Documenting a GPR Utility Survey Before Anyone Starts Digging

A GPR scan only helps protect the job if the findings can still be understood after the paint fades, the trench is backfilled, or a different contractor arrives on site. The record should show where the scan was done, what was found, what was not confirmed, and exactly where the proposed digging area sat in relation to the house, driveway, meter, fence, and other fixed features.

For a homeowner planning a new water line, sewer repair, footing drain, fence post, or patio excavation, ask for the survey record before the machine starts. It should not be a single photo of spray paint on dirt. It should be a usable package of marked-up photos, a sketch or site plan, field notes, and clear limits on what the scan could identify. Our page on what it costs in practice explains the kind of site conditions that affect the work involved.

The first thing to preserve is the condition of the ground on the day of the scan. Fresh asphalt, saturated clay, thick concrete, standing water, dense gravel, and heavy surface clutter can all affect what the radar sees. A useful survey record gives the excavation crew something specific to work from instead of leaving them with a vague instruction to “be careful around utilities.”

Utility survey documentation from GPR starts with the work area

Start with the limits of the area surveyed. Record the street address, date, scan time, weather, and the exact work zone. On a residential job, that might be “north side yard from the front hose bib to the rear property fence, 48 feet long by 12 feet wide.” If only a 6-foot-wide trench route was scanned, the paperwork should say that. It should not imply that the entire yard was cleared.

Tie the scan area to permanent reference points. Use the front-left house corner, water meter box, electrical meter, driveway edge, cleanout cap, retaining wall corner, or a mature tree with a distinct trunk. Measurements do not need to be complicated to be useful. A note such as “suspected line crosses 9 feet 4 inches east of the rear cleanout and runs toward the garage slab” gives the crew a starting point they can relocate.

Photograph the area before marks are placed, after marks are placed, and again after any utility exposure. Take one wide photo showing the whole side yard or driveway, then close photos of each mark. Include a tape measure, folding rule, or painted scale line where possible. A 24-inch scale beside a marked target is far more useful than a close-up image that could have been taken anywhere.

Homeowners can check one part of this without tools today: walk the planned route and look for fixed reference points that will still be there next week. A lawn chair, garden hose, trash bin, and loose paver are not reference points. A meter box, house corner, foundation vent, curb, or driveway seam is. If the marks in the yard cannot be related to something permanent, photograph them before rain, mowing, or foot traffic removes them.

Record what the radar showed, not what someone hoped was there

Ground-penetrating radar detects changes in material below the surface. It may show a pipe-shaped response, disturbed trench backfill, a void, a buried structure, or a strong linear feature. It does not automatically identify every target as water, sewer, gas, electrical, or communications. The documentation needs to separate a confirmed utility from a suspected target.

Use plain labels in the field notes. “Probable 4-inch PVC sewer line,” “possible metallic service line,” “linear anomaly at approximately 28 inches,” and “no clear GPR response beneath 5-inch concrete slab” are better than calling every line a utility. If a line was identified from a cleanout direction, pipe locator trace, exposed fitting, or CCTV result, say how it was identified.

Depth needs the same care. A radar depth estimate is commonly based on an assumed soil condition and signal response. A note reading “target estimated at 30 to 36 inches below current grade” is more honest and more useful than “pipe at 33 inches” when the line has not been exposed. Grade can change across a yard, especially near a retaining wall, filled trench, or driveway addition.

GPR utility survey documentation should also state where the scan had reduced confidence. Heavy reinforcing steel, wire mesh, wet soil, utility congestion, and a 6-inch concrete slab can obscure or distort targets. If a section beside the garage foundation was difficult to read, record that section by location and length, such as “limited interpretation for 14 feet along west slab edge.” That tells the excavator where hand exposure or slower digging is needed.

Match field markings to photos, sketches, and verified exposures

Paint and flags are temporary. The paperwork is what connects those temporary marks to the actual work plan. Number each target or route on the sketch, then use the same number in the photos and notes. For example, Target U-1 might be a suspected line running from the front meter toward the house, while U-2 is a possible abandoned line crossing the proposed trench.

A simple plan view is often enough for a house lot. Draw the building footprint, driveway, fence line, meter, cleanout, and planned excavation route. Add arrows for detected routes and show offsets from fixed objects. If the planned trench is 18 inches wide and a suspected line is 20 inches from the trench centerline, put both measurements on the sketch. That distance matters more than a generic statement that utilities were “nearby.”

If a utility is exposed, document the exposure before work continues. Photograph the pipe or cable in the hole, show its direction of travel, and measure from the exposed item to the planned route. Record the actual outside diameter where visible: 3-inch ABS, 4-inch PVC, 1-inch copper, or a conduit roughly 2 inches across. A verified exposure can confirm, correct, or disprove the GPR interpretation.

This is where GPR CCTV sewer inspection can add useful detail on drainage work. A camera inspection may show pipe material, direction of travel, fittings, root intrusion, offsets, or a damaged section that helps explain a radar response. Keep the camera findings and GPR sketch together, but label the source of each finding so nobody mistakes an inferred route for a visually confirmed pipe.

Keep a change record until the excavation is closed

The documentation should continue after the initial scan. If the trench shifts 3 feet to avoid a target, note the new route. If the excavation goes from a planned 30 inches deep to 54 inches deep because of a grade change or repair depth, add that to the record. A survey made for one route and one depth cannot automatically be treated as coverage for a larger excavation.

Keep dated photos of each exposed utility, including the condition before and after the work. Show bedding, pipe crossings, service connections, and any abandoned line encountered. If an unknown pipe is capped, broken, or already crushed when uncovered, photograph it before moving it. A later question about damage is much easier to address when there is a time-stamped sequence showing what was found.

The final file should include the survey scope, site plan, photos, target list, depth estimates, limitations, exposed-utility records, and any route changes. Save the original photos rather than only screenshots or compressed copies. File names such as “2025-06-14_U-2_rear-yard-exposure_32in-deep” are easier to sort than a phone gallery full of unnamed images.

The cost behind radar work includes equipment, training time, field time, and the time required to turn marks into a defensible job record; GPR equipment cost for plumbers is one reason survey paperwork should be treated as part of the service rather than an afterthought. The useful record is the one a homeowner, excavator, adjuster, or plumber can open six months later and use to understand exactly what was scanned and what happened next.