Our understanding of the crustal section at any drill site is generally impeded by incomplete core recovery. In many cases, the material that is recovered is biased toward particular lithologies. Geophysical, geochemical, and electrical imaging logs are especially useful in these poorly sampled environments. Case Study ODP Site 831 is located on Bougainville Guyot, which has collided with the New Hebrides Island Arc. The hole was logged using an electrical imaging tool, the Formation MicroScanner.
Resistance is measured using each electrode independently.
The data are then processed into an electrical image, which is effectively a map of the borehole wall shaded according to the measured resistance. Because the pads cover only a limited area of the borehole wall, the tool is often run twice to increase the coverage. This example of the electrical imaging log, based on two logging passes, clearly shows a well-defined boundary between limestone and volcanic breccia. The exact location of this boundary was not apparent from examination of the recovered cores. The textural information recorded in this electrical image includes individual clasts, fractures, and other features that are clearly distinguishable in the borehole wall.