Page | 175 6. In-Field Referencing ................................................................................................................................................ 38 6.1 Measuring Crustal Anomalies using In-field Referencing ............................................................................... 38 6.1.1 IFR Survey Maps ............................................................................................................................................. 39 6.2 Interpolated In-field Referencing.................................................................................................................... 40 7. Survey Calculation Methods ................................................................................................................................... 42 7.1 Examples of current methods ............................................................................................................................... 42 8. Survey Frequency.................................................................................................................................................... 46 8.1 Determining TVD................................................................................................................................................... 46 9. Gyro Surveying ........................................................................................................................................................ 48 9.1 Background and History of Gyros ......................................................................................................................... 48 What is a Gyro? ....................................................................................................................................................... 48 9.2 Oilfield Applications – A Brief History ................................................................................................................... 49 9.3 Chronology of gyro development ......................................................................................................................... 49 9.4 Improving Performance and Service Capability.................................................................................................... 50 9.5 The Gyro Survey Process....................................................................................................................................... 50 9.5.1 Surface Reference Orientation ...................................................................................................................... 50 9.5.2 Gyro Drift – Precession Correction ................................................................................................................ 51 9.5.3 True Centre Correction (or Offset Centre Correction)................................................................................... 51 9.5.4 Tool Centralization ......................................................................................................................................... 52 9.6 ERD and Horizontal Transit ................................................................................................................................... 52 9.7 Ring Laser Gyro ..................................................................................................................................................... 53 9.8 Fibre Optic Gyro .................................................................................................................................................... 54 9.9 Vibrating Structure Gyroscope.............................................................................................................................. 54 Coriolis Effect .......................................................................................................................................................... 54 9.10 Coriolis Vibratory Gyros (CVG) ............................................................................................................................ 54 9.10.1 Micro electro mechanical System (MEMS) – Tuning Fork Gyro .................................................................. 55 9.10.2 Hemispherical Resonator Gyro (HRG).......................................................................................................... 55 10. Basic Gyro Theory ............................................................................................................................................... 56 10.1 Fundamental principles....................................................................................................................................... 56 10.1.1 Gyroscopic inertia ........................................................................................................................................ 56 10.1.2 Angular momentum ..................................................................................................................................... 57 10.1.3 Precession .................................................................................................................................................... 58 10.1.4 The application of the precession principle ................................................................................................. 58 10.2 Gyrodata Rate Gyro ............................................................................................................................................ 59 10.3 E h’s of o on....................................................................................................................................... 60 10.4 How to Measure Azimuth ................................................................................................................................... 61 10.5 Sensor Errors ....................................................................................................................................................... 62 10.6 Survey Tool Calibration ....................................................................................................................................... 64 10.7 Survey Tool Operating Modes ............................................................................................................................ 64 10.7.1 Gyro-compassing mode ............................................................................................................................... 64 10.7.2 Continuous mode......................................................................................................................................... 65 10.7.3 Summary ...................................................................................................................................................... 65 11. When to run Gyros.............................................................................................................................................. 66
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