Logging Data Processing and QA/QC

Logging-While-Drilling Data Processing

Depth adjustments
The main depth adjustment is the depth shift from the rig floor to sea floor, which is determined from the step in the natural gamma log seen at the sediment-water interface. Depth discrepancies between the logs recorded by the different LWD tools are rare.

Environmental corrections
Environmental corrections are designed to remove any effect from the collars, borehole, and drilling fluids that may partially mask or disrupt the log response from the formation. The logs from the gamma ray and neutron porosity tools are corrected in near-real timeduring log acquisition. Density data are corrected for the irregular borehole using a technique called “image-derived” processing, which is particularly useful in deviated or enlarged borehole with irregular or elliptical shape.

Sonic logs
Sonic slowness logs are converted into sonic velocities.

Quality control and documentation
The quality of the data is assessed in terms of reasonable values for the logged formation, repeatability between the gamma ray and porosity curves recorded by the different tools, and correspondence between logs affected by the same formation property (e.g., the resistivity log should show similar features to the velocity and density logs). Depth shifts, corrections, and data quality are documented in the processing report.

Data output
The processed data are saved in ASCII and DLIS format.

FMS Image Processing

The Formation MicroScanner (FMS) maps the conductivity of the borehole wall with a dense array of sensors. FMS Processing is required to convert the 64 electrical current traces recorded into a color-scale image representative of the conductivity changes in the formation.

BorEID corrections
Several corrections are applied using the BorEID module of GeoFrame:

Speed correction. The data from the z-axis accelerometer are used to correct the vertical position of the data for variations in the speed of the tool (“GPIT speed correction”), including “stick and slip.” In addition, “image-based speed correction” is also applied to the data, based on reducing any offset between the data from two rows of button electrodes on each FMS pad.

Equalization. The responses of the button electrodes on the pads of the tool are equalized to correct for various tool and borehole effects, which affect individual buttons differently.

Button correction. If the measurements from a button electrode are unreasonably different from its neighbors (e.g., “dead buttons”), the defective trace is replaced by traces from adjacent good buttons.

EMEX voltage correction. During logging, the voltage that drives the current is continuously regulated so that current flows even through very resistive formations. The button response is divided by the EMEX voltage so that the response corresponds more closely to the conductivity of the formation.

Depth adjustments
The natural gamma log resulting from the BorEID speed correction is matched to the gamma ray log from the same pass after conventional log depth shifting. The logs are checked for a good match, and then the resulting depth shifts are applied to FMS images and their associated logs (pad azimuth, etc.). The resulting FMS images are then on a depth scale comparable to the conventional logs.

Conductivity average
A high resolution conductivity log is then produced from the FMS data by averaging the conductivity values from the 64 button electrodes. This enables the FMS data to be plotted using common graphing applications and more easily used in numerical analyses (e.g. spectral analysis).

These conductivity values are unscaled; more accurate (but lower vertical resolution) values are given by the resistivity logs from the DIT and DLL resistivity tools.

Image normalization
Using the BorNor module of GeoFrame, “static” and “dynamic” normalizations of the image are applied. In the static normalization, the resistivity range of the entire interval of data is computed, and is partitioned into 256 color levels; this image is good for examining large-scale resistivity variations. In the dynamic normalization, the full range of color levels is assigned to a resistivity range of short intervals (e.g., 2 m); thus the color contrast is increased, enhancing the fine details of the resistivity structure.

Oriented presentation
The image is displayed as an unwrapped borehole cylinder (its circumference is derived from the bit size). Several passes can be oriented and merged together on the same presentation to give additional borehole coverage if the tool pads followed a different track. 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.

Data output
The data is saved in DLIS and GIF format (Static and dynamic image files in 50 m and full interval).

UBI Image Processing
The Ultrasonic Borehole Imager (UBI) provides an acoustic image of the borehole wall by scanning it with a narrow pulsed acoustic beam from a rotating transducer while the tool is pulled up the hole. Processing is required to improve accuracy, eliminate cycle skips, and reduces echo losses.

Corrections
Several corrections are applied using the BorEID module in GeoFrame:
Speed correction.The data from the z-axis accelerometer is used to correct the vertical position of the data for variations in the speed of the tool, including ‘stick and slip’.Transit time–radius conversion. The transit time measurement from the UBI scanner is converted to a borehole radius measurement given, the velocity of ultrasound in the borehole fluid and the tool radius.

Amplitude eccentering correction. When the tool is eccentered in a circular borehole, the amplitude is increased in the directions where the distance to the borehole wall is decreased and vice versa. This change in amplitude can often be larger than the changes in amplitude produced by features on the borehole wall that we wish to image. To correct for the effects of eccentering, low order angular harmonic components of the signal with a periodicity equal to 1 and 1/2 revolution are removed.

Transit-time eccentering correction. The transit time signal is corrected in the same way as the amplitude.

Radius eccentering correction. The distance and direction of points on the borehole wall are initially given with the tool axis as the origin. The geometrical centre of the points on the borehole wall is calculated, and the distance to those points is recalculated relative to the geometrical borehole centre. Both corrected and uncorrected radius images are output. The uncorrected image should be used for analyses such as breakouts and dip computations.

Azimuth equalization. The background response for all azimuths over a large window (e.g. 3 m) is equalized, removing preferential enlargement at a particular azimuth.
Image rotation. A tool specific rotation is necessary for the UBI to account for the alignment of the transducer (-17° in the case of the current tool).

Depth adjustments
Typically, the UBI is run in hard rock holes where there is also good FMS data coverage. In this case, features such as fractures in the UBI images are matched to the same features in the FMS images. The FMS images can then be overlaid on the UBI: the greater resolution of the FMS combined with the 360° coverage of the UBI makes features in the borehole wall much easier to see and interpret. Where no FMS is available, the UBI images are depth adjusted by matching the UBI tool string gamma ray log to the reference gamma ray log from the standard processing.

Image normalization
Using BorNor module in GeoFrame, “static” and “dynamic” normalizations of the images are applied. In the static normalization, the amplitude or radius range of the entire interval of good data is computed and partitioned into 256 color levels. In the dynamic normalization, the color intensity is rescaled over a smaller interval, thus enhancing the contrast between adjacent amplitude or radius levels.

Oriented presentation
The image is displayed as an unwrapped borehole cylinder (its circumference is derived from the bit size). 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 the North; hence the strike of dipping features can also be determined.

Data output
The data is saved in DLIS and GIF format (Static and dynamic image files in 50 m and full interval).

Resistivity-at-the-Bit Image Processing

The geoVISION LWD (Logging While Drilling) tool maps the electrical resistivity of the borehole wall with three depths of investigation. Because the tool is rotating while drilling, its three electrodes (one for each penetration depth) provide a 360° electrical image of the borehole wall.

Processing is required to convert the electrical current in the formation, emitted by the geoVISION button electrodes, into a gray or color-scale image representative of the resistivity changes. This is achieved through two main processing phases, the first performed aboard the JOIDES Resolution, and the second post-cruise at BRG.

Ship-based processing
Azimuthal orientation and conversion to depth, The main processing steps are performed by the Schlumberger engineer using the Schlumberger ‘Ideal’ software package, just after the raw data is downloaded from the tool. 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 6.4°, because the resistivity measurements are assigned to 56 radial bins. A full revolution (360°) of the resistivity data is sampled every 5 (or 10) seconds, therefore the data density in terms of depth depends upon the rate of penetration into the formation – the slower the penetration, the more densely sampled the formation will be.The geoVISION 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.

Ship heave effect. The effect of ship heave is sometimes apparent as horizontal discontinuities in the image. They exist because it can be difficult, with a long drill string, to accurately determine the exact depth of the bit based on measurements on the rig floor.

Shore-based processing
Image Normalization. The depth-based image for each depth of penetration (shallow, medium, and deep) is normalized both statically and dynamically. In static normalization, the resistivity range of the entire interval of good data is computed and the image is rescaled unto 64 color levels, enabling different parts of the hole to be compared. In dynamic normalization, the color intensity is rescaled over 2 m intervals, thus enhancing the local contrast.

Depth adjustment. The normalized images are finally shifted to a sea-floor reference. They are presented on this web site.

Oriented presentation
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 the North; hence the strike of dipping features can also be determined.

Data output
The data is saved in DLIS and GIF format (Static and dynamic image files in 100 m and full interval).

Other LWD Image Processing
The best known of the Logging While Drilling (LWD) image logs are the Resistivity-at-the bit images; some of the other LWD tools, however, such as the EcoScope and the adnVISION tools, provide images as well. These include density, PEF, gamma radiation, borehole radius, and tool standoff. In contrast to Resistivity-at-the-bit images, the other LWD images are scaled manually and are not dynamically normalized. Otherwise, shipboard and shore-based processing are similar to that applied to the Resisitivity-at-the-bit data.

Data output
The images in GIF format are provided in 100 m and full interval for all data. After processing they are transmitted via satellite back to the ship. Also, they are available online in the online log database LogDB. The processed data are also available in DLIS format.

Processing of Other Log Data

Sonic waveform data
During logging, sonic travel-times are picked from the waveform data acquired by the DSI sonic tool; these picks are used in the conventional log processing. Waveform data are routinely converted into binary format and made available online in the LDEO-BRG online log database LogDB.

Seismic data
Seismic data, both individual shot records and stacks for each station, are converted into SEGY format and made available online in the online log database LogDB. First arrival times are picked on the ship and are not generally re-picked onshore. Shot and stack summary tables are available online in ASCII format. Where there are enough stations for a vertical seismic profile, a corridor stack can be produced and compared to the synthetic seismogram and seismic section.

Other data
Processing of data from tools that have been used only occasionally, such as the Azimuthal Resistivity Imager or the LWD sonicVISION tool, has been carried out at the Schlumberger processing facility.