What is QL-B?
QL-B (Quality Level B) represents surface geophysics or "designating" to determine horizontal position of underground utilities. It's the primary workhorse method for subsurface utility engineering investigations, generating the majority of designations in most SUE programs.
Method: Surface Geophysics • Purpose: Horizontal Positioning • Standard: ASCE 38-22 • Also Known As: "Designating"
Primary QL-B Methods (Surface Geophysics)
Electromagnetic (EM) Pipe & Cable Locating
EM is the workhorse method for QL-B; many programs report it generates the majority of designations.
Active Locating (Preferred)
Direct Connection
Transmitter leads on the target, independent ground
Inductive Clamp
Around accessible conductor
Inductive (Broadcast) Mode
Transmitter placed on the surface above line
Best Practice: Select frequencies appropriate to target/setting; verify trace continuity and avoid bleed-over.
Passive Locating
Detect ambient 50/60 Hz power or VLF radio returns when active methods aren't possible, then corroborate.
When to Use
- • Active methods not possible
- • Detect ambient electrical signals
- • Always corroborate findings
Industry Note: EM is the workhorse method for QL-B; many programs report it generates the majority of designations.
Ground-Penetrating Radar (GPR)
GPR Technology
Equipment Options
- • Single-channel carts
- • Multi-channel arrays
- • 3D imaging systems
Target Detection
- • Images dielectric contrasts
- • Detects metallic utilities
- • Detects non-metallic utilities
Important Limitations
Depth and detectability depend on soil conductivity/moisture; clays/saturated soils attenuate signal. Use appropriate antenna frequency for target size/depth.
Sonde / Tracer Techniques for Non-Metallic Pipes
Non-Metallic Pipe Location
Sonde Methods
- • Insert sonde/transmitter into pipe/conduit
- • Use traceable CCTV push-cable
- • Track EM signal at surface
Tracer Wire
- • Energize existing tracer wire
- • Where present and accessible
- • Provides continuous trace
Supplemental/Conditional Geophysical Methods
Use when justified by site conditions or target characteristics
Metal Detectors
Very shallow, small ferrous/non-ferrous targets; noisy in rebar/metal-rich environments.
Magnetometers
Ferrous pipelines, tanks; effective to moderate depths.
Terrain Conductivity
EM conductivity mapping for broad anomalies; limited in congested corridors.
Electrical Resistivity
Search tool; rarely a tracing tool.
Acoustic Methods
Active/passive sonic or leak noise where pressurized flow or vibration exists.
Professional Standards
ASCE 38 includes an appendix of surface geophysical techniques; FHWA stresses that correct selection and competent interpretation are critical to achieving QL-B.
What Does Not Qualify by Itself
One-Call/811 Paint Marks Alone
One-Call/811 paint marks alone without field verification by acceptable geophysics do not meet QL-B. Agencies explicitly reject "Miss Utility" marks as the basis for QL-B unless verified.
Minimum Workflow to Claim QL-B (Practical)
Plan & Correlate
Review records (QL-D) and surveyed appurtenances (QL-C) to target the search.
Designate with Geophysics
Apply EM first where feasible; supplement with GPR and other methods as site conditions demand (rails/traction power, rebar, clay, congestion, etc.). Document method, instrument, frequency, setup.
Mark & Survey
Mark the approximate horizontal position on the ground and survey to project control (grid/chainage) for drafting.
Attribute & Qualify
In your utility base, tag features QL-B (ASCE 38-22), include method(s), date, operator, and limitations (e.g., "GPR ineffective in clay here").
Escalate to QL-A
Escalate to QL-A (test holes) at conflicts or where vertical/clearance certainty is required.
Tips for Rail/LRT Corridors (Common Pitfalls)
EM Interference
EM interference from traction power/return rails → favor direct connection/appropriate frequencies; validate with second method.
GPR Challenges
Ballast and metal clutter can degrade GPR; consider multi-channel GPR or rely more on EM/sonde for target tracing, then confirm with selective QL-A.
Why Invest in Proper QL-B?
Proven Return on Investment
Documented studies show strong benefit–cost for doing QL-B/QL-A versus relying on QL-C/D alone.
Reduced Utility Conflicts
Fewer Change Orders
Lower Project Risk
Ready to Implement Professional QL-B Standards?
Contact us to learn how our Visual Ground Disturbance System can help you document and manage QL-B investigations with proper attribution and workflow management.