UBI Image Data
Processing
IODP logging
contractor: USIO/LDEO
Hole: U1362A
Expedition: 327
Location: NE Pacific Ocean
Latitude: 47° 45.6628' N
Longitude: 127° 45.6720' W
Logging date: August 12, 2010
Sea floor
depth (driller's):
2672 m DRF
Sea floor depth (logger's): 2673.5 m WRF
Casing shoe (driller's):
Casing shoe (logger's):
Total
penetration: 3200 m DRF (528 m DSF)
Total core
recovered: 44.4 m (
30 % of cored section)
Oldest sediment recovered: N/A
Lithologies: Highly fractured and altered basalt, both pillow and massive.
UBI Pass 1:
UBI Pass 2: 308-506 m WSF
UBI Pass 3: 372-505 m WSF
Magnetic declination: 17.7213°
The Ultrasonic Borehole Imager (UBI) provides an acoustic image of
the borehole wall by scanning it with narrow pulsed acoustic beam from rotating
transducer while the tool is pulled up the hole. The same transducer acts as a
receiver and measures both the amplitude and transit time of the ultrasonic
pulse. The tool is relatively insensitive to eccentralization up to 1/4 in. and
in hard formations yields images that are clean and easy to interpret.
The
purpose of this report is to describe the images from Hole U1362A and the
different steps used to generate them from the raw UBI measurements. The median
amplitude and radius measured by the UBI, together with inclinometry and gamma
radiation logs from tools on the same tool string are presented with the
'standard' data.
UBI Logging Parameters
Pass
|
Logging Speed (ft/hr) | Frequency (KHz) |
Vertical Resolution (inches) |
Samples/Rotation |
Image Quality |
Pass 1
|
250 |
1.0 |
140 |
Poor |
|
Pass 2
|
400 and 800 |
250 |
0.4 |
180 |
Intermediate |
Pass 3
|
400 |
0.2 |
180 |
Good |
Data Quality
Generally poor to intermediate quality UBI images were obtained from Hole U1362A due to large hole diameter (12-18") for most of the hole section. Smears and truncations in the images resulted largely from downhole tool motion. The images resolved relatively well over the narrower hole section of 456-572 m WMSF. No meaningful UBI image was aquired in the hole section of 388-438.5 m WMSF during Pass 3, due to a bad configuration of the UBI with a 5-1/2" bit size.
The UBI images were depth-shifted to seafloor (2673.5 m WRF) but no depth-match was applied to the images due to technical problems associated with the normalization of depth-matched images using GeoFrame. As a result, the processed UBI images from Passes 1 and 3 are offset about 0.5 m from that of Pass 2 (reference) for few short intervals of the hole.
High angle fractures and foliations can be easily identified in a 360° image display. Moreover, the UBI responds to contrasting physical properties, enabling (for example) differentiation of open and filled fractures.
Image
Processing
The
following corrections are applied in GeoFrame's BorEID module:
Depth
Shift to Sea Foor
The
image data is shifted to the sea floor based on the loggers' sea floor depth
determined from the step in gamma ray values during the standard data
processing.
Speed
correction corrects for irregular changes in tool speed. Initially, depths are
assigned based on the near-constant speed of the cable at the rig floor. The data
from the z-axis accelerometer is used to correct the vertical position of the
data for variations in the speed of the tool, including 'stick and slip'.
The
transit time measurement from the UBI scanner is converted to a borehole radius
measurement given the velocity of ultrasound in the borehole fluid, and the
tool radius.
When
the tool is eccentered in a circular borehole, the amplitude is increased in
the directions where the distance to the borehole wall is decreased and vice
versa. This change in amplitude can often be larger than the changes in
amplitude produced by features on the borehole wall that we wish to image. To
correct for the effects of eccentering, low order angular harmonic components
of the signal with a periodicity equal to 1 and 1/2 revolution are removed.
The
transit time signal is corrected in the same way as the amplitude.
The
distance and direction of points on the borehole wall are initially given with
the tool axis as the origin. The geometrical center of the points on the
borehole wall is calculated, and the distance to those points is recalculated
relative to the geometrical borehole center. Both corrected (IRBK) and
uncorrected (XRBK) radius images are output. The uncorrected image should be
used for analyses such as breakouts and dip computations.
The
background response for all azimuths over a large window (e.g. 3m) is
equalized, removing preferential enlargement at a particular azimuth, e.g. the
keyseat effect.
The
following three corrections are minor and will only be apparent in good
(circular) borehole sections where the signal shows very small real variations:
EMEX
Noise Filter
Gains
Calibration
Sampling
Bias Correction
A
tool specific rotation is necessary for the UBI to account for the alignment of
the transducer (-17° in the case of the current tool).
No depth-match was applied to the images due to technical problems associated with the normalization of depth-matched images using GeoFrame.
Image
normalization is applied using GeoFrame's BorNor module.
In
the 'static normalization', histogram equalization is used to obtain
the maximum quality image. The amplitude or radius range of the entire interval
of good data is computed and partitioned into 256 color levels.
The image can be enhanced when it is
desirable to highlight features in sections of the well where amplitude or
radius events are relatively subdued when compared with the overall amplitude
or radius range in the section. This enhancement is called 'dynamic
normalization'. By rescaling the color intensity over a smaller interval,
the contrast between adjacent amplitude or radius levels is enhanced. It is
important to note that with dynamic normalization, amplitude or radius in two
distant sections of the hole cannot be directly compared with each other. A 2-m
normalization interval is used.
The normalized images are shifted to a
sea-floor reference and converted to gif files using in-house software. They
are presented on this website. The image is displayed as an unwrapped borehole
cylinder. A dipping plane in the borehole will be displayed as a sinusoid on
the image; the amplitude of this sinusoid is proportional to the dip of the
plane. The images are oriented with respect to the North; hence the strike of
dipping features can also be determined.
Interested scientists are welcome to visit the log
interpretation center at /LDEO if they wish to use the
image generation and interpretation software.
Additional information about the drilling and logging operations can be found in the Operations and Downhole Measurements sections of the expedition report, Proceedings of the Integrated Ocean Drilling Program, Expedition 327. For further questions about the logs, please contact:
Tanzhuo Liu
Phone: 845-365-8630
Fax: 845-365-3182
E-mail: Tanzhuo Liu
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia