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Downhole measurements

During Expedition 332, LWD and MWD were conducted in Hole C0002G and used to confirm and correlate the location of lithologic boundaries identified through previous drilling of Hole C0002A during IODP Expedition 314. LWD technology has been successfully employed during multiple previous Ocean Drilling Program and IODP expeditions using several generations of tools to obtain measurements of bulk density, gamma radiation, resistivity, and sonic velocity (Mikada, Becker, Moore, Klaus, et al., 2002). In particular, Expedition 314 focused exclusively on in situ measurements using LWD and MWD tools to evaluate subseafloor physical properties, stress state, lithology, and geomechanics. During Expedition 332, we used a similar suite of logging tools to confirm the depth of lithologic boundaries anticipated from previous drilling and identify favorable conditions for placement of the LTBMS. Here we describe the technology used to obtain the geophysical measurements, as well as the methods used to interpret and analyze data included later in this report.

Logging while drilling

LWD and MWD in Hole C0002G were conducted under contract by Mantle Quest Japan, Inc., in conjunction with Schlumberger Drilling and Measurements Services. The tools included LWD and MWD capabilities to enable real-time measurement of drilling parameters (MWD PowerPulse/TeleScope) as well as storing data for retrieval when tools were recovered on the drill floor. Drilling parameters were monitored with the MWD PowerPulse/TeleScope, located on the main body of the arcVISION825 tool string. LWD measurements for Expedition 332 included only resistivity and natural gamma radiation, which were supplemented by previous logging in Hole C0002A, located 50 m east-northeast of Hole C0002G. LWD was used to identify the boundaries between lithologic Units I and IV, following the nomenclature used during Expedition 314 (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009). The Unit II/III boundary marks the transition from Quaternary lower forearc basin sediments to Pleistocene forearc basin sediments, and identification of this transition was critical to this expedition, as it determined the location of the casing shoe, the downhole casing plan, and the configuration of the LTBMS. Previous logging of this boundary indicates it is marked by a decrease in electrical resistivity fluctuations, decreasing gamma radiation, and high photoelectric absorption, and it was anticipated that this transition would be encountered at ~830 mbsf. The target depth for the strainmeter was proximal to the Unit III/IV boundary, or the top of the Miocene accretionary prism. In contrast to Unit III, this transition is marked by low natural gamma ray values and larger fluctuations in electrical resistivity, with an expected depth of ~935 mbsf.

Systems and tools


The LWD tool used during Expedition 332 is the arcVISION Array Resistivity Compensated (ARC8) tool, developed by Schlumberger. The tool string was equipped with the ARC8 tool, which has a diameter of 6.75 inches (15.24 cm) with five transmitters and two receivers located along its antenna array (Fig. F1). The transmitters emit a magnetic field that travels through the formation and arrives at the receivers, which register a change in amplitude and phase. By varying the distance between the receivers and transmitter, multiple depths of investigation are accomplished; a larger distance between a transmitter and receiver increases the depth of investigation at the expense of lowering the resolution. The variable depth capability allows differentiation between anisotropy, borehole effects, shoulder beds, and invasion (i.e., mud entering the formation).

The arcVISION825 includes an array resistivity compensated tool based on propagation resistivity, meaning that it can make multiple borehole compensated phase-shift and attenuation resistivity measurements (​TOOLS_LABS/​SPECIAL/​lwd_arc.html). Attenuation resistivity measurements have deeper penetration and are less affected by anisotropy than phase-shift resistivity measurements, but vertical resolution is poorer. Conversely, phase-shift measurements are more distorted by surrounding beds and apparent dip but less influenced by invasion. The two frequencies used for Expedition 332 are 2 MHz and 400 kHz, with the latter providing higher penetration depth but lower resolution. In addition, the usage of water-based mud, with a resistivity of ~1 Ωm, resulted in increased vertical resolution and decreased penetration depth.

In addition to the resistivity tools, the arcVISION825 provides nonazimuthal gamma ray measurements. Using a NaI detector, the naturally occurring gamma radiation emitted by rocks and sediment in different amounts is measured and used to identify changes in lithology. The gamma ray sensor has a measurement range of 0–250 gAPI, with an accuracy of 3% and a theoretical vertical resolution of ±2 gAPI at 100 ft/h (i.e., 31 m/h).

Depth tracking systems

LWD data interpretation requires specific and precise depth measurements that connect the logging data to lithologies in the subseafloor. Because LWD tools record data only as a function of time, an integrated drilling evaluation and logging surface system from Schlumberger, Ltd. is installed onboard the D/V Chikyu to record the time and depth of the drill string below the drill floor, as well as the rate of penetration (ROP).

Accurate and precise depth tracking requires independent measurements of the (1) position of the traveling block and top drive system in the derrick, (2) heave of the vessel because of wave action, swells, or tides, and (3) activity of the motion compensator. These measurements are automatically recorded during drilling, and the depth of the drill string and ROP are determined from the length of the bottom-hole assembly (BHA) and drill pipe and the position of the top drive in the derrick. The configuration of these components is illustrated in Figure F2.

Measurement while drilling (PowerPulse, updated TeleScope)

The arcVISION825 tool is integrated with an MWD PowerPulse/TeleScope and an annular pressure while drilling tool. MWD was critical to successful drilling during Expedition 332, as it provided real-time two-way communication between LWD tools and the surface and enabled scientists to monitor drilling operations and, if needed, stop drilling (e.g., when horizons selected for the casing shoe were reached). The data are transferred by fluid pulse telemetry through the fluid within the drill pipe. Continuous pressure waves are generated by a rotary valve, and analog information from the paired strain gauges, accelerometers, and lateral shock sensors near the base of the MWD collar are transformed into binary numbers and transmitted by the pressure waves as a series of “0s” and “1s.” By changing the phase of the pressure wave signal, continuous transmission is obtained, which is sensed by a pressure transducer within the mud-flow line. In order to increase the amount of data and the speed of transmission, the TeleScope tool was used during Expedition 332, which is similar to the PowerPulse but allows data transfer to occur up to 4× faster by using the Orion telemetry platform.

The distance between the bottom of the drill bit and the logging bands for gamma ray and resistivity is typically several meters, which (during Expedition 314) translated to a 10–15 min recording delay before data were transferred into onboard memory. By comparison, Expedition 332 used fewer measurement sensors on the drill string, and the distance between the drill bit bottom and the logging ports was much shorter, so the rate of data updates was correspondingly shorter. Moreover, the ROP was slowed to provide higher data density than previously. The time after bit for gamma ray and resistivity data is 10–15 min if ROP is maintained at 30 m/h. This means that the bit was ~14 and 3.5 m ahead of the resistivity and gamma ray measurements on the monitoring screen in real time for the 12¼ and 10⅝ inch portions of the hole, respectively. Additional tool specifications appear in Figure F1.

Log characterization and lithologic characterization

LWD is commonly used to identify lithologic boundaries that reflect changes in the composition, texture, and structure of the rocks and pore fluids. During Expedition 332, the LWD logs were essential for identification of the boundaries targeted for casing planning and for LTBMS emplacement.

Data analysis

During Expedition 332, both resistivity and natural gamma ray measurements were employed to confirm the anticipated depths of lithologic boundaries identified during drilling of nearby Hole C0002A (Expedition 314). Changes in composition and texture were identified by variations in natural gamma ray values that coincided with amplitude changes in the electrical resistivity data. LWD data obtained from Hole C0002G were compared with data from Hole C0002A to confirm that both the depth and character of the transition were consistent with previous logging. In particular, a marked decrease in resistivity fluctuations coupled with an increase in natural gamma ray values defined the Unit II/III boundary. The lower boundary of Unit III was identified in the same manner in order to position the strainmeter in a relatively continuous zone of elevated gamma ray and resistivity values.