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 Leg 200 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.
Location: Hawaii-2
Observatory (central tropical N Pacific)
FMS Pass 1: 36 - 176 mbsf
FMS Pass 2: 39 - 174 mbsf
FMS Pass 3: 37 - 174 mbsf
Magnetic declination: 12.6971°
Water depth: -4978 mbrf
The FMS images of basalts from Hole 1224F are of very good quality. Above 102 mbsf the hole is about 12 inches in diameter with only a few thin wider intervals; below this depth, there are more rough sections of wider hole, which reach 17 inches between 137-142 mbsf, according to the caliper arm on the HLDT density tool. The FMS caliper arms appear not to have fully opened - Pass 1 reaches a maximum of 10.7 inches, Pass 2 12.3 inches, and Pass 3 11.8 inches. Therefore pad contact with the borehole wall is not always ideal; though usually at least one of the four pads has a clear image. Pass 2 is probably the best of the three passes. The image depths from the three passes match to within 1 m. The FMS caliper arms were closed at about 66 mbsf for each pass; from 66 to 40 mbsf there is still information in the dynamically normalized images, in spite of the bad pad contact. Above 40 mbsf, the tool is close to the pipe and the images are unreliable.
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
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 '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 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.
2) Image Display: 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 and on CD-ROM.
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.
Interested scientists are
welcome to visit one of the log interpretation centers at LDEO, Aachen,
Leicester, Montpellier or Tokyo (http://www.ldeo.columbia.edu/BRG/ODP/STAFF) if
they wish to use the image generation and interpretation software.
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 or questions about the processing,
please contact:
Trevor Williams
Phone: 845-365-8626
Fax: 845-365-3182
E-mail: Trevor Williams
Cristina
Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia