Hole C0011A, Expedition 322: GVR Image Processing Report
IODP logging
contractor: CDEX-JAMSTEC
Hole: C0011A
Expedition: 322
Location: Nankai Trough (NW Pacific Ocean)
Latitude: 32° 49.73' N
Longitude: 136° 52.89' E
Logging-while-drilling date: August 26, 2009
Total penetration: 5030 m LRF (952 m LSF)
Total core recovered: None (LWD/MWD used)
Image interval: 39 - 950 mLSF
Azimuth reference (P1AZ): 219.14°
Overall, the resistivity images are of poor qualitt. The amount of stick-slip, caused by unstable bit rotation, increases with depth in the borehole. Between 596 and 629 m LSF, the stable ROP following a wiper trip resulted in high-quality resistivity images. Elsewhere in the borehole, the variation in rotations per minute and high angular acceleration index indicate that the bit rotation was not stable. Sections of missing images are the result of the sudden rotation caused by stick-slip, consistent with high angular acceleration index. The ship's heave also produced some discontinuities in the resistivity images, locally resulting in an overlap of images at the same depth. The close correspondence of deep- and
shallow-button resistivity values indicates little or no significant drilling mud invasion into the formation.
Image
Processing
Processing is required to convert the electrical current in the formation, emitted by the GVR button electrodes, into a gray or color-scale image representative of the resistivity changes. This is achieved through two steps:
1) Azimuthal
orientation and conversion to depth
The main processing steps are performed using Schlumberger 'Ideal' software package, just after the raw data is downloaded from the tool. An azimuth and a depth are assigned to each measurement based on measurements of the pipe orientation and position at the rig floor. The resolution of the azimuth is about 6.4°, because the resistivity measurements are assigned to 56 radial bins. The resistivity data is sampled every 10 (or 20) seconds, therefore the data density in terms of depth depends upon the rate of penetration (ROP) into the formation- the slower the penetration, the more densely sampled the formation will be. For this hole, the ROP was generally in the range of 35-50 m/hr.
The
GVR tool does not move with a constant velocity down the hole: new sections of
drill pipe have to be added every 10 m and ship heave is never completely
compensated. This means that there will often be repeat measurements for one
particular depth in the borehole. The measurement that is used is the first one
taken at a particular point, before the borehole has had time to deteriorate.
The
effects of ship heave are sometimes apparent as horizontal discontinuities in
the image. They exist because it can be difficult, with a long drill string, to
accurately determine the depth of the bit based on measurements on the rig
floor.
The
GVR data is output from the Ideal software as a depth-indexed DLIS file.
2)
Image normalization:
The DLIS file is loaded into the Schlumberger GeoQuest GeoFrame software, where the depth-based image for each depth of penetration (shallow, medium, and deep) is normalized both statically and dynamically.
In "static normalization", a histogram equalization technique is used to obtain the maximum quality image. In this technique, the resistivity range of the entire interval of good data is computed and partitioned into 256 color levels. This type of normalization is best suited for large-scale resistivity variations. The image can be enhanced when it is desirable to highlight features in sections of the well where resistivity events are relatively subdued when compared with the overall resistivity range in the section. This enhancement is called "dynamic normalization". By rescaling the color intensity over a smaller interval, the contrast between adjacent resistivity levels is enhanced. It is important to note that with dynamic normalization, resistivities in two distant sections of the hole cannot be directly compared with each other.
The images are normalized with a 2-m interval for dynamic normalization. The normalized images are then converted to gif files using in-house code (no depth shifts to sea floor are needed). Thery are posted online and 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.
This report was prepared at the Borehole Research Group of the Lamont Doherty Earth Oservatory, based on the processing notes and additional information provided by the logging staff scientists of Expedition 319. Additional information about the drilling and logging operations can be found in the Operations and Downhole Measurements sections of the expedition reports, Proceedings of the Integrated Drilling Program, Expeditions 319 and 32.2
For further information or questions about the processing, please contact
sio7-info@jamstec.go.jp
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