RAB Image Data Processing
IODP-USIO logging contractor: LDEO-BRG
Location: Cascadia Margin (NE Pacific)
Latitude: 48° 37.6350' N
Longitude: 127° 3.0289' W
Logging date: September 22-23, 2005
Water Depth (as seen on logs): 1838 mbrf
Total penetration: 300 mbsf
RAB depth range: 0-300 mbsf
Azimuth reference (P1AZ): 242°
The RAB (Resistivity At Bit) LWD (Logging While Drilling) tool maps the electrical resistivity of the borehole wall at three depths of penetration. Because the tool is rotating while drilling, its three electrodes (one for each penetration depth) provide 360° data coverage of the borehole wall. These data are displayed as an electrical image of the formation in either gray or color scale. The purpose of this report is to describe the images from Expedition 311 and the different steps used to generate them from the raw RAB measurements. The RAB tool also takes total gamma radiation and resistivity logs, which are presented with the 'standard' data.
The images are of moderate to good quality. The deep RAB images are probably the most reliable, as shallow and medium are more affected by small-scale variations in hole diameter. The ARC tool gives good conventional resistivity logs as a counterpart to the high resolution RAB images. Dipping beds and gas hydrates can be observed in the images.
Processing is required to convert the electrical current in the formation, emitted by the RAB button electrodes, into a gray or color-scale image representative of the resistivity changes. This is achieved through two main processing phases, the first shortly after the data is downloaded from the tool by the Schlumberger engineer, and the second post-cruise at LDEO-BRG.
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 30-100 m/hr range, and RPM approximately 60 rpm.
The RAB 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 RAB 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 at LDEO-BRG, 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. 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
E-mail: Gilles Guerin