How to Locate a Camera Sonde and Mark Its Depth

Why this matters

A camera inspection that finds a broken joint is only worth something if the excavator can dig to it. Mark the spot 3 ft off and the crew opens a hole that misses, widens it by hand in a trench that keeps getting deeper, and the customer pays for the second hole. Mark the depth wrong and the backhoe operator either digs carefully for an extra hour or lands a bucket on the pipe. Locating a sonde takes ten minutes. Doing it carelessly is how a good inspection turns into a bad excavation.

This card is the locate: finding the sonde above ground, confirming it, reading and marking depth. Running the camera is in How to Perform a Sewer Camera Inspection, and judging what it saw is in Sewer Camera Image Interpretation.

Safety at the access point and on the street

Opening the cleanout. A sewer that is backed up holds sewage under head behind the cleanout cap. Loosen the cap slowly with your body to the side, wearing gloves and eye protection, and let any head bleed off before the cap comes free. Treat everything that comes out of the line, and the camera cable as it is pulled back, as contaminated: gloves on, wipe the cable as it retracts, wash hands before eating, and keep the cable off the customer's walkway.

Sewer gas. An open cleanout or manhole can vent hydrogen sulfide and methane. Work from outside the opening, never put your head into a manhole, and never enter one; manhole entry is permit-required confined-space work under 29 CFR 1910.146 (29 CFR 1926 Subpart AA on construction work) with an attendant and atmospheric testing, and it is not part of this procedure.

Traffic. A locate that crosses into the street or the right-of-way puts you in traffic with your head down looking at a screen. Wear high-visibility clothing and set cones or traffic control as the locality requires before you walk out.

Power. Plug the camera reel and monitor into a GFCI-protected outlet or use a battery unit; the reel and the wet cable are on the ground beside an open drain.

How the sonde's field looks from above

The sonde is a small transmitter in or just behind the camera head, broadcasting at a set frequency (512 Hz is common for camera sondes because it carries through cast iron and clay; match the locator to whatever frequency your sonde uses). Its field is shaped like a bar magnet lying along the pipe. A locator held with its mast vertical, in the common peak mode, shows three features along the pipe's line:

  locator mast vertical, bottom of locator on grade
  reading:   null         PEAK         null
              |            |            |
  grade ===== x ========== X ========== x =====
              |<-- 37 in ->|<-- 37 in ->|
              |    null to null: 74 in  |
              |            |
              |            | 52 in, grade to sonde
              |            |
  sewer  -----------------[sonde]----------------->
  line           camera head riding on the invert

• The peak, the strongest signal, directly over the sonde.
• Two nulls, sharp drops to near nothing, one ahead of and one behind the sonde on the pipe's line.
• Beyond each null the signal rises again into a weaker ghost peak, which is the trap: a locator swept fast across a yard can settle on a ghost and mark the wrong spot.

For a sonde lying level in straight pipe, each null sits about 0.7 times the sonde's depth from the peak, so the null-to-null distance is about 1.4 times the depth. That geometry is the free check on the locator's depth number. It holds only for a level sonde: a head tilted on a slope, in a bend or climbing over an offset joint pushes the nulls to unequal distances, and the null spacing stops meaning depth.

Steps

1. Park the camera at the point of interest and write down the footage. Stop the head at the defect on the monitor, note the cable counter reading from the access point, and make sure the head is lying flat in the pipe, not perched on the defect. Why: a head tilted on an offset or a broken bell distorts the field (see the second case below), and the footage is your sanity check on where the sonde should be.

2. Set the locator to the sonde's frequency and confirm the signal near the access point. With the head just inside the cleanout, hold the locator over it and confirm a strong signal. A locator that cannot hear the sonde a foot from the cleanout has the wrong frequency, a sonde that is off, or a dead battery, and you want to find that out here, not 40 ft out in the yard.

3. Walk out along the footage and find the peak. Start beyond where the footage says the sonde is and sweep across the probable line of the pipe in a grid, mast vertical, at a steady walking pace. Turn the gain down as the signal rises so the peak stays sharp instead of reading full scale across a wide patch. The peak is where the signal is strongest and falls off in every direction you move. Keep the mast plumb: a tilted locator moves the apparent peak sideways.

4. Confirm with the two nulls. From the peak, walk slowly along the pipe's line in one direction until the signal drops to a null, and mark it with a flag. Go back through the peak and find the null on the other side, and flag it. The sonde is under the peak and halfway between the nulls. If the peak is not roughly midway between them, the head is tilted or something nearby is distorting the field; do not mark yet.

5. Read the depth at the peak. Set the bottom of the locator on the ground directly over the sonde, mast vertical, still, and read the depth. Most locators measure from the bottom of the locator housing; if yours is held at a height instead, the maker's instructions say how to correct. The reading is the depth to the sonde, which rides near the bottom of the pipe, not to the top of the pipe. The crown of the pipe sits higher than the reading by roughly the pipe's diameter less the height of the sonde above the invert; on a 4 in line, call it about 3 in shallower.

6. Take the second reading. Step away, come back in from the opposite direction, find the peak again and take another depth reading. The two readings should agree; a common shop standard is within 2 in of each other at the 3 to 6 ft depths most house sewers run, tuned to your locator's stated accuracy. Then cross-check against the null spacing times 0.7.

7. Mark it. Paint the sonde position on the ground with an X and the depth beside it, and the pipe's line through the flags. Green is the APWA uniform color for sewers and drain lines; white marks a proposed excavation. On a hard surface the paint is the record. On grass, a flag survives a mower better than paint. Write the footage, the depth, and the distance to two fixed points (a house corner, a fence post) on the work order, because paint fades before some excavations happen.

One gate, two locates

The gate is the same on both: two depth readings within 2 in of each other, with the null spacing agreeing.

Locate A: a cracked pipe 26 ft out, under lawn. Head parked flat at 26 ft by the counter, 4 in PVC. Peak found at about 25-1/2 ft from the cleanout along the line. Nulls flagged 37 in either side of the peak, 74 in apart. Depth readings: 52 in coming from the house side, 53 in coming back from the street side, 1 in apart. Null cross-check: 74 times 0.7 is 51.8 in, call it 52 in. Both checks pass. Painted: X, "52 in to sonde, about 49 in to crown," and the line through the flags. The excavator dug to 49 in and found the pipe at the crack.

Locate B: an offset joint 31 ft out, under the driveway edge. The head stopped at the offset by the counter at 31 ft, and on the monitor it was half up on the lip of the next pipe. Peak found, but the nulls came in unequal: 26 in on one side, 40 in on the other. Depth readings: 41 in, then 49 in from the other direction, 8 in apart. The gate failed on both counts, and the unequal nulls named the cause: the head was tilted on the offset, so the field was tilted with it.

The fix was physical, not a better reading. The head was pulled back 1 ft so it lay flat in straight pipe before the offset, at 30 ft by the counter. New locate: nulls 34 in and 33 in from the peak, 67 in apart. Depth readings 47 in and 48 in, 1 in apart. Null cross-check: 67 times 0.7 is 46.9 in. Both pass. The sonde position was painted, and the offset joint was marked 1 ft further out along the painted line, with a note that the mark is offset from the locate by the 1 ft the head was pulled back. The rebar in the driveway edge was a second suspect for the distorted field; it was ruled out when the flat head gave a clean, symmetrical locate within a foot of the same spot.

When the locate will not settle

• Interference. Buried power lines, metal fences, rebar mesh in a slab and parallel metallic lines can bend or swamp the signal. Move the head a few feet, relocate, and see whether the error moves with the sonde (a sonde problem) or stays in one place (an interference problem).
• Very deep or very noisy. The locator's depth accuracy falls off with depth and with interference. Where the readings will not agree, mark the position, write the depth as approximate with both readings, and tell the excavator to pothole carefully rather than trust a single number.
• Under a structure. A sonde under a slab can be located through it, but the depth reads to the sonde from the top of the slab; write which surface the depth is measured from.

Before anyone digs on your mark, the excavating contractor still calls the one-call center (811) for the other buried utilities. A sewer locate tells them where the sewer is, not where the gas line is.

References

• 29 CFR 1910.146 and 29 CFR 1926 Subpart AA, permit-required confined spaces, which this procedure does not cover; manhole entry is a permit crew.
• APWA uniform color code for marking underground utilities (green for sewers and drain lines, white for proposed excavation).
• The locator and camera makers' instructions for frequency, depth measurement point and stated accuracy.
• See related: How to Perform a Sewer Camera Inspection
• See related: Sewer Camera Image Interpretation