IODP-USIO logging contractor: LDEO-BRG
Hole: U1309D (Phase 1)
Expedition: 304
Location: Atlantis Massif-Mid-Atlantic Ridge (central N Atlantic)
Latitude: 30° 10.120' N
Longitude: 42° 7.113’ W
Logging date: January 2, 2005
Sea floor depth (driller’s): 1656 mbrf
Total penetration: 401.3 mbsf
Total core recovered: 257 m (64 % of cored section)
Oldest sediment cored: none
Lithologies: Basalt, diabase, gabbro, oxide gabbro, peridotite, troctolite
FMS Pass 1 (Phase 1): 85-400 mbsf
FMS Pass 2 (Phase 1): 60-400 mbsf
Magnetic
declination: -15.71°
Water depth: 1656 mbrf
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 304 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 were obtained from Hole U1309D. Additionally, for the two passes, the FMS pads followed different paths up the borehole wall, effectively doubling the image coverage for much of the hole. The second pass was chosen as the reference pass, because the tool speed up the hole was smoother than for Pass 1. The images from Pass 1 were then depth-matched to Pass 2, so that the features in the images (fractures, foliation, etc.) typically match to within 10 cm.
The borehole was in very good condition, with a width of 9-10 inches below 292 mbsf. Between 95-292 mbsf, the borehole was generally 10-12 inches wide, with some wider sections to >15 inches, e.g. at 228-229 mbsf.
The lithology of the logged interval includes basalt, diabase, gabbro, oxide gabbro, peridotite, and troctolite.
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.
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 and
DLL resistivity tools.
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 the log interpretation center at LDEO 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