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

 

Image Processing

 

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.

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:


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