FMS Image Data Processing
ODP logging contractor: LDEO-BRG
Hole: 907A
Leg: 151
Location: Iceland Plateau (Greenland Sea)
Latitude: 69° 14.988' N
Longitude: 12° 41.892' W
Logging date: August, 1993
Bottom felt: 1811.6 mbrf (used for depth shift to sea floor)
Total penetration: 224.1 mbsf
Total core recovered: 229.98 m (102 %)
FMS Main Pass: 92-190 mbsf
FMS Repeat Pass: 74-139 mbsf
Magnetic declination: -17.2796°
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 151 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.
The FMS images in Hole 907A are of
good quality, except where there was poor pad contact due to the enlarged hole,
particularly at the bottom of the hole. The images show an overall mottled
texture due to the abundant bioturbation in the sediments. High resistive features organized in
levels or patches can be interpreted as ash layers or ash pods. The ash pods
originated from bioturbated ash layers. Further studies of resistive layers
versus resistive patches could provide an estimate of the intensity of
bioturbation. Furthermore, diffuse limits of the resistive features could also
map the extent of bioturbation and the dispersion of ashes by the bottom
dwelling organism. Weak resistive limits could be due to grain-size variations.
Obvious ash layers are noticed on
the FMS images at 95.8, 98.8, 99.1, 100.2, 101.6, and 131.6.136.8, 131.6mbsf.
These ash layers allow one to accurately correlate between the main and repeat
passes of FMS (2 cm differences) and to cores.
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