Hole C0002A, Expedition 314: GVR Image Processing Report

The GVR (or RAB, Resistence 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 314 and the different steps used to generate them from the raw GVR measurements. The GVR tool also takes total gamma radiation and resistivity logs, which are presented with the 'standard' data.

IODP logging contractor: CDEX

Hole: C0002A

Expedition: 314

Location: Nankai Trough (NW Pacific Ocean)

Latitude: 33° 18.0192'

Longitude: 136° 38.1810' E

Logging-while-drilling date: October 13, 2007

Sea floor depth (as seen on logs): 1964.5 mLRF

Total penetration: 1401.5 mLSF (3366 mLRF)

Image interval: 67 - 1394 mLSF

Azimuth reference (P1AZ): 220.6°

Data Quality

The GVR images are gernerally of good quality. The deep GVR images seem to be the most reliable, as the shallow and medium ones are more affected by small-scale variations in hole diameter. Breakouts and dips of the formation can be observed in the images.

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 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 30-50 m/hr range, and RPM approximately 85-120 rpm.

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 vertical bands in the images are due to the stick-slip motions of the LWD tool.

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 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.

Since the GVR image collections from this hole contain all of the needed parameter arrays such as TOOT, the GVR images are therefore normalized using the standard BorNor module of the GeoFrame software, 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.

Interested scientists are welcome to visit one of the log interpretation center at LDEO (http://iodp.tamu.edu/staffdir/ldeo.html) if they wish to use the image generation and interpretation software.

Additional information about the drilling and logging operations can be found in the Site Chapter of the expedition report, Proceedings of the Integrated Drilling Program, Expedition 314 . For further questions about the logs, please contact:

Yoshinori Sanada
IODP Department, CDEX
Japan Agency for Marine Science Technology(JAMSTEC)
3137-25, Showama-machi, Kanazawa-ku
Yokohama, 236-0001, Japan
tel: +81-45-778-5649

fax: +81-45-778-5704
e-mail: sanada@jamstec.go.jp

For any web site-related problem please contact:
E-mail: logdb@ldeo.columbia.edu