Definition of Surveying Terms from accelerometer to wireline
Accelerometer
A device for measuring the acceleration of a body in a particular direction. Accelerometers are used in solid state magnetic tools, surface read out gyros, North Seeking Gyro tools and Inertial Navigation systems to sense changes of direction of the tool with respect to the Earth's gravity vector.
Air gap
The distance between the bottom of a drilling barge or platform and the Mean Sea Level.
Along-hole Depth (AHD)
The distance measured along the actual course of the borehole from the surface reference point to the point of interest, e.g. as measured by drill pipe or wireline. (Also known as measured depth).
Along-hole position uncertainty
The position uncertainty for a selected point in an axial direction of the borehole. Wireline depth errors contribute to the along-hole position uncertainty.
Angle unit
A mechanical instrument, used in film-type survey systems, which measures inclination over a specified range, e.g. 0-10 degrees. For magnetic single and multi-shots the compass is an integrated part of the angle unit.
Austenitic steel
A type of non-magnetic stainless steel used for non-magnetic drill collars (NMDC's), stabilisers, instrument components and spacer bars.
Azimuth
Azimuth is the angle between the horizontal component of the borehole direction at a particular point and the direction of North. The angle should be expressed in a 0-360 degree system measured clockwise from North. The angle may refer to either Magnetic, True (geographic) or Grid North. Whichever is referred to must always be clearly indicated. (Outside the Group also known as bearing or direction).
Baffle plate
See landing ring.
Bearing
Non standard term used for azimuth.
Below Derrick Floor (BDF)
Depth (either AHD or TVD) measured below the derrick floor. This depth is the same as below rotary table (BRT) only if the rotary table is flush with the derrick floor.
Below Kelly Bushing (BKB)
Depth (either AHD or TVD) measured below the kelly bushing (See also BRT and BDF).
Below Rotary Table (BRT)
Depth (either AHD or TVD) measured below the rotary table. (See also BDF and BKB).
Benchmark
A checkpoint just below the last casing shoe where a survey is taken to verify correct operation when tripping MWD tools.
Bent sub
A sub used above a mud motor to give a non-straight bottom hole assembly. One of the connection threads is cut at an angle to the axis of the sub. (See also Universal Bottom Hole Orientation sub.
British Geological Survey
A United Kingdom government department that, amongst other things, provides a computer model of the Earth's magnetic field (See also British Geological Survey Global Geomagnetic Model).
British Geological Survey global geomagnetic model
A computer program available from the British Geological Survey for calculating declination and other magnetic parameters of the Earth's magnetic field. This model is updated every year.
Build-up rate
The rate at which the inclination increases in a build-up section expressed in degrees per 10 m (°/10 m) AHD or in degrees per 100 ft (°/100 ft) AHD.
Build-up section
A section of borehole where the inclination is continuously increasing, i.e. building up.
Cage
Mechanical alignment of the gyro spin axis with a reference direction on the tool case and hence the instrument barrel.
Calibration
The determination of fixed reference points on the scale of an instrument by comparison with a known standard and the subsequent subdivision or graduation of the scale to enable measurements in definite units to be made with it.
Central Meridian (CM)
The central line of longitude in a map projection. In the Universal Transverse Mercator (UTM) grid system a square grid is aligned along the central meridian. (See also Transverse Mercator projection, TM).
Chart Datum (CD)
A permanent datum from which measurements of vertical depth are referenced.
Circular arc
See Minimum Curvature
Closure
The difference (in degrees) between the total calculated drift and the total observed drift of a conventional gyroscopic multi-shot and G2 survey. Outside the Group, also meaning the horizontal displacement (HD) from the wellhead to the last station of a survey.
Comparison plot
Plots of azimuth and inclination for different types of surveys against along-hole depth (e.g. comparing magnetic and gyro surveys or gyro in/outruns) used to help evaluate survey quality and decide upon the best representation of the well path.
Compass
An instrument measuring the angle of direction (azimuth) from a known reference direction.
Continuous survey mode
Where a tool takes a survey while the tool is moving, without the need to stop and make measurements at discrete survey stations.
Cubic splines
A mathematical method for converting values of horizontal displacements into along-hole depths, inclinations and azimuths. The method is employed when converting survey data obtained by an Inertial Navigation System, e.g. FINDS.
Datum
A permanent common depth reference defined for a specific area. (See also chart datum).
Departure
Non-standard term used for East.
Declination (D) formula: ATN = AMN+ D
The angular difference in azimuth readings of Magnetic North and True North. The declination varies with time and place. By definition, the declination is positive when moving clockwise from True North to Magnetic North, and negative when moving anti-clockwise from True North to Magnetic North. Note that this standard definition is not used throughout the Group. (Also known as variation in the marine world).
Deviated drilling
A method of drilling whereby a well is drilled not vertically but to a point some distance horizontal from the start of the borehole. (See Directional Drilling.)
Deviated hole
A well bore that is off the vertical, either by design or accident.
Deviation
See Inclination.
Deviation direction
See Azimuth.
Dip (See dip angle and formation dip).
Dip angle The angle between a tangent to the Earth's magnetic field vector at a particular location and the horizontal.
Direction
See Azimuth.
Directional drilling
A method of drilling whereby a well is drilled along a predetermined trajectory to intersect a designated subsurface target. (See Deviated Drilling.) Examples are multiple wells from offshore platforms or relief wells drilled to intersect a blowout well.
Displacement
See horizontal displacement, East and North.
Dogleg (DL)
The total angular change between the tangent to the borehole at one point and the tangent to the borehole at another point. A dogleg may result from changes of inclination and/or azimuth.
Dogleg Severity (DLS)
The rate of total angular change of the borehole direction between two consecutive borehole survey stations, expressed in degrees per 10 m (o/10 m) AHD or in degrees per 100 ft (°/100 ft) AHD.
Downhole
A term to describe tools, equipment and instruments that are used inside the well.
Drift
The movement of the gyro spin axis in the horizontal plane. A non-Group standard term used for inclination.
Drift angle
See inclination.
Drift direction
See Azimuth.
Drift check
A measurement of the rate at which a gyro is drifting. Measurements are used to improve obtained survey data.
Driller's depth
The along-hole depth (AHD) as measured by the drillpipe and recorded on the pipe tally.
Drop-off rate
The rate of decrease of inclination in a drop-off section, expressed in degrees per 10 m (°/10 m) AHD or degrees per 100 ft (°/100 ft) AHD.
Drop-off section
A section of borehole where the inclination is continuously decreasing, i.e. dropping off.
East
The horizontal displacement, from a vertical line through the wellhead to a selected point in the well, in an East-West direction. East is defined positive and West negative. It is sometimes labelled 'X' on plots and listings. (Outside the Group also known as departure).
Ellipsoid of position uncertainty
An ellipsoid-shaped space wherein a specified point of the wellbore is likely to be found. The surface of the ellipsoid represents the maximal expected position error. (Also see envelope of position uncertainty).
Envelope of position uncertainty
An envelope in space wherein the wellbore is likely to be found. The envelope is produced by connecting the outer limits of the ellipsoids of uncertainty for an infinite number of stations along the wellbore. The surface of the envelope represents the expected position uncertainty.
Foresight
A fixed mark of known position and orientation to which 'free' gyroscopes are aligned in order to obtain a reference for azimuth readings.
Formation dip
The angle between a geological stratum and the horizontal.
'Free' Gyroscope
A gyro where the spinning wheel is free inside gimbals so that it must be oriented relative to a known direction (foresight) to provide an azimuth reference. This is opposed to a North Seeking Gyro where the gyro measures the direction of North.
Geographic North
See True North.
Geosteering
Directional drilling and steering using data from near bit petrophysical logging sensors. Geosteering has been primarily developed for horizontal wells and thin reservoirs, where it is necessary to remain in a particular area of a reservoir, e.g. away from an oil water contact etc.
Gimbal
A frame mounted between two objects, allowing independent rotation of both objects along two axes in the plane of the gimbal.
Gravity toolface
The angle between high-side and toolface. (Also known as high-side toolface).
Grid convergence (G)
Formula: ATN= AGN + D The angular correction converting azimuth readings of Grid North to True North. By definition, the grid convergence is positive when moving clockwise from True North to Grid North, and negative when moving anti-clockwise from True North to Grid North. Note that this standard definition is not used throughout the industry.
Grid correction
See grid convergence. Grid correction is the negative of grid convergence. (In other words Grid correction=-Grid convergence).
Grid North (GN)
Within a rectangular grid system, the direction which is parallel to the central meridian of longitude through the grid origin.
Gyroscope
Comprises a spinning mass mounted within a gimbal system. In the absence of friction and unbalance, the spinning mass would remain stationary in inertial space and ideally act as a portable reference direction. Note that a rate gyro measures the rate of rotation at right angles to its spin axis.
Hot-spot
See magnetic hot-spot.
High-side
The upward direction of a line perpendicular to the borehole axis and which lies in a vertical plane. Low-side is in opposite direction of high-side.
High-side toolface
The angle between high-side and toolface. (Also known as gravity toolface).
Horizontal Displacement (HD)
The horizontal distance from a vertical line through the wellhead to a selected point along the wellbore. It is often quoted as a distance with a direction (azimuth). (Not to be confused with projected horizontal displacement, PHD).
Horizontal position uncertainty
The projection of the ellipse of position uncertainty onto a horizontal plane. The dimensions of the ellipse are usually quoted as half-axis dimensions (i.e. Radii).
Inclination The angle (in degrees) between the tangent of the borehole at a selected point and the vertical axis through the same point. (Outside the Group also known as drift or deviation).
Inclinometer
See angle unit.
Inertial Navigation System (INS)
A system which measures components of acceleration. By integrating these components of acceleration twice, horizontal and vertical displacements can be calculated. The accelerometers can be mounted on a gyro-stabilised platform to cancel out the effects of tool rotation. For strapdown INS the rotation is measured and accounted for.
Induced magnetism
Magnetism exhibited by a component due to the magnetic field in which the component is situated. When the external magnetic field is removed, the induced magnetism also disappears.
Inter-cardinal error
See inter-gimbal error.
Inter-gimbal angle
The angle between the inner and outer gimbals of a gyroscope.
Inter-gimbal error
Due to geometric properties of the gimbal assembly, a difference in observed azimuth and actual azimuth will appear. (Also known as inter-cardinal error).
Jetting
Circulating on bottom with one nozzle removed from the bit and oriented in the desired direction of kick-off.
Kick-Off Point (KOP)
The depth or point in a hole at which a build-up section is started.
Landing ring
A ring installed between tool joints in a drill string to land a survey instrument on. Also known as a TOTCO ring or baffle plate.
Latitude
Lines of latitude are angular distances North or South of the Equator and parallel to it. Expressed in degrees North or South from the Equator to the point of interest. A non-Group standard term for North.
Lateral position uncertainty
The position uncertainty in the horizontal plane to the left or right about a selected point in the wellbore. Azimuth uncertainties contribute to the lateral position uncertainty.
Lead angle
A deliberate offset from the target azimuth when kicking off a well. The lead angle is chosen to account for expected bit walk.
Level rotor gyro
A type of 'free' gyroscope that has a sensor to maintain the spin axis horizontal and hence convert any tilt into drift. (See Tilt Scale Gyro).
Longitude
The set of co-ordinates defined by equally spaced circles through the poles of rotation of the Earth. Expressed in degrees East or West from the Greenwich meridian to the point of interest.
Low-side
The opposite direction of high-side. The downward direction of a line perpendicular to the borehole axis and which lies in a vertical plane. A free ball would roll along the low-side of the wellbore.
Magnetic azimuth correction
A calculation method for use with Solid State Magnetic Tools that uses knowledge of the Earth's Magnetic Field to correct the azimuth for errors caused by magnetic fields in the drillstring. A number of Survey Companies offer this service and Shell also have their own magnetic azimuth correction principle called SUCOP.
Magnetic convergence
The angular difference in azimuth between Magnetic North and Grid North. It is the combination of Declination and Grid Correction.
Magnetic declination
See Declination.
Magnetic hot-spot
A localised area in a non-magnetic material which can disturb measurements of the Earth's magnetic field. Usually due to inhomogeneous material or embedded magnetic material.
Magnetic North (MN)
The direction of the horizontal component of the Earth's magnetic field at a particular point on the Earth's surface. A compass will align itself in the direction of the field with the positive pole of the compass pointing to the Magnetic North.
Magnetic storm
Major irregular fluctuations in both direction and magnitude of the Earth's magnetic field.
Magnetic toolface
The angle between Magnetic North and toolface projected onto a horizontal plane.
Magnetometer
An instrument which measures the strength of a magnetic field in a particular direction.
Mean Sea Level (MSL)
The mean elevation of the sea level as locally measured over a monthly period.
Measured depth
See along-hole depth.
Measurements While Drilling (MWD)
Measuring and transmitting real time downhole information to surface, e.g. directional data, turbine speed, downhole WOB/torque or formation radioactivity/resistivity.
Minimum curvature (Circular Arc)
The mathematical method recommended in the Group to calculate the horizontal and vertical co-ordinates of the measured values of along-hole depth (AHD), inclination (I) and azimuth (A).
Misalignment error
The error caused in survey tool readings due to the physical misalignment between the survey tool axis and the borehole axis.
Monel Monel or K-monel
A non-magnetic alloy used for Non-Magnetic Drill Collars. It is a softer material than the more usual Stainless Steels used in non-magnetic components, but has better corrosion resistance in mixed salt mud. Sometimes used to mean any Non-Magnetic Drill Collar even if not made from monel.
Monel sensor
An activating device for magnetic single shot surveys. Senses non-magnetic material and activates the camera after a short pre-set time.
Motion sensor
An activating device for magnetic single shot surveys. Senses motion and will not activate the camera until a short pre-set time after motion ceases.
Mule shoe
A device used on the bottom of orienting tools to position the tool. The shaped end of the shoe forms a wedge to rotate the tool when lowered into a mating seat.
Mudline Suspension (MLS)
Hanging off the casings just below the sea bed (mudline). Makes it possible to suspend, cap and re-enter a well.
Non-magnetic
A material is considered to be non-magnetic if its relative magnetic permeability is less than 1.010. The value in a vacuum is precisely 1, but for steel the value can vary between 600 and 1,000,000.
Non-Magnetic Drill Collar (NMDC)
A drill collar made from non-magnetic material such as austenitic stainless steel or monel so that magnetic survey instruments can be run inside the bottom hole assembly. Although not specifically made from monel, they are often called monels.
North seeking gyro
A gyro that measures the rotation of the Earth to calculate the direction of True North. The gyro does not need a foresight or reference direction as a basis for azimuth readings.
North
The horizontal displacement, from a vertical line through the wellhead to a selected point in the well, in a North-South direction. North is defined to be positive and South negative. It is sometimes labelled 'Y' on plots and listings. (Outside the Group also known as latitude).
Oriented coring
The marking downhole of core samples with a key so that their orientation relative to a magnetic multi-shot instrument is known.
Ouija board
A slide rule used in directional drilling showing the combined dogleg, inclination and azimuth change as result of a directional change in toolface.
Permanent magnetism
The magnetism exhibited by an object even when it is not presently situated within an external magnetic field. (Also known as remanent magnetism).
Piano wire
See Wireline
Position uncertainty
The range of the actual possible positions of a survey station from its calculated position due to uncertainties in the survey methods.
Positional closure The difference in the calculated bottom hole position comparing the in-run and out-run of a survey. The difference in the calculated surface location made by calculating the survey down on the in-run and up on the out-run.
Precession
A property of the spinning mass of a gyroscope which, when a force is applied, causes it to move in a direction at right angles to both the applied force and the spin axis.
Principal hole direction
The average wellbore azimuth over a specified interval.
Projected Horizontal Displacement (PHD)
The horizontal displacement projected onto the target plane. Projected horizontal displacement PHD) plotted against true vertical depth (TVD) are the co-ordinates in the vertical section of a well plot. Not to be confused with horizontal displacement (HD). (Also known as projection on target plane and vertical section).
Projection on target plane
See projected horizontal displacement, PHD.
Quadrant System
A system where azimuths are expressed as angles from 0° to 90° measured from North in the two Northern quadrants and from South in the Southern quadrants. For instance S20E in the quadrant system equals 160° in the azimuth system.
Radial position uncertainty
Half the maximum width of the ellipsoid of uncertainty when projected onto a vertical plane. (See also vertical position uncertainty).
Ragland diagram
A graphical method for finding the toolface setting and final inclination after a turn from one azimuth to another.
Random error
Errors uniformly distributed around the true value. The average error of an infinite number of readings would be zero. (Also see systematic error).
Relative position uncertainty
The position uncertainty expressed in metres (feet) per 1000 m (1000 ft) of along-hole depth (AHD) for sections of constant inclination.
Reach
The maximum obtainable horizontal displacement. Kick-off point, build-up rate and maximum inclination are considered in its calculation.
Relative magnetic permeability (mr)
A measure of a material's ability of holding and contributing to a magnetic field. (See also Non-magnetic).
Relief well
A well drilled directionally to intersect a blowout well. The original purpose was to produce the relief well to reduce the bottom hole pressure and hence help in the killing the blowout. The more usual purpose is to use the well to inject fluid to kill the blowout.
Remanent magnetism
See permanent magnetism.
Rotational shot
A number of surveys (usually 6) taken at the same depth but with different toolface orientations to allow the calculation and correction of misalignment error or magnetic azimuth correction.
Sail angle
See Inclination.
Scribe line
A marker line on bottom hole assembly components defining the direction of toolface.
Side entry sub
A sub which allows the wireline to enter the drillstring via a sealing arrangement. Allows drillpipe to be added without pulling the wireline steering tool.
Slabbed plot
A projection of wells, between two selected planes, at right angles to the viewing direction.
Survey station
Any position in the borehole at which survey measurements are made. Often used for surveys that are made at discrete points by stopping the tool to take measurements.
Systematic error
When measurements are not uniformly distributed around the true value, a difference between the average of the measurements and the true value will appear. This difference is known as the systematic error. (See also random error).
Tangent section
A borehole section held at constant inclination and azimuth.
Target azimuth
The azimuth to the centre of a subsurface target, usually measured from the wellhead for a new well, or from the kick-off point for a re-drill from an existing well.
Target direction
See Target Azimuth.
Target plane
The vertical plane in the direction of the target azimuth of a well.
Tattle tail
A lead pin indicator showing if a mule shoe has been correctly seated into the mule shoe sleeve.
Tilt
The movement or drift of the spin axis of a 'free' gyro in the vertical plane. This only applies to a tilt scale gyro and not to a level rotor gyro.
Tilt scale gyro
A 'free' gyro where the spin axis is free in the gimbal system to precess both in the vertical and horizontal direction. This type of gyro is offered by Sperry-Sun. (See also Level Rotor Gyro).
Toolface (TF)
Defining a fixed reference direction on a tool. Used to orient tools contained in the drillstring to the desired direction.
Toolface Offset Centre correction (TOC)
A method to calculate and correct survey data misalignment errors. It can be done either with rotational shots taken during the survey, or, in certain circumstances, with in-run/out-run comparisons.
Total Depth (TD)
The greatest Along-Hole Depth reached by a well.
Total observed drift
The difference in azimuth of the gyro spin axis as measured before and after a survey.
TOTCO
A trade name for inclination only survey tools manufactured by TOTCO. A general name for inclination only survey tools.
TOTCO ring
See landing ring.
Transverse Mercator projection (TM)
A projection of the Earth's globe onto a transverse cylinder, so that the cylinder makes a line of contact along one meridian of longitude. The meridian of contact is known as the central meridian and will appear as a straight line on the map. The other meridians of longitude on the map will converge towards the poles. (See also Universal Transverse Mercator).
True North
The direction to the geographic North Pole which lies on the axis of rotation of the Earth. Also known as Geographic North.
True Vertical Depth (TVD)
The vertical distance from the surface reference level to a particular underground horizon. TVD plotted against projected horizontal displacement (PHD) gives the co-ordinates in the vertical section of a well plot.
Universal Bottom Hole Orientating (UBHO) sub
An orienting sub with an adjustable mule shoe sleeve, so that when made up with a bent sub the key in the mule shoe sleeve is aligned with the toolface scribe line of the bent sub.
Universal Transverse Mercator (UTM)
A standardised system of superimposing rectangular grids upon map projections (Transverse Mercator) of selected zones. The world is divided into 60 zones of 6° width, with central meridians in the middle of each zone. The zones are numbered 0 to 60 with zone 31 having the 0° (Greenwich) meridian on the left and 6° East on the right. Each zone is further divided into grid sectors. A sector covers 8° of latitude starting from 80° South to 80° North. The sectors are given the letters C to X, excluding I and O. Co-ordinates are measured in metres. The Northings are measured from the equator. For the Northern hemisphere the equator is taken as 0 m, whereas for the Southern hemisphere the equator is 10,000,000 m North to avoid negative numbers. The East co-ordinates are measured from a line 500,000 m West of the central Meridian (i.e. the Central Meridian has a Easting of 500,000 m. Again this avoids negative co-ordinates.
Upward position uncertainty
Defined as the position uncertainty at right angles to both the along-hole and the lateral position uncertainties.
Variation
See magnetic declination.
Vector
A quantity which has both magnitude and direction.
Vertical position uncertainty
The maximum height of the ellipsoid of uncertainty when projected onto a vertical plane. (See also radial position uncertainty). It is often quoted as the half-axis dimension (i.e. radii).
Vertical section azimuth
The azimuth defining the direction of the Vertical Section. The azimuth may or may not be the same as the target azimuth.
Vertical section
The Vertical plane in the direction of the vertical section azimuth of a well. A plot of true vertical depth and the projected horizontal displacement on the vertical plane in the direction of the vertical section azimuth. A plot of True Vertical Depth vs. the horizontal displacement (closure). The azimuth defining a vertical plane for plotting.
Wireline
Any line of wire or cable used for downhole operations. Usually one of three types: piano, braided (sandline) or electric wireline.
Wireline Steering Tool (ST)
A directional survey tool located close to the bit. It measures and transmits survey data to surface via a wireline. (See also bent sub, universal bottom hole orientation (UBHO) and mule shoe).