Currently, the problem of detecting unmapped metalliferous objects, including those of non-ferrous metals, is an urgent one. One of the possible approaches to solving this problem is the use of small-size inductive devices, combining the functions of radiating an original electromagnetic field and receiving the secondary electromagnetic field induced by the metalliferous object irradiated by the original electromagnetic field of the device.
Known in the art are devices for detecting underground metalliferous objects (high frequency M 480 detector from Seba Dynatronic, FRG, and the TW-5 pipe and cable locator from Grigsby Co., USA) of like design and each comprising: a generator loaded by a driving loop, a receiver loop positioned orthogonally to this driving loop and loaded by connected in series selective amplifier, data processor and acoustic signalling unit. The device uses geometric cancelling of the signal induced in the receiving loop by the original electromagnetic field of the driving loop.
Functioning of these devices is based on creating an original electromagnetic field with the aid of the driving loop, this field propagating in the area being searched, and reception of a secondary electromagnetic field generated by the metalliferous object.
Such devices are highly susceptible to external interfering electromagnetic fields, including those from metalliferous objects and service lines in the vicinity of the search area, because they also generate secondary fields in response to irradiation by the original electromagnetic field of the driving loop.
Moreover, the reliability of detecting service lines located at an angle less than 20.degree. to the direction of device movement is low, due to the low signal induced in the service lines by the driving loop.
Also widely known in the art are metalliferous objects detectors (V. F. Bakhmutsky. "Induktsionnye kabeleiskateli" (Inductive cable locators), 1970, Svyaz Publishers, pp. 75-78.--In Russian) comprising a driving loop and two rigidly fitted to it orthogonally and oppositely connected receiving loops mounted symmetrically to both sides of the driving loop. In one embodiment the driving loop is horizontal and the receiving loops are vertical, in another embodiment the loop orientation is opposite; the electric signal recording circuitry in both embodiments is identical and similar to that described herein above.
It should be noted, that the direction of the devices movement coincides with that of the rod connecting the receiving loops.
Such devices are characterized by a low immunity to external interfering electromagnetic fields and metalliferous objects in the proximity of the search area. This is due to the wide spacing of the receiving loops, necessary to attain sufficient sensitivity and degree of geometric cancelling. Furthermore, the reliability of locating service lines positioned at angles less than 20.degree. is low, due in the first embodiment to low secondary fields from the service lines and in the second embodiment--to a low signal induced in the service line by the driving loop.
It should be noted, that the low interference immunity of all the herein above described detecting devices makes it impossible to use them installed onto a vehicle, which itself is a source of interference for such devices, so that the productivity of search work is low.
A higher productivity and interference immunity is featured by the metalliferous objects detector (SU, A, 1190330), comprising a driving loop, two rigidly fitted to it receiving loops mounted coaxially and parallel to one another and symmetrically and orthogonal to the driving loop located in the plane of the common axis of the receiving loops, an alternating voltage generator electrically coupled to the driving loop, and connected in series signal balancer for the receiving loop signals with the inputs thereof connected to the receiving loops, and differential amplifier with the output thereof connected to the data input of an electric signal compensator to cancel the effects of the original electromagnetic field of the driving loop on the receiving loops with the control input thereof electrically coupled to the alternating voltage generator and with the output thereof connected to the input of a selective amplifier, the output whereof is connected to a recorder and electrically coupled to a signalling unit.
The above mutual arrangement of the driving and receiving loops, constituting the inductive system of the device, allows a significant improvement in its interference immunity due to a practically equal signal level induced in the receiving loops by external electromagnetic field sources (power distribution lines, electrified railways, communications lines, etc.) and cancelling out these interference signals by oppositely connecting the two receiving loops.
This known in the art device features a narrow coverage, i.e. the area in which the receiving loops are capable of picking up the secondary electromagnetic field induced by a metalliferous object. As is known, the width of the coverage area is a function of the spacing between the receiving loops and is limited by a falling off of the interference immunity at greater spacings, accompanied by a higher sensitivity.
Moreover, if metalliferous objects (service lines) are at an angle less than 20.degree. relative to the direction of searching, which is normal to the rod connecting the receiving and driving loops, the detection reliability is low due to the weak signal induced in the service line by the original electromagnetic field of the driving loop.
To improve the detection probability, the search area has to be scanned twice, at different orientation of the induction system relative to the direction of searching. However, even then the probability of detection is insufficient, because it is difficult to maintain the same search direction when scanning in the opposite direction, this being true both with manual scanning and when the detector is mounted onto a vehicle.