Abstract: A resistivity sub including a resistivity sensor forms part of a bottom hole drilling assembly. The sensor is maintained at a substantially fixed offset from the wall of a borehole during drilling operations by, for example, a stabilizer. In WBM, galvanic sensors may be used, with or without commonly used focusing methods, for obtaining a resistivity image of the borehole wall. In OBM, capacitive coupling may be used. The apparatus is capable of using other types of sensors, such as induction, MPR, shielded dipole, quadrupole, and GPR sensors. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 37 CFR 1.72(b).

Abstract: A resistivity sub including a resistivity sensor forms part of a bottom hole drilling assembly. The sensor is maintained at a substantially fixed offset from the wall of a borehole during drilling operations by, for example, a stabilizer. In WBM, galvanic sensors may be used, with or without commonly used focusing methods, for obtaining a resistivity image of the borehole wall. In OBM, capacitive coupling may be used. The apparatus is capable of using other types of sensors, such as induction, MPR, shielded dipole, quadrupole, and GPR sensors.

Abstract: A method and apparatus for obtaining a resistivity measurement of an earth formation surrounding a borehole in an MWD device uses an electrode for injecting current into the earth formation and an electrode for obtaining a responsive signal from the borehole. The electrodes are located on the drill bit arm or blade. Measured resistivity values are obtained at the location of the drill bit. Measurements can be taken in both oil-based mud and water-based mud environments. Maximum or minimum resistivity can be used to best represent the resistivity of the surrounding formation.

Abstract: A method and apparatus for obtaining a resistivity measurement of an earth formation surrounding a borehole in an MWD device uses an electrode for injecting current into the earth formation and an electrode for obtaining a responsive signal from the borehole. The electrodes are located on the drill bit arm or blade. Measured resistivity values are obtained at the location of the drill bit. Measurements can be taken in both oil-based mud and water-based mud environments. Maximum or minimum resistivity can be used to best represent the resistivity of the surrounding formation.

Abstract: An apparatus for obtaining resistivity images of a borehole includes an array of measure electrodes separated from a pad or the body of the instrument by a focusing electrode. The focusing electrode is maintained at a slightly higher potential than the measure electrodes. The apparatus is primarily intended for use with conducting fluids but may also be used with non-conducting fluids. A modulated electrical current with a carrier frequency of 1 MHz is injected into the formation. When used with a non-conducting fluid, capacitive coupling between the electrode and the conductive formation is provided by the dielectric of fluid. When used with a conducting borehole fluid, an additional capacitor may be incorporated into the circuit. The current in the measure electrode is indicative of the conductivity of the formation.

Abstract: An apparatus for obtaining resistivity images of a borehole having an oil-based mud includes a rotating measure electrode on a measurement-while-drilling apparatus. A modulated measure current is capacitively coupled to the formation through the nonconducting mud. The electrode may be carried on a stabilizer of the drilling assembly. An extendable arm may be used to maintain a desired distance between the electrode and the formation.

Abstract: An apparatus for obtaining resistivity images of a borehole having an oil-based mud includes a rotating measure electrode on a measurement-while-drilling apparatus. A modulated measure current is capacitively coupled to the formation through the non-conducting mud. The electrode may be carried on a stabilizer of the drilling assembly. An extendable arm may be used to maintain a desired distance between the electrode and the formation.

Abstract: An apparatus for obtaining resistivity images of a borehole includes an array of measure electrodes separated from a pad or the body of the instrument by a focusing electrode. The focusing electrode is maintained at a slightly higher potential than the measure electrodes. The apparatus is primarily intended for use with conducting fluids but may also be used with non-conducting fluids. A modulated electrical current with a carrier frequency of 1 MHz is injected into the formation. When used with a non-conducting fluid, capacitive coupling between the electrode and the conductive formation is provided by the dielectric of fluid. When used with a conducting borehole fluid, an additional capacitor may be incorporated into the circuit. The current in the measure electrode is indicative of the conductivity of the formation.

Abstract: An apparatus for obtaining resistivity images of a borehole includes an array of measure electrodes separated from a pad or the body of the instrument by a guard electrode. The guard electrode is maintained at a slightly lower potential than the pad and the measure electrode is at an intermediate potential thereto. With this arrangement, the current from the guard electrode defocuses the measure current from the measure electrode as it enters the formation, and at greater distances, the current from the measure electrode is refocused by the effect of the current from the pad. This defocusing and refocusing defines a region of investigation away from the borehole wall that is relatively insensitive to borehole rugosity. Circumferential and vertical overlap may be obtained either by the arrangement of the measure electrodes, or by relying on the broadening of the measure beam as it enters the formation.

Abstract: An apparatus for obtaining resistivity images of a borehole includes an array of measure electrodes separated from a pad or the body of the instrument by a guard electrode. The guard electrode is maintained at a slightly lower potential than the pad and the measure electrode is at an intermediate potential thereto. With this arrangement, the current from the guard electrode defocuses the measure current from the measure electrode as it enters the formation, and at greater distances, the current from the measure electrode is refocused by the effect of the current from the pad. This defocusing and refocusing defines a region of investigation away from the borehole wall that is relatively insensitive to borehole rugosity. Circumferential and vertical overlap may be obtained either by the arrangement of the measure electrodes, or by relying on the broadening of the measure beam as it enters the formation.