FMS Image Data Processing (re-entry, phase 2)


IODP-USIO logging contractor: LDEO-BRG

Hole: 1256D

Expedition: 309

Location: Guatemala Basin (NE equatorial Pacific)

Latitude: 6° 44.163' N

Longitude: 91° 56.061' W

Logging date: August 21-24, 2005

Sea floor depth (driller's): 3645.4 mbrf

Sea floor depth (logger's): 3643.5 mbrf

Total penetration: 1255.1 mbsf

Total core recovered: 143.2 m (34.7%; on Exp 309)

Oldest sediment recovered: Calcareous nannofossil ooze (Middle Miocene) at Hole 1256B during ODP Leg 206

Lithologies: Clay-rich sediments and calcareous nannofossil ooze (sediments); basalt flows and dykes (basement)


FMS Pass 1: 652 - 1217 mbsf

FMS Pass 2: 300 - 1211 mbsf

Magnetic declination:  4.569°


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 Exp 309 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.

Data Quality


Excellent borehole images of basaltic crust were obtained from the phase 2 logging of Hole 1256D at the end of Expedition 309. The different formations (massive basalt, pillows, breccia) are all apparent in the image, as well as inclined fractures. High resolution (to better than ~10cm) was carried out on the data. In the upper part (300 - 750 mbsf), the images from pass 2 were depth matched to the images from pass 1 of the Leg 206 logging of this hole. In the lower part (750-1216 mbsf), the images from pass 2 were matched to the reference run for phase 2 logging, the main pass of the Triple Combo tool string. The pass 1 images were then matched to the resulting pass 2 images. This enables the pass 1 and pass 2 images to be overlaid, thus achieving greater coverage of the borehole wall and easing interpretation of features such as fractures and pillows. The overlay of pass1, pass 2, and pass 1 from Leg 206 is presented in the 'merged' files.


The hole was in good condition, generally reading between 10-12 inches in diameter. Slightly wider sections with thin washouts occur at 348-403, 418-435, 450-473, 530-605, 678-694, 816-824, 919-930, 1050-1061 mbsf. Below 968 mbsf, the hole is smoother and thin washouts occur less frequently.


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: 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 one 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.


Interested scientists are welcome to visit the log interpretation center at LDEO if they wish to use the image generation and interpretation software.


For further information or questions about the processing, please contact:


Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia