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(“Additional file 1”): The Nobeoka Thrust Drilling Project Coring and Logging
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Operation－Methodology for Core-log Integration
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Drilling and coring down to 255 m depth across the Nobeoka Thrust was conducted
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from 27 July to 15 September 2011 by Sumiko Resources Exploration & Development,
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Co., Ltd (SRED). Prior to coring, the S-wave velocity structure of the study area was
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investigated using a micro-tremor survey to determine the depth of the fault zones and
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to select casing programs and coring techniques. Cores with a diameter of 86 mm were
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retrieved with 99.81% recovery. The azimuths of the cores were measured during
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drilling, and reference lines were marked on every core. Cores were then scanned with a
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digital camera and set out for visual observation at the drilling site. Lithological
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descriptions and structural analyses of cleavage, fractures, faults, mineral veins, bedding,
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and folding were conducted for every 1-m core. Image fitting of the cores and the
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borehole wall was carried out to better determine core orientation, eliminating the effect
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of rotation due to drilling.
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Geophysical wireline logs were acquired continuously across the Nobeoka Thrust
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along the borehole at a depth of 11.5–254.5 m during 17–18 September 2011 by SRED
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and Raax Co., Ltd. The logging recorded neutron porosity, resistivity, acoustic wave
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velocity (Vp, Vs), natural gamma rays, density, caliper, spontaneous potential, and
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temperature for every 10 cm. Acoustic and optical images were obtained along the
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borehole to evaluate the presence of bedding, fractures, and faults.
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The neutron logging tool (SYSTEM VI, CNL-9073) measures neutron porosity
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values, in which neutrons are emitted into the formation from a source (AmBe241; 16
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Cu, ~5 MeV), and detectors at short and long distances from the neutron source (32.4
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cm and 61 cm, respectively) measure the attenuation of the neutrons [Ellis and Singer,
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2007]. Neutrons emitted from a source collide with various atoms and become
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attenuated, losing energy by releasing gamma rays. Since neutrons lose the most energy
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when they collide with hydrogen, which is nearly the same weight as the neutrons (i.e.,
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the billiard effect), the measured energy loss may be caused by pores filled with
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hydrogen compounds such as water, oil, and gas. Thus, porosity is calculated from the
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measured attenuation by neutron signals. The sensor that measures the intensity of total
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natural gamma rays (NGR) from the formation is combined with the neutron porosity
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logging tool. When radioactive materials such as
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gamma radiation with a unique energy spectrum is emitted. The NGR detector consists
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of a sodium iodide (NaI) scintillator through which the gamma rays pass to be detected.
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Energy windows measure each spectrum, and applications such as the indication of
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U,
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Th, and
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K isotopes decay,
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shaliness and the classification of lithology are possible.
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The resistivity log (ELM-204, SCM-304) was obtained from the electrical survey,
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which consists of two electrodes (>150 V, 0–50 mA) lowered down the borehole, from
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which currents (>20~300 Hz, shortwaves) travel through the stratum. The two
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electrodes are connected to an ammeter and voltmeter on the ground, respectively,
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detecting the current and voltage. The equipotential surface around the ammeter would
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ideally be a sphere, and resistivity is obtained from Ohm’s Law. The measurement depth
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can be controlled by changing the distance L between the two electrodes and the radius
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of the equipotential sphere. In this measurement, two types of resistivity (long normal
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[LN] resistivity and short normal [SN] resistivity) were obtained, corresponding to L =
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100 cm and L = 25 cm, respectively, in which the electric current travels a longer
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distance (LN) and shorter distance (SN). Although LN measurements can measure
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greater depths, the vertical resolution is relatively lower compared to SN measurements.
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The acoustic log (MATRIX Logger, FWS 50) calculates acoustic wave velocity by
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the travel time difference between first arrivals of primary waves from three receiver
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points; 80, 120, and 160 cm from a single transmitter. Here, an electric monopole source
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generates a pressure wave (>25 kHz) that impinges on the borehole wall and causes a
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flexural wave to propagate through the wall. Since shear waves cannot be transmitted in
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water, the S-wave velocity can be detected only when its velocity is above the acoustic
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velocity through water (~1.5 km s-1) where rocks do not have major voids and fractures.
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Four sets of arrival points of the same phase were picked for each depth, and an
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approximated line was fitted using a least-squares method to obtain wave slowness and
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velocity.
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The density log tool (SYSTEM VI, CDL-9139) consists of a gamma ray transmitter
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for a Cs137 (1.7 Cu, 662 keV) source, which shoots gamma rays into the formation and
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far-field and near-field detectors (30.9 cm and 14.9 cm from the transmitter,
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respectively) that measure the attenuation of the gamma rays as counts per second in the
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region near the borehole at each depth. Gamma rays shot into a material will lose energy
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due to collisions with electrons (known as “Compton scattering”). The coefficient of
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attenuation is linear with the density of the electrons. The coefficient corresponds to the
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formation density, and thus density is obtained from the log-linear relationship with the
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counts measured in the two detectors (near, far). The transmitter and detectors are
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attached directly to the borehole to minimize the effect of mud water, and a caliper arm
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of the tool extends to the other end of the borehole, measuring the caliper at the same
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time. An electrode that measures another type of resistivity called guard resistivity (GD)
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is also included in the density log tool.
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Optical and ultrasonic wave images were scanned through the BIP-V system
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operated by SRED and Raax. The optical scanner digitally recorded 360° borehole
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images with measured azimuth using an optical camera and a specialized mirror
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included in the probe. A beam of ultrasonic waves helically scans the borehole wall at a
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frequency of 1 MHz, recording the reflectance time and strength along the borehole wall.
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The optical images were then analyzed using the imaging tool “BIPS Image Viewer”
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and the structural analysis tool “StereoWin” (trademarked by Raax) to measure the dip
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directions and dip angles of fractures, faults, mineral veins, cleavage, bedding, and
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breccia seen within the image.
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Core images were laid out side-by-side with logs and borehole images to produce a
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visual correlation between core and log data. Thus, better determination of in situ or
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drilling-induced structures and calibration of depth and core orientation eliminating the
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effect of rotation due to drilling are possible. The precise location of the fault core (the
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boundary between the hanging wall and footwall) was recognized at ~41 m below the
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ground surface by comparing the lithology and structures observed in surface outcrops
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and cores, and is also shown clearly by the contrasts in geophysical logs (Hamahashi et
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al., 2013).