Abstract: In order to identify a cable buried underground, a very low frequency voltage signal is applied to the cable and an electric field sensor is brought into proximity with the cable. The sensor thus detects the voltage signal on the cable and so identifies the cable. The sensor is unaffected by one or more additional cables carrying voltage signals, which are proximate the cable of interest, as the electric filters from such additional cables do not pass to the cable of interest. The sensor is mounted on a probe which is mounted into a bore in the soil around the cable of interest. The probes may also carry a magnetometer for detecting magnetic fields generated by low frequency alternating current signals on the cable of interest.

Abstract: If a solenoid is mounted on an underground object, such as a boring tool, magnetic fields generated by an electric current flowing through that solenoid can be detected by a suitable detector at or above the surface. If the axis of the solenoid is tilted, the maximum value of the field is not directly above the solenoid. Therefore, the present invention makes use of measured values of horizontal and vertical components of the magnetic field to determine the separation of the detector and the solenoid, and also, by making use of a tilt sensor associated with the solenoid to derive a prediction of the ratio of the horizontal and vertical components of the field at a position vertically above or below the solenoid. If that predicted value of the ratio is then compared with the measured value of the ratio, the two will coincide when the detector is vertically above the solenoid. Thus, by moving the detector until such coincidence is obtained, the position of the solenoid can be determined.

Abstract: A locator such as a ground penetration probe (24) has spaced antennae (21,22,23) therein which detect electromagnetic signals from an object (26) such as a buried cable. By analyzing the electromagnetic signals using a suitable processor (25) it is possible to determine the separation of the locator and object (26), both in terms of the direction (X) corresponding to the spacing of the antennae (21,22,23) and the perpendicular direction (Y) to the object (26). This then permits a display to be generated showing visually the separation of the locator and the object (26). If the locator incorporates a tilt sensor, the processor (25) can then compensate for tilting of the locator, and determine the vertical and horizontal separation of the locator and the object (26).

Abstract: In order to locate an inaccessible object, such as an underground boring tool (30), a solenoid (10) on or in the tool generates a magnetic field which is detected at two measuring locations (20, 21). Information relating to the relationship between the axial and radial components of the field from the solenoid (10) are stored, and comparison of the measured values of the axial and radial components at the measurement locations (20, 21) enables the direction of the solenoid (10) from the measurement locations (20, 21) to be determined. The known attenuation of the magnetic field from the solenoid (10) enables the distance between the solenoid (10) and the measurement locations to be determined from the absolute value of the field at the measurement locations (20, 21) and the direction to the solenoid (10).

Abstract: Two vertical, parallel, horizontally spaced signal coils are connected in series but in opposite phase across an a.c. source. Thus a buried conductor located centrally beneath the coils experiences an alternating magnetic field whose strength is due to the addition of the fields induced by the two coils. The field strength may be further enhanced by one or more of: providing a ferromagnetic flux diverter between the upper ends of the coils; placing a conductive nonmagnetic screen between them; connecting a tuning capacitor in parallel with the coils; or providing a tuning circuit having series-connected secondary coils which are flux-linked to respective signal coils and are in parallel with a capacitor. There may be a switch arrangement for switching the signal coils to in-phase connection, for use when they are oriented horizontally.

Abstract: An angular displacement sensor for limited angle applications (e.g., for sensing automotive throttle positions) comprising first (14) and second (16) relatively rotatable components arranged to confront each other axially. The first component (14) provides a plurality of poles (14A, B, C) which are angularly disposed about the rotation axis and extend towards the second component. These poles (14A, B, C) have axes which extend in the same direction as the rotation axis. Some poles have windings (14A, B), while others (14B) provide flux return paths. The second component comprises an inductance affecting component (16) which overlies only some of the wound poles at any given time, the relative rotation varying the poles which are overlaid. The sensor includes an output unit (17) for providing output signal data related to the inductances of the excitation poles and thus related to the rotary configuration of the components.

Abstract: Geographical positional data is obtained for an underground object by locating the object using a device that locates underground objects. An apparatus that receives data from a global position system ("GPS") provides data identifying global position of the device, and thus also of the located underground object. A data storage unit stores such positional data, which may be used in the future to re-locate the underground object. The GPS-data receiving apparatus can preferably receive data from at least one overhead GPS satellite and from a secondary stationary receiver. Data from the secondary stationary receiver may be modulated on to a signal transmitted along a detected underground cable object.

Abstract: An elongate vessel, preferably cylindrical, is partially filled with a conductive and/or ferromagnetic fluid. At least two coils are wound on the vessel in longitudinally different regions so that tilting the vessel varies the amount of the fluid inside the coils differently. This affects their inductances. The variation in inductances is monitored, directly or indirectly, e.g. by applying an AC signal to the coils and monitoring the voltage drops across them. This provides an electrical output related to the inclination of the sensor's axis. It is unaffected by rotation about the axis. Thus the sensor can be used to monitor the orientation of the axis of a drilling tool, e.g. a mole, which rotates about its axis.

Abstract: An inductive displacement sensor has first and second elements which define a path for movement. The first element has coil portions which interact with an inductance affecting part of the second element so that the inductances of the coil portions vary with movement of the inductance affecting part, only some of the coil portions being affected by the inductance affecting part at any time. The coil portions are arranged in two series connections, each series connection being at least two coil portions electrically connected in series. Coil portions of the two series connections are then arranged alternately. In this way, substantially sinusoidal output signals may be obtained.

Abstract: A system for tracing routes of conductors is disclosed. An alternating signal is applied to the object to be traced (12), and the field produced by this signal is detected remotely from the object. In order to distinguish between signals produced by the object being traced and those produced by nearby conductors (14) due to capacitive coupling, the alternating signal has first and second components, related in frequency and phase, and the field is detected at a plurality of positions. The phase relationship of the detected signals is investigated to determine unambiguously the position of the object concerned. In one embodiment, the second frequency component is a harmonic of the other. In another embodiment the frequency of the second component is the frequency of the first, plus or minus a sub-harmonic of the first.

Abstract: Displacement along a linear or rotary path causes relative displacement of a pair of elements (101,102) that confront each other across the path. One element (101) provides a series of coil portions (AB,BC), while the other (102) has a portion which increases the inductance of the fraction of the coil portions that it lies adjacent at any instant. The coil portions are homopolar. Typically each coil portion has an axis which intersects the path, and all are wound in the same sense about their axes. They are generally connected in series as a single winding on a core (101). The core has unwound portions (104) for providing a flux return path.

Abstract: A temporarily inaccessible object such as a locator in a buried pipe has a first transmitter (1) which produces an electromagnetic field rotating about a horizontal axis (A--A) and a second transmitter (7) which produces a solenoidal field centered on that axis. The rotating field is remotely detected at a receiver using a vertical axis coil (6) which locates the object and a reference signal is obtained by a horizontal coil (9) at the receiver. The roll .alpha. of the object may be determined by detecting the phase difference between the detected signal and the reference signal. The yaw or angle .beta. of departure from a desired horizontal direction is obtained from a maximum output of the horizontal receiver coil (9). With a third transmitter (15) producing another phase related field rotating about a horizontal axis transverse to the first one (A--A) a similar technique at the receiver gives the pitch angle .gamma..

Abstract: Two members 21,22 are relatively displaceable along a linear or circular path. One member 21 provides a plurality of coil portions along the path. Their individual inductances are dependent on the configuration of the second member 22. For example, the first member 21 may be an annular stator with inward radial projections k,l,m . . . on which the coil portions are wound, alternately in different senses so that the flux path tends to loop in and out. The second member 22 may be an annular rotor, one section 23 being ferromagnetic and the other section 24 being non-ferromagnetic and/or having a conductive screening layer. Alternatively the second member may be mercury 42 half-filling an annular conduit 40 which surrounds a static ferromagnetic core 44 (FIG. 9). A particular characteristic is that for any relative position of 21 and 22, half the coil portions of 21 have a different inductance from the other half.

Abstract: A high frequency and two low frequency electromagnetic signals are generated (1), the low ones being coherent with the second being a sub-harmonic of the first. The high frequency is modulated (4) by the second low frequency and applied (7,9) as a composite signal to an underground conductor, while the first low frequency is applied (9,10) additively. A receiver traversed over the line has aerials (12,21) or ground probes (24,25) for picking up these signals in separate channels. The high frequency is demodulated (18) to provide a reference signal for a synchronous detector (20) to which the first low frequency is fed. That suffers a phase reversal if there is a fault, and the output of the synchronous detector is arranged to indicate that.