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doi:10.2204/iodp.proc.332.104.2011

Downhole measurements

Logging-while-drilling results

Gamma ray and attenuation/phase shift resistivity measurements were made using the LWD arcVISION825 tool described in “Downhole measurements” in the “Methods” chapter (Expedition 332 Scientists, 2011b). The four units described here and identified by changes in frequency, amplitude, or magnitude of the data are believed to reflect changes in lithology. Suspected boundaries were compared with data from Expedition 314 to confirm that the signature and depth of the transition was consistent with logging results from Hole C0002A:

1. The Unit I/II boundary was reached at 129 mbsf, marked by a notable, consistent increase in both gamma ray and resistivity.

2. The Unit II/III boundary was identified at 827 mbsf, directly below distinct negative gamma ray and resistivity peaks. Gamma ray values do not show any significant change in amplitude of oscillations, in contrast to phase shift resistivity, which is characterized by a sudden quiescence or amplitude reduction after passing the boundary (Fig. F6).

3. The Unit III/IV boundary was identified at 931 mbsf. The transition is clearly visible through the sudden decrease in gamma ray and attenuation resistivity data at the boundary. Moreover, both data sets show strong oscillations downhole, indicating a highly heterogeneous material.

In addition to these boundaries, two smaller zones were identified in Unit II: Zone A from 215.5 to 399 mbsf and Zone B from 481.5 to 546.5 mbsf. Zone A shows no significant variations in gamma radiation but steadily increases below the Unit I/II boundary. Attenuation resistivity, however, decreases for a short interval, after which both resistivity measurements show strong variations and distinct peaks with very high values. Below Zone A at 399 mbsf, the strong variations suddenly disappear, synchronous with a sharp drop in overall resistivity and gamma ray. Resistivity continues to exhibit lower values with less variation until the transition to Zone B (481.5 mbsf) is reached, where gamma ray drops again significantly and resistivity values become highly variable. Throughout Zone A, both resistivity measurements and gamma ray are characterized by higher amplitude oscillations (relative to Zone B), beginning at 481.5 mbsf until 546.5 mbsf.

The main objective for the upper interval (i.e., Unit II) of LWD was to identify an appropriate zone for hydrologic monitoring of ambient pore pressure and transience. To this end, a relatively continuous zone of estimated low permeability (i.e., clay- or mudstone) was required, as identified by a zone of elevated resistivity and gamma ray measurements. Based on these criteria, four adequate locations for the third pressure port (PP3) of the LTBMS were identified: (1) 424–473 mbsf, (2) 549–574 mbsf, (3) 614–649 mbsf, or (4) 759–784 mbsf. Interval iv was chosen because it enabled the most favorable configuration of the instrument package. The final location of pressure Port PP3 was ~760 mbsf.

For measuring hydrostatic reference pressure, pressure Port PP4 was situated at the wellhead, whereas pressure Port PP2 was installed within the cemented region directly below the strainmeter at ~917 mbsf. There, it is to be used as a proxy for strain or confirm successful cementing of the strainmeter. Pressure Port PP1 was situated below the cement port at ~947 mbsf, with the primary objective of monitoring ambient and transient pore pressure within the accretionary prism.

Correlation with Hole C0002A

LWD/MWD measurements were conducted in Hole C0002A during Expedition 314 (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009), and here these data are compared to the new downhole measurements from Hole C0002G. Hole C0002A is located 50 m east-northeast of Hole C0002G, in the same (Kumano) basin, and it was anticipated that the same lithologic units would be encountered at similar depths. The Expedition 314 scientists identified four lithologic units from LWD measurements: Quaternary upper forearc unit (Unit I), lower forearc unit (Unit II), basal forearc unit (Unit III), and the accretionary prism (Unit IV) (for details, see Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009). The definition of these units was based on a more comprehensive suite of LWD measurements, which included gamma ray, resistivity, bulk density, porosity, etc. The transitions between these units (gamma ray and resistivity) show the same signatures but more pronounced than were described for Hole C0002G (Fig. F7). Moreover, the boundaries are situated at similar depths: the Unit I/II boundary is at 135.5 mbsf (6.5 m deeper), the Unit II/III boundary is at 830.4 mbsf (3.4 m deeper), and the Unit III/IV boundary is at 935.6 mbsf (4.6 m deeper).

The division of smaller zones within Unit II is, with the exception of Zone A, similar to our interpretation, as well as the general character of the gamma ray and resistivity data that helped to define the zones (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009). Consistent with the unit boundaries, the transitions of the different zones are also shifted. Zone A was defined as a slightly thinner section from 218.1 to 400.4 mbsf, whereas Zone B is slightly thicker, ranging from 481.6 to 547.1 mbsf. These boundaries also correlate well with geologic features identified in the 3-D seismic data set (Moore et al., 2007). The sudden decrease in both resistivity measurements in Holes C0002G and C0002A, for example, corresponds with the appearance of a reflector simulating the seafloor but showing a reversed polarity, which cuts the other seismic reflectors, usually referred to as the BSR (Fig. F8).

Because of the similarity in LWD logs, the nearly identical position of the boundaries, and the small distance between the boreholes, a reasonable conclusion is that the units identified in Hole C0002G are the same units defined in Hole C0002A.

The absolute differences in depth between the LWD/MWD measurements from Holes C0002A and C0002G are only minor. At ~6 m difference, the first boundary shows the largest difference, which is probably related to the normal faulting observed in the Quaternary forearc unit of the Kumano Basin. The 4.6 m difference of the Unit III/IV boundary is probably related to folding of the accretionary prism and the lowermost part of Unit III. Therefore, it seems that despite deformation and faulting that are abundant in the upper accretionary prism, their effect on the location of the boundaries is negligible within such a short distance. Also, the boundaries are not significantly affected by the depositional history of the Kumano Basin because the turbiditic sequences appear fairly homogeneous.

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