10th Asian Regional Conference of IAEG (2015)

Development of directional drilling system and measurement method in the borehole-Application of seismic tomography between surface and the boreholeKenji KUBOTA(1), Kenzo KIHO(1), Shigeharu MIZOHATA(2) and Fumitoshi MURAKAMI(2)(1) Central Research Institute of Electric Power Industry, Japan
E-mail:kubota@criepi.denken.or.jp(2) JGI, Inc., Japan
Abstract
Preliminary investigations in site selection for High Level Waste disposal requireevaluation of geological characteristics from the ground surface and/or several boreholes,
and seismic reflection is one investigation method used. To obtain a more precise reflection
profile, it is necessary to obtain a highly accurate P-wave velocity profile. The Central
Research Institute of Electric Power Industry has investigated directional borehole drilling
and technologies for logging and measurement. Seismic tomography can be conductedusing boreholes and the ground surface, resulting in P-wave velocity profiles and more
precise reflection profiles. In an in-situ test we drilled a 1000 m borehole, drilling
horizontally at lengths exceeding 700 m. Hydrophones were installed every 10 m at drilling
lengths of 100 m to 785 m. Vibrator points were installed every 20 m at the ground surface
above the borehole. We acquired waveform data by hydrophones when the vibrator
oscillated a seismic wave. We obtained a P-wave velocity profile between the groundsurface and the borehole by 2-D tomographic inversion. Using this P-wave velocity profile,
we recalculate the reflection profile obtained at the ground surface to be the same as at thevibrator points. The results demonstrated that we could interpret geological structures fromthe reflection profile more accurately than before.
Keywords: High Level Waste disposal, directional drilling, seismic tomography, MDRS
method1.

Introduction

results. Furthermore, the reflection profile will be
improved by using this P-wave velocity profile. In
this study, we conducted seismic tomography using aborehole drilled in an in-situ test.

In site selection for High Level Waste disposal,
preliminary investigations are conducted to assess thegeological characteristics from the ground surface
and/or several boreholes. The seismic reflection
method is one methods used in such investigations,but to obtain more precise reflection profiles andevaluate the geological characteristics in detail, it isnecessary to obtain the P-wave velocity profiles withhigh accuracy.
Central Research Institute of Electric Power
Industry (CRIEPI) has investigated directional
borehole drilling and technologies for logging andmeasurement,
andhasdeveloped
drilling
technologies for fault zone drilling, horizontal drilling,
gentle slope drilling, coring, and locality detection,
and applications of these methods have been verified
in in-situ tests (Kiho et al., 2015). Seismic
tomography can be conducted using borehole andground surface data. More precise P-wave velocity
profiles can be obtained from the seismic tomography2.

Directional drilling

2.1 Study area
The study area, Horonobe town, is located about
50 km south of Wakkanai, the northernmost city inJapan. The geology around the study area mainly
consists of the Koetoi and Wakkanai formations of
the Neogene. Rock facies of these formations aremainly diatomaceous mudstone (Koetoi Formation)and hard shale (Wakkanai Formation). Eachformation is more than several hundred meters thick(Fukusawa, 1985). The Horonobe area ischaracterized by the prevalence of folded structuresand reverse Omagari faults. Fold axes have anorth-northwest trend, dipping to the north at thenorthern part of Horonobe. The Omagari fault has asimilar trend and a length of 25 km. It intersects1

10th Asian Regional Conference of IAEG (2015)

through the town center, with the fault outcrop clearer
in the southern part of the town (Ishii et. al., 2006).
The porosity of the diatomaceous mudstone of the
Koetoi and Wakkanai formations are respectively
about 60% and 30%, and their uniaxial strengths arerespectively about 6 MPa and 15 MPa.

GDAPS-4A digital telemetry system (JGI, Inc.) fordata acquisition.
3.2 Layout and measurement conditions
Figure 2 shows a location map of thehydrophones and vibrator points. Hydrophone array
positions were divided into three sections (section 1:
555–785 m; section 2: 340–570 m; section 3:
110–330 m). Vibrator points (VP) were installed at
odd-numbered points (20 m intervals) from VP 15(near the HCD-3 well head) to VP 117 (about 1000 mfrom the well head). Table 1 shows the vibration
parameters, and Table 2 shows the recording
parameters.
The cable was suspended and not crimped whenhydrophones were set in section 3, so the waveform
S/N was very low and we could not distinguish theinitial shock. Therefore, we did not use analysis datafrom this section.
We also conducted a seismic refraction method at
the ground surface to obtain the P-wave velocity
profile in shallow areas and conduct full waveform
inversion.

2.2 In-situ directional drilling test
In the directional drilling project, a 1000 m
borehole was drilled with horizontal drilling at
lengths of 700 m and beyond. Casing was set at
drilling lengths of 200 m and shallower. The borehole
passed the Omagari fault at drilling lengths of about
330 m to 450 m,.
3.

Seismic tomography

3.1 Equipment for the measurements
There are generally two types of receiver used inboreholes, those that crimp to the borehole wall(geophones) and those that do not (hydrophones).
Geophones can reduce the influence of tube waves,
but it is necessary to install the unit to crimp to theborehole wall, making multiple installations difficultand measurements time-consuming. In contrast,hydrophones do not require crimping, so it is easy toinstall multiple units. In this study, we used ahydrophone array for the seismic tomography. Figure
1 shows a schematic layout of the hydrophone array.
We used 24 hydrophones (HTI-96-MIN/V 5/8, HighTech, Inc.) installed at 10 m intervals. The array tools
can be inserted into the borehole by self-weight, butmust be inserted at horizontal drilling depths by adriving force. We therefore installed a feeding
apparatus at the bottom end of the hydrophone array
tools.
We used Enviro VIB / Minibuggy (IVI, Inc.) for
the seismic source from the surface and the

3.3 2-D tomographic inversion
We conducted 2-D tomographic inversion
between the surface and the borehole. First, a P-wave
velocity profile was calculated using the results ofseismic tomography between the surface and theborehole, and then the profile was updated by fullwaveform inversion using the results of the seismic
refraction method at the ground surface. The analysis
procedure was as follows:(1) The initial velocity profile was updated using
the recorded travel time. The back-projection method
was used for tomographic inversion.
(2) The refraction ray path was calculated using
the updated velocity profile calculated in (1). The

Fig. 1 Schematic layout of the hydrophone array.

2

10th Asian Regional Conference of IAEG (2015)

Fig. 2 Hydrophone and vibrator point locations.
Table 1 Vibration parameters.
Item
Sweep length
Frequency
Stacking
Sweep wave

Table 2 Recording parameters.

Parameter
12 s
8 to 80 Hz
10Linear up sweep

Item
Sampling interval
Recording length
Number of channels
Preamplifier gain
Noise edit

linear travel-time interpolation (LTI) method wasused for the calculation.
(3) The velocity profile was updated using therefraction waveform calculated in (2). The conjugate
gradient method was used for the tomographic
inversion.
(4) Using the velocity profile calculated in (3), thevelocity profile was updated by frequency domain
full-waveform inversion. We used TOY2DAC
software (SEISCOPE Consortium, http://seiscope2.
osug.fr/?lang=en) for full-waveform inversion.

Parameter
1 ms
3s
24
18 dB
Factor 2, Gate 15 sshows a comparison of P-wave velocities fromseismic tomography and rock samples. As that figure
shows, the measurements are highly consistent. On
the other hand, there was a difference at drilling
lengths between about 300 m and about 400 m. The
borehole passed through the Omagari fault at thisdrilling length, and these fractures may have caused
the observed decrease in velocity. Note thatlaboratory experiments were conducted using intact
rocks, so any velocity decrease caused by fractures
would not have been reproduced, making P-wave
velocities from seismic tomography lower than thatfrom rock samples. The results of laboratory
experiments demonstrated that the rapid decrease invelocity occurred at drilling lengths around 850 m.
Rock facies changed (from Wakkanai to Koetoi
formations) at this drilling length, so there is apossibility that this velocity change occurred due tothe change of rock facies. However, no velocitychange was obtained in the seismic tomography

3.4 P-wave velocity profile from seismic
tomography
Figure 3 shows the P-wave velocity profile fromseismic tomography. To verify the effectiveness ofthe velocity profile from seismic tomography, we
compared the P-wave velocity from seismic
tomography with that from laboratory experiments
using rock samples from the borehole. Figure 43

10th Asian Regional Conference of IAEG (2015)

Fig. 3 P-wave velocity profile from seismic tomography.
4.1 Outline of the recalculation
We recalculate the reflection profile obtained at
the ground surface in the same way as the vibrator
points, using a P-wave velocity profile obtained fromseismic tomography between the surface and theborehole. In this study, the multi-dip reflection
surface (MDRS) method (Aoki et al., 2011), an
improvement on the common reflection surface(CRS) stack method (Jager et al., 2001; Mann et al.,2007; Schleicher et al., 1993), was used for the
inversion to derive a more precise reflection profile.
Figure 5 shows a schematic representation of the
MDRS method (Aoki et al., 2011). The top figures
show stacking methods such as common midpoint(CMP) (Fig. 5a), CRS (Fig. 5b), and MDRS (Fig. 5c).
Each triangle represents rays from a CMP gather. The
CRS method utilizes several CMP gathers byconsidering the slope and curvature of a reflection
surface (Fig. 5b). MDRS acts the same way, exceptthat it simultaneously considers reflections fromdifferent surfaces. The bottom figures show
zero-offset sections. The CMP section (Fig. 5d)
suffers from noise (dashed line). The CRS section(Fig. 5e) obtains enhanced events (bold lines), butloses an event at a conflicting dip part. The MDRS
section (Fig. 5f) achieves not only enhanced events
but also a higher fidelity wave field representation.

Fig. 4 Comparison of P-wave velocity fromseismic tomography (open circles) and rocksamples (filled squares).

4.2 Reflection profile
Figure 6 shows the reflection profile calculated byconventional CMP stacking (Fig. 6a) and the MDRS
method (Fig. 6b). In comparison with CMP stacking,the MDRS method improved in the continuity ofreflection events, making it easier to distinguish theboundary of rock facies. We therefore conclude that areflection profile can be acquired with high accuracy
and that the geological structure is easily interpretedby using the highly accurate P-wave velocity profile
calculated by the MDRS method.
Figure 7 shows the geological section estimatedfrom surface geology and rock samples. The
amplitude and continuity of reflection events
decreased at drilling lengths of about 300–400 m(Fig. 6). The decrease occurred because fault

Fig. 5 Schematic representation of three stacking
methods (a–c) and their zero-offset sections (d–f)(Aoki et al., 2011).
results, because hydrophones were not set at thisboundary (the deepest hydrophone was at 785 m).
4.

Recalculation of the reflection profile using
P-wave velocity profile4

10th Asian Regional Conference of IAEG (2015)

Fig. 6. Reflection profiles: (a) conventional CMP stacking, (b) MDRS method.

Fig. 7 Geological section estimated from surface geology and rock samples.
fractures scattered the reflected waves. Using the
profile calculated by the MDRS method made thisdecreased zone easier to distinguish. High-amplitudereflection events appeared at depths of 300–500 m,
CMP distances of 280–400 m in MDRS method (Fig.
6b). This zone was consistent with the underlying
anticline. This high-amplitude event (the anticline)
did not appear in the reflection profile from CMP

stacking (Fig. 6a), suggesting that a more precise
reflection profile than conventional CMP stacking
can be obtained by using the P-wave velocity profile
obtained from seismic tomography between thesurface and the borehole in directional drilling.
5.

5Conclusions

10th Asian Regional Conference of IAEG (2015)

We conducted seismic tomography between thesurface and a borehole, and calculated the reflection
profile using the obtained P-wave velocity profile andthe MDRS method. The results demonstrated that areflection profile could be obtained more precisely
than when using the conventional stacking method.
This suggests that it becomes easier to evaluategeological characteristics by seismic tomographyusing a directional drilling borehole and the MDRS
method.

Fukusawa, H. (1985): Late Neogene formation in the
Tenpoku-Horonobe region, Hokkaido, Japan-Stratigraphic reinvestigation on the“Wakkanai”
and “Koitoi” Formations-, Jour. Geol. Soc. Japan,
91, pp. 833-849. (written in Japanese with English
abstract)
Ishii, E., Yasue, K., Tanaka, T., Tsukui, R., Matsuo,
K., Sugiyama, K. and Matsuo, S. (2006): Threedimensional distribution and hydrogeologicalproperties of the Omagari Fault in the Horonobe
area, northern Hokkaido, Japan, Jour. Geol. Soc.
Japan, 112, pp. 301-314. (written in Japanese withEnglish abstract)
Jager, R., Mann, J., Hocht, G. and Hubral, P. (2001):
Common-reflection-surface stack: Image andattributes, Geophysics, 66, pp. 97-109.
Kiho, K., Sunaga, T. and Hase, K. (2015):
Development of directional drilling system -Outlineof the system and its application for Omagari Fault
in Hokkaido-, Proc. 10th Asian Regional
Conference of IAEG.
Mann, J., Schleicher, J. and Hertweck, T. (2007):CRS Stacking -A Simplified Explanation, Ext. Abst.
69th EAGE Conference & Exhibition, B044.
Schleicher, J., Tygel, m. and dan Hubral, P. (1993):
Parabolic and hyperbolic paraxial twopoint
traveltimes in 3d media, Geophys. Prosp., 41, 495-5.

Acknowledgements
This research was done under contracts awarded
from the Agency of Natural Resources and Energy(ANRE) of the Japanese Ministry of Economy, Tradeand Industry (METI). The in-situ drilling and survey
was conducted as a collaboration work with theHoronobe Underground Research Center of Japan
Atomic Energy Agency (JAEA).
References
Aoki, N., Narahara, S., Nishiki, T., Takahashi, A. andKikuchi, S. (2011): Structural delineation ingeologically complex area using MDRS (multi-dip
reflection surfaces), Proc. 10th SEGJ International
Symposium, pp. 167-170.

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