UBI Image Data
Processing
Science operator: Texas A&M University
Hole: U1556B
Expedition: 390
Location: Mid-Atlantic Ridge (S Atlantic Ocean)
Latitude: 30° 56.5244' S
Longitude: 26° 41.9472' W
Logging date: May 5, 2022
Sea floor depth (driller's): 5013.1 m DRF
Sea floor depth (logger's): 5016.0 m WRF
Total penetration: 5646 m DRF (633.2 m DSF)
Total core recovered: 342.2 m (56.1% of cored section)
Oldest sediment recovered:
Lithology: Silty clay and calcareous nannofossil ooze, calcareous nannofossil chalk, basement basalt and sedimentary breccia
UBI Main:
UBI Repeat:
Magnetic Declination: -23.9726°
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 U1556B 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.
Data Quality
The UBI image of hole U1556B is of excellent quality and displays features that are closely tied to the ones in the FMS images. The UBI and FMS images are best interpreted side-by-side or in overlay, as the 360 degree coverage of the UBI images complement the higher resolution of the FMS resistivity images. Moreover, the UBI and FMS respond to contrasting physical properties, enabling (for example) differentiation of open and filled fractures. Fractures and foliations can easily be identified; high angle fractures are easier to identify in the UBI images than in the FMS images.
Image
Processing
The following corrections are applied using Techlog's UBI image processing software package.
Depth
Shift to Sea Floor
The
UBI images are shifted to the sea floor based on the loggers' sea floor depth
(5016 m WRF) 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 (xrad) and
uncorrected (rad) 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. 3 m) 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).
Typically, the UBI is run in hard rock holes where there is also good FMS data coverage. In this case, features such as fractures in the UBI images are depth-matched to the same features in the FMS images. The FMS images can then be overlaid on the UBI: the greater resolution of the FMS combined with the 360° coverage of the UBI images makes features in the borehole wall much easier to see and interpret. During this processing, the UBI image (main pass) was depth-matched with the reference run of FMS image (pass 2) via the gamma ray log, and the UBI image (repeat pass) was then depth-matched with that of main pass.
Image normalization is applied using in-house image processing code.
In
the 'static normalization', histogram equalization is used to obtain
the maximum quality image. The amplitude 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 events are relatively subdued when compared with the overall amplitude
range in the section. This enhancement is called 'dynamic
normalization'. By rescaling the color intensity over a smaller interval,
the contrast between adjacent amplitude levels is enhanced. It is
important to note that with dynamic normalization, amplitude in two
distant sections of the hole cannot be directly compared with each other. A 1-m
normalization interval is used.
The UBI radius images are processed by Techlog software but not normalized due to the nature of radius.
The processed amplitude and radius images presented on this website are shifted to a
sea-floor reference and converted to gif files using in-house software. They are 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 390. For further questions about the logs, please contact:
Tanzhuo Liu
Phone: 845-365-8630
Fax: 845-365-8777
E-mail: Tanzhuo Liu
Gilles Guerin
Phone: 845-365-8671
Fax: 845-365-3182
E-mail: Gilles Guerin