FMS Image Data Processing
ODP logging contractor: LDEO-BRG
Hole: 1072B
Leg: 174A
Location: New Jersey Rise (NW Atlantic)
Latitude: 39° 21.9305' N
Longitude: 72° 41.6647' W
Logging date: July, 1997
Bottom felt: 109.5 mbrf
Total penetration: 358.6 mbsf
Total core recovered: none
Water depth: 2303 mbrf
FMS Run: 50-297 mbsf
Magnetic declination: -13.69413°
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 174A 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 recorded in Hole 1072B are of good
quality, except in the upper part of the hole. A progressive degradation of
hole conditions is observed above 130 mbsf; interestingly, however, the topmost
ten meters (50-60 mbsf) provide reliable images.
Good quality FMS images were obtained in the lower section from 100 to 297 mbsf. The 259-297 mbsf interval shows conductive electrical images with the occurrence of two main and thick resistive layers (276-279 and 265.5267.5 mbsf).
The overall matrix resistivity increases at 259
mbsf and above, associated with the development of thin conductive laminated
features. These conductive features are more common in the 246-249 and 237-241
mbsf intervals and above 211 mbsf. The transition between lithologic Units I
and II is marked by an increase of the resistivity from 147 mbsf upward. This
boundary is also marked between 150 and 152 mbsf by highly resistive levels
limited by two thick conductive layers (with a thickness of 3 and 6 m above and
below respectively). From 100 mbsf upward the images are degraded by the bad
hole condition, except in the 50-60 mbsf interval, which shows stratified
conductive sediments limited by two resistive layers (56-57 and 54-55 mbsf).
The FMS electrical images generally exhibit a mottled
structure with either high or low resistive spots, pods or traces. These
features are possibly related to burrows or concretions. Electrically
contrasted spots occur in the following intervals: 173-174, 220-225, and
240-252 conductive features), and 57.5-58.5, 268-272, and 293-294 (resistive
features). Both resistive and conductive features are detected on the images,
but are difficult to map. For instance, the 136-137 mbsf interval shows
resistive levels with moderate dip angle to the E. Between 185 and 187 mbsf,
thin conductive bands present low angle dip to the W-SW.
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