FMS Image Data
Processing
IODP-MSP
drilling and logging contractor: ESO
Hole: M0004B
Expedition: 302
Location: Lomonosov Ridge (
Latitude: 87° 52.018' N
Longitude: 136° 10.475' E
Logging
date: September 4,
2004
Sea floor
depth (driller's): 1289.70 mbrf
Sea
floor depth (logger's): 1291.00 mbrf
Total
penetration: 218.00 mbsf
Total
core recovered:
7.31 m (66.50 % of cored section)
Oldest
sediment recovered: Middle Eocene
Lithologies: Clays to silty muds
Magnetic
declination: 89.64°
The FMS
(Formation MicroScanner) maps the conductivity of the borehole wall with a
dense array of sensors. This provides a high resolution electrical image of the
formation which can be displayed in either gray or color scale. The purpose of
this report is to describe the images from IODP-ESO Expedition 302 and the
steps used to generate them from the raw FMS measurements.
The FMS tool records electrical images using four
pads, each with an array of 16 buttons, which are pressed against the borehole
wall. The tool provides approximately 25% coverage of the borehole wall in a
10-inch diameter borehole. The tool string also contains a triaxial
accelerometer and three flux-gate magnetometers (in the GPIT, General Purpose
Inclinometry Tool) whose results are used to accurately orient and
position the images. Measurements of hole size, cable speed, and natural gamma
ray intensity also contribute to the processing.
Data Quality
The
collection of FMS images from Hole M0004B was improved by the lack of heave and
good borehole conditions. However,
the borehole wall was 'marked' during the drilling/reaming/pipe pulling procedure
and this drilling artifact (fine dark lines running at constant angle across
the image) is clearly visible on the FMS images, which in places obscures the
details of interest.
Image Processing
Processing is
required to convert the electrical current in the formation, emitted by the FMS
button electrodes, into a gray or color-scale image representative of the
conductivity changes. This is achieved through two main processing phases: data
restoration and image display.
1) Data
Restoration
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 ('GPIT speed correction'), including 'stick
and slip'. In addition, 'image-based speed correction' is also applied to the
data: the principle behind this is that if the GPIT speed correction is
successful, the readings from the two rows of buttons on the pads will line up,
and if not, they will be offset from each other (a zigzag effect on the image).
Equalization
Equalization is the process whereby the average response of
all the buttons of the tool are rendered approximately the same over large
intervals, to correct for various tool and borehole effects which affect
individual buttons differently. These effects include differences in the gain
and offset of the pre-amplification circuits associated with each button, and
differences in contact with the borehole wall between buttons on a pad, and
between pads.
Button Correction
If the measurements from a button are unreasonably
different from its neighbors (e.g. 'dead buttons') over a particular interval, they are declared
faulty, and the defective trace is replaced by traces from adjacent good
buttons.
EMEX voltage correction.
The button response (current) is controlled by the
EMEX voltage, which is applied between the button electrode and the return
electrode. The EMEX voltage is regulated to keep the current response within
the operating range. The button current response is divided by the EMEX voltage
to give the relative conductivity of the formation.
Depth-shifting
Each of the
logging runs are 'depth-matched' to a common scale by means of lining up
distinctive features of the natural gamma log from each of the tool strings. Fro Expedition 302, the caliper logs were used due to containing a greater level of detail enabling a better depth match to be performed. If
the reference logging run is not the FMS tool string, the depth shifts
determined during the standard data processing are applied to the FMS images.
The position of data located between picks is computed by linear interpolation.
Often, for short logged intervals, a single 'block shift' is sufficient to
depth-match the FMS data to the reference log.
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). Specifically, the FMS
conductivity values are averaged over each of the four pads and over five
0.254-cm depth levels to produce a file with 1.27-cm sample interval containing
the total (4-pad, 64-button) average conductivity value, plus the 16-button
averages from each of the four pads. Note that the conductivity values are
un-scaled and more accurate (but lower vertical resolution) values are given by
the resistivity logs from the DIT and DLL resistivity tools (where these tools are run).
2) Image Display:
Normalization
Once the data is
processed, both 'static' and 'dynamic' images are generated; the differences
between these two types of image are explained below. Both types are provided
online.
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.
Oriented
Presentation
The image is
displayed as an unwrapped borehole cylinder. 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
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
north; hence the strike of dipping features can also be determined.
Interested
scientists are welcome to visit one of the log interpretation centers at USIO/LDEO,
Additional
information about the drilling and logging operations can be found in the
Operations section of the Site Chapter in IODP Proceedings of Expedition 310.
For further questions about the data, please contact:
Jennifer Inwood
Phone:
011-44-116-252-3327
Fax:
011-44--116-252-3918
E-mail: iodp@le.ac.uk
For any web
site-related problem please contact:
E-mail: logdb@ldeo.columbia.edu