How to Prepare for a Close Interval Survey

A successful close interval survey (CIS survey) starts well before you arrive in the field. Reviewing pipeline and cathodic protection system information, selecting the right survey settings, checking equipment, and verifying interruption and waveform performance helps identify potential issues before they delay fieldwork.

In a recent American Innovations Virtual Brew, Lon Duke shared practical considerations for preparing for a close interval survey, including equipment checks, interruption setup, stationing methods, and GPS configuration. This article highlights key takeaways from that discussion.

1. Review Pipeline and Cathodic Protection Information

Before beginning a close interval survey, gather information about the pipeline and survey area. Understanding existing conditions helps you anticipate changes in readings and identify areas that may require additional attention.

Important information to review includes:

• Pipe Coating: Identify coating types and transitions, such as older coal tar coating and newer fusion-bonded epoxy (FBE), which can affect on/off readings.
• Pipe Size and Age: Document the basic characteristics of the pipeline being surveyed.
• Soil Conditions: Review known soil conditions and resistivity, which may vary along the pipeline.
• Right-of-Way Conditions: Identify urban areas, difficult terrain, or other conditions that could affect the survey.
• Depth of Cover: Review expected depth of cover along the pipeline.
• Cathodic Protection System: Identify galvanic anodes, impressed current systems, rectifiers, and ground beds.
• Operational and Construction History: Review repairs, new construction, previous survey results, and other records that may help explain conditions encountered in the field.

Having this information ahead of time gives your survey team a better understanding of what to expect and where potential issues may occur.

2. Determine Your Close Interval Survey Settings

Establish your survey configuration before collecting data in the field. Several settings affect how readings are collected and how stationing is recorded. Determine:

• Will you perform an on or interrupted survey
• If you perform an interrupted survey, is it AC or DC?
• Will you use fixed increment or flag mode
• The appropriate survey increment
• Whether you require submeter GPS accuracy
• Field notes and observations to capture

For fixed increment surveys, increments such as 2.5 or 5 feet may be used. The PHMSA Mega Rule specifies increments of no more than 5 feet.

Also consider what information you need to record for pipe exposures, indications, depth of cover, and other field observations.

Your survey configuration can also affect the field data collection workflow. For example, Field Data Collector (FDC) supports both AC and DC current interruption surveys, however you cannot change the survey type after starting your survey. To change the type, you must start a new survey.

3. Prepare and Test Close Interval Survey Equipment

Equipment problems are easier to address before deployment than in the field. Before starting your CIS survey, inspect and test all equipment you will use, including:

• Field computers or tablets
• GPS equipment
• Digital voltmeter (DVM)
• Survey wires and cables
• Reference cells
• Batteries and chargers
• Survey software

Make sure equipment is fully charged, and test cables for continuity. Carrying spare cables helps to prevent a minor equipment issue from becoming a field delay.

In addition, confirm that your software is up to date. Updates may include bug fixes, new capabilities, and other changes that affect the survey workflow.

Check the Reference Cell

Before the survey, confirm that the reference cell is:

• Clean
• The correct type for the application
• Properly charged
• In good condition

For a copper/copper sulfate reference cell, check that the copper rod is clean and shiny and that the solution has adequate saturation. We recommend confirming that copper sulfate crystals remain at the bottom when the solution is shaken.

Check the DVM and Strain Relief

If your DVM is mounted to a tablet holder, make sure the cable is routed through the strain relief correctly. This reduces constant pulling on the DVM cable while working in the field.

4. Verify DVM Calibration

Check DVM calibration before heading into the field and make sure the appropriate calibration documentation is available.

American Innovations offers DVM calibration with a calibration certificate. The calibration process tests the instrument across a range of voltages and identifies when readings are outside of specification.

Addressing a calibration issue before deployment prevents unnecessary delays once the survey is underway.

5. Confirm Cathodic Protection Interruption Settings

For an interrupted close interval survey, verify your interruption setup before collecting data. Confirm:

• On/off cycle
• Cycle starting point
• Relay type
• Normally open or normally closed configuration
• Foreign lines and bonds
• AC mitigation systems

You may also need to coordinate with other pipeline operators. If other systems are bonded to or affecting the pipeline, you may need to interrupt them as well. Operators should coordinate their systems so the appropriate interrupters operate on the same cycle.

Consider AC Mitigation Systems

If AC mitigation systems are present in the area, they can affect readings collected during an interrupted survey. Capacitive discharge and recharge associated with PCRs can complicate the waveform, while zinc ribbon associated with AC mitigation can further affect the signal.

These conditions can make it difficult to obtain a true instant-off reading, making waveform verification especially important before beginning your survey.

6. Verify the Interruption Waveform

Before you begin the survey, use the DVM waveform display to confirm that the interruption works as expected.

Verify that:

• The on and off portions of the cycle are correctly identified
• The transition points are properly configured
• You are receiving the expected on and polarized potential or instant-off readings
• Interrupters are synchronized

Check Cycle Synchronization

Your survey settings need to match what the interrupters are actually doing.

For example, if the survey is configured to begin with the off portion of the cycle but the interrupters actually begin with on, readings can shift by one second. The important consideration is that the survey configuration and interrupter timing are aligned.

Know What Problematic Waveforms Look Like

Waveforms can also reveal conditions that may compromise survey readings. Examples include:

• AC Mitigation: Capacitive discharge and recharge can create a waveform that makes a true instant-off difficult to obtain.
• Out-of-Sync Interrupters: An interrupter operating on a different portion of the cycle can affect both on and off readings.
• DC Transit Systems: Electrical noise from a DC transit system can bleed onto the pipeline and appear in the collected readings.

Checking the waveform before collecting significant amounts of field data gives you an opportunity to resolve these issues early.

7. Choose a Stationing Method

The stationing method you use—how you track distance along the pipeline so each reading ties back to a known location—affects the accuracy and consistency of your CIS survey data.

Fixed Increment Mode

Fixed increment mode collects readings at a predetermined interval, such as 2.5 or 5 feet.

Pay attention to the physical path of the survey wire. If the pipeline changes direction and the wire is not anchored, the wire may take a shorter path than the pipeline. This can result in inaccurate stationing.

Anchoring the wire at a bend or other appropriate location helps maintain the intended distance and keep stationing aligned with the pipeline route.

Flag Mode

Flag mode uses predetermined distances marked along the survey route. For example, a survey team may place flags every 100 feet before beginning the survey.

Each flag establishes a known distance and resets stationing as the survey progresses. You can then use the readings collected between flags to determine the appropriate increments.

Using known distances throughout the survey helps maintain consistent and accurate stationing.

8. Configure GPS Sync or GPS Real Time

GPS configuration is another consideration when preparing for a close interval survey, particularly when working with longer interruption cycles.

• GPS sync collects an on/off pair during each cycle. For example, with a five-second cycle, a new on/off pair is available every five seconds. If you are walking faster than new pairs are generated, you may see duplicate readings between cycles.
• GPS real time works differently. Rather than waiting for a complete on/off pair, it captures the reading occurring when the survey triggers a measurement. This allows the survey to collect current on or off readings while moving through the survey area.

When using GPS real time, configure on and off delays to create windows around transitions during which readings are not recorded. This helps prevent capturing measurements during the transition between on and off states.

Avoid making these windows too narrow. If the window for capturing an off reading is too short, you may not collect enough valid off readings.

As a general consideration, GPS real time may be more useful with longer interruption cycles, while faster cycles work well with GPS sync.

Prepare Before You Deploy

A close interval survey involves more than collecting readings along a pipeline. The quality and efficiency of your cathodic protection survey depends on decisions made before the first measurement is taken.

Reviewing pipeline and CP system information, preparing your close interval survey equipment, verifying calibration, confirming interruption settings, and checking waveforms help you identify problems before they become field delays. Choosing the appropriate stationing and GPS configuration also help ensure that collected data accurately reflects the survey route and conditions.

For teams using digital tools for cathodic protection data collection and analysis, American Innovations’ Field Data Collector and Pipeline Compliance System (PCS) support survey workflows from the field to the office.

Want to see these concepts in action? Watch the full American Innovations Virtual Brew session for Lon Duke's walkthrough of close interval survey preparation, setup, and field considerations.