FMS Image Data Processing
ODP logging
contractor: LDEO-BRG
Hole: 1061A
Leg: 172
Location: Blake Ridge (NW Atlantic)
Latitude: 29° 58.4976' N
Longitude: 73° 35.9929' W
Logging date: March, 1997
Bottom felt: 4048.2 mbrf
Total penetration: 350.3 mbsf
Total core recovered: 298.2 m (85.1 %)
Water depth: 4046.5 mbrf
FMS Pass 1: 124-350 mbsf
FMS Pass 2: 103-350 mbsf
Magnetic declination: -10.20242°
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 172
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
The FMS images of Hole 1061A are of moderate quality. The quality of a large part of the logged interval is affected by poor pad contact, due to the rugosity of the borehole; in fact the borehole is enlarged and irregular throughout most of the logged interval and particularly in the upper part. FMS images show that conductive layers are often associated with washouts; interpretation of FMS images is therefore recommended in close connection with caliper data. Cyclical patterns observed in the hole-size variations recorded by the two perpendicular FMS calipers seem to be lithologically controlled; in fact, the carbonate-rich layers are supposed to be more easily washed out than the clay-rich sediments.
The overall observation of Hole 1061A FMS images shows two main features. The first one is represented by resistive sections which correspond to locally good hole conditions and which present an homogenous inner structure (for instance in the 110-117, 126-132, 205-210, and 345-350 mbsf intervals). The second main feature consists of highly contrasted images which often exhibits clear cyclical patterns (as described in details below).
The calipers show a normal 11-13 inches hole diameter above 327 mbsf and a narrow 8-9 inches hole diameter below 327 mbsf. The upper part of the logged section is marked by a deterioration of the borehole condition and the resulting FMS images are of bad quality. Above 260 mbsf, due to the roughness of the borehole, conductive layers systematically correlate with hole enlargements. Between 180 and 250 mbsf, 10 to 20 cm-thick conductive layers are observed in a resistive matrix, with a spacing of approximately 50 cm. From 260 to 327 mbsf, the electrical laminations are thinner (5 cm) and very regularly spaced except at about 270 mbsf, where they are thicker ( 10-15 cm) and irregularly spaced. The 327-350 mbsf interval is characterized by alternation of 10 cm-thick conductive and resistive layers.
Image
Processing
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