1. Towards an Enhanced Telluric Compensation Methodology
Chijioke Ukiwe & Shamus McDonnell
Hunter McDonnell Pipelines Services Inc.
Presented at AUCSC 2011 by Gord Parker, C.E.T., CP2
Edmonton, Alberta, Canada

2. This Work
This paper is a progress report of an ongoing R&D project to develop an enhanced Telluric and AC interference compensation methodology.
Research commissioned by Spectrum XLI
Partial funding from the Government of Canada
This presentation was previously delivered at the NACE Northern Area Western Conference.

3. Telluric Current
A telluric current (from Latin tellūs, "earth") is an electric current which moves underground or through the sea.
The currents are extremely low frequency and travel over large areas at or near the surface of Earth.
Telluric currents are also observed in the Earth's crust and mantle.

4. Telluric currents are primarily induced by changes in the outer part of the Earth's magnetic field, usually caused by interactions between the solar wind and the magnetosphere or solar radiation effects on the ionosphere.

5. Some Telluric Current Facts
The magnitude varies, but can be in the order of 100's to 1000's of Amperes.
Local variations in the conductivity of the Earth's crust affect the density of these currents as they follow the path of least resistance.  
This effect can be used to detect the presence of low-conductivity ore bodies beneath the crust in geological surveys.
Along with the currents induced in the Earth's crust, any man-made conducting objects on or beneath the Earth's surface will also have large currents induced in them.  
This can cause power grid systems to trip from the overload.
High sun-spot activities can buffet the Earth's magnetic field, causing fluctuations which in turn induces telluric currents on earth conductors, increasing the likelihood of such interruptions.

6. How do telluric currents affect pipelines?
Magnetic field
Earth’s geo-magnetic field
The sun emits energy & Particles
Voltage swings in CP systems
during close interval survey CIS
Induced electric field
The DSTL theory proposes that telluric currents are produced when an induced electric field, caused by changing magnetic field outside the earth, drives an electric charge along the pipeline, which causes a movement of charges from end and a build-up at the other.
Induced electric currents on pipelines
Source: space weather Canada

7. V Telluric currents: economic importance = Possibility of Corrosion
Voltage
Time V
Test Post
CSE
Difficulty to pipeline integrity assessment – if uncorrected could result in incorrect interpretation and improper adjustment of CP system
Pipeline
= Possibility of Corrosion
Pipe-to-soil potential measurement

8. Telluric on pipe-to-soil potentials: practical example
5 Seconds grid
2 Seconds CP Interruption Cycle
Second Telluric Current Cycle
Low frequency
Temporal
Voltage swing
= DV at t for
any pipeline
location
Telluric current frequency varies; cycle times of a few seconds to minutes.

9. AC Interference on pipe-to-soil potentials
Multiple Voltage swings superimposed
on the pipe-to-soil
Potential:
High frequency AC
50/60 Hz
Low Frequency telluric 1/40th Hz
4 second grid
2 second CP Interruption Cycle

10. Telluric characteristics: Temporal AND Regional effects
At the same moment in time
Telluric Magnitude
500mV
400mV
300mV
100mV
PIPELINE DISTANCE C D A B
Regional Effects on pipeline structures on telluric variation:
Valves, bends, rectifiers, anodes, insulating flanges
Telluric Potential Variation up to ~100 mV/Km at same moment

11. Telluric characteristics: Temporal effects
2. Space weather effects
At a given location
along the pipeline,
PSP changes with time
20+ mV/sec
> 100mV / cycle telluric effect has been recorded!

12. What is “Telluric Compensation”?
Voltage
Time
Time
Voltage
Exclusion of
“Noise” on PSP
Data during CIS

13. Basis for Telluric Compensation
Stationary PSP data (from stationary
data logger (SDL)) summed over
time yields “stable” average PSP
Comparable regional effects of
telluric on PSP within several kms
SDL PSP data can be used to correct
for any PSP variation in close
proximity
PSP
PSP

14. Average PSP?
Use statistics carefully, considering duration of temporal effects, and size of data pool.
Using data from a short time period, other than the actual survey period can result in error.
There are three kinds of lies: lies, damned lies, and statistics.
Benjamin Disraeli ( )

15. + - Current Field Practices for Telluric Compensation
Real-time telluric compensation; based on stationary reference electrode placed at start of survey.
Data Logger + - V V
SRE
Telluric Pipe-to-soil potential variations are corrected real-time

16. X Current Field Practices for Telluric Compensation
1. Real-Time Compensation: pit-falls
Unreliable representative average PSP
Difference in regional telluric effects on pipe can render correction useless X
High Telluric magnitude at A
Error in compensation increases away from Stationary electrode
SRE
Suppressed Telluric magnitude at B A B

17. X Current Field Practices for Telluric Compensation
Single Stationary Data Logger Compensation
Post Processing For Compensation
Reliable representative average PSP during survey
Difference in regional telluric effects on pipe can render correction useless X
Differences in regional
telluric currents at A & B
affect correction method A B
SRE + SDL
Suppression of Telluric magnitude at B

18. 3. Multiple SDL Telluric Correction
Current Field Practices for Telluric Compensation
3. Multiple SDL Telluric Correction A B C
Approximately linear telluric profile at any
Given time, “t” within pipe section, AB

19. Multiple SDL Telluric Correction: Underlying Assumptions
Based on Distributed Source Transmission Line
(DSTL) Theory
SDL placement spacing ~ 6 – 8 km
Electrical continuity of telluric currents =
uniformity of soil and pipeline characteristics
Statistical average PSP is calculated from pool of readings that span entire survey period.

20. Multiple SDL Telluric Correction
SDL-B
SDL-A

21. Multiple SDL Telluric Correction
Enhancements
Field work
Enhancements
Easier and faster
Data
Post-processing
3-4 SDLs
Waveform
Log =
Strategic
SDL
Positioning
And more reliable results!

22. Enhanced Telluric Compensation: unique advantage ~ capturing FULL telluric variation trend along pipeline
Telluric Magnitude
Pipeline A B C D

23. 4 SDL Telluric Compensation Method: example

24. 4 SDL Telluric Compensation Method: Practical safeguard against error!
PSP
Variation
(mV)
496.1
500.0
502.3
503.5
SDL Placement (miles)

25. Enhanced Telluric Compensation: Fast Interruption cycle
Telluric Variation can exceed
50 mV per second
times cycle length = significant!
Use of Fast Interruption
Cycle of about ~1 second = ON:OFF (~3:1) = <50mV per cycle (simple 1 potential / cycle correction)
Longer cycle makes it necessary for independent Telluric correction for ON and Instant Off PSP; different magnitude at start of cycle and end of cycle!

26. Multiple SDL Telluric Correction Enhancements:
(2) Post-processing
(a) Continuous waveform logging of the PSP
Need to ensure the same readings are selected from both survey data and SDL – if different technologies used, errors can result

27. Multiple SDL Telluric Correction Enhancements:
(2) Post-processing
(b) Simple moving average system on SDL data before telluric
compensation
Careful and thorough
Choice of each ON and
OFF PSP data before
Telluric correction using
Simple moving averages
Of telluric and high
Frequency AC interferences

28. Multiple SDL Telluric Correction Enhancements: Final Post-processed data

29. … Even further telluric enhancements
Recent advances in research holds promise for the use of telluric trends to forecast telluric magnitude over longer sections of pipelines = more strategic SDL placements and time of survey.
Ability to compensate for high frequency interference; 50/60 Hz AC using high frequency stationary data loggers.

30. Conclusions
The use of 3 ~ 4 Stationary data loggers to track telluric pattern
and ensure accurate compensation
Fast cycles are preferred to avoid under/over-compensation
to either ON or OFF pipe-to-soil potential data
Continuous waveform logging can enhance data accuracy
Advanced statistical approaches can enhance post-processing
and hence, overall pipeline integrity

31. The Authors are grateful to the National Research Council of Canada
Acknowledgement
The Authors are grateful to the
National Research Council of Canada
for financial support through an Industrial
Research Assistantship fund

32. Questions?