Gyro surveys will be taken when the casing has been run if it was not possible to perform an MMS in the open hole. There are two distinct groups of slim, surface reading, gyro surveying tools:
- those that require orientation to a foresight reference bearing on the surface (‘surface referenced’) and,
- those that seek True North.
Gyro Single Shots (GSS) surveys shall be used in place of magnetic based instruments where magnetic interference is suspected.
A GMS survey shall be taken whenever MMS surveys have shown uncorrectable magnetic interference over a significant part of the survey interval. The interval between stations shall not exceed 100 ft.
Foresight Reference
Prior to conducting any surface referenced gyro based survey, the Drilling Engineer shall provide telexed/faxed confirmation of the foresight reference to be used.
Surface Referenced Gyros
Azimuth Referencing Requirement
For land rigs, the surface co-ordinates and the elevation of the rig floor are used for the surface referenced Gyros.
Reporting Requirements
The survey report listing gyro stations and calculated co-ordinates should include
- the instrument serial number
- downhole gyro drift rates (if any)
- drift closure rate
- in-run/out-run differences
- foresight information including method of calculation
- type of wireline used
- any relevant Survey Engineer’s comments.
North Seeking Gyros
Applications
Due to its inherently very low drift rates, this type of gyro may be used at all depths and is only limited by its temperature range.
Azimuth Reference Requirements
A foresight reference bearing for this type of surface read-out gyro is not required. The gyro tool references either the individual survey stations or the entire survey to True North by sensing the Earth’s rotation. Therefore, the Grid Convergence between Grid North and True North must be applied to the measured azimuth readings by the survey engineer.
Survey Reporting Requirements
Along with the survey results of inclination and azimuth and the calculated co-ordinates, the Survey Engineer shall also present a report detailing:
- the instrument serial number
- centralisation scheme
- in-run/out-run differences of inclinations and azimuth
- shop calibration dates
- calibration check data
- quality control information (also temperature range and whether continuously increasing)
- type of wireline used
- depth counter slippage
- any relevant survey engineer’s remarks.