Pre-excavation locating reduces uncertainty; it does not certify that the ground is empty. The distinction matters because a clean paint line can represent a well-verified facility, a coupled signal on the wrong conductor, an approximate corridor, or only one asset in a crowded trench.
A Vivax-Metrotech utility locator can provide multi-frequency electromagnetic evidence, depth and current readings, and distortion-related views. Those capabilities are useful inside a damage-prevention system. They are not a substitute for 811 communication, records, qualified interpretation, GPR where appropriate, survey, selective exposure, or safe excavation practice.
In 2026, strike prevention is a system—not a scan
The current Common Ground Alliance Best Practices Guide makes the system view explicit. Version 22.0, published in March 2026, contains more than 160 consensus practices covering planning and design, 811 centers, locating and marking, excavation, mapping, compliance, education, and reporting. It also adds a practice for large or complex project locate requests.
CGA describes its guide as general guidance. Employer procedures and applicable federal, state, and local requirements still govern the work. Ticket timing, tolerance zones, response codes, private-facility obligations, and permitted excavation methods vary by jurisdiction.
Within that framework, an excavation release should connect seven controls:
- define the exact work area and planned excavation method;
- notify through the applicable 811 process and meet current timing requirements;
- review positive responses from the notified facility owners;
- investigate expected public and relevant private facilities;
- resolve critical identity, position, and depth questions;
- preserve, maintain, and refresh marks and records as the job changes;
- excavate with the care required by law, owner rules, and site conditions.
FHWA’s current Subsurface Utility Engineering guidance follows the same logic at project scale. SUE combines records, engineering, surveying, surface geophysics, utility coordination, and selective exposure. No single instrument performs that entire process.
A valid ticket and fresh marks may document that listed operators responded. They do not automatically account for private lines, unknown or abandoned facilities, nonconductive services without tracers, or a depth-critical crossing that needs physical confirmation.
Where electromagnetic locating works well
Active electromagnetic locating is strongest when the operator can create and follow a controlled current path on the intended facility.
The most favorable setup has:
- a continuous conductive pipe, cable, tracer wire, or trackable sonde;
- a known access point tied to the intended facility;
- a safe, selective application such as direct connection or an appropriate clamp;
- enough separation from other conductors to limit coupling;
- a response that stays consistent across current, route, peak/null, and depth behavior;
- correlation with records, surface features, and known points.
The receiver detects the electromagnetic field produced by current on the conductive path. It does not image the pipe or cable itself. That is why signal application matters. A direct connection, when safe and performed by qualified personnel, generally provides a clearer relationship between transmitter and target than broad induction.
Frequency is a control, not an accuracy setting. Start at a lower practical frequency and move upward only when the response is not strong or stable enough. Higher frequencies can help energize short or difficult conductors, but they also couple more readily to neighboring lines.
The vLoc3-Pro specification lists configured frequencies from 98 Hz to 200 kHz, depending on transmitter, mode, accessories, and configuration. Relevant interpretation aids include continuous depth and current information, peak/null comparison, distortion indication, Vector Locate, Transverse Graph, and Plan View. None of those displays independently establishes facility identity.
Consider two traces:
- A direct connection at an identified pedestal produces stable current, matching peak and null positions, and a route that reaches the next known access point.
- An induced high-frequency signal produces one strong peak that branches toward several parallel services.
The first supports a stronger trace hypothesis because several observations agree. The second is a search result that still needs identity work.
Where electromagnetic locating fails or misleads
EM locating fails outright when there is no usable conductive path. A plastic pipe without a functioning tracer or inserted sonde may return no active signal. A broken tracer wire, inaccessible access point, poor grounding, or discontinuous cable can produce the same result.
It can also return a convincing signal on the wrong route.
Coupling and shared bonds
Current can transfer to a nearby parallel conductor or split through a common bond. The wrong line may produce a smooth, continuous, strong response. Two frequencies can agree because both follow the same unintended path, so frequency agreement is not independent proof.
Field distortion
Nearby metal, reinforced structures, congested crossings, poor grounding, and multiple conductors can distort the field. Peak and null positions may separate, depth may become unstable, and the apparent centerline can shift. Electronic depth should not be treated as an exposed elevation when field geometry is inconsistent.
Passive-mode gaps
Passive power and radio searches detect compatible ambient signals. They do not energize every utility and should not be used as an all-clear sweep.
Identity failure
The receiver reports field behavior, not ownership, material, pressure, service, or operational status. A trace must be reconciled with access points, records, owner response, and other field evidence.
The correct conclusion after non-detection is narrow: the selected setup did not produce a recognizable signal. “No utility is present” is a much stronger statement and requires additional evidence.
Where GPR adds evidence—and where it loses visibility
Ground-penetrating radar can add evidence for both conductive and nonconductive targets. It sends electromagnetic energy into the ground and records reflections from interfaces with different electrical properties. A pipe may appear as a hyperbolic feature in the data, but the reflection does not name the owner or service.
GPR is generally more favorable when the ground is relatively dry and nonclay, the target has useful dielectric contrast with the surrounding material, surface access supports systematic scanning, and clutter is manageable.
Its limitations are physical:
| Condition | Practical effect |
| Moist or clay-rich soil | Energy attenuates, reducing useful penetration and target visibility |
| Metal sheet, plate, or continuous layer | Energy does not penetrate the metal, so features beneath it are hidden |
| Small-diameter or deep target | The response may be too weak relative to background noise |
| Reinforcement and dense clutter | Many reflections can mask or imitate utility features |
| Insufficient scan orientation or spacing | A target may not be crossed at a geometry that produces a clear response |
FHWA’s general acquisition guidance gives 5 ft (1.5 m) as an example grid spacing and 2 ft (0.6 m) for higher-resolution imaging, while emphasizing documented coordinates, site conditions, and scans in both grid directions. Those numbers are not a universal project specification; the grid must match the target and required confidence.
A plastic service that EM cannot trace may produce a GPR anomaly in favorable, dry soil. The same service may disappear in wet clay. Even when visible, the anomaly needs route correlation, records, or exposure before it can control a critical excavation decision.
EM and GPR therefore fail differently. Used together, they can reveal discrepancies. They still do not guarantee complete detection.
Tickets and paint can fail as project controls
Locating technology may perform correctly while the project-control system fails around it.
Incomplete positive response
CGA 22.0 says the excavator should review positive responses before work begins. Seeing some paint does not show that every notified owner responded. An expected but unmarked facility is a conflict to escalate, not a blank area to cross.
Private and unknown facilities
The standard 811 process may not cover every private asset on a property. Project planning should name responsibility for private investigation and for facilities that appear in records or field observations without a confirmed owner.
Misread markings
Marks may represent a centerline, outside edges, several separate facilities, a corridor, or an offset. A crew that does not understand the marking convention can convert accurate locator information into the wrong excavation assumption.
Damaged, stale, or expired information
Paint fades, stakes move, milling removes markings, weather changes the surface, and construction installs new facilities. Tickets and marks also have jurisdiction-specific life and update rules. The field package needs a named owner for review, protection, and refresh.
Long and complex projects
CGA’s new Practice 5-35 addresses work that is larger, longer, or more complex than a standard request can reasonably cover. It calls for enhanced periodic communication and, where available, project-specific processing and marking agreements. A multi-month corridor should be staged around active work fronts rather than relying on one static set of marks.
Use layered evidence and explicit release gates
The prevention plan should distinguish three tasks:
- Search: look for conductive and nonconductive evidence throughout the defined area.
- Trace and identify: follow a specific facility from known points and reconcile it with the owner and records.
- Clear a critical conflict: obtain the position and identity needed for the planned excavation, bore, or design decision.
Build the evidence in layers:
- Work definition and 811 communication. Define the limits, method, depth, schedule, and affected work fronts; obtain and review responses.
- Records and site walk. Identify expected networks, private facilities, access points, recent construction, and inconsistencies.
- Active and passive EM. Trace known conductive paths selectively and run a separate search for additional conductors.
- GPR or another suitable geophysical method. Add evidence where EM cannot address material or where route coverage remains uncertain.
- Survey and documentation. Preserve coordinates, methods, dates, confidence, limitations, and unresolved conflicts.
- Selective exposure. Resolve exact identity and elevation at crossings whose consequence or geometry cannot tolerate the remaining uncertainty.
- Controlled excavation. Follow the applicable tolerance-zone and safe-excavation requirements, owner conditions, and field observations.
CGA 22.0 describes a general tolerance-zone best practice as the facility width plus 18 in. on either side of the outside edge. It explicitly states that this does not preempt state or provincial requirements specifying a larger zone. Treat 18 in. as CGA guidance, not as a universal legal distance.
Within the applicable zone, CGA discusses reasonable-care methods such as potholing, hand digging when practical, soft digging, vacuum excavation, and other approved methods. The permitted sequence depends on current jurisdiction, owner requirements, geology, surface, and employer procedure.
A release gate should name the evidence required. “Located” is not enough. A critical bore crossing might require all operator responses, a reproducible EM trace, a documented GPR anomaly review, and surveyed exposure of the controlling facility before the final profile is approved.
Stop work when the evidence changes
A good prevention plan includes stop conditions before production starts. Stop, protect the area, and escalate under the applicable process when:
- an expected facility has no mark or positive response;
- an unmarked line, tracer, structure, or anomaly appears;
- the exposed utility does not match the marked route, identity, or expected depth;
- peak/null, current, depth, or frequency behavior becomes inconsistent;
- GPR and EM evidence conflict at a consequential crossing;
- marks are missing, moved, obscured, or outside their valid period;
- the excavation limits, method, depth, or alignment change;
- ground movement, fluid return, or other construction behavior suggests an unknown path or facility;
- any contact, scrape, dent, coating damage, or suspected utility damage occurs.
PHMSA’s Part 196 requirements, within their scope, include using the available one-call system, waiting for pipeline marking, taking practicable steps with proper regard for the marks, using one-call again as necessary, and promptly reporting pipeline damage. State enforcement and additional requirements vary.
The goal is not to produce more paint. It is to prevent a weak inference from becoming an excavation decision. Pre-excavation locating works when its physical method matches the target, its results are checked against independent evidence, and the project stops at unresolved conflicts. It fails when a ticket, display, or mark is treated as certainty beyond what it actually proves.