Apparatus and methods operable to determine a resistivity of a subterranean formation surrounding a wellbore. One such method includes using an apparent impedance function depending on a frequency variable and a plurality of unknown parameters, at least one of the unknown parameters depending on a formation impedance of the subterranean formation. The method also includes applying a voltage, at each of a plurality of frequency values, between electrodes of a resistivity tool that is disposed in the wellbore. The method also includes measuring, across the electrodes, a plurality of apparent impedance values, each corresponding to a different one of the frequency values. The method still further includes determining the unknown parameters based on the frequency values and the apparent impedance values, and estimating the formation resistivity based on an expression that includes at least one of the unknown parameters.

A method of imaging electrical conductivity distribution of a subsurface containing metallic structures with known locations and dimensions is disclosed. Current is injected into the subsurface to measure electrical potentials using multiple sets of electrodes, thus generating electrical resistivity tomography measurements. A numeric code is applied to simulate the measured potentials in the presence of the metallic structures. An inversion code is applied that utilizes the electrical resistivity tomography measurements and the simulated measured potentials to image the subsurface electrical conductivity distribution and remove effects of the subsurface metallic structures with known locations and dimensions.

A method of imaging electrical conductivity distribution of a subsurface containing metallic structures with known locations and dimensions is disclosed. Current is injected into the subsurface to measure electrical potentials using multiple sets of electrodes, thus generating electrical resistivity tomography measurements. A numeric code is applied to simulate the measured potentials in the presence of the metallic structures. An inversion code is applied that utilizes the electrical resistivity tomography measurements and the simulated measured potentials to image the subsurface electrical conductivity distribution and remove effects of the subsurface metallic structures with known locations and dimensions.

In one aspect, a resistivity logging tool for a fluid-producing formation if provided. The resistivity logging tool includes at least one transmitter device connected to at least one excitation electrode, multiple receiver devices respectively connected to monitoring electrodes, and a controller. The transmitter can inject an excitation current into the formation via the excitation electrode. Each receiver device can determine a respective voltage level induced by the excitation current. The controller can determine whether a voltage level measured by at least one receiver device is within a specified range. Based on the measured voltage level, the controller can select a global amplitude adjustment algorithm for modifying excitation currents or a localized amplitude adjustment algorithm for modifying one or more gains of one or more receiver devices. The controller can modify the excitation current or gains by executing the selected algorithm.

In one aspect, a resistivity logging tool for a fluid-producing formation if provided. The resistivity logging tool includes at least one transmitter device connected to at least one excitation electrode, multiple receiver devices respectively connected to monitoring electrodes, and a controller. The transmitter can inject an excitation current into the formation via the excitation electrode. Each receiver device can determine a respective voltage level induced by the excitation current. The controller can determine whether a voltage level measured by at least one receiver device is within a specified range. Based on the measured voltage level, the controller can select a global amplitude adjustment algorithm for modifying excitation currents or a localized amplitude adjustment algorithm for modifying one or more gains of one or more receiver devices. The controller can modify the excitation current or gains by executing the selected algorithm.

The present invention provides a methodology and system for determining formation resistivity beyond a casing. Using devices to generate, direct, and measure electric signals, the travel speed, travel time, attenuation and phase shift of an electric signal traveling through the casing can be measured. Applying a relationship between the travel speed or travel time of an electric signal and the formation resistivity beyond a casing, it is then possible to determine the formation resistivity beyond a casing by measuring the travel speed or travel time or phase shift or attenuation of an electric signal.

A method and apparatus for downhole data transmission. The method comprising monitoring electrical impedance properties of a downhole formation by applying an electrical signal between a first portion and a second portion of a drill string that are electrically insulated from each other by an insulating portion, selecting a frequency for transmitting data by electromagnetic telemetry based at least in part on the monitored electrical impedance and transmitting the data by encoding the data in a waveform having the selected frequency and applying the waveform between the first portion and the second portion of the drill string.

A method and apparatus for downhole data transmission. The method comprising monitoring electrical impedance properties of a downhole formation by applying an electrical signal between a first portion and a second portion of a drill string that are electrically insulated from each other by an insulating portion, selecting a frequency for transmitting data by electromagnetic telemetry based at least in part on the monitored electrical impedance and transmitting the data by encoding the data in a waveform having the selected frequency and applying the waveform between the first portion and the second portion of the drill string.

An example system includes an electrode-based tool for deployment in a downhole environment. The electrode-based tool has a plurality of current electrodes, at least one voltage monitoring electrode, at least one return electrode, and electronics to collect one or more voltage measurements from the at least one voltage monitoring electrode as current from at least one of the plurality of current electrodes is injected into the downhole environment and flows to the at least one return electrode. The system also includes at least one processor to estimate leakage current between at least one of the plurality of current electrodes and the at least one return electrode based on the collected one or more voltage measurements and a predetermined internal impedance model or table. The at least one processor is configured to derive a corrected downhole environment parameter based at least in part on the estimated leakage current.

An example system includes an electrode-based tool for deployment in a downhole environment. The electrode-based tool has a plurality of current electrodes, at least one voltage monitoring electrode, at least one return electrode, and electronics to collect one or more voltage measurements from the at least one voltage monitoring electrode as current from at least one of the plurality of current electrodes is injected into the downhole environment and flows to the at least one return electrode. The system also includes at least one processor to estimate leakage current between at least one of the plurality of current electrodes and the at least one return electrode based on the collected one or more voltage measurements and a predetermined internal impedance model or table. The at least one processor is configured to derive a corrected downhole environment parameter based at least in part on the estimated leakage current.