Inclinometer Data & Recognition of System ErrorsBy

P. Erik Mikkelsen, P.E.Consulting Engineer GeoMetron Inc PS Bellevue, WA USA (Graphics: Courtesy of Rick Monroe, Durham Geo-Slope Indicator) GeoUniv. 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 CouplingsAdvantages 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 CouplingsAdvantages 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 DeviationL x sine

L

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Surveying the Casing

Plotting Lateral Deviation

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Deviation vs DisplacementDeviation: Casing profile Displacement: Casing movement

Deviation

Displacement

Displacement Graphs

Displacement vs Depth

Displacement vs Time

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Orientation of Probe, Casing, & PlotAA+

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 20000L= 24 in

= 24 in x 349 20000 = 0.4188 in

English Units

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Difference & Probe BiasA0 at 1 Ideal: 349 A180 at 1 Ideal: -349

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

Difference vs ChecksumsA0 at 1 A180 at 1

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

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DisplacementDisplacement = Deviation = Deviation n Deviation 0 = L x Algebraic Difference 40000

English Units

Graphing Inclinometer DataPresentation Graphs Displacement vs Depth Displacement vs Time

Diagnostic Graphs Checksums 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 TipKeep 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 TipChoose scales that reveal appropriate information and hide noise.

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AccuracyTotal 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 ErrorProbe Readout Cable Bias shift* Depth control* Set up Rotation* Poor storage Operation Connectors AgingWheels

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 DMMGenerate 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 ProfileExample 1

Displacement A

Casing Profile B

Calculating a Correction FactorFactor = Displacement Error A Profile B Displacement Error A = 35 mm Profile B = 4500 mm Factor = 35 4500 = 0.007835 4500

Displacement A

Profile B

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Calculating a Correction FactorCorrection = Displacement Error A Profile B = 35 mm 4500 mmB04 0 5035 mm

mm

A0

= 0.0078

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

Probe is higher if Casing was added. Use of pulley started. Cable twisted. Cable shortened.

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Depth Error

Curvature vs DisplacementExample 1

Curvature

Displacement

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Curvature vs DisplacementExample 2

Curvature

Displacement

Curvature vs DisplacementExample 2 Scale changed

Curvature

Displacement

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Depth Error in S-Shaped CasingLow High

Curvature

Displacement

Curvature

Displacement

Depth Error in J-Shaped CasingLow High

Curvature

Displacement

Curvature

Displacement

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Depth Correction in DMM

Potential Systematic Error

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