RAB Image Data Processing

Science operator: Texas A&M University

Hole: U1517A

Expedition: 372

Location: Hikurangi Subduction Margin (SW Pacific Ocean)

Latitude: 38° 49.7722' S

Longitude: 178° 28.5574' E

Logging date: December 16-17, 2017

Sea floor depth (driller's): 736.2 m DRF

Sea floor depth (logger's): 735.5 m LRF

Total penetration: 205 m LSF

RAB image intervl: 0-203 mbsf

Azimuth reference (P1NO): 246.2°

Magnetic declination: 21.166°

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 372 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 RAB images of Hole U1517A range from low to moderate quality. In order to obatin good quality images, the tool rotations per minute (RPM) generally need to be greater than 30 c/min to generate a good quality image, while the rate of penetration (ROP) needs to be less than 15.24 m/h. At this hole RPM is 30-60 c/min for most of the logging section with negligible effect on the image quality. At discrete intervals, it drops below 30 c/min. ROP shows values higher than 15.24 m/h for most of the logging interval. Higher ROP has affected the image quality which is also severely affected by the stick and slip motion of the tool string. Neverthless, natural fractures and structural features are still recognizable in the logging interval.

Image Processing

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.

1) Azimuthal orientation and conversion to depth

The main processing steps are performed using Schlumberger's TechLog software package (v. 2016). 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 1.6°, because the resistivity measurements are assigned to 224 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.

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.

2) Image Normalization

The DLIS file of RAB images is loaded into Schlumberger's TechLog software, where the depth-based image for each depth of penetration (shallow, medium, and 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 are shifted to the sea floor (735.5 m DRF) and converted to gif files using the LDEO's 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.

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 372. 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