29.8 Concluding remarks

1. A combination of dedicated QC processes involving georeference checks relative to the local Earth’s gravity, magnetic and rotation fields, multi-station tests, repeated measurements, and independent verification surveys are needed to ensure a survey reliability level consistent with the demands embedded in instrument performance/error models.

2. These QC procedures, together with a standardized 99.7% confidence level, should be incorporated into common practice for the design and verification of most wellbore survey operations.

3. Georeference checks, multi-station tests, and repeated measurements may protect against a wide range of potential gross errors and should be used to maximize the survey reliability whenever possible during a drilling process.

4. Georeference checks, multi-station tests, and repeated measurements with a single tool/BHA cannot detect gross errors in a few critical sources, like BHA sag and magnetic declination errors.

5. Multi-station tests can only be fully used after a long wellbore section has been drilled and several stations have been surveyed in significantly different attitudes.

6. Single-shot/rotation-shot georeference checks are usually weaker than multi-station checks, but are sometimes the only available QC options, for example, during kick-off operations and therefore, they should be used.

7. A data check, consisting of combined comparisons of inclinations, azimuths, and coordinates relative to an independent verification survey, is the most powerful QC test method available. Georeference checks usually have to be supplemented with data checks based on a comparison of inclinations, azimuths, and/or coordinates relative to independent verification surveys to ensure optimal survey reliability.

8. The value of checks relative to independent verification surveys depends on the degree of independence between the surveys. A verification survey is independent only as long as it is performed with a different survey method, a different survey tool, different running gear/BHA, and a different depth measurement system.