Abstract: A system for setting a tool in a downhole environment includes a communicator configured to broadcast a signal; a downhole tool configured to receive the signal from the communicator and execute on an instruction conveyed by the signal, a radio wave inhibiting structure being interposed between the communicator and the downhole tool and method.

Abstract: Exemplary systems and methods are directed to measuring temperature in a borehole located below a surface of a landform using high-energy transmission. A sequence of electromagnetic (EM) energy pulses are generated from a signal generator located at the surface of the landform. The energy pulses are reflected at a ring frequency by one or more downhole transducers. The reflected energy pulses are received at a receiver, which is located at the surface, during a predetermined time interval. The receiver detects the received energy pulses through a time domain or frequency domain technique. Each detected ring frequency is correlated to an inductance of the associated downhole transducer. The inductance is correlated to a temperature of the borehole.

Abstract: The present invention provides a method and system in which signals from the surface are sent by changing flow rate of the drilling fluid supplied to the drill string during drilling of a wellbore. The signals are sent based on a fixed or dynamic time period schemes so that the sent signals cross a threshold value in a known manner. A detector measures the changes in the flow rate. A controller downhole determines the number of times a downhole parameter, such as voltage, relating to the changes in the flow rate crosses a predefined threshold value. Based on the number of the crossings and the timing of such crossings, signals are assigned to commands. The controller controls or operates a steering device based on the commands.

Abstract: An apparatus for detecting mud pulse telemetry signals includes a differential transducer. In various exemplary embodiments, a high-pressure side of the differential transducer is in fluid communication with either drilling fluid in a standpipe (which is in fluid communication with drilling fluid in the borehole) or a gas chamber of a pulsation dampener. Exemplary embodiments typically further include a pressure delay module in fluid communication with the low-pressure side of the differential transducer and the gas chamber of the pulsation dampener. The invention is intended to advantageously improve the reliability and bandwidth of mud pulse telemetry communications in oilfield drilling applications.

Abstract: A downhole communication apparatus has a signal coupler disposed in an end of a downhole tubular component. The signal coupler has a transceiver portion and a conductor portion. A data conductor integrated into the downhole tubular component is operably connected at a junction to the conductor portion of the signal coupler. A peripheral electronic device is in communication with the conductor portion of the signal coupler, either through an electrical coil wrapped around at least part of the conductor portion of the signal coupler or direct electrical contact with the conductor portion of the signal coupler.

Abstract: A data communication technique for use in producing wells. Signals are transmitted through the product (p) by varying and monitoring flow rate at respective ends of the communication channel. At low data rates robust communication can be provided.

Abstract: A logging radar system and method for measuring propped fractures and down-hole formation conditions in a subterranean formation including: a radar source; an optical source; an optical modulator for modulating an optical signal from the optical source according to a signal from the radar source; a photodiode for converting the modulated optical signal output from the optical modulator to the source radar signal; a transmitter and receiver unit; and a mixer. The transmitter and receiver unit receives the source radar signal from the photodiode, transmits the source radar signal into the formation and receives a reflected radar signal. The mixer mixes the reflected radar signal with the source radar signal to provide an output. This technology can be used to describe all fractures connected to the wellbore and differentiate between the dimensions of the two vertical wings of a propped fracture.

Abstract: Disclosed herein is a sensor system having a sensor and at least one communication line operable with the sensor. The transmission medium is configured to convey data transmitted by the sensor to a remote location. The sensor transmits data on the communication line a plurality of times by at least two methods of transmission or modulation. Further disclosed herein is a method based on the foregoing. Further disclosed is a method of communicating by modifying a voltage amplitude of a signal and receiving the communication signal by employing a variable threshold detection circuit in the downhole device. Further disclosed is a system for communicating data between a downhole device and a remote location including a remote device for generating a communication signal, the remote device configured to modify a voltage amplitude of said communication signal.

Abstract: A mud pulse landing assembly (10) which allows for the removal of the mud pulse generator (16) containing the mud pulse orifice (106), thus creating an unobstructed passageway for any tool or instrumentation that may need to be passed through the drill string. The retainer (14) of the mud pulse landing assembly also provides a universal mount for engaging alternate tools or instrumentation for use in analyzing the borehole geology. The retainer (14) is positioned between the mud pulse generator (16) and the landing sub body, so as to protect the retainer from the turbulent and abrasive mud flow, and to prevent obstruction of the area downstream of the compact muleshoe (60).

Abstract: A communication system for communicating information with a compliant medium is disclosed, the communication device includes a constrained fluid, a valve, a modulator, a sensor and a demodulator. The constrained is fluid distributed along a length. The valve is configured to operatively engage a second point relative to the length. The modulator configured to actuate the valve according to information. The sensor configured to measure pressure at a first point relative to the length, where the first point is distant from the second point. The demodulator is coupled to the sensor to recover the information.

Abstract: A method for communicating a signal to an instrument in a wellbore includes axially accelerating the instrument in a preselected pattern of acceleration. The predetermined pattern corresponds to the signal to be communicated. The axial acceleration of the instrument is detected, and the signal is decoded from the detected axial acceleration. A signal detection system for an instrument in a wellbore includes an accelerometer oriented along a longitudinal axis of the instrument and means for comparing measurements made by the accelerometer to at least one predetermined pattern.

Abstract: Apparatus and methods for terminating and anchoring fiber optic connections are disclosed. One apparatus includes a sealed body enclosing a passage for one or more optical fibers terminating in connections proximate a first end of the sealed body, the sealed body having a diameter less than an internal diameter of an oilfield tubular, the sealed body having a second end adapted to sealingly engage and anchor an end of a fiber optic carrier conduit, the second end connecting the sealed body to the fiber optic carrier conduit.

Abstract: The present invention concerns application of a unique conductive electrode geometry used to form an efficient wideband, one- or two-way wireless data link between autonomous systems separated by some distance along a bore hole drill string. One objective is the establishment of an efficient, high bandwidth communication link between such separated systems, using a unique electrode configuration that also aids in maintaining a physically robust drill string. Insulated or floating electrodes of various selected geometries provide a means for sustaining or maintaining a modulated electric potential adapted for injecting modulated electrical current into the surrounding sub-surface medium. Such modulated current conveys information to the systems located along the drill string by establishing a potential across a receiving insulated or floating electrode.

Abstract: A gap sub assembly can be used to form an electrical isolation in a drill string, across which a voltage is applied to generate a carrier signal for an electromagnetic (EM) telemetry system. The assembly comprises two conductive generally cylindrical components fashioned with a matching set of male and female rounded coarse threads, held such that a relatively uniform interstitial space is formed in the overlap space between them. The third component is a substantially dielectric electrical isolator component placed into the gap between the threads that effectively electrically isolates the two conductive components. Injecting the dielectric material under high pressure forms a tight bond resistant to the ingress of conductive drilling fluids (liquids, gases or foam), thus forming a high pressure insulating seal.

Abstract: A system for communicating between a first location and a second location comprises a jointed tubular string having a first section and a second section connected at a connection joint, with the tubular string having a fluid in an internal passage thereof. A first acoustic transducer is mounted in the internal passage of the first section proximate the connection joint, and a second acoustic transducer is mounted in the internal passage of the second section proximate the connection joint. A signal transmitted from the first location to the second location is transmitted across the connection joint as an acoustic signal in the fluid from the first acoustic transducer to the second acoustic transducer.

Abstract: A network includes a plurality of wirelessly interconnected self-organizing network (SON) devices for relaying signals in a self-organizing network and a field device for originating output signals. The sensor is configured to transmit the output signals to at least one of the SON devices, and the SON devices do not originate signals but only relay signals originated externally. At least one of the SON devices is self-powered by harvesting energy from an adjacent energy source.

Abstract: A communication system for down hole use and comprising a drill collar (101) comprising a first portion (103) and a second portion (105) separated from each other by an electrically insulating material (67) and means (22, 63, 77, 81) for generating an electrical signal and for applying the electrical signal to the drill collar (101) such that the electrical signal is transmitted into a geological formation being drilled, characterized in that the means for generating the electrical signal comprises an alternator (22, 63, 77, 81) and means (2, 4, 10) responsive to an electrical output of the alternator for regulating rotation of the alternator.

Abstract: A data telemetry system for multi-conductor wirelines to enhance transmission of analog and digital signals with frequency content up to 100 kHz by assigning at least two conductors to a first terminal of a data transmittal means and to a first terminal of a data receiver means and connecting at least one additional conductor to a second terminal of a data transmittal means and to a second terminal of a data receiver means. Pairing conductors reduces attenuation and crosstalk and permits the signal to contain multiple frequency modulated carrier bands with individual channel bandwidths of up to 3 kHz or higher.

Abstract: A technique that is usable with a well includes using at least one downhole sensor to establish telemetry within the well. The sensor(s) are used as a permanent sensing device.

Abstract: A surface mud pipeline system is described with a mechanical acoustic filter (1245) tuned to a predetermined frequency band so as to attenuate pump noise (132) within this band. The filter can be combined with a Venturi constriction to provide enhanced attenuation and is used to improve signal transmission of a mud pulse telemetry system (113-2).

Abstract: One embodiment includes an apparatus that includes a piezoelectric transducer to generate an acoustic signal that is to modulate along a mandrel, wherein the piezoelectric transducer includes at least one piezoelectric element and at least one electrode that is without non-permanent joints.

Abstract: A mud-pulse telemetry tool includes a tool housing, a motor disposed in the tool housing, and a magnetic coupling coupled to the motor and having an inner shaft and an outer shaft. The tool may also include a stator coupled to the tool housing, a restrictor disposed proximate the stator and coupled to the magnetic coupling, so that the restrictor and the stator adapted to generate selected pulses in a drilling fluid when the restrictor is selectively rotated. The tool may also include a first anti-jam magnet coupled to the too housing, and an second anti-jam magnet disposed proximate the first anti-jam magnet and coupled to the inner shaft and/or the outer shaft, wherein at least one of the first anti-jam magnet and the second anti-jam magnet is an electromagnet, and wherein the first anti-jam magnet and the second anti-jam magnet are positioned with adjacent like poles.

Abstract: Thin flexible piezoelectric transducers are bonded to or imbedded into oilfield tubular members or structural members. The transducers may be used to telemeter data as acoustic waves through the members. By proper spacing of transducers and phasing of driving signals, the transmitted signals can be directionally enhanced or encoded to improve transmission efficiency. The transducers may be used for health monitoring of the tubular or structural members to detect cracks, delaminations, or other defects. The flexible transducers are very thin so that overall dimensions of tubular or structural members are essentially unchanged by incorporation of the transducers.

Abstract: A rotary pulser for transmitting information to the surface from down hole in a well by generating pressure pulses encoded to contain information. The pressure pulses travel to the surface where they are decoded so as to decipher the information. The pulser includes housing containing a stator forming passages through which drilling fluid flows on its way to the drill bit, a rotor, and a replaceable wear sleeve enclosing the rotor. The rotor has blades that are capable of imparting a varying obstruction to the flow of drilling fluid through the stator passages depending on the circumferential orientation of the rotor, so that rotation of the rotor by a motor generates the encoded pressure pulses.

Abstract: A pressure release encoding system for communicating downhole information through a wellbore to a surface location has a valve for providing a fluid restriction to fluid passing in the wellbore, a sensor positioned in the wellbore for sensing a downhole condition in the wellbore, an actuator cooperative with the valve for moving the valve between at least two positions in timed relation to the downhole condition sensed by the sensor, and a detector positioned at the surface location for providing a measurement value at the surface location correlative to the time between the changes of the pressure of the fluid in the wellbore. The sensor is an inclination sensor for sensing an angle of inclination of a drill bit in the wellbore.

Abstract: A system and methods of transmitting information between a first location and a second location comprise transmitting a data signal and a known signal from one of the first location and the second location over a signal channel having a first noise component. A second noise component is measured in a noise channel adjacent the signal channel. The data signal, the known signal, and the first noise component are received at the other location. The first noise component is estimated based on the second noise component. The estimated noise component is combined with the received data signal and the received known signal to generate noise-cancelled received data and received known signals. The noise-cancelled received known signal is processed to obtain an estimate of the channel transfer function. The estimated channel transfer function is combined with the noise-cancelled received data signal to reconstruct the transmitted data signal.

Abstract: A downhole communication apparatus has a signal coupler disposed in an end of a downhole tubular component. The signal coupler has a transceiver portion and a conductor portion. A data conductor integrated into the downhole tubular component is operably connected at a junction to the conductor portion of the signal coupler. A peripheral electronic device is in communication with the conductor portion of the signal coupler, either through an electrical coil wrapped around at least part of the conductor portion of the signal coupler or direct electrical contact with the conductor portion of the signal coupler.

Abstract: A system and method in accordance with the invention communicator remotely with remotely controllable down hole tools in a well bore at a drilling installation. At the surface, high energy pressure impulses directed into the tubing or the annulus, or both, being at a level to propagate through an interface between very different impedances zones, such as an upper level gas zone and a lower level of mobile fluid media extending down into the desired downhole location. The pressure impulses, provided by directionally gating along the longitudinal confining path a pressure impulse initially having sharp leading and trailing edges, reach the downhole location as physical perturbations forming a discernible pattern that can be detected by one or more energy responsive transducers. With combinations of these signals, one of a number of separate control devices can be remotely actuated.

Abstract: An oscillating shear valve system for generating pressure fluctuations in a flowing drilling fluid comprising a stationary stator and an oscillating rotor, both with axial flow passages. The rotor oscillates in close proximity to the stator, at least partially blocking the flow through the stator and generating oscillating pressure pulses. The rotor passes through two zero speed positions during each cycle, facilitating rapid changes in signal phase, frequency, and/or amplitude facilitating enhanced, multivalent data encoding. The rotor is driven by a motorized gear drive. In one embodiment, a torsional spring is attached to the motor and the resulting spring mass system is designed to be near resonance at the desired pulse frequency. The system enables the use of multivalent encoding schemes for increasing data rates.

Abstract: A data transmission apparatus having first and second electrical conductors is disclosed. The first and second electrical conductors are disposed within recesses of a first and second complementary surfaces that are magnetically conducting and electrically insulating. The first and second surfaces are in close proximity to each other. The first surface is translatable along the length of the second surface. The first and second electrical conductors are in electromagnetic communication and provide for the transmission of data or power from the first electrical conductor to the second electrical conductor as the first surface overlaps the second surface. The data transmission apparatus may be located in one or more downhole tools.

Abstract: A decision feedback equalizer (DFE) structure which uses a reference signal based adaptive equalizer as forward filter and a blind adaptive equalizer as feedback filter is used for surface processing of mud pulse telemetry data.

Abstract: A hybrid, hardwired and wireless well communication system having a fiber optical, electrical or other signal transmission conduit extending from the wellhead into the well and one or more wireless signal transducers that are located at a distance from the conduit and transmit wireless signals to one or more signal converters which are coupled to the conduit, and which are located near branchpoints of a multilateral well.

Abstract: A system and method is provided for communicating with a device disposed in a wellbore. Signals are communicated between a surface location and the device via a hardwired section of wellbore and a wireless section of wellbore. The signal is sent downhole or uphole over a portion of the distance via a communication line and over another portion of the distance via wireless communication.

Abstract: A mud pulse telemetry system comprises a surface located fluid supply line having a drilling fluid flowing therein. A non-venting pulser is disposed in the drilling fluid in the supply line, and the non-venting pulser is adapted to generate pressure fluctuations in the flowing drilling fluid. A downhole receiver in hydraulic communication with the non-venting pulser is adapted to detect the pressure fluctuations in the drilling fluid. A method for transmitting pressure signals from a surface location to a downhole location in a wellbore, comprises disposing a non-venting pulser in a surface located fluid supply line having a drilling fluid flowing therein. The non-venting pulser is actuated to generate pressure fluctuations in the flowing drilling fluid according to a predetermined encoding scheme. The pressure fluctuations are detected with a downhole receiver in hydraulic communication with the non-venting pulser.

Abstract: A transmission system in a downhole component comprises a data transmission element in both ends of the downhole component. Each data transmission element houses an electrically conducting coil in a MCEI circular trough. The electrically conducting coil comprises at least two generally fractional loops. In the preferred embodiment, the transmission elements are connected by an electrical conductor. Preferably, the electrical conductor is a coaxial cable. Preferably, the MCEI trough comprises ferrite. In the preferred embodiment, the fractional loops are connected by a connecting cable. In one aspect of the present invention, the connecting cable is a pair of twisted wires. In one embodiment the connecting cable is a shielded pair of twisted wires. In another aspect of the present invention, the connecting cable is a coaxial cable. The connecting cable may be disposed outside of the MCEI circular trough.

Abstract: Thin flexible piezoelectric transducers are bonded to or imbedded into oilfield tubular members or structural members. The transducers may be used to telemeter data as acoustic waves through the members. By proper spacing of transducers and phasing of driving signals, the transmitted signals can be directionally enhanced or encoded to improve transmission efficiency. The transducers may be used for health monitoring of the tubular or structural members to detect cracks, delaminations, or other defects. The flexible transducers are very thin so that overall dimensions of tubular or structural members are essentially unchanged by incorporation of the transducers.

Abstract: Geophysical measurement system employed during the drilling of a well borehole. The system employs a reference clock disposed within equipment at the surface of the earth and a borehole assembly which houses a downhole clock and at least one sensor. The borehole assembly is operationally connected to a drill string, which advances the borehole. At least one synchronization shuttle apparatus containing a shuttle clock is conveyed downhole to the borehole assembly to synchronize the borehole clock with the reference clock. Reference and borehole clock synchronization is maintained at one millisecond or less over a period of days. Outputs from the reference clock and borehole clock and sensor are combined to obtain a measure of a geophysical parameter of interest. Although the measurement system is particularly applicable to seismic-while-drilling measurements, it can be used in a wide variety of clock driven geophysical measurements.

Abstract: A swivel assembly for a downhole tool string comprises a first and second coaxial housing cooperatively arranged. The first housing comprises a first transmission element in communication with surface equipment. The second housing comprises a second transmission element in communication with the first transmission element. The second housing further comprises a third transmission element adapted for communication with a network integrated into the downhole tool string. The second housing may be rotational and adapted to transmit a signal between the downhole network and the first housing. Electronic circuitry is in communication with at least one of the transmission elements. The electronic circuitry may be externally mounted to the first or second housing. Further, the electronic circuitry may be internally mounted in the second housing. The electronic circuitry may be disposed in a recess in either first or second housing of the swivel.

Abstract: A device, method, and system for creating a pressure pulse from drilling fluid within a drill string in a down hole drill collar for enabling measurement-while drilling. The device and system are designed such that primarily laminar flow exists in the area surrounding the pulser apparatus. The method associated with the reproducible and essentially noise-free pulses occurs when a pulser bell is manipulated in an upward and downward direction by a combination of the solenoid activation of a bi-directional poppet to redirect the fluid flow from the pressure reservoir to and from a sliding pressure chamber and associated upper and lower flow connecting channels. The pulse or non-pulse is converted into a digital signal uphole by a pressure transducer in conjunction with a decoding algorithm. It is then displayed to the driller and oilfield operators as useful directional and formation information that help the oilfield operator for uphole decision making regarding directional drilling.

Abstract: Acoustic telemetry devices and methods that provide directional detection. In one embodiment, a disclosed acoustic telemetry device comprises at least two acoustic sensors and an electronics module. A first of the acoustic sensors detects a communication signal that propagates along a tubing string in a first direction. A second of the acoustic sensors is configured to detect the communication signal before the first acoustic sensor. The electronics module combines the detection signals from the acoustic sensors to obtain a combined signal that substantially excludes signals propagating in a direction opposite to the communication signal. Such signal suppression may significantly enhance the communication signal's signal-to-noise ratio, thereby increasing channel capacity. The acoustic telemetry device may be configured to support logging while drilling and/or full-duplex communication.

Abstract: The invention includes a wireless communication circuit and a method of communicating with wireless signals that feature a tank circuit having an inductor connected in parallel with a capacitor circuit. The capacitor circuit includes a pair of capacitors coupled in series. Each of the capacitors is connected in common to ground and has a capacitive value associated. A feedback network is connected to selectively bias the tank circuit to produce a signal having an amplitude. The amplitude of the signal is a function of a ratio of the capacitive values associated with the capacitor circuit. The frequency of the signal is defined by the inductor and the capacitor circuit. The tank circuit is multifunctional in that it may be biased to function as a transmitter and a receiver.

Abstract: An electromagnetic borehole telemetry system for transmitting information between a borehole transceiver and a surface transceiver located at or near the surface of the earth. Tubulars, such as steel casing and liners, are typically set within the well borehole to stabilize the wall of the borehole and to assist in hydraulically isolating penetrated formations. The invention utilizes these tubulars cooperating with one or more signal wires to reduce attenuation and noise in signals transmitted between the borehole and surface transceivers. The one or more signal wires are typically disposed within an annulus formed by the borehole wall and the outside surface of the casing. The one or more signal wires are connected at one end to one or more connection terminals positioned preferably near the bottom a tubular string. Opposing ends of the one or more signal wires are connected to one or more terminals of the surface transceiver.

Abstract: A method and a downhole telemetry system a downhole telemetry system using discrete multitone modulation for transmitting a data stream over a plurality of transmission modes is provided. The downhole telemetry system includes a downhole transmitter, a cable, and a surface receiver. The downhole transmitter is configured to transmit the data stream over the plurality of transmission modes. The surface receiver is coupled to the downhole transmitter via the cable and is configured to combine signals from each of the plurality of transmission modes to obtain the data stream transmitted by the downhole receiver. The method includes receiving a plurality of signals over the plurality of transmission modes from a downhole package, and may further include combining the plurality of signals to form at least one data stream.

Abstract: An electromagnetic borehole telemetry system for transmitting information between a borehole transceiver and a surface transceiver located at or near the surface of the earth. Tubulars, such as steel casing and liners, are typically set within the well borehole to stabilize the wall of the borehole and to assist in hydraulically isolating penetrated formations. The invention utilizes these tubulars cooperating with one or more signal wires to reduce attenuation and noise in signals transmitted between the borehole and surface transceivers. The one or more signal wires are typically disposed within an annulus formed by the borehole wall and the outside surface of the casing. The one or more signal wires are connected at one end to one or more connection terminals positioned preferably near the bottom a tubular string. Opposing ends of the one or more signal wires are connected to one or more terminals of the surface transceiver.

Abstract: A method and system for telemetry through a drilling fluid during drilling is disclosed. A reflector (110) is positioned downstream from the drilling mud pumps (80) and causes reflected pressure waves having the same pressure polarity as incident pressure waves traveling upwards. At least one pressure sensor (92) is positioned below the reflector (110) to sense pressure in the drilling fluid. The reflector can be a fixed orifice plate or an adjustable aperture.

Abstract: A petroleum well having a borehole extending into a formation is provided. A piping structure is positioned within the borehole, and an induction choke is positioned around the piping structure downhole. A communication system is provided along the piping structure between a surface of the well and the induction choke. A downhole module is positioned on an exterior surface of the piping structure and is configured to measure characteristics of the formation. The formation characteristics, such as pressure and resistivity, are communicated to the surface of the well along the piping structure.

Abstract: The invention relates to a pressure-measuring device (M1) with a controller (C) for controlling the functions of the measuring device (M1) and/or for processing measured data, a sensor (S1) for measuring a variable (p1) to provide first measured data (I1), and an output interface (IA1, D1) for outputting the measured data (I1).

Abstract: A signal transmission connection for a tool string used in exploration and production of natural resources, namely: oil, gas, and geothermal energy resources. The connection comprises first and second annular elements deployed in cooperative association with each other. The respective elements comprise inductive transducers that are capable of two-way signal transmission between each other, with downhole components of the tool string, and with ground-level equipment. The respective inductive transducers comprise one or more conductive loops housed within ferrite troughs, or within ferrite trough segments. When energized, the conductive loops produce a magnetic field suitable for transmitting the signal. The second element may be rotational in drilling applications. The respective elements may be fitted with electronic equipment to aid and manipulate the transmission of the signal. The first element may also be in communication with the World Wide Web.

Abstract: A telemetry system and method for communicating information axially along a drill string comprised of a drive train supported within a housing. The system includes an axial conducting loop formed by the drill string for conducting an axial electrical signal embodying the information between a first axial position and a second axial position in the drill string, which axial conducting loop extends between the first and second axial positions. A transmitter transmits information to the axial conducting loop. The drive train includes a downhole end which extends from and is located below the housing. At least one of the first and second axial positions is located in the downhole end. The method includes the step of conducting the axial electrical signal between the first and second axial positions through the axial conducting loop which extends between the first and second axial positions.

Abstract: Data transmission in pipeline systems A first set of apparatus is arranged for transmitting data from a point in a cased section of a well 1, 3 to a remote location. The apparatus may be used as a relay station 6 to increase operational depth. Signals are applied to and received from the string 1 at the relay station 6 and a selected length of the string 1 is provided with insulating spacer means 9 on either side of the relay station to ensure that the string 1 and casing 3 are effectively isolated for a selected minimum distance. This enables potential differences to be both applied to and detected from the string 1, thus allowing data transmission and reception. A second set of apparatus is arranged for transmitting from an internal unit 408 inside a cased section of the well 401, 403 to the immediate surrounding area outside the casing 403. The internal unit 408 injects current into the string 401. A toroid 415 which surrounds the casing 403 is provided to pick up signals.