Inclinometer Mikkelsen Inclinometer Errors

Inclinometer Data & Recognition of System Errors By

P. Erik Mikkelsen, P.E. Consulting Engineer GeoMetron Inc PS Bellevue, WA USA (Graphics: Courtesy of Rick Monroe, Durham GeoGeo-Slope Indicator) Univ. of Florida Short Course, March 19, 2007

Installed Casing

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Inclinometer Casing

The Function of Casing • Provides sub-surface access for measurements. • Maintains orientation of probe. • Provides flat surface for repeatable measurements. •Grooves compatible with probe wheels • Deforms with the surrounding ground.

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Glue & Rivet Couplings Advantages • Very strong. High tolerance for twisting and bending. • Easy to extend or repair. Disadvantages • Needs glue, rivets, and tape. • 2.57 / 70 mm diameter only. Other • Simple settlement couplings. EPIC Casing

Shear-Wire Couplings Advantages • Rapid assembly. • Can be disassembled. • Very strong. • Self-sealing. • Good in cold weather. Disadvantage Oversize couplings. Other: 3 diameters. CPI Casing

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Inclinometer Probe, Cable, Readout

From Tilt to Lateral Deviation L x sine θ

L θ

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Surveying the Casing

Plotting Lateral Deviation

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Deviation vs Displacement Deviation: Casing profile Displacement: Casing movement

Deviation

Displacement

Displacement Graphs

Displacement vs Depth

Displacement vs Time

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Orientation of Probe, Casing, & Plot A-

A+

A180

A0

A–

A+

Reading Units

English Units

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Reading Units

English Units

Example: Reading Units to Deviation

Deviation = L x

A 20000

= 24 in x 349 20000

L= 24 in

θ

= 0.4188 in English Units

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Difference & Probe Bias A0 at 1° Ideal: 349

A180 at 1° Ideal: -349

Ideal Difference = 349 – (349) = 698 Actual Difference = 359 – (-339) = (349 +10) – (-349 +10) = 698 Actual: 359 Actual: -339 Bias: +10 Bias: +10

English Units

Difference vs Checksums A0 at 1°

A180 at 1°

Difference = A0 – A180 = 698 Checksum = A0 + A180 = 359 + (-339) = 20 Actual: 359 Ideal: 349 Bias: +10

Actual: -339 Ideal: -349 Bias: +10 English Units

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Displacement Displacement = Δ Deviation = Deviation n – Deviation 0 = L x Δ Algebraic Difference 40000

English Units

Graphing Inclinometer Data Presentation Graphs

Diagnostic Graphs

• Displacement vs Depth

• Checksums

• Displacement vs Time

• Casing Profile • Casing curvature

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Displacement vs Depth • Incremental • Cumulative

Incremental

Cumulative

Displacement vs Time

Movement at selected zone, plotted against time.

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Presentation Tip Keep your graphs simple and easy to understand.

Checksum Graphs

Checksum

Difference Checksum

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Diagnostic Graphs • Casing Curvature • Casing Profile

Curvature

Profile

Graphing Tip Choose scales that reveal appropriate information and hide noise.

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Accuracy Total Error ± 0.3 inch per 100 ft. Random Error ± 0.05 inch per 100 ft. Systematic Error ± 0.25 inch per 100 ft. Limit of Precision ± 0.01 inch per reading. ± 0.05 inch per 100 ft. English Units

Sources of Error Readout Cable Bias shift* Depth control* Set up Rotation* Poor storage Operation Connectors Aging

Probe

Wheels

Casing Inclination* Curvature* Backfill Grooves Couplings

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Bias-Shift Error

Example 1

Example 2

Bias Shift and Checksums

Displacement

Checksums

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Bias-Shift Analysis in DMM

-6.625 –

-1.3

Bias-Shift Analysis in DMM Generate Report 1. Display graph. 2. Check box to Include Bias shift. 3. Choose zone. 4. Click Report.

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Rotation Error & Cross-Axis Profile Example 1

Displacement A

Casing Profile B

Calculating a Correction Factor Factor = Displacement Error A Profile B

35

4500

Displacement Error A = 35 mm Profile B = 4500 mm Factor = 35 4500 = 0.0078

Displacement A

Profile B

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Calculating a Correction Factor 35 mm

Correction = Displacement Error A Profile B =

35 mm 4500 mm

= 0.0078

4

0 50

mm

A0

B0

Correction value is the sine of the angle of rotation and is normally < 0.0175 ( sin 1° ).

Probe Position: High or Low? Probe is lower if …

Probe is higher if …

• Casing was cut off. • Use of pulley halted. • Casing settled. • Cable lengthened.

• Casing was added. • Use of pulley started. • Cable twisted. • Cable shortened.

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Depth Error

Curvature vs Displacement Example 1

Curvature

Displacement

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Curvature vs Displacement Example 2

Curvature

Displacement

Curvature

Displacement

Curvature vs Displacement Example 2 Scale changed

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Depth Error in S-Shaped Casing Low High

Curvature

Displacement

Curvature

Displacement

Depth Error in J-Shaped Casing Low High

Curvature

Displacement

Curvature

Displacement

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Depth Correction in DMM

Potential Systematic Error

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