Induction, Blind Sweeps and Nulling · Handbook

Induction

Printed page 18

Induction should be used after all the previous techniques have been exhausted. The Genny should be placed on the ground with the handle in line with any suspected or known buried services. The CAT should then be used a minimum of 10 metres away from the Genny.

Fig 01Genny stood upright on the footway in line with a chamber cover: induction position.

Fig 02Genny upright in the centre of a paved chamber cover in the footway.

Fig 03Genny lying flat beside the chamber cover: the horizontal position couples along a different axis.

Fig 04Genny upright in the middle of a patched trench line running along the road.

Blind Induction Sweep

Printed page 19

The Genny should be placed at regular intervals a minimum of 10 metres away from the intended excavation. At each position the CAT should then be used to locate any detected signal.

Fig 01Blind induction sweep: the Genny placed at the left of the search area at an alley mouth (position 1 of 6).

Fig 02Blind induction sweep: the Genny stepped across the search area (position 2 of 6).

Fig 03Blind induction sweep: the Genny stepped across the search area (position 3 of 6).

Fig 04Blind induction sweep: the Genny stepped across the search area (position 4 of 6).

Fig 05Blind induction sweep: the Genny stepped across the search area (position 5 of 6).

Fig 06Blind induction sweep: the Genny at the far side of the search area (position 6 of 6).

Induction Signal Nulling

Printed page 20

Nulling out a previously detected buried utility can be carried out by placing the Genny on its side directly over the previously detected utility.

Airborne signal from the Genny can lead to confusing results. To confirm whether the CAT is detecting airborne signal or signal from below, use the technique outlined here.

01With the Genny switched on and inducing a signal into the ground, adjust the CAT sensitivity so that a low but clearly audible signal is received. Move the CAT through the signal and ensure that the signal increases and decreases.
02Keeping the CAT across the assumed route of the buried utility, aim the CAT directly towards the Genny. If the signal stays the same or increases, the signal is airborne.
03If the signal decreases or disappears, the signal is coming from a buried utility.

Fig 01Genny upright and centred on a paved square in the footway.

Fig 02Genny laid over at an angle on the same spot: changing its attitude to null the signal.

Fig 03Line drawing: a lit match and radiating waves reaching a receiver, the airborne-signal sketch.

Line drawing: the CAT held upright beside the buried line with signal waves arriving from the Genny. — Fig 04With the Genny inducing into the ground, set the CAT sensitivity so a low but clearly audible signal is received.

Line drawing: the CAT held low and level across the route of the buried utility, waves arriving from the Genny along the ground. — Fig 05If the signal decreases or disappears as you cross the route, it is coming from the buried utility.

Filed under

Induction Airborne signal Search distance Signal nulling