FMS Image Data Processing
Science operator: Texas A&M University
Location: Brothers Volcano SW Pacific Ocean
Latitude: 34° 51.6588' S
Longitude: 179° 3.4572' E
Logging date: June 19-20, 2018
Sea floor depth (driller's): 1606 m DRF
Sea floor depth (logger's): 1601 m WRF
Total penetration: 2054.1 m DRF (453.1 m DSF)
Total core recovered: 76.77 m (16.9 % of cored section)
Oldest sediment recovered: n/a
Lithology: volcaniclastic rocks (lapillistone, lapilli-tuff and tuff-breccia)
FMS Pass 1: 122-443.5 m WMSF
FMS Pass 2: 96-444.25 m WMSF
Magnetic declination: 18.8286°
The FMS (Formation MicroScanner) maps 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 IODP Expedition 376 and the steps used to generate them from the raw FMS measurements.
The FMS tool records electrical images using four pads, each with an array of 16 buttons, which are pressed against the borehole wall. The tool provides approximately 25% coverage of the borehole wall in a 10-inch diameter borehole. 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 of hole U1530A are generally of excellent quality below 346 m WMSF and of good-to-intermediate quality above this depth due to the large borehole size (>16"). The pad1 azimuth logs (P1AZ) were different between passes by 90° to 180°, suggesting that FMS pads followed different tracks while logging, thus providing more coverage of the borehole wall. Note that the FMS images from pass 2 were distorted at 176 m and 233 m WMSF due to the quick close of the calipers caused likely by large ship heave (>2 m) at this time.
Average heave was estimated at 0.5-1 m while logging. The heave compensator was utilized whenever the tools were in open hole.
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
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 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.
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 current response is divided by the EMEX voltage to give the relative conductivity of the formation.
Depth-matching and Depth-shifting
Each of the logging runs is '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 depth shifts determined during the standard data processing are applied to the FMS images. The position of data located between picks is computed by linear interpolation. Often, for short logged intervals, a single 'block shift' is sufficient to depth-match the FMS data to the reference log. During this processing, FMS images from passes 1 and 2 were depth-matched with the TripleCom main pass (reference) via their depth-matched P1AZ logs before they were depth-shifted to the sea floor (1601 m WRF).
A high-resolution conductivity log is then produced from the FMS data by averaging the conductivity values from the 64 button electrodes. This enables the FMS data to be plotted using common graphing applications and more easily used in numerical analyses (e.g. spectral analysis). Specifically, the FMS conductivity values are averaged over each of the four pads and over five 0.254-cm depth levels to produce a file with 1.27-cm sample interval containing the total (4-pad, 64-button) average conductivity value, plus the 16-button averages from each of the four pads. Note that the FMS conductivity values are un-scaled and more accurate but lower vertical resolution values of HRLA resistivity logs are avaialable for this hole.
2) Image Display
Once the data is processed, both 'static' and 'dynamic' images are generated; the differences between these two types of image are explained below. Both types are provided online.
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.
The image is displayed as an unwrapped borehole cylinder. 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.
Interested scientists are welcome to visit the log interpretation center at LDEO if they wish to use the image generation and interpretation software.
Additional information about the drilling and logging operations can be found in the Operations and Downhole Measurements sections of the expedition report, Proceedings of the International Ocean Discovery Program, Expedition 376. For further questions about the logs, please contact:
E-mail: Tanzhuo Liu
E-mail: Cristina Broglia