FMS Image Data Processing
Operator
and logging contractor: LDEO-BRG
Hole: 19B (proposed site MNGH Gap)
Expedition: NGHP-1
Location: Manahadi Basin, Eastern India (Bay of Bengal)
Latitude: 18° 58.6532' N
Longitude: 85° 39.5160' E
Logging date: August 8, 2006
Sea floor
depth (drillersÕ): 1433
mbrf
Sea floor
depth (loggersÕ): 1436
mbrf
Total
penetration: 1713 mbrf
(280 mbsf )
Total core
recovered: 271.28 m (90.4 % of cored section, from Hole
NGHP-19A)
Oldest
sediment cored: n/a
Lithologies: Nanofossil-rich clay (from Hole NGHP-19A)
FMS Pass 1: 100-260 mbsf
FMS Pass 2: 80-260 mbsf
Magnetic declination: -0.81°
The FMS images from Hole 19B are
generally of good quality. The hole is rough and irregular, with bridges at
103-105, 130-136, 150-152, 168-170, and 198-200 mbsf. Some of these bridges
caused increased cable tension, and consequently some small depth offsets may
remain in the vicinity of the bridges. Small-scale washouts occur throughout
and are more prevalent towards the top of the logged section.
The
basic principle of the FMS (Formation MicroScanner) is to map 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
Expedition NGHP-1, Hole 19B and the different steps used to generate them from
the raw FMS measurements.
The FMS tool
records 4 perpendicular electrical images, using four pads, which are pressed
against the borehole wall. Each pad has 16 buttons and the tool provides
approximately 25% coverage of the borehole wall. 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.
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 (ÔGPIT
speed correctionÕ), including Ôstick and slipÕ. In addition, Ôimage-based speed
correctionÕ 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. Ôdead buttonsÕ) 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 response is divided by the EMEX voltage so that the response
corresponds more closely to the conductivity of the formation.
Depth-shifting:
Each of the logging runs are Ôdepth-matchedÕ 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 FMS tool string, the specified
depth shifts are applied to the FMS images. The position of data located
between picks is computed by linear interpolation.
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 resistivity tool.
2) Image Display:
Once the data is processed, both ÔstaticÕ and ÔdynamicÕ images are generated; the differences between these two types of image are explained below.
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 (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 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.
For further information about
the processing, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail:
Cristina Broglia