IODP -USIO logging contractor: LDEO-BRG
Location: Atlantis Massif - Mid Atlantic Ridge (central N Atlantic))
Latitude: 30° 10.108' N
Longitude: 42° 7.11’ W
Logging date: November 30, 2004
Sea floor depth (driller's): 1653.4 mbrf
Total penetration: 103.8 mbsf
Total core recovered: 48.7 m (46.9 % of cored section)
Oldest sediment cored: Quaternary
Lithologies: Basalt, diabase, gabbro, peridotite
FMS Pass 1: 20-96 mbsf
FMS Pass 2: 20-96 mbsf
Magnetic declination: 15.69°
Water depth: 1653.4 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 Expedition 304 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.
Excellent borehole images were obtained from Hole U1309B in the section where the FMS arms were open (54-95 mbsf). In the 20-54 mbsf interval, the images (particularly the dynamic normalization) are more blurry but still usable. The borehole is about 10 inches wide, apart from thin wider zones (e.g. 61, 72 mbsf). The lithologies logged are basalts, diabase, gabbros, and peridotite.
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.
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 conductivity values are un-scaled and more accurate (but lower vertical resolution) values are given by the resistivity logs from the DIT and DLL resistivity tools.
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 and on CD-ROM.
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.
Interested scientists are welcome to visit the log interpretation center at LDEO if they wish to use the image generation and interpretation software.
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