ODP logging contractor: LDEO-BRG
Hole: 1265A
Leg: 208
Location: Walvis Ridge (tropical SE
Atlantic)
Latitude: 28° 50.101' S
Longitude: 2° 38.360' E
Logging date: April 1, 2003
Bottom felt: 3071 mbrf
Total penetration: 321 mbsf
Total core recovered: 300 m (93.5 %)
FMS Main Pass: 70-325 mbsf
FMS Repeat Pass: 257-325 mbsf
Magnetic declination: -23.04°
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 208 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.
Data Quality
Good borehole images were obtained from the lower half of Hole 1265A. Where the hole is wider than 15.5 inches (the maximum extent of the FMS caliper arms), pad contact with the borehole wall deteriorates. In spite of this, even where the hole is wider than 18 inches at least two of the pads generally retain good contact with the borehole wall, and the images still contain useful information.
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.
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 or questions about the processing,
please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia