Abstract: A data transmission system for use in extracting data from a downhole location in a well is disclosed. The well has electrically conductive downhole structure (2,C) which together with an earth return forms a signal circuit which is used to transmit data. A current source (34) is provided at the wellhead for applying a reference signal to the downhole structure (2,C). Effective impedance varying means (48) are located downhole and used for varying the effective impedance of the signal circuit in dependence on data to be transmitted to the wellhead. Monitoring means (35) for monitoring changes in the reference signal caused by varying the effective impedance of the signal circuit are located at the wellhead. Monitoring these changes allows data to be extracted. A separate reference earthing point may be provided for the monitoring means to alleviate the effect of imperfections in the earth connection (S) used to complete the signal circuit.

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.

Abstract: An anode monitoring system for monitoring the integrity of anodes 2 provided on a metallic structure such as a pipeline system 1. A signal circuit Sn including the pipeline 1 and one of the anodes 2 is set up and appropriate signals are looked for at a central station 4. Reception of the signal at the central station 4 is dependent on the integrity of the anode 2 under inspection at that time. If the anode 2 is missing or defective the expected signal is not received and thus the fault in the anode 2 can be detected. A notch filter 5 is inserted in series between each anode 2 and the pipeline 1. The filter 5 provides a high impedance which can be signalled across but does not interfere with the cathodic protection system.

Abstract: A subsurface object location apparatus for determining the position of an object 2 located below a surface S such as a land surface, the surface of the sea or a seabed. A subsurface unit 1 having transmitter means for transmitting inductive signals is mounted on the object 2. A plurality of surface units 3 are disposed at spaced locations at the surface S. Each surface unit 3 has receiver means arranged for receiving the inductive signals. Processing means 4 are provided for determining the location of the object 2 using time difference of arrival techniques in respect of the signals received at the surface units. Cyclic correlation is used in determining the times of reception of signals at the surface units.

Abstract: An apparatus and method for detecting and locating electric current leakage paths along the length of an electrically conducting elongate member. The apparatus has a variable supply device and a meter device for monitoring the impedance of the member (9, 11), the supply device being operable to drive current in opposite directions along the member, so that a movable node can be established, the node being where the net change in potential between the member and its surroundings is substantially zero. When the apparatus is in use, the position of the node is varied by varying the current driven along the member and the resultant impedance is monitored by the monitoring meter device, whereby the position of the node at which there is a local minimum in the impedance indicates the position of a current leakage path.