Technical Practice Guide

Close-Interval Surveys: Field Method, Data Quality, Interpretation, and Common Errors

A close-interval survey, or CIS, is a detailed pipe-to-soil potential survey performed at closely spaced intervals along a pipeline route to evaluate cathodic protection conditions between permanent test stations.

Overview

A close-interval survey is a detailed field survey used to evaluate cathodic protection along a pipeline at closely spaced locations instead of only at permanent test stations.

A CIS provides a structure-to-electrolyte potential profile along the pipeline route.

Close-interval surveys can identify localized low-potential areas that may not be visible from widely spaced test station readings.

CIS data is commonly used to evaluate CP effectiveness, identify potential dips, support coating condition investigations, evaluate current distribution, and support troubleshooting.

ON CIS data may include IR drop. Interrupted CIS data can provide instant-off values intended to better represent polarized potential.

Core concept:

A test station survey tells you what is happening at selected access points. A close-interval survey helps show how potentials change between those points.

Technical Basis

A close-interval survey measures structure-to-electrolyte potential along a pipeline route at close spacing.

The pipeline is electrically connected through a test lead, trailing wire, or other approved connection method. A reference electrode is placed on the soil surface over or near the pipeline at frequent intervals. The measured value becomes part of a potential profile along the route.

  • ON CIS potentials are measured with CP current applied and may include IR drop.
  • Instant-off CIS potentials are measured immediately after CP current interruption and are intended to better represent polarized potential.
  • If multiple current sources influence the pipeline, interrupters should be synchronized where interruption is required.
  • Poor synchronization, poor reference electrode contact, or incorrect alignment with the pipeline route can make the profile misleading.
  • Potential changes may indicate changes in CP current distribution, coating condition, soil condition, geometry, or interference.

A CIS is not simply “more readings.” It is a controlled method for observing how the structure responds along the route.

When Close-Interval Surveys Are Used

  • Evaluating CP effectiveness between permanent test stations
  • Investigating low or inconsistent pipe-to-soil potentials
  • Identifying potential dips along a pipeline route
  • Supporting coating condition investigations
  • Evaluating the effect of rectifier adjustments
  • Assessing current distribution from an impressed current CP system
  • Investigating suspected interference or foreign current influence
  • Evaluating pipelines near casings, bonds, isolations, and crossings
  • Documenting CP performance for compliance or engineering review
  • Supporting repair prioritization or additional testing plans

Equipment Typically Used

Equipment Purpose
High-impedance voltmeter or CIS data logger Measures and records pipe-to-soil potentials along the route.
Copper-copper sulfate reference electrode Provides a stable reference potential for soil measurements.
Test station or pipeline connection Provides electrical connection to the pipeline.
Trailing wire or approved connection method Maintains electrical connection during route walking where applicable.
Current interrupters Cycle CP current sources ON and OFF for instant-off measurements.
GPS, stationing wheel, or route stationing method Documents where each reading was obtained.
Field notes or survey software Records route conditions, access issues, crossings, and anomalies.
Rectifier access or RMU data Confirms current source operation and interruption status where applicable.

General Field Method

  1. Review the survey limits, pipeline route, test stations, CP current sources, bonds, crossings, and known access constraints.
  2. Confirm the pipeline connection point and verify electrical continuity where required.
  3. Confirm reference electrode condition and meter or data logger operation.
  4. Establish the survey interval and stationing method.
  5. Place the reference electrode at each survey location over or near the pipeline route as required by the survey objective.
  6. Record ON potentials with CP current applied, if ON data is part of the survey.
  7. Interrupt applicable CP current sources where instant-off data is required.
  8. Verify interrupter timing and synchronization before relying on instant-off data.
  9. Record instant-off potentials immediately after interruption at each survey point.
  10. Document route conditions, pavement, dry soil, standing water, crossings, casings, bonds, foreign structures, and access limitations.
  11. Check questionable readings before leaving the area.
  12. Confirm interrupted current sources are restored to normal operation after the survey.

Exact procedures vary by owner specification, structure type, survey objective, and equipment configuration.

Valid Data Conditions

  • Correct pipeline connection
  • Known structure identity and electrical continuity
  • Proper reference electrode condition
  • Consistent reference electrode placement
  • Known survey interval and stationing
  • Stable meter or data logger operation
  • Known current condition for each reading: ON, instant-off, or other documented condition
  • Proper current interruption where instant-off data is required
  • Synchronized interruption of significant current sources where applicable
  • Documentation of route conditions and access limitations
  • Awareness of nearby foreign structures, bonds, casings, and interference sources
  • Verification of unusual dips, spikes, or unstable readings

A CIS profile can be distorted by incorrect current interruption.

A potential dip may indicate a coating holiday, current shielding, interference, poor contact, route misalignment, or another local condition.

Field notes are part of the data. The numbers alone may not be enough.

Common Errors and Misinterpretations

Error Why It Matters
Treating ON CIS data as polarized potential data ON readings may include IR drop and can overstate protection.
Using unsynchronized interrupters Instant-off readings may not represent the intended current-off condition.
Placing the reference electrode inconsistently Can create artificial dips, spikes, or trends in the profile.
Failing to document route conditions Makes later interpretation of anomalies difficult.
Assuming every potential dip is a coating holiday Dips can also result from interference, geometry, soil changes, poor contact, or shielding.
Surveying the wrong alignment Readings may not represent the intended pipeline.
Ignoring bonds, casings, and foreign structures These can affect current distribution and measured potentials.
Failing to verify questionable readings Bad contact, bad location, or equipment issues can be mistaken for CP problems.
Not restoring interrupted current sources Can leave CP systems out of service after the survey.

Interpretation

CIS data is interpreted as a potential profile, not only as individual readings.

  • A single test station value may miss localized low-potential areas.
  • CIS helps identify where potentials change along the route.
  • Instant-off CIS values may be compared to the −850 mVCSE polarized potential criterion where valid.
  • ON CIS values can help show current influence but may include IR drop.
  • Potential dips should be interpreted with field notes and route features.
  • CIS data may identify areas needing additional testing, coating evaluation, excavation, interference testing, or CP adjustment.
  • CIS does not replace engineering judgment or data validation.
Observation General Interpretation
Instant-off profile consistently more electro-negative than −850 mVCSE Surveyed locations may satisfy the polarized potential criterion, assuming valid interruption and data quality.
Localized instant-off dip less electro-negative than −850 mVCSE May indicate a local area requiring further evaluation.
ON profile much more electro-negative than instant-off profile May indicate significant IR drop in the ON readings.
Sharp potential changes near crossings, casings, or bonds May reflect current distribution changes, interference, or electrical continuity effects.
Unstable values in dry soil, pavement, or poor contact areas Verify reference electrode contact and document limitations before interpreting.

Worked Example

A short interrupted CIS segment records the following values:

Station ON Potential Instant-Off Potential Field Note
10+00 −1,080 mVCSE −910 mVCSE Normal soil contact
10+25 −1,040 mVCSE −880 mVCSE Normal soil contact
10+50 −970 mVCSE −815 mVCSE Near road crossing
10+75 −1,020 mVCSE −865 mVCSE Normal soil contact

The ON values are all more electro-negative than −850 mVCSE, but ON readings may include IR drop.

The instant-off values at 10+00, 10+25, and 10+75 satisfied the −850 mVCSE polarized potential criterion.

The instant-off value at 10+50 failed to satisfy the −850 mVCSE polarized potential criterion.

The localized dip near the road crossing should be verified and evaluated with route features, coating condition, electrical continuity, shielding, casing/interference conditions, and historical data.

Correct conclusion:

The correct conclusion is not that the entire pipeline failed. The correct conclusion is that the CIS identified a localized area requiring additional evaluation.

Practice Questions

Question 1

What does a close-interval survey provide that widely spaced test station readings may not?

  1. Coating thickness measurements
  2. Rectifier efficiency calculations
  3. A detailed potential profile between test stations
  4. Direct corrosion-rate measurements

Answer: C

Question 2

Why can ON CIS readings be misleading?

  1. Because ON readings may include IR drop
  2. Because ON readings always indicate overprotection
  3. Because ON readings are always unstable
  4. Because ON readings cannot be recorded electronically

Answer: A

Question 3

What does synchronized interruption help accomplish?

  1. Improved coating adhesion
  2. Reduced pipeline resistance
  3. Automatic polarization correction
  4. More reliable instant-off conditions when multiple current sources exist

Answer: D

Question 4

What is the best interpretation of a localized instant-off dip below the −850 mVCSE criterion?

  1. The entire pipeline automatically fails
  2. The location may require additional evaluation
  3. The rectifier has completely failed
  4. The reference electrode is definitely defective

Answer: B

Question 5

Why are field notes important during a CIS?

  1. They replace potential measurements
  2. They automatically determine coating condition
  3. They help explain anomalies, access issues, and route conditions affecting interpretation
  4. They eliminate the need for synchronized interruption

Answer: C

Related Pages

ACVG Survey Applied AC signal coating defect detection and interpretation. DCVG Survey Coating defect detection, voltage gradients, and field interpretation. Current Requirement Testing Temporary current application, current distribution, and CP design interpretation. Interference Testing Foreign current influence, bonds, stray current, and interpretation. Casing Testing Carrier pipe isolation, casing shorts, and electrolytic contact interpretation. Electrical Isolation Testing Isolation joints, continuity checks, and CP current distribution. Current Interruption Rectifier interruption, synchronization, and instant-off timing. Instant-Off Potential Interrupted structure-to-electrolyte potential measurements. Pipe-to-Soil Potential Surveys Field potential survey methods and interpretation. Depolarization Testing 100 mV polarization criterion evaluation. Reference Electrode Use Reference electrode placement and data quality. Rectifier Inspection Rectifier output evaluation and interruption considerations.