IODP-USIO logging contractor: LDEO-BRG
Location: Guatemala Basin (NE equatorial Pacific)
Latitude: 6° 44.163' N
Longitude: 91° 56.061' W
Logging date: August 21-24, 2005
Sea floor depth (driller's): 3645.4 mbrf
Sea floor depth (logger's): 3643.5 mbrf
Total penetration: 1255.1 mbsf
Total core recovered: 143.2 m (34.7%; on Exp 309)
Oldest sediment recovered: Calcareous nannofossil ooze (Middle Miocene) at Hole 1256B during ODP Leg 206
Lithologies: Clay-rich sediments and calcareous nannofossil ooze (sediments); basalt flows and dykes (basement)
UBI Main Run: 705-1020 mbsf
UBI Repeat Run: 1006-1216 mbsf
Magnetic declination: 4.569°
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 Expedition 309 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 toolstring are presented with the 'standard' data.
Images of generally excellent quality were obtained during the second phase of logging in Hole 1256D. Both passes were depth matched (generally to within 10-cm) to the corresponding FMS images from Hole 1256D. The main pass actually recorded down 1216 mbsf, but the interval from 998-1216 contained inaccurate amplitude and radius data, which seem to be a result of the faster speed at which this pass was run (800 ft/hr compared to 400 ft/hour). 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 of the FMS resistivity images. Moreover, the UBI and FMS respond to contrasting physical properties, enabling (for example) differentiation of open and filled fractures.
The following corrections are applied in GeoFrame's BorEID module:
GPIT Speed Correction
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'.
Transit-Time – Radius Conversion
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.
Amplitude Eccentring Correction
When the tool is eccentred 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 eccentring, low order angular harmonic components of the signal with a periodicity equal to 1 and 1/2 revolution are removed.
Transit-Time Eccentring Correction
The transit time signal is corrected in the same way as the ampitude.
Radius Eccentring Correction
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
Sampling Bias Correction
A rotation of –17 degrees is necessary for the UBI to account for the alignment of the transducer. (note that there appears to be a small rotation of the UBI image relative to the FMS images in some intervals – the origin of this rotation is unclear).
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 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.
For further information or questions about the processing, please contact:
E-mail: Cristina Broglia