UBI Image Data
Processing
Science operator: Texas A&M University
Hole: U1473A
Expedition: 360
Location: SW Indian Ridge (SW Indian Ocean)
Latitude: 32° 42.3622' S
Longitude: 57° 16.688' E
Logging date: January 23-24, 2016
Sea floor depth (driller's): 721 m DRF
Sea floor depth (logger's): 721 m WRF (FMS/DSI/GPIT/EDTC/HNGS Pass 3)
Total penetration: 1510.2 m DRF (789.2 m DSF)
Total core recovered: 469.15 m (63.2 % of cored section)
Oldest sediment recovered: N/A
Lithology: Olivine gabbro
UBI Uplog:
Magnetic declination: -35.7696°
The
purpose of this report is to describe the images from Hole U1473A 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 images obtained from Hole U1473A are generally of good quality. Fractures and foliations can easily be identified; high angle fractures are easier to identify in the UBI images than the FMS images. The FMS and UBI images are best interpreted side-by-side or in overlay, as the 360 degree coverage of the UBI images complement the higher-resolution FMS resistivity images. Moreover, the UBI and FMS images respond to contrasting physical properties, thus enabling differentiation of open and filled fractures.
The average ship's heave was 1.5 m. with a maximum of 3 m prior to logging. The Wireline Heave compensator was used during the logging operations.
Image
Processing
The
following corrections are often applied in GeoFrame's BorEID module:
Depth
Shift to Sea Foor and Depth Match
The
image data are depth matched after they are depth shifted to the sea floor (721 m WRF); the amount of depth is based on the logger's sea floor depth
determined from the step in gamma ray value during the standard data
processing.
Depth Match
The
image data are shifted to the sea floor (721 m WRF) 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 are 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. 3 m) is equalized, removing preferential enlargement at a particular azimuth, e.g. the keyseat effect.
In order to enhance possible breakouts of the basement hole section in the UBI images, the "Equalization" option was not used during the BorEID processing.
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. In the WellEdit module of GeoFrame, 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. Where no FMS image is available, the UBI images are depth adjusted by
matching the UBI tool string gamma ray log to the reference gamma ray log from
the standard processing.
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 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 2-m
normalization interval is used.
The UBI radius images from IRBK and XRBK logs are not normalized due to the nature of the radius, and are thus processed only by the BorEid module.
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 360. For further questions about the logs, please contact:
Tanzhuo Liu
Phone: 845-365-8630
Fax: 845-365-8777
E-mail: Tanzhuo Liu
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-8777
E-mail: Cristina Broglia