GeoVISION Image Data Processing
Operator and logging contractor: LDEO-BRG
Expedition: NGHP-1
Holes: all LWD holes
The geoVISION Resistivity-at-the-Bit tool (geoVISION) maps the electrical resistivity of the borehole wall at three depths of penetration. Because the tool is rotating while drilling, its three electrodes (one for each penetration depth) provide 360° data coverage of the borehole wall. These data are displayed as an electrical image of the formation in either gray or color scale. The purpose of this report is to describe the images from Expedition NGHP-1 and the different steps used to generate them from the raw geoVISION measurements. The geoVISION tool also measures total gamma radiation and resistivity logs, which are presented with the 'standard' data.
| Hole |
Depth |
Water Depth |
Azimuth Reference |
| 2A |
2-53 |
1066 |
223.7 |
| 2B |
2-252 |
1066 |
223.7 |
| 3A | 17-303 |
1087 |
223.7 |
| 4A |
5-306 |
1083.5 |
223.7 |
| 5A |
0-207 |
956 |
223.7 |
| 5B | 0-198 |
954.7 |
223.7 |
| 6A |
0-350 |
1168 |
223.7 |
| 7A |
0-258 |
1296.5 |
223.8 |
| 8A |
0-348 |
1701 |
224.1 |
| 9A |
0-340 |
1935 |
224.1 |
| 10A |
0-200 |
1049.3 |
224.1 |
| 11A |
0-199 |
1019 |
223.7 |
Data
Quality
The
quality of the GVR resistivity images is, in general, good, with clear
identification of features such as dipping fractures and major resistive zones.
In low resistivity formations (the bulk of the data outside of the gas hydrate
zones), howvere, there are differences between the shallow, medium, and deep
GVR resistivity values, manifested as mismatches in the base resistivity level
and abrupt jumps in resistivity values. It is recommended that the images be
used in conjunction with the GVR Ring resistivity or the EcoScope
resistivities, which repeat well in low resistivity formations.
Some
speckling is present in the resistivity images from Holes 2B and 3A –
this is an artifact.
Image
Processing
Processing is required to convert the initial measurements into a gray or color-scale image. This is achieved through two processing phases; the first shortly after the data is downloaded from the tool by the Schlumberger engineer and the second at LDEO-BRG.
1)
Azimuthal orientation and conversion to depth
The
main processing steps are performed using Schlumberger's '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. The resistivity data is sampled every 10 (or 20) 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. For this hole, the rate of penetration was generally in the
5-20 m/hr range.
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.
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
accurately determine the depth of the bit based on measurements on the rig
floor.
The
geoVISION data is output from the Ideal software as a depth-indexed DLIS file.
2)
Image Normalization:
The DLIS file is loaded into the
Schlumberger GeoQuest GeoFrame software at LDEO-BRG, where the depth-based
image for each depth of penetration (shallow, medium, and deep) is normalized
both statically and dynamically.
In "static
normalization", a histogram equalization technique is used to obtain the
maximum quality image. In this technique, the resistivity range of the entire
interval of good data is computed and partitioned into 256 color levels. This
type of normalization is best suited for large-scale resistivity variations.
The image can be enhanced when it is
desirable to highlight features in sections of the well where resistivity
events are relatively subdued when compared with the overall resistivity range
in the section. This enhancement is called "dynamic normalization".
By rescaling the color intensity over a smaller interval, the contrast between
adjacent resistivity levels is enhanced. It is important to note that with
dynamic normalization, resistivities in two distant sections of the hole cannot
be directly compared with each other. A 2-m normalization interval is used.
The normalized images are shifted to a
sea-floor reference and converted to gif files using in-house software. They
are presented on this web site. The image is displayed as an unwrapped borehole
cylinder. A dipping plane in the borehole will be displayed 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.
For further information or
questions about the processing, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail:
Cristina Broglia