Miscellaneous LWD Image Data Processing

 

IODP logging contractor: CDEX

Expedition:  314

 

The best known of the Logging While Drilling (LWD) image logs are the GVR Resistivity At Bit (GVR) images (formerly known as RAB images), which are generated as the tool rotates while drilling, providing 360° data coverage of the borehole wall. In addition to the GVR resistivity images, some of the other LWD measurements, made by the GVR and EcoScope tools provide equivalent images. These include density, photoelectric effect, gamma radiation, borehole radius, and tool standoff. In contrast to the GVR resistivity images, the other LWD images are scaled manually and not dynamically normalized (in order to retain the original data units, e.g., g/cm3 for density). Otherwise, processing is similar to that applied to the GVR data. Gif images are provided in both full and 100 m intervals for all data sampled at 3.048 cm.

 

The following table provides a summary of the images available, their sampling rates, and the codes used in the GeoFrame processing and online:

 

Property LDEO-BRG Code GeoFrame Code Sample Interval (cm) Number of Radial Bins Tool
Density RHOB ROSI 3.048 16 ADN
Photoelectric Effect PEF PESI 3.048 16 ADN
Volumetric Photoelectric Effect VPE USI 3.048 16 ADN
Borehole Radius RADIUS DCRA 15.24 16 ADN
Tool Standoff STOF UOIM 15.24 16 ADN
Gamma Radiation GR GR_RAB_IMG 15.24 56 GVR

 

 

The following table provides a summary of the borehole intervals logged at each hole as well as technical information about the logging-while-drilling. It should be noted that, because adnVISION real-time data was lost during data transfer to the surface, no adnVision data is available for hole C0001D below 510 mLSF. Also, because the only radioactive source on board was lost and cemented at hole C0003A, no adnVision images were collected afterward for holes C0004B and C0006B.

 

 

 

Hole Depth Range (m)

Approx. Rate of

Penetration (m/hr)

 Approx. Rotation Rate (rpm) Comment
C0001D 65 -968 15 - 30 50 - 100 GVR
  40 - 507 15 - 30 50 - 100 ADN
C0002A 67 - 1394 30 - 50 85 - 120 GVR
  40 - 1372 30 - 50 85 - 120 ADN
C0004B 56 - 396 30 - 50 30 - 80 GVR
C0006B 40 - 883 30 - 50 50 - 120 GVR

 

Image Processing

 

Processing is required to convert the initial measurements into a gray or color-scale image. 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's 'Ideal' software, just after the raw data is downloaded from the tool. The azimuth of the sensors relative to north is set at the rig floor, and subsequent pipe rotation is tracked during drilling so that the image is oriented correctly. For the measurements made by the density sensor in the EcoScope or ADN tools, a full revolution (360°) of data is sampled every 10 seconds, so the vertical resolution depends upon the rate of penetration (ROP) into the formation - the slower the penetration, the more densely sampled the formation will be. ROP is typically in the 25-50 m/hr range, and the rotation is typically around 50-80 rpm.

 

The depth assigned to LWD data is derived from the known length of pipe and the vertical position of the top drive at the rig floor. After the LWD data is downloaded from the tool, the depth data are merged with it based on accurately synchronized time data. 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 exactly determine the depth of the bit based on measurements on the rig floor.

 

The LWD tools do 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 LWD data is output from the Ideal software as a depth-indexed DLIS file.

 

2) Depth Shift and Image Generation

 

The DLIS file of the depth-shifted log data is loaded into the Schlumberger GeoFrame software at LDEO-BRG. The GVR images are normalized statically and dynamically (with a 2-m interval for the dynamic normaliztion) and the output gif files are produced. The image is displayed as an unwrapped borehole cylinder. A dipping plane in the borehole appears 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.

 

Both raw and processed DLIS files posted online are the same DLIS files downloaded from CDEX's website (http://sio7.jamstec.go.jp/expedition-index.html#exp314).

 

Interested scientists are welcome to visit the log interpretation center at LDEOif 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