1. Field of the Invention
The invention disclosed herein relates to subterranean imaging and, in particular, to arrangements of electrodes for resistivity imaging within a wellbore.
2. Description of the Related Art
Imaging of formations surrounding boreholes provides valuable information for describing geologic features. Some of the features include structural framework, fracture patterns, sedimentary feature, and in-situ stress orientation. High-resolution borehole images are used as an aid in providing conventional core description and determining orientation. While various technologies are used for imaging, one technology that is particularly useful involves resistivity measurements.
Information obtained by performing resistivity measurements is useful for planning formation testing, sampling, perforating and other such tasks. An exemplary instrument for making resistivity measurements is available from Baker Hughes, Incorporated of Houston, Tex. The instrument, referred to as an “Earth Imager,” has provided for a variety of resistivity images.
With regard to the exemplary instrument, reference may be had to FIG. 1. In FIG. 1, there is shown a depiction of the prior art instrument for performing resistivity imaging. In this example, the instrument 21 is disposed within a wellbore 11 (also referred to as a “borehole”). The instrument 21 includes pads 3 mounted on articulating arms 2. The articulated pads 3 are typically pressed up against a wall of the wellbore 11 and make firm contact therewith. Current I flows from at least one transmitter electrode 6 on the pad 3 to the return electrode 4. The return electrode 4 is electrically separated from each transmitter electrode 6 by an insulator 5. The current I is typically alternating current (AC). Accordingly, it is recognized that the current I actually flows in two directions as polarity changes.
Other arrangements of electrodes may be used. For example, a sensor may additionally include sensor electrodes, focusing electrodes, and other such electrodes.
During operation of the instrument, current measurement for each combination of the return electrode 4 and the transmitter electrode 6 is a function of the formation conductivity and the voltage applied. High resolution images are achieved by sampling at a high rate (for example, about 120 samples per foot).
While this instrument produces valuable data, one skilled in the art of data interpretation may recognize certain limitations. Accordingly, a variety of combinations and relationships are known for providing a sensor having a combination of the return electrode 4 and at least one transmitter electrode 6. Some of these embodiments include ones having the return electrode 4 on the pad 3 in a close relationship to the transmitter electrode 6.
Resistivity images taken within a borehole may be acquired with a one-side transmitter. That is, current emitted from a transmitter electrode will flow to one or return electrodes located on one side of the sensor. As a result, an image can appear asymmetric over a symmetric feature. For imaging devices operating in conductive muds, the return electrode 4 is usually located above the transmitter electrodes 6 mounted on several pads 3 (as shown in FIG. 1). However, because of the great relative distance between electrodes, the asymmetry is often not significant. For non-conductive mud imaging, transmitter electrodes 6 and return electrodes 4 may need to be placed close to each other, such as both being “on pad.” In embodiments of sensors having on pad transmitter electrode(s) and return electrode(s), the image response will be significantly asymmetric because of the proximity of the transmitter electrode to the return electrode.
Such is the case regardless of whether the transmitter electrode 6 is located above or below the return electrode 4. Symmetric images may be obtained with symmetric transmitter electrodes placed on the opposite sides of return electrodes and activated simultaneously to receive current emitted from the return electrodes.
Therefore, what are needed are methods and apparatus for obtaining symmetric images through imaging with asymmetric arrangements of electrodes.