FMS Image Data Processing
ODP logging contractor: LDEO-BRG
Location: Mid Atlantic Ridge (central tropical N Atlantic)
Latitude: 22° 45.35' N
Longitude: 46° 04.9' E
Logging date: July, 1997
Bottom felt: 4494 mbrf (used for depth shift to sea floor).
Total penetration: 664 mbsf (drilled during DSDP Leg 45)
Total core recovered: 106 m (18%)
FMS Pass 1: 120-607 mbsf
FMS Pass 2: 119-607 mbsf
FMS Pass 3: 118-606 mbsf
Magnetic declination: -17.26°
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 174B 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 recorded in Hole 395A Pass 2 are of good to excellent quality. The conductive signal is generated mainly by fractures, either open or mineralized, and lithological boundaries. While the former might be smaller in size than the electrical signature on FMS images (< cm in thickness), the latter are generally larger (> dm in thickness), as indicated by ARI (Azimuthal Resistivity Imager) images as well. The thick aquifers encountered in hole 395A at 169.5-173.5, 176.5-182.5, 230-232.5, and 419-429 mbsf are generally the site of hole enlargements, hence the lower quality of the images. The images of such intervals are in marked contrast with those observed in flows at 18 -192, 196-204, and 29.5-295.5 mbsf.
Occasional problems occur with a sharp apparent rotation of the tool in the hole (at 257.2, 258.1, and 269.5 mbsf). These apparent rotations could be due either to a sudden torque release, in this case related to real mechanical movements in the hole, or to the presence of a strongly magnetized layer in the formation. Such a layer could contain such alteration minerals as pyrite or chalcopyrite, for example, which may mislead the Z-axis magnetometer in the identification of North.
Passes 1 and 3: while the images are of comparable quality to those of Pass 2, the orientation with respect to North is not possible due to a problem with the inclinometry tool. The data from FMS Pass 3 are of lower quality than those of the other two passes due to a failure of the heave motion compensator.
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