Abstract: An induced polarization tool using focused electrodes that uses direct coupling in the focusing loop to prevent spurious phase shifts. The invention uses a differential-in differential-out amplifier and voltage-controlled resistances to provide zero potential difference between pairs of monitoring electrodes. The measured current and voltage are digitized downhole and transmitted to the surface. At the surface means are provided for determining in-phase and quadrature response.

Abstract: A method and apparatus are described for reducing "Groeningen" effect and other anomaly induced errors in resistivity measurements of an earth formation. Such errors are typically noted during resistivity measurements of an earth layer and are generally referred to as "Groeningen" effect or "casing" effect. A sonde having a plurality of voltage measuring electrodes and current emitting electrodes is provided and suspended in a borehold by means of a conductive cable. The sonde is attached to the conductive cable by means of a length of insulated cable and two reference electrodes are disposed above the sonde. A survey current and at least two focusing currents are emitted from the sonde at alternate times or on alternate frequencies and various voltages induced by each current are measured and utilized to calculate a corresponding set of transfer impedances for that current.

Abstract: An induced polarization tool using focused electrodes that uses direct coupling in the focusing loop to prevent spurious phase shifts. The invention uses a differential-in differential-out amplifier and voltage-controlled resistances to provide zero potential difference between pairs of monitoring electrodes. The measured current and voltage are digitized downhole and transmitted to the surface. At the surface means are provided for determining in-phase and quadrature response.

Abstract: In the preferred and illustrated embodiment of a downhole oil well resistivity logging tool having a focused electrode, the current ideally flows from the electrode radially outwardly in a thin sheet deep into the earth's formation adjacent to the well. There is an undesirable vertical current component along the electrodes which distorts the measurement of resistivity. First and second alternate embodiment are set forth which utilize additional balanced electrodes in the electrode array. They establish a vertical current in the adjacent formation. The vertical current is varied by means of an oscillator connected to the balanced electrodes, and the voltage of the electrodes is monitored to detect zero crossing. Zero crossing is indicative of reduced vertical current flowing in the formation.

Abstract: The method of the invention comprises determining the variation of the electric potential on both sides of a central electrode in a borehole, detecting the two levels of the borehole where the potential gradient is zero, and measuring the electric resistivity of the geological formation between these two levels.

Abstract: A method and apparatus are described for detecting the presence of an anomaly, such as the Groningen effect, whereby an error in a resistivity measurement of an earth formation layer is noted when such layer is located below a highly resistive bed. The resistivity is measured with a resistivity measuring sonde with the aid of a reference electrode located between the sonde and a conductive sheath around the cable from which the sonde is suspended in a borehole. A phase sensitive signal is measured and used to detect the presence of an error. In one embodiment, the out-of-phase potential of an electrode is recorded alongside a resistivity measurement as a function of depth. In another embodiment, the out-of-phase potential deviation from a normal value is determined and used to derive a corresponding in-phase potential deviation from which a measurement of the magnitude of the error in the resistivity measurement is made. Other embodiments are shown and described.

Abstract: A logging tool in accordance with the invention comprises a unitized electrode assembly adapted to be removably mounted on a central mandrel. The unitized electrode assembly comprises a metallic tube that is encased within an electrically-insulating sheath. An insulating sleeve surrounding the sheath carries various electrodes that are separated from each other by insulating spacers.