well as coring data (chlorinity and electrical resistivity) have been used to estimate gas hydrate concentrations (e.g., Hyndman et al., 1999, 2001; Yuan et al., 1999). Based on measurements of core electrical resistivity and porosity, Hyndman et al. (1999) derived a set of empirical Archie coefficients (a = 1.4, m = 1.76) that were used to calculate gas hydrate concentrations from electrical resistivity logging data. The gas hydrate concentration was found to vary between 25% and 35% of the pore volume over a 100 m thick interval just above the BSR. Slightly lower concentrations of ~20% were found using acoustic velocity data (sonic log, MCS interval velocity, and VSP). Chlorinity data also provided an estimate of gas hydrate concentration. Combining the derived Archie coefficients from discrete core measurements with the chlorinity data, gas hydrate concentrations were calculated to approach 30% of the pore space just above the BSR.
These concentration estimates were recently reevaluated by defining different baselines for chlorinity and acoustic velocity and a different set of empirical Archie coefficients (Riedel et al., 2005; Ussler and Paull, 2001; Collett, 2000). Using these new reference log values, gas hydrate concentrations were calculated to be much lower; <10% over the 100 m thick interval above the BSR. Significant uncertainty remains, however, in the interpretation of the data depending on which pore water resistivity baseline is assumed.
The primary research objectives for this site are linked to the overriding transect concept of this expedition. Site U1327 is near Site 889, which provided critical baseline data for development of the objectives of this expedition. Site U1327 site is located near the middle of the expedition's coring transect on a broad uplifted ridge of accreted sediments, about mid-slope up the margin.
The concentration of gas hydrate in sediments is mainly determined using
Results from Leg 146 left many questions unanswered (Riedel et al., 2005), such as
Answers to these questions are needed to calibrate remote sensing techniques, such as seismics and CSEM, and these answers can be obtained through coring and downhole logging.
The operational plan to achieve these objectives was based on a general three-hole concept, which included