The extension of dipmeter design to the measurement of many microresistivity traces resulted in the production of electrical images of the borehole wall. The first electrical imaging device was introduced in 1986 by Schlumberger as the Formation Microscanner (FMS). The multiple microresistivity traces were produced either by a two-pad tool with 27 measurement buttons on each pad, or a four-pad arrangement, each with 16 buttons. The borehole wall coverage was 20% with the two-pad and 40% with the four-pad device. Improvements continue to be made in resolution and coverage. The vertical resolution is on the order of an inch, although much finer features (such as hairline fractures) can be detected if they have a sufficiently strong resistivity contrast with their surroundings.
The microresistivity curves recorded by the buttons are converted into gray levels and plotted as a map of the borehole wall. By convention, high resistivity features appear white, low-resistivity features black, and the intervening resistivity range as intermediate shades of gray. Image processing is generally run interactively on workstations using customized software for image enhancement and the geometrical analysis of features. Inclined planar features such as fractures or crossbedding surfaces will appear as sinusoidal elements on the images. A useful review of the technology with numerous geological examples is given by Bourke and others (1989).
A typical example of an FMS electrical image is shown in Figure 11. The imaged section is located above a productive Morrow Sandstone interval (Pennsylvanian) from a well in southwest Kansas.The interval consists