IODP-USIO logging contractor: LDEO-BRG
Hole: 1256D
Expedition: 309
Location: Guatemala Basin (NE equatorial Pacific)
Latitude: 6° 44.163' N
Longitude: 91° 56.061' W
Logging date: July 17-18, 2005
Sea floor depth (driller's): 3645.4 mbrf
Sea floor depth (logger's): 3643.5 mbrf
Total penetration: 752 mbsf (Leg 206)
Total core recovered: 227m (47.8%) (Leg 206)
Oldest sediment recovered: Calcareous nannofossil ooze (Middle Miocene) at Hole 1256B during the ODP
Lithologies: Clay-rich sediments and calcareous nannofossil ooze (sediments); basalt (basement)
FMS uplog: 305 - 725 mbsf
Magnetic declination: 4.569
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 Exp 309 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
Excellent borehole images of basaltic crust were obtained in Hole 1256D. The different formations (massive basalt, pillows, breccia) are all apparent in the image, as well as inclined fractures. The FMS images were depth matched to FMS Pass 1 from the Leg 206 logging of this hole.
The hole was in very good condition, generally reading between 10-12 inches in diameter. Slightly wider sections with thin washouts occur at 348-403, 418-435, 450-473, 530-605, and 678-694 mbsf. The washouts appear as conductive (dark) in the image.
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://iodp.tamu.edu/staffdir/ldeo.html) 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:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia