FMS Image Data Processing
ODP logging contractor: LDEO-BRG
Location: Great Bahama Bank (tropical NW Atlantic)
Latitude: 24° 32.763' N
Longitude: 79° 15.650' W
Logging date: March, 1996
Bottom felt: 494.5 mbrf
Total penetration: 677 mbsf
Total core recovered: none.
FMS Pass: 115-1052 mbsf
Magnetic declination: -4.926141°
Water depth: 493.5 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 166 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 1003D are of moderate quality. Reliable data were recorded in the lower half of the logged interval, below 598 mbsf, where hole enlargements affect the quality of the FMS images in the 657-662 and 930-945 mbsf intervals. The hole is considerably degraded above this depth; over this interval, resistive bands correspond to hole collapse, conductivity bands to hole enlargement. These patterns associated to the hole shape may be directly related to lithologic changes, therefore any interpretation must be made with caution. Hole conditions are particularly bad above 230 mbsf: no reliable FMS images were recorded in this upper interval.
The best quality images, obtained in the lower section from 598 to 1052 mbsf, display fine scale bedding. The FMS images show abrupt changes at 645, 738, and 973 mbsf, which match closely the major lithologic boundaries. At 645 mbsf, the limit between Units IV and V is marked by the downward transition to more resistive electrical images presenting a gentler contrast. At 738 mbsf, the limit between Units VA and VB is characterized by the downward decrease of the electrical contrast of the FMS images. The bottom of Unit VA shows thin conductive layers (10 cm thick) in a highly resistive matrix (see also at 800-807 mbsf), whereas the top of Unit VB display large conductive bands with a more gentle electrical contrast with the surrounding matrix. At 973 mbsf, the limit between Unit VIA and VIB is marked by an increase in the resistivity of the sediments. This feature may correspond to the presence of turbidites just above the limit.
The main feature observed on the FMS images consists of cyclic alternations of conductive and resistive beds. This recurrence is not regular. For instance, the occurrence of conductive layers is more spaced above 835 mbsf (lower wavelength); the layer thickness changes from tiny laminations (835-840 and 1010-1015 mbsf intervals) to meter-thick layers (840-930 and 1018-1020 mbsf intervals). Detailed observation of the FMS images reveals cm-scale conductive spots (such as at 963.5 mbsf), possibly related to the presence of burrows or vugs. Between 1010 and 1015 mbsf thin conductive laminations show a slight dip to the N.
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
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.
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.
For further information or questions about the processing, please contact:
E-mail: Cristina Broglia