User's Manual

Appendix B

68 DigiTrak

SE™ Operator’s Manual

Due to the shape of the transmitter’s signal field (flux lines), when it is at a pitch greater than ±30% (±17°)

and/or a depth of 15 ft (4.6 m) or more, the position of the locate line will be some distance ahead of or

behind the transmitter’s actual position. In this case, the depth displayed on the receiver becomes what is

called the projected depth. The transmitter’s distance ahead of or behind the locate line is called the

fore/aft offset.

The projected depth and fore/aft offset, shown in Figure B2, must be accounted for when the transmitter

is steep and/or deep. See the tables provided later in this appendix (Tables B1 and B2) to determine the

actual depth and fore/aft offset when you know the displayed (projected) depth and pitch of the

transmitter.

Fore/Aft Offset

Projected Depth

Actual Depth

30% (17°)

Pitch

Figure B2. Projected Depth vs. Actual Depth and Fore/Aft Offset When Steep and Deep

Figure B2 above shows a transmitter positioned in a drill string that is meant to illustrate drilling at either a

positive or a negative pitch—the pitch is positive if you are drilling left to right, and it is negative if you are

drilling right to left. The transmitter’s signal field is also pitched at the same angle as the transmitter. The

locate line (LL), which is where the depth measurement is taken, is the horizontal component of the

transmitter’s signal field flux lines. That is, the LL is found where the flux lines are horizontal, as illustrated

with short horizontal yellow lines in the figure above.