Dip magnitude is routinely assessed by analyzing the image of the intersection of a geological boundary with the borehole wall. Planar geological interfaces yield characteristic sinusoidal patterns as the well bore image is unfolded on a flat surface or displayed in 2-D on a screen. The magnitude of the dip is directly computed from the amplitude of the corresponding sinusoid. One source of uncertainty in this seemingly simple calculation is the so-called "Depth of Investigation." Simply put, borehole images do not originate at the borehole wall, but somewhat deeper, inside the formation. The actual location of the image is sometimes erroneously designated as "Depth of Investigation" or D.O.I. This nomenclature has created confusion and misled many users. Focused resistivity tools have specific depths of investigation represented by the mid-point on the Integrated Radial Geometrical Factor and denoted as D.O.I. The Depth of Investigation of the focused resistivity measurement can reach over 10 inches into the formation; by comparison the actual location of the associated image is usually within 1 or 2 inches from the borehole wall. The implication for apparent dip computation and for geosteering and real time decision is significant.
The object of this publication is to clarify and to quantify through modeling the various parameters erroneously designated as "Depths of Investigation." We introduce the concept of "Depth-Of-Electrical Image" for determining dips from sinusoidal patterns on borehole images. It is the location where the apparent images emanate. Modeling results as well as field examples demonstrate that the "Depth-of-Electrical Image" is vastly different from the "Depth of Investigation." A recently released Azimuthal Focused Resistivity tool produces three sharp images with significantly different Depths of Investigation, but nearly identical "Depths-Of-Electrical Image," of less than one inch. Similarly, a new Azimuthal Deep Resistivity is shown to produce low resolution, deep looking images with "Depth-Of-Electrical Image" of several feet.
Another useful concept is that of "Depth-Of-Detection" which characterizes the capability of identifying approaching boundaries from a distance. Recently introduced azimuthal deep resistivity instruments are capable of early detection of an approaching boundary and are especially valuable for effective geosteering.
Depth of Investigation, Depth of Detection, and Depth-Of-Electrical Image are modeled and illustrated on actual log examples, leading to improved accuracy in dip calculation.
One important application of electrical micro images is the determination of dip. Dip calculation from high definition logs and images originated with dipmeter processing and then gained accuracy and confidence with the use of full images. There remain, however, various sources of uncertainty in dip computation from images and from dipmeter logs. The most noted one is the imprecise knowledge of the effective location of the micro-image with respect to the borehole wall. While micro-imagers attempt to map geological events precisely at the borehole wall, they are effectively mapping resistivity variations a few tenths of an inch from the surface of the borehole. It will be shown that an error of 0.5 inch on the depth of the image.
