FMI
Image Data Processing
Science operator: University of Rhode Island-University of Milano
Logging operator: Schlumberger Italy
Hole: Cismon (APTICORE Program)
Location: Monte Grappa State Road 50 (km 53), Veneto, Italy
Latitude: 46° 2' 43.46" N
Longitude: 11° 45' 46.85" E
Logging dates: January 17-18, 1996
Elevation (ground level): 398 m
Total penetration: 131.8 m
FMI Pass 1:
FMI Pass 2: 2-132 m
FMI Pass 3: 2-132 m
Magnetic declination: 0.74° E
The
FMI (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 steps
used to generate the images from the raw FMI measurements.
The FMI 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 image quality is excellent.
Image
Processing
Processing
is required to convert the electrical current in the formation, emitted by the
FMI 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. If
the reference logging run is not the FMI tool string, the depth shifts
determined during the standard data processing are applied to the FMI images.
The position of data located between picks is computed by linear interpolation.
Often, for short logged intervals, a single 'block shift' wis sufficient to
depth-match the FMI data to the reference log.
A high-resolution conductivity log is then produced from the FMI
data by averaging the conductivity values from the 64 button electrodes. This
enables the FMI data to be plotted using common graphing applications and more
easily used in numerical analyses (e.g. spectral analysis). Specifically, the
FMI 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.
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.
Additional information about the Cismon hole is available at:
https://riviste.unimi.it/index.php/RIPS/article/view/5330/5382
https://people.clas.ufl.edu/jetc/files/Channelletal2000GSAB.pdf
https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2009PA001769
Questions about the data available on line can be sent to:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia
Gilles Guerin
Phone: 845-365-8671
Fax: 845-365-3182
E-mail: Gilles Guerin