Technical Developments
Borehole Imaging Tools - Principles and Applications
by Philippe Gaillot, Tim Brewer, Philippe Pezard, and En-Chao Yeh
doi:10.2204/iodp.sd.5.07S1.2007
Industry standard wireline imaging tools: Industry-standard
wireline image tools, top-combinable with open-hole wireline tools commonly used in the framework of IODP-ICDP,
measure either the electrical conductivity of the borehole
wall or the sonic travel time and amplitude of
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The wireline resistivity image tools are pad
tools that measure the formation micro conductivity directly
through an array of resistivity buttons mounted on pads that
are pressed against the borehole wall (Fig. 1a; Ekstrom et al.,
1987). Such tools normally provide the best high-resolution
borehole images in conductive (water-based) muds.
Examples are the Schlumberger Formation MicroScanner
(FMS) and Fullbore Formation MicroImager (FMI) with
azimuthal resolution of 64 (FMS) vs 192 (FMI), and capable
of radial microresistivity measurements (vertical resolution
0.2”, vertical sampling: 0.1”, depth of investigation: 30”;
These tools can be used in a wide variety of geological and
drilling environments, providing borehole images of rock
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Schlumberger, Ltd., 1994). The wireline acoustic tools (or
acoustic televiewer, ATV) send sound pulses out to the formation and measure both the amplitude and the travel time
of the returning signals (Fig. 1B). Because ATVs rely on
sound pulses, they can work in resistive (oil based) muds,
where electrical conductivity is very poor. Their disadvantages when compared to electrical images are a heightened
sensitivity to borehole roughness or washouts and a generally poorer quality image overall in part or all of some holes.
One example is the Schlumberger Ultra Sonic Borehole
Imager (UBI): azimuthal resolution: ~2°; vertical resolution
from 0.2” to 1.0” depending on pulse frequency (500 to 250
kHz); depth of investigation: borehole wall).
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Figure 1. Schematic of electrical, acoustic and optical imaging tools. [A] Schlumberger Formation MicroScanner (FMS) with four pads, 16 buttons
each pad, covering 25%-40% of hole diameter and Schlumberger Fullbore Formation MicroImager (FMI) with four pads and four hinged flap and 24
buttons on each pad and flap. The hinged flap is able to increase coverage of up to 80% (Modified from Ekstrom et al., 1987; Schlumberger, 1994).
The individual buttons are aligned in two rows; processes for depth corrections shift the recorded resistivity to one row. Each button consists of
an electrode surrounded by an insulation. [B] The Ultrasonic Borehole Imager features a high-resolution transducer that provides acoustic images
of the borehole wall. The transducer emits ultrasonic pulses at a frequency of 250 or 500 kHz (low and high resolution, respectively), which are
reflected by the borehole wall and then received by the same transducer. Amplitude and traveltime of the reflected signal are then determined.
[C] Optical televiewers generate a continuous oriented 360° image of the borehole wall unwrapped using an optical imaging system (downhole
CCD camera which views a reflection of the borehole wall in a conic mirror-sketch of advanced logic technology ALT OB140). Like electrical
imaging tools and acoustic televiewers, the optical televiewers include a full orientation device consisting of a precision 3 axis magnetometer and 2
accelerometers thus allowing for both accurate borehole deviation data to be obtained during the same logging run, and for accurate and precise
orientation of the image.
Scientific Drilling, No. 5, September 2007
Table 1. Tectonic, sedimentary and diagenetic features usually recognized on borehole images.
Tectonic
Sedimentary
Diagenetic
Self-evident
Structural dip
Natural Fractures Drilling
induced fracture/
breakout
Folds
Stylotites (high amplitude peaks)
Ambiguous
Faults
Bedding surfaces
Laminations
Cross-bedding
Grading
Erosional surfaces
Deformation features
Lithology changes
Cobbles, pebbles, Breccia
Detrial shales, ripples
Cherts
Bioturbation
Grain size/texture
Bioturbation
Thin lamination
Limestone textures
Vugs
Needs Core
Small fractures
Horizontal fractures
and fluid properties in formations ranging from fractured /
karstic carbonates to soft, thinly laminated sand/shale
sequences. High resolution and often nearly complete borehole coverage images are interpreted at an interactive graphics workstation. When the image is “unrolled” and displayed from 0° to 360°, linear features intersecting the
borehole appear as sinusoids (Rider, 1996). Assuming that
the images are properly oriented to geographic north, the
amplitude and minimum of the sinusoids can be related to
the dip and azimuth of the associated feature, respectively,
and consequently provide fundamental information regarding the encountered formation (Fig. 2). Bedding, fracture
features, faults, stratigraphic features, and many other features can often be manually or (semi-) automatically identified and quantified (Ye and Rabiller, 1998). In addition to
identifying fractures and faults, borehole imaging tools are
routinely used in support of detailed core analysis for a
variety of other applications such as sequence stratigraphy,
facies reconstruction, stratigraphy, and diagenetic analysis
(Table 1).
E
0
90
180
270
Stylotites (low amplitude peaks)
In a general manner, electrical images appear to be sensitive to variations in mineralogy, porosity, and fluid content
that highlight both natural fractures and rock fabrics.
Acoustic image logs reveal a similar natural fracture population, but generally image slightly fewer fractures and do not
reveal rock fabric due to their lower resolution. However, due
to their full coverage, acoustic images can reveal drillinginduced borehole wall tensile fractures, breakouts, and petalcenterline fractures (Fig. 3). Both images can record textural properties of deformed materials within fractures; these
textures can be related to variations in mineralogy, alteration, or porosity using the electrical log and can be used to
infer slip history. Drilling-induced fractures, breakouts, and
petal-centerline fractures that are thought to form just ahead
of the drill bit then provide additional constraints on the orientation of the minimum horizontal regional stress (Zoback
et al., 1985).
MN
N
Nodular concretions
Amplitude
A�V �wo-way
MN
�raveltime
FMS
�N
Paired tensile
cracks
360
Image log
5 ft
h
d
N E S W N
Figure 2. Projection of a planar intersection with a cylindrical
borehole. Dip direction of the planar feature is given by the orientation
of the sinusoid minimum; dip angle = arctan (h/d) where h = height of
sinusoid and d = borehole diameter.
Paired breakouts
Figure 3. Borehole wall tensile fracture and breakouts in acoustic
televiewer (AT V). [A] amplitude, [B] travel time images, and
[C] Formation MicroScanner (FMS) electrical images The ATV and
FMS logs are oriented with respect to magnetic North (MN) and true
(TN), respectively (Modified from Zoback et al., 1985).
Scientific Drilling, No. 5, September 2007 Rate (m/H) 0 to 40
Pullback pressure (bar)
0 to 40
Torque (bar) 0 to 100
Core
Core
Depth (m)
Technical Developments
65
66
67
68
Figure 4. Integration of optical and acoustic image logs with drilling
parameters and digital line scan image of cores from IODP Expedition
310 (Tahiti sea level) Hole M0005D. Example of repositioning core to
its correct position in relation to the log-drill datasets. If no attempt
if made to position core data accurately, errors are in proportion to
the percentage of recovery per length of core drilled. Thus potential
errors increase around voids where recovery is lower, especially as
core may be collected by subsequent drill runs (Figure courtesy of
Jenny Inwood).
Wireline slim-hole imaging tools: In the framework of fractured-rock and karstic aquifer studies, slim borehole-wall
imaging has undergone significant advancements since
Paillet et al. (1990) provided a state-of-the-art overview. Due
to their small diameter and advanced
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focusing system (acoustic reflector and rotating mirror), s�������������������������
limhole ATVs and optical
televiewers (OTVs) offer incredibly high azimuthal and vertical resolution (e.g. Advanced Logging Technology televiewers: acoustic borehole imager (ABI40) and optical borehole imager (OBI40); azimuthal resolution: 1° or less, vertical
resolution: 1 mm). ��������������������������������������
ATV slim-hole tools use an ultrasonic
pulse-echo configuration with a 0.5–1.5 MHz transducer
(Fig. 1B). The transit time and amplitude of the reflected
Scientific Drilling, No. 5, September 2007
acoustic signal are recorded as photograph-like images, and
the transit-time data can be used to generate high-resolution
caliper logs.��������������������������������������������������
OTV tools use a ring of lights to illuminate the
borehole, a CCD (charge-coupled device) camera, and a
conical or hyperbolic reflector housed in a transparent
cylindrical window (Fig. 1C). The CCD camera measures
the intensity of the color spectrum in red, green, and blue. If
the medium is transparent (air or clean water), the reflector
focuses a 360° slice of the borehole wall in the camera’s
lens.
Like industry-standard wireline imaging tools, slim-hole�
OTV and ATV provide high-resolution, continuous, and oriented 360° views of the borehole wall from which the character, relation, and orientation of lithologic and structural planar features can be defined. The combined application of
optical and acoustic imaging provides critical information for
water-supply development and source-water protection, as
well as characterization and remediation of contamination of
water reservoirs (Cunningham et al., 2004). Such types of
televiewers have been successfully used in IODP Expedition
310 to image Tahiti Pleistocene-Holocene reefs (Camoin et
al., 2005). There, imaging of the borehole wall using acoustic
and optical geophysical methods complimented the sedimentological work and allowed an unambiguous correlation
of cores (coral assemblages), core logs (Multi-Sensors Core
Loggers), downhole logs, and drilling parameters resulting
in a more precise depth assignation of core sections (Fig. 4)
and proper identification of macro-porosity regions and estimation of true core recovery.
Logging While Drilling (LWD) imaging tools: Back to oilbusiness industry standards, a major step in downhole geology includes logging while drilling, or LWD, where the formation properties are measured just after they have been
drilled. Electrodes on a rotating LWD collar produce
360° images of the formation resistivity. These images can
be analyzed in real time by geologists and the driller to make
timely decisions about difficult drilling operations (unstable
hole conditions). The Resistivity at Bit (Schlumberger RAB)
tool is one of the LWD tools that can provide azimuthal-oriented images of the borehole. Connected directly to the drilling bit, this tool uses its lower portion and the bit as a
measuring electrode. Button electrodes provide shallow,
medium, and deep resistivity measurements as well as azimuthally oriented images acquired as the RAB rotates, with
a ~6º resolution. Low resolution (16 azimuthal sectors, with
one-foot axial resolution) density images can also be measured by the rotating LWD collar.
Such LWD resistivity and density images were used to
estimate the concentration of gas hydrate in several boreholes drilled in the vicinity of Hydrate Ridge during ODP
Leg 204 (Tréhu et al., 2003). Density at azimuthal positions
and formation resistivity around each borehole as a function
of depth were used to compute porosity and water (Sw) and
hydrate (1-Sw) saturation, respectively, at each azimuthal
position in these holes. The results allow for delineation of
the shape, geometrical distribution, and azimuthal orientation of the porous sediment structures that are saturated
with gas hydrate as a function of depth. This 360° approach
contrasts with conventional methods that use wireline logs
or core data and produces a single saturation value at each
measurement depth. In addition, LWD data are acquired
only minutes after the formation is drilled, limiting the extent
of hydrate dissociation on the measured in situ properties
resulting into a significant increase of the understanding of
the original spatial distribution of hydrate in these formations (Janik et al., 2003).
References
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Scientists, 2005. Expedition 310 of the mission-specific
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Authors
Philippe Gaillot, CDEX (Center for Deep Earth
Exploration)—IFREE (Institute for Research on Earth
Evolution), JAMSTEC (Japan Agency for Marine-Earth
Science and Technology), Yokohama Institute for Earth
Science, 3173-25 Showa-machi, Kanazawa-ku, Yokohama,
Kanagawa, 236-0001 Japan, e-mail: gaillotp@jamstec.go.jp.
Tim Brewer, Department of Geology – Geophysics and
Borehole Research, University of Leicester, University Road,
Leicester, LE1 7RH, U.K.
Philippe Pezard, LGHF (Laboratoire de Géophysique et
d’Hydrodynamique en Forage) – Geosciences Montpellier,
University of Montpellier 2, France.
En-Chao Yeh, Department of Geosciences, National Taiwan
University, No.1, Sec. 4, Roosevelt Road., Taipei 106,
Taiwan.
Related Web Links
http://publications.iodp.org/proceedings/310/310toc.htm
http://www-odp.tamu.edu/publications/176_IR/176TOC.
HTM
http://www.scec.org/instanet/01news/es_abstracts/Iturrino_et_al.pdf
Texas, (Scociety of Professional Well Log Analylsts), pp.
3–23.
Rider, M.H., 1996. The Geological Interpretation of Well Logs, (Second
Edition):
Cambridge,
England
(Rider-French
Consulting,Ltd).
Schlumberger, Ltd., 1994. FMI Fullbore Formation MicroImager:
Houston, Texas (Schlumberger Educational Services).
Tréhu, A.M., Bohrmann, G., Rack, F.R., Torres, M.E., Bangs, N.L.,
Barr, S.R., Borowski, W.S., Claypool, G.E., Collett, T.S.,
Delwiche, M.E., Dickens, G.R., Goldberg, D.S., Gràcia, E.,
Guèrin, G., Holland, M., Johnson, J.E., Lee, Y.-J., Liu, C.-S.,
Long, P.-E., Milkov, A.V., Riedel, M., Schultheiss, P., Su, X..,
Teichert, B., Tomaru, H., Vanneste, M., Watanabe, M.,
Weinberger, J.L., 2003. Proc. ODP, Init. Repts., 204: College
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proc.ir.204.2003.
Scientific Drilling, No. 5, September 2007