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: August 21-24, 2005
Sea floor depth (driller's): 3645.4 mbrf
Sea floor depth (logger's): 3643.5 mbrf
Total penetration: 1255.1 mbsf
Total core recovered: 143.2 m (34.7%; on Exp 309)
Oldest sediment recovered: Calcareous nannofossil ooze (Middle Miocene) at Hole 1256B during ODP Leg 206
Lithologies: Clay-rich sediments and calcareous nannofossil ooze (sediments); basalt flows and dykes (basement)
FMS Pass 1: 652 - 1217 mbsf
FMS Pass 2: 300 - 1211 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 from the phase 2 logging of Hole 1256D at the end of Expedition 309. The different formations (massive basalt, pillows, breccia) are all apparent in the image, as well as inclined fractures. High resolution (to better than ~10cm) was carried out on the data. In the upper part (300 - 750 mbsf), the images from pass 2 were depth matched to the images from pass 1 of the Leg 206 logging of this hole. In the lower part (750-1216 mbsf), the images from pass 2 were matched to the reference run for phase 2 logging, the main pass of the Triple Combo tool string. The pass 1 images were then matched to the resulting pass 2 images. This enables the pass 1 and pass 2 images to be overlaid, thus achieving greater coverage of the borehole wall and easing interpretation of features such as fractures and pillows. The overlay of pass1, pass 2, and pass 1 from Leg 206 is presented in the 'merged' files.
The hole was
in 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,
678-694, 816-824, 919-930, 1050-1061 mbsf. Below 968 mbsf, the hole is smoother
and thin washouts occur less frequently.
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 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.
Interested scientists are welcome to visit the log
interpretation center at LDEO if they wish to use the image generation and
interpretation software.
For further information or questions about the processing,
please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia