UBI Image Data Processing
ODP logging contractor: LDEO-BRG
Hole: 1256D
Leg: 206
Location: Guatemala Basin (NE equatorial Pacific)
Latitude: 6° 44.163' N
Longitude: 91° 56.061' W
Logging date: December 28-30, 2002
Water depth (as seen on logs): 3643.5 mbrf
Total penetration: 752 mbsf
Total core recovered: 227.3 (47.8%)
UBI main pass: 265-747 mbsf
UBI repeat pass: 641-737 mbsf
The Ultrasonic Borehole Imager (UBI) measures amplitude and
transit time. It 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. An innovative processing
technique improves accuracy, avoids cycle skips and reduces echo losses, which
makes the UBI transit-time measurement as reliable as that of the amplitude.
The tool is relatively insensitive to eccentralization up to 1/4 in. and yields
images that are clean and easy to interpret.
The purpose
of this report is to describe the images from Leg 206 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.
Excellent quality images were obtained from Hole 1256D. Massive basalt, breccia, and pillows can all be identified in the images, especially in the amplitude image. Fractures can easily be identified; high angle fractures are much easier to identify in the UBI images than in the FMS images. The FMS and UBI images are best interpreted side-by-side, 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.
Image Processing
The following corrections are applied in GeoFrame's BorEID module:
Speed correction corrects for the assumption that measurements are acquired at the cable depth; they are actually acquired at a somewhat different depth, because the tool speed is usually irregular compared with 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 v, 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 centre of the points on the borehole wall is calculated, and the distance to those points is recalculated relative to the geometrical borehole centre. 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 rotation of -17 degrees is necessary for the UBI to account for the alignment of the transducer.
Image normalization is applied using GeoFrame's BorNor module.
In "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. This type of normalization enables large-scale resistivity variations to be clearly visualized.
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 web site. 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.
For further information or questions about the processing,
please contact:
Cristina
Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia