Definitions of various surveying terms:
Survey
A series of measurements made to determine the borehole trajectory. A survey comprises position (North, East and True Vertical Depth) and/or direction (along-hole depth, inclination and azimuth) from which position is calculated.
Survey tool accuracy
The potential of a survey tool to determine the actual borehole position. The borehole position uncertainty is the potential difference between the measured borehole position and actual borehole position.
Survey tool quality
he potential of a survey tool to measure the actual borehole position is modelled by its tool uncertainty model. This model represents the average survey accuracy of a fleet of tools. A highly accurate survey tool does not imply a high quality survey tool. On the other hand a low accuracy survey tool may indeed be a high quality survey tool.
Survey requirements
Survey requirements are defined as geometrical targets in space. They originate from geol ogical, BOC, anti-collision and local legislation requirements.
Independent surveys
Two surveys are defined independent if the physical principles exploited are independent, e.g. magnetic versus gyroscopic.
Definitive survey
A definitive survey is defined as the final and most accurate survey of a hole section. This survey is stored in the database.
Toolface:
The orientation of the bottom hole assembly in the cross-borehole plane is defined as toolface.
Dogleg:
The dogleg angle between consecutive survey stations is calculated as starting point for the wellpath calculations. The corresponding dogleg severity is the dogleg angle over a certain length of borehole.