Miscellaneous LWD Image Data Processing
Science operator: Texas A&M University
Expedition: 372
The best known of the
Logging While Drilling (LWD) image logs are the geoVISION Resistivity At Bit
(GVR) images (formerly know 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 NeoScope tools provide equivalent images. In expedition 372 these include 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, eg. g/cm3 for density). Otherwise, processing is similar to
that applied to the GVR data. GIF images are provided for the full interval logged for
all data.
The following table
provides a summary of the images available, their sampling rate, and the codes
used in the GeoFrame processing and online:
| Property | LDEO-BRG Code | Sample Interval (cm) | Number of Radial Bins | Tool |
| Borehole radius | Radius | 15.24 | 16 | NeoScope |
| Tool Standoff | STOF | 15.24 | 16 | NeoScope |
| Gamma radiation | GR | 15.24 | 56 | geoVISION/NeoScope |
The following table
provides a summary of the intervals logged at each hole as well as technical
information about the logging-while-drilling.
| Hole | Depth Range (m) | Approx. Rate of Penetration (m/hr) |
Approx. Rotation Rate (rpm) | Comment |
| U1517A | 0 -205 | n.a. | 25-60 | geoVISION/NeoScope |
| U1518A | 0 - 123 | 10-40 | 20-60 | geoVISION/NeoScope |
| U1518B | 54-601 | 10-40 | 30-60 | geoVISION/NeoScope |
| U1519A | 0 - 652 | 20-40 | 60-70 | geoVISION/NeoScope |
| U1520A | 0 -104 | 20-60 | 20-60 | geoVISION/NeoScope |
| U1520B | 0 -757 | 20-60 | 20-60 | geoVISION/NeoScope |
Processing is required to convert the initial measurements into a 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.
1) Azimuthal orientation and conversion to depth
The main processing steps are performed onboard by Schlumberger personnel, 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 NeoScope tool, 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.
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.
2) Depth Shift and Image Generation
The DLIS file is loaded into the Schlumberger Techlog software at LDEO, where the image for each measurement is shifted so that the depth is relative to sea floor, and output GIF and DLIS files are produced. The color scale ranges for the gif images are chosen manually.
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.
After the moratorium period you may direct your questions to:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia
Tanzhuo Liu
Phone: 845-365-8630
Fax: 845-365-3182
E-mail: Tanzhuo Liu