Expedition 311: LWD Image Data Processing
The best known of the Logging While Drilling (LWD) image logs are the Resistivity At Bit (RAB) images (from the geoVISION tool), which are generated as the tool rotates while drilling, providing 360° data coverage of the borehole wall. In addition to the RAB resistivity images, some of the other LWD measurements, made by the RAB, EcoScope, and adnVISION tools provide equivalent images. These include density, photoelectric effect, gamma radiation, borehole radius, and tool standoff. In contrast to the RAB resistivity images, the other LWD images are scaled manually and not dynamically normalized. Otherwise, processing is similar to that applied to the RAB data. GIF images are provided in 100 m and full intervals 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||GeoFrame Code||Sample Interval (cm)||Number of Radial Bins||Tool|
|Volumetric Photoelectric Effect||VPE||USI||3.048||16||Ecoscope/adnVISION|
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
|Approx. Rotation Rate (rpm)||Comment|
|U1325A||9 - 351||20 - 50||60 - 70||geoVISION/adnVISION/EcoScope|
|U1326A||0 - 293||30 - 100||~60||geoVISION/adnVISION/EcoScope|
|U1327A||10 - 309||30 - 70||60 - 70||geoVISION/adnVISION/EcoScope|
|U1328A||14 - 293||30 - 80||~60||geoVISION/adnVISION/EcoScope|
|U1329A||5 - 218||20 - 50||60||geoVISION/EcoScope|
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.
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 adnVISION 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 60 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 is loaded into the Schlumberger GeoFrame 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.
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 Integrated Ocean Drilling Program, Expedition 311.
For further information or questions about the processing, please contact:
E-mail: Gilles Guerin
E-mail: Cristina Broglia