ODP logging
contractor: LDEO-BRG
Hole: 1253A
Leg: 205
Location: Costa Rica Convergent Margin (equatorial NE Pacific)
Latitude: 9° 38.8583' N
Longitude: 86° 11.4337' W
Logging date: October 7-8, 2002
Bottom felt: 4387 mbrf
Total penetration: 600 mbsf
Total core recovered: 148.6 m (64.6 %)
FMS Pass 1: 410 - 528 mbsf
FMS Pass 2: 410 - 560 mbsf
Magnetic declination: 1.393°
Water depth: 4390 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 Leg 205 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
Hole conditions were variable because of the contrasting lithologies encountered. The hole was wider than 18 inches in the sediment section (429-459 mbsf), with at least two of the pads having little contact with the borehole wall. The hole was in excellent condition in the gabbro (414-428 mbsf and 458-558 mbsf), the diameter being mostly 12 inches, except for isolated rougher sections where the diameter could reach 18 inches. Consequently, the FMS images are generally of very good quality in the gabbro. The hole was cased to 414 mbsf.
In Pass 2, the tool string stuck in three places, leading to localized depth
discrepancies (around 468, 475,
542-546 mbsf). In Pass 1 at 486
and 491 mbsf, the high resistivity bands in the image are artifacts. Only one
of the caliper logs (C1) recorded properly
for both passes. Although the C2 caliper was open, as is seen from the good
contact in the images, it did not record good measurements.
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:
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.
For further information or questions about the processing,
please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-3182
E-mail: Cristina Broglia