MicroScope Image Data Processing
Science operator: CDEX-JAMSTEC
Hole: C0002Q
Expedition: 358
Location: Nankai Trough (NW Pacific Ocean)
Latitude: 33° 18.050' N
Longitude: 136°38.2029 ' E
Logging-while-drilling date: November 17-December 14, 2018
Total
penetration: 5225.38 m LRF (3257.88 m LSF)
Image interval: 2890-2927 m LSF
The MicroScope LWD (Logging While Drilling) tool maps the electrical resistivity of the borehole wall at four 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 358 and the different steps used to generate them from the raw MiscroScope measurements. The MicroScope tool also takes total gamma radiation and resistivity logs, which are presented with the 'standard' data.
Image Processing
Processing is required to convert the electrical current in the formation, emitted by the MicroScope button electrodes, into a gray or color-scale image representative of the resistivity changes.
1) Azimuthal orientation and conversion to depth
The main processing steps were performed using Schlumberger's TechLog software package by logging scientists aboard the Chikyu. An azimuth and a depth are assigned to each measurement based on measurements of the pipe orientation and position at the rig floor. The sampling of the azimuth is about 6.4°, because the resistivity measurements are assigned to 56 radial bins (1.7° and 208 radial bins for the high resolution resistivity data). 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) in the formation – the slower the penetration, the more densely sampled the formation will be.
The MicroScope 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.
2) Image Normalization
The DLIS file of MicroScope images is loaded into Schlumberger's TechLog software, where the depth-based image for each depth of penetration (shallow, medium, deep and extra deep) is converted. The images are then normalized both statically and dynamically using LDEO's in-house code.
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 during the data processing.
The normalized images were shifted to the sea floor (1967.5 m LRF) abord the Chikyu and converted to gif files at LDEO using in-house software. 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.
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 International Ocean Discovery Program, Expedition 358. For further questions about the logs, if the hole is still under moratorium please contact the staff scientist of the expedition.
After the moratorium period you may direct your questions to:
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