Abstract: A method of determining resistivity of an earth formation through a conductive wellbore casing. The resistance of conductors within an armored electrical cable for a wellbore sonde is measured and the sonde is inserted into the wellbore. The sonde has a plurality of electrodes at axially spaced apart locations for electrically contacting the casing. A first electrical current is applied between a first electrode and a surface return electrode and a first current amplitude and phase is measured. A second phase shifted electrical current is applied between a second electrode and the surface return electrode and the second current amplitude and phase is measured. A first voltage amplitude and phase between a first pair of electrodes axially disposed between the first and second electrodes is measured, and a second voltage amplitude and phase between a second pair of electrodes axially disposed between the first and second electrodes is measured.

Abstract: A system for evaluating multiphase fluid flow in a wellbore. Two distinct AC signals are injected into a fluid. One AC signal originates from a transmitter electrode in ohmic contact with the fluid, and the other AC signal originates from a transmitter electrode capacitively coupled to the fluid. The geometric factors between the electrode pairs are selected to distinguish between different constituents within the wellbore fluids. The output electrical impedance of the ohmic transmitter is low so that the AC signal from the capacitive transmitter is shunted to signal ground in electrically conductive fluids. A single detector can measure the fraction of electrically conductive water covering the detector electrode as well as the dielectric constant of an oil and gas mixture. Arrays of such detector electrodes can measure the holdups and velocities of the constituent fluids in a flowing multiphase fluid.

Abstract: An improved method and apparatus for initiating seismic data storage in an Isolated Distributed Recording System. An encoded acoustic signal is generated by an acoustic energy source which is detected at an isolated distributed recorder where it is correlated with a preselected correlating signal. Upon sufficient correlation between the encoded acoustic signal and the preselected correlating signal, seismic data are stored. In the preferred embodiment, the invention relates to a method and apparatus for generating an encoded seismic signal utilizing a seismic energy source. The encoded seismic signal is correlated with a preselected correlating signal at an isolated distributed recorder; upon sufficient correlation between said signals, the isolated distributed recorder is triggered to store incoming seismic data commencing at the time the seismic data were generated and extending for a predetermined time interval.

Abstract: A method and apparatus for selectively controlling isolated distributed recorders in an Isolated Distributed Recording System. Selection and control of isolated distributed recording units comprising a seismic data gathering Isolated Distributed Recording System are effected by transmitting preselected encoded acoustic seismic signals to which the isolated distributed recorders have been preconditioned to respond by effecting a desired control action. The same encoded acoustic seismic signal may also provide the seismic data for the experiment. Each isolated distributed recorder is conditioned to respond to several encoded acoustic signals, to discriminate against noise, and/or to respond to one such encoded acoustic signal by effecting a plurality of control actions at the isolated distributed recorder.

Abstract: The invention is a system for reducing offset in an analog to digital conversion system. A signal representing the system offset is stored in digital form. A digital to analog converter generates an analog feedback signal in response to the stored digital signal. The output of the system analog to digital converter is periodically sampled to update the stored digital word representing system offset.

Abstract: A system is disclosed for removing a periodic interfering signal from an electrical signal which may also include a data signal such as a seismic signal. A digital representation of the interfering signal, comprising a series of digital words, is generated and a nulling signal formed from this digital signal is subtracted from said electrical signal. The amplitude difference between said electrical signal and the nulling signal is determined and the digital words comprising the digital representation of the interfering signal are changed by a function of the difference between said electrical signal and said nulling signal, to reduce said difference.

Abstract: A system for performing wellbore telemetry operations wherein a communication link between a subsurface location in a well and the location substantially at the surface is established and maintained through electromagnetic coupling between two insulated electric conductors. The first conductor is electrically coupled to a transmitter, normally located at the subsurface location, and extends upwardly in the well. The second conductor is electrically coupled to a receiver, normally at the surface location, and extends down into the well. Normally, the two conductors will overlap along a substantial portion of the lengths thereof. As the well is drilled deeper and the drill string is lengthened, the transmitter and the first conductor follow the progression of the wellbore into the earth, thereby reducing the overlap between the first and second conductors. When the two conductors cease to overlap, the second conductor may be lengthened to re-establish the overlap.

Abstract: Apparatus is described for eliminating monochromatic interfering frequency components of seismic signals detected during the course of seismic exploration comprising an electrical circuit for producing a monochromatic nulling signal that has the same frequency as the interfering signal. The monochromatic nulling signal is produced by generating a truncated triangular wave having the same period as the interfering signal and filtering the truncated triangular wave to remove all components thereof except those having the period of the interfering signal. Circuit means are provided for controlling the amount of truncation of the truncated triangular wave, and for controlling the time phase thereof. The filtered signal is combined with the geophone signal in such a manner that the filtered signal at least partially cancels out the interfering component of the seismic signal.