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doi:10.2204/iodp.proc.319.102.2010

Downhole measurements

Modular Formation Dynamics Tester

The MDT wireline logging tool was used to measure in situ stress, permeability, and pore fluid pressure. Its modular design allowed it to be customized for several applications. The configuration we used during Expedition 319 included the gamma ray sonde, pumpout module (MRPO), single probe module (MRPS), and dual packer module (MRPA) (Figs. F15, F16). We used the MDT tool to conduct three types of tests: (1) single probe drawdown tests to measure pore fluid pressure (Pf), fluid mobility, and permeability (k); (2) dual packer drawdown tests to measure Pf and k; and (3) dual packer hydraulic fracture tests to estimate the least principal stress (σ3).

Single probe tests

Single probe pore pressure measurements are made by pushing a probe against the sidewall and withdrawing pore fluid (Figs. F15, F17). Pressure in the isolated zone is recorded during and after pore fluid extraction with a sampling period of 300 ms. For single probe measurements during Expedition 319, a fluid volume of 5–10 cm3 was extracted at a rate of 30–80 cm3/min. The extracted volume is usually chosen based on anticipated formation permeability, and it is commonly adjusted during multiple drawdown tests at a single measurement location. Three probe types are available for single probe measurements: (1) conventional, (2) large-diameter probe, and (3) large-diameter packer. For all Expedition 319 deployments, the large-diameter probe was used.

We estimated in situ pore fluid pressure from the last pressure recorded during the pore pressure recovery in single probe tests. More sophisticated approaches that extrapolate from the recovery curve of pore pressure to in situ pressure were deferred to postexpedition research. During Expedition 319, we used Schlumberger's standard approach to estimate fluid mobility for single probe measurements. Pore pressure was drawn down for a specified time (typically 15 s), and then the pressure was allowed to partially or fully recover. The mobility is calculated by

kD/µ = Cq/ΔP. (36)

Equation 36 is dimensionally dependent. kD is the drawdown permeability (md), µ is viscosity (centipoise), q is the flow rate (volume of fluid extracted during the drawdown divided by the time [cm3/s]), and ΔP (psi) is the difference between drawdown pressure and in situ pressure (often approximated as the final build-up pressure). C is a constant that depends on the probe type (C = 5360, 2395, and 1107 for conventional, large-diameter probe, and large-diameter packer, respectively); C = 2395 for all Expedition 319 deployments.

Dual packer tests

The dual packer module on the MDT tool was configured to isolate a 1 m section of the borehole (Fig. F18). Pressures in the packers and test interval were recorded simultaneously and could be displayed in real time. As with the single probe tests, the data sampling interval was 300 ms. The MRPO is used to pump fluid from the mud column to the packers or into the test interval. The MRPO can either withdraw or inject fluid into the test interval. The flow rate depends on the differential pressure (the difference between the mud pressure and the flowing pressure). At a differential pressure greater than several thousand pounds per square inch (several MPa), flow rates are as low as 0.1 cm3/s, whereas at differential pressures of a few hundred pounds per square inch (MPa), flow rates are as high as 31.6 cm3/s. The maximum differential pressure is 3500 psi (24 MPa) for the displacement pump. The packers have a 10 inch diameter prior to inflation and are designed for 12¼ to 14¾ inch boreholes.

The packers seal most effectively if they are deployed in a zone of the borehole that is not washed out and where there are no preexisting fractures. During Expedition 319, logging data and core information were used to find optimal zones for deploying the dual packer tool. Our minimum criteria for choosing a location were a 5 m thick zone <14¾ inches in diameter with hole ovality (max diameter/min diameter) of <130%. The test section was sealed off by inflation of the dual packers. After sealing off the test interval, either a drawdown test or a hydraulic fracturing test was performed.

Dual packer pressure drawdown test

During a drawdown test, fluid is rapidly extracted from the isolated section causing a drop in pressure within the interval. The pump is then stopped and the pressure begins to recover as fluid flows from the formation into the sealed borehole interval. The time necessary for pressure stabilization is longer for formations with low permeability. Permeability and the coefficient of consolidation can be estimated from comparison with theoretically derived curves (e.g., Papadopulos et al., 1973). As for the single probe tests, formation pore fluid pressure was estimated as the pressure at the end of the pressure recovery phase.

Dual packer hydraulic fracture test

The hydraulic fracturing, or "minifrac," test is carried out following a standard procedure (Haimson and Cornet, 2003). Fluid is pumped into the isolated interval under a constant flow rate (Fig. F18). This gradually raises the pressure within the interval until a fracture is initiated within the formation. Pumping is stopped ("shut in") some time after fracture initiation, and the pressure decays. Several minutes after shut in, pressure is released and allowed to return to ambient conditions. The pressure cycle is repeated several times, maintaining the same flow rate. Key pressure values used in the computation of in situ stress are picked from this pressure-time record. Among these, the most useful is the instantaneous shut in pressure (ISIP), which is thought to be close to the least principal stress (σ3). ISIP is determined from the first break in slope in a plot of pressure versus time after the shut in (Zoback, 2007). If repeated cycles provide consistent values of the key pressures (e.g., the ISIP), this confirms that the fracture has grown sufficiently and the least principal stress is being measured.

The hydraulic fracture data can be integrated with other information to obtain further constraints on the in situ stress state. For example, if an image log is run after the hydraulic fracturing test, the orientation of the induced fracture (and therefore of σ3) can be defined. Borehole breakout information can be used to define the orientation of the maximum and minimum horizontal compressive stresses (see "Structural geology").

Vertical seismic profile in scientific drilling

The VSP is a high-resolution seismic imaging technique often employed in industry. In past scientific ocean drilling, zero-offset VSP (check shot) experiments were conducted to improve depth resolution and seismic velocity near the borehole (Holbrook et al., 1996). A VSP involves geophones lowered from the drillship into the drill hole to receive signals from a sea-surface source. The zero-offset VSP, which receives signals from a source next to the drillship, provides traveltime more precisely than seismic survey data and improves the accuracy of the location of strata and faults. The VSP technique can be expanded with a line of shots fired at increasing distances from the drill hole by a surface ship (walkaway VSP) to acquire 2-D seismic reflection images and refraction data.

There have been several previous VSP experiments in IODP designed to record seismic waves generated from an air gun towed by a shooting vessel. One example is an experiment using a broadband seismometer lowered to 714.5 mbsf by wireline in a borehole in the Japan Sea during ODP Leg 128 (Kanazawa et al., 1992; Shinohara et al., 1992). Later experiments used broadband seismometers cemented permanently at the bottom of boreholes in the northwestern Pacific Ocean and the Philippine Sea (Salisbury et al., 2006; Shinohara et al., 2008). These seismometers were all intended as part of a permanent or long-term borehole observatory.

For a VSP experiment, the seismic source signal is commonly recorded at an array of receivers placed in the drill hole. A walkaway VSP produces higher quality data than data obtained from a surface ship using hydrophones at the sea surface because the VSP array employs three-axis geophones that are clamped to the walls of the borehole. These clamps couple the receivers firmly to the formation, and the receiver can resolve the ground motion caused by the seismic source with high fidelity. With three-axis data, the direction from which reflected waves arrive is also determined. Seismic signals with higher frequencies are acquired because the seismic signal travels through the water and the seafloor interface only once, whereas sea-surface source and receiver systems record signals that have passed through the water and seafloor twice. The seafloor interface degrades signals and attenuates the higher frequencies that provide better spatial resolution. In addition, the shortened ray path reduces the Fresnel zone. The much better signal-to-noise ratio achieved in a quiet borehole environment enhances recording of low-amplitude signals.

Seismic anisotropy from VSP experiment

During the walkaway VSP experiment during Leg 128, circle shooting was conducted for analysis of seismic anisotropy of the Yamato Basin crust (Hirata et al., 1992). In this experiment, two circular lines (radius of 9 and 18 km) were shot to define the seismic anisotropy of refracted waves in the lower crust. Seismic anisotropy can also be studied from linear seismic lines of different azimuths crossing at the borehole (Shinohara et al., 2008). During the walkaway VSP experiment in Expedition 319, a circle with a 3.5 km radius was shot around Site C0009 (Fig. F19) to document seismic anisotropy in the Kumano Basin sediment.

Walkaway vertical seismic profile

A walkaway VSP experiment was conducted in Hole C0009A after installation and cementing of the 13⅜ inch casing to ~1600 m DRF, with a seismometer array (VSI tool, Schlumberger, 2002) deployed near the bottom of the hole to observe air gun shots from a separate shooting vessel (Fig. F20). The VSI tool consists of an array of separate sensor shuttles, a VSI cartridge, and a telemetry module. Each sensor shuttle consists of triaxial (three component) geophone accelerometers, a shaker, and a locking arm to provide mechanical coupling to the casing interior (Table T11). The shooting vessel (JAMSTEC R/V Kairei) shot a single 53.4 km long offset transect in the dip direction of the subducting plate and a 3.5 km radius circular shooting offset around the borehole (Fig. F19). Opening the hole to 17 inches, installing the 13⅜ inch casing, and cementing the casing were completed before the walkaway VSP experiment.

The objective of the walkaway VSP experiment was to improve our understanding of the structure around the megasplay fault and the master décollement including

  • Spatial variation of S-wave velocity,

  • Attenuation of P- and S-waves,

  • Anisotropy of P- and S-waves, and

  • Contrasts in physical properties across the faults.

Better coupling of the VSI seismometer with the formation improves the quality of the S-wave data from a wide-offset range to provide reliable shear wave velocity estimates. Direct comparison of seismic data obtained from the ocean bottom and the downhole environment are valuable for attenuation parameter estimation. Azimuthal dependence of arrival times and amplitudes of seismic waves provide estimates of the direction and magnitude of anisotropy in compressional (P) and shear (S) wave velocities, which may be correlated to the stress field around the site. Seismic records obtained from the downhole site are expected to be rich in high-frequency content and thus will enable us to examine the fine structure of the plate boundary fault system. Measurements of reflection coefficients (not only P to P but also for S to S, P to S, and S to P reflections) will clarify contrasts in elastic moduli and density across reflectors.

Survey design

The design of the walkaway VSP experiment during Expedition 319 was guided by recommendations from an industry contractor (Seismic 2020), whose staff modeled the experiment with proprietary 3-D VSP modeling software. Modeling showed that deployment of the receivers at the bottom of the drill hole (~1600 mbsf) was optimal, but that good results could also be obtained further up the hole. The critical angle for reflections from the plate interface would be reached ~15 km from the hole, and a maximum number of receivers (20) would optimize results.

Location and survey lines

The location of the survey line, ocean-bottom seismometers (OBSs), and broadband ocean-bottom seismometers (BBOBSs) are shown in Figure F19 and Tables T12, T13, and T14.

Data acquisition

The shooting ship Kairei shot air guns while traveling along the survey line (Fig. F19). The air guns towed by Kairei consisted of four tuned subarrays of air guns (Fig. F21) designed to generate high-amplitude broadband acoustic waves. A total of 16 VSI shuttles with 15.12 m intervals were lowered as deep as possible into the casing and individually clamped in the casing with mechanical arms. The acquisition parameters are shown in Tables T15, T11, and T16. The triaxial geophones in the VSI tool (Table T11) recorded direct, reflected, refracted, and converted seismic waves.

Prior to air gun shooting, the Kairei deployed eight OBSs and communicated acoustically to three BBOBSs deployed in a previous survey. The Chikyu and the Kairei conducted a radio communication test at maximum range of 50.7 km to confirm shot and recording synchronization before air gun shooting began. For Line 1, the Kairei shot every 60 m from southeast to northwest starting 24.1 km southeast of the Chikyu (B in Fig. F19). In the vicinity of the Chikyu, the Kairei turned at a point 500 m from Hole C0009A and maintained a distance of 350 m from the Chikyu to avoid collision before returning to the shooting course. After passing the Chikyu, the Kairei switched shooting from Line 1 to Circle 1, with an offset of 3.5 km from the Chikyu (Fig. F19). In circle shooting, the Kairei shot every 30 s while cruising clockwise. After completing circle shooting, the Kairei resumed shooting Line 1 to 29.3 km northwest of the Chikyu (A in Fig. F19).

Accurate synchronization between shooting and receiving timing is the most important factor in successful walkaway VSP data acquisition. A very high frequency radio signal from the blaster controller on the Kairei was used to start data recording aboard the Chikyu. A predetermined constant time interval for shooting was prepared in case of radio communication breakdown over long distances or in bad radio communication conditions. Accurate time from the Global Positioning System (GPS) was recorded for all shots for both the shooting and recording systems. Another system, Schlumberger's VSI recording system (MAXIS), which records GPS time with an accuracy of 1 s, was used in conjunction with time synchronization and the accurate time recording system as shown in Table T17.

Acquisition parameters

Seismic source parameters are given in Table T15, and recording parameters are given in Table T18. Navigation and positioning parameters are given in Table T16. Specifications for time sychronization and time recording are given in Table T17.

Onboard data flow, quality check, and processing

The data were stored in MAXIS (Schlumberger's VSI acquisition system) and an initial quality control check was conducted on the ship. The data were converted to SEGY (Society of Exploration Geophysicists standard Y) format and delivered to the scientists. Further processing will be completed postexpedition.

Zero-offset VSP

A zero-offset VSP (check shot) was conducted after the walkaway VSP experiment using the same VSI tools (Table T11). In this configuration, the VSI tools in the borehole receive seismic energy from a conventional air gun suspended from a crane on the drillship.

The main objectives of the zero-offset VSP are to

  • Obtain a depth-time-velocity profile (check shot),

  • Conduct sonic calibration,

  • Document seismic reflectivity along/below the borehole (corridor stack), and

  • Serve as a reference for the walkaway VSP in the vicinity of the borehole.

Tool and system

Schlumberger's VSI receiver tools and MAXIS surface seismic recording system were used as in the walkaway VSP experiment. The three-component geophone accelerometers detect vertical and horizontal particle motions and provide a linear and flat response from 3 to 200 Hz. The receivers consisted of eight shuttle-array VSI systems spaced at 15.12 m. The receivers record acoustic waves fired by air guns shot from 6 mbsl and ~60 m away from the borehole with a chamber pressure of 1700–2000 psi (Fig. F22). We used three sets of 250 in3 air guns as a seismic source (Fig. F23). Time correlations of the shots are ensured with a surface hydrophone suspended 5 m below the air guns (11 m below mean sea level).

Data acquisition

The VSI receivers were checked by air gun shooting from the Chikyu before the zero-offset VSP survey, and the waveforms of VSI Shuttles 9 and 10 indicated that coupling to the formation was poor. The positions of the VSI shuttles were changed and rechecked, but their coupling did not improve. Consequently, we decided to use only eight shuttles for zero-offset VSP. The VSIs were clamped to the casing and the air gun was fired between 6 and 11 times at each station to improve the signal-to-noise ratio by stacking. After stacking, the arm crutches (clamps) were released and the VSIs were raised by ~121 m (8 shuttle intervals) and then clamped again and shots fired. These procedures were repeated until reaching 2227.4 m WRF. During acquisition, the seismic signals were recorded in MAXIS. The acquisition parameters are shown in Table T19.

Onboard data flow and quality check

The VSI raw seismic data, depth-time/-velocity tables (check shot results), and quick report were delivered by Schlumberger engineers and distributed to the Shipboard Science Party for immediate processing and analysis. The waveforms were converted to SEGY format.