Device to measure the acceleration in three axes of the borehole imager, including the acceleration of gravity which is used to find up-down of the tool from the ratio of x and y acceleration. The acceleration of the tool along the wellbore is used to calculate the exact z-position of measurement at a specific time. Usually of the piezoelectric type.

acoustic imager

Acoustic imager
A spiral-type borehole imager based on ultrasound reflectance of the wellbore surface. Commercial acronyms CBIL, UBI, CAST.


For sediments, this usually refers to rotational symmetry where the symmetry axis is normal to the bedding plane. Acoustic velocities, permeability, mechanical properties etc. are the same in any direction perpendicular to the axis but different from these properties along the axis. Higher degrees of anisotropy imply lower symmetry: Two fracture sets orthogonal to bedding give orthorhombic symmetry, a cross-set has monoclinic symmetry, a fractured laminated rock has triclinic symmetry.


The statistically best fitting line of intersection of planes thought to be of common origin, such as cross-lamination of varying paleodip belonging to the same set, bedding variably rotated by cylindrical folding, or intersection of two conjugate fracture sets.


Angle in a plane tangent to the earth, with reference to the projection on this plane of the direction to either geographic or magnetic pole, or UTM north (grid north).

borehole image (or image, or BHI)

Borehole image (or image, or BHI)
An oriented 2D image of the interior surface of a borehole or a zone just behind it. the image may be constructed (if necessary by interpolation) from any physical quantity which can be measured with the necessary resolution and which has sufficient dynamic range.

borehole imager

Borehole imager
A logging tool designed for acquiring measurements which can be used to compose a borehole image. There are two basic types: spiral imagers acquiring data while the sensor is spinning around the tool axis while moving along the borehole, and linear imagers where sensor arrays glide along the wellbore.


Compressive failure (“crumbling”) of rock along the wellbore axis in the directions of maximum tangential stress. Equivalent to rock burst in tunnels.


Diameter of borehole calculated from extension of sensor arms on a logging tool. Diameters in several relative bearings may be calculated.

CBIL or UltrasonicXplorer

CBIL or UltrasonicXplorer
Acoustic imager similar to UBI but the rotating transducer is enclosed in a diesel-filled teflon tube.


An axisymmetric fabric, for example 1) caused by a polar force such as gravity, arising from the parallel arrangement of platy minerals (lamination), in the absence of any horizontal traction; 2) caused by an axial force such as stress, forcing platy mineral grains to become parallel (foliation). A perfect cluster has eigenvalues (1,0,0).


Within a window of measured depth, the standard deviation of the distances between adjacent events f.x. fracture intersections.

CMI or Compact microimager

CMI or Compact microimager
An 8-arm electrical imager for water base mud.


A function of 2 variables, for example the shift between windows of curves 1 and 3 and the shift betwen curves 2 and 4 in a 4-arm dipmeter. The function is the sum of the correlation coefficients of the 6 pairs of curves. Each point in the correlogram is an orientation in the tool coordinate system. The global maximum is the best fitting dip (2) for the window.

depth increment

Depth increment
The distance in the z direction between each sensor measurement. A characteristic of a logging tool.

depth of investigation

Depth of investigation
The depth behind the borehole wall where the rock outside it contributes the same as the rock inside it to the value read by the imaging sensor. To investigate the impact of different DOI values on dip data Eriksfiord offers its Java based DOI Sensitivity Analyzer tool : Eriksfiord DOI Sensitivity Analyzer. Note: Java 6 or higher is required on your computer to run this tool.


Rotating dips back to their orientation relative to their continental plate at the time of deposition, by subtracting the PHZ. For paleoclimatology, a correction for plate rotation may be applied.


Angle between vertical and a line tangent to the borehole.

dip 1

Dip 1
The (smallest) angle between a horizontal plane and the tangent plane of a geological surface, measured in the plane orthogonal to their intersection line.

dip 2

Dip 2
Informal expression for a tangent plane to a geological surface, defined on an image by its dip (1) and dip azimuth.

dip azimuth

Dip azimuth
The azimuth of the vector in the dip (2) orthogonal to the strike.


Precursor to the linear borehole imager, acquiring curves of electric conductivity by eletrodes mounted on pads arranged in 3, 4 or 6 different directions (commercial acronyms HDT, SHDT, Diplog…).

Ecoscope, ADN

Ecoscope, ADN
Rotating density/neutron sensor density data are sampled to a 16-column image while drilling, in both WBM and OBM.

EI or Earth Imager

EI or Earth Imager
Electrical imager for OBM, with 5 sensors on 6 pads. Replaced by GXPL or GeoXplorer.


For a matrix A, solutions to det(A-λI)=0 where I is the identity matrix. An orientation matrix of dips has 3 eigenvalues, whereof the greatest indicates the tightness of a dip cluster.


For a matrix A, substituting eigenvalues λ , the solutions x to Ax=λx. The eigenvector of the greatest eigenvalue is an estimator of the mean dip.

equatorial plane 1

Equatorial plane 1
The x-y plane of a logging tool containing the reference sensor.

equatorial plane 2

Equatorial plane 2
The horizontal plane through the centre of the orientation sphere, the plane of the stereographic projection.


The pattern of poles to planes in a stereographic projection or an equal-area projection. The symmetry of the fabric is composed of the symmetries of all causes (deformational or depositional mechanisms) having acted on the rock.

facies (image)

Facies (image)
The appearance of a sedimentary facies on a borehole image. Obtained from certain diagnostic features. For example, the truncation of cross-laminae by the following cross-set proves that the image represents cross-bedded facies.

FLF (fault-like feature)

FLF (fault-like feature)
A plane identified on a borehole image which may be the reflection of a fault plane intersected by the borehole. May be elevated to “fault plane” or “fault zone” if associated beyond doubt by evidence of shear motion, e.g. drag folding of adjacent lamination.

FMI or Formation micro-imager

FMI or Formation micro-imager
Four arm, eight pad imager for WBM. Measures rock conductivity through conductive mudcake. A long-term reference tool for high detail fracture or sedimentology interpretation. Successor to FMS (Formation microscanner).


The space between two fracture surfaces once adjacent, separated by a fluid (tar, gas, oil, carbon dioxide…) or a solid, like clay smear, rock flour, glass, or crystals (quartz, calcite, anhydrite, pyrite…) precipitated in situ.

fracture surface

Fracture surface
Surface within a solid generated by rupture, either shear, torsion or tension.

fracture statistics

Fracture statistics
Linear or spherical statistics apply. Linear statistics describe the frequency and clustering of fractures observed along a wellbore, whereas spherical statistics describe the shape and symmetry of the distribution of poles to fracture planes on the directional sphere, e.g. as a function of the paleo-stress-field.

galvanic imager

Galvanic imager
A borehole imager working on the principle of measuring the amount of current injected by a given voltage pulse. Commercial acronyms FMI, FMS, EMI, XRMI.


Senior Eriksfiord employee.


An axisymmetric, orthorhombic or monoclinic fabric, caused by for example a) a combination of gravity and horizontal traction during sedimentation, b) folding of a foliated or laminated rock, c) the transposition of lamination/foliation by shear fractures or d) extrusion. A perfect girdle has eigenvalues of (0,.5,.5).

GXPL or GeoXplorer

GXPL or GeoXplorer
Six-arm imager for OBM. Measures resistivity through capacitive mud film, by transmitting a megahertz electrical impulse. Provides geological detail similar to NGI or Quanta Geo.

gyroscopic survey

Gyroscopic survey
A downhole gyroscope measures the orientation of the borehole relative to the universe. by fitting a line through successive measurements, e.g. by splining, the welltrace in 3 dimensions is reconstructed (survey).

high side (image high side, HS)

High side (image high side, HS)
The point on an image which would be at HAZI if the well were rotated back to vertical about an axis which would preserve direction of HAZI. HS is equal to HAZI on a north-referenced image.

hole azimuth (HAZI)

Hole azimuth (HAZI)
The azimuth of the projection of a tangent to the welltrace on the horizontal plane.


Four arm, single electrode dipmeter with electro-mechanical navigation and an extra electrode permitting limited speed-correction. Replaced by SHDT.

induced fracture

Induced fracture
A tensile fracture in the sector of the wellbore which has tensile effective stress greater than tensile strength, and caused by the drilling (replacement of a cylinder of solid by a liquid).


Some trace fossils like Diplocraterion or Thalassinoides are sufficiently large and characteristic to be recognized on galvanic borehole images.

inversion (of stress)

Inversion (of stress)
From the evidence of stress-induced instability on a borehole image and knowledge of the mechanical properties of the rock, determination by iteration of the stress tensor which caused this instability.

LWD imager

LWD imager
Spiral type imager based on electrical resistivity (e.g. RAB) or density (e.g. ADN) acquired while drilling. Data are either stored downhole or transmitted in a reduced form to surface by mudpulsing.


Device used to measure the magnetic azimuth of the imaging tool. Usually a triaxial fluxgate type is used. Magnetometer readings are verified by comparison with the gyro survey.

magnetic declination

Magnetic declination
The azimuth of the magnetic vector.

magnetic inclination

Magnetic inclination
The angle between a tangent surface to the earth and the geomagnetic vector.

measured depth (MD)

Measured depth (MD)
The length of cable or drillstring between the reference point of a tool and the drillfloor.

NGI or Quanta Geo

NGI or Quanta Geo
Eight-arm imager for OBM. Measures rock resistivity in high detail through capacitive mudcake with radio-frequency impulses. Logs up or down equally well.

normalisation 1

Normalisation 1
Manipulating a timeseries so it gets a Gaussian distribution over the interval in question.

normalization 2

Normalisation 2
Manipulating a piece of image so pixel values get a flat distribution. The piece may be sliding, leading to a “dynamic” normalisation, or fixed, leading to a “static” normalisation.

north (image north)

North (image north)
The point which would be geographic or magnetic north on a borehole image if the well were rotated back to vertical about an axis which would preserve the direction of hole azimuth.


Oil-base mud imager measuring potential difference between two electrodes. Superseded by NGI or Quanta Geo. Often, two 4-pad OBMIs were run together at 45deg offset.

oil base mud

Oil base mud (OBM)
Drilling fluid which is an emulsion of water in oil. Electrically resistive as the water is the discontinuous phase. Low viscosity and low density is an advantage to acoustic imagers. Chemical interaction with the host rock is limited compared to water base mud.


Electrical imager for OBM, with 6 double sensors per pad. Similar to OBMI or OBMI2.

orientation matrix

Orientation matrix
For a set of directions, the matrix of the sums of the cross-products of direction cosines.


Dip and azimuth of a plane, relative to the paleohorizontal.

paleohorizontal (PHZ)

Paleohorizontal (PHZ)
The orientation of a geological surface considered to have been horizontal at the time of its deposition.


The stress state before deformation took place, e.g. the stress required to initiate a fold-and-thrust belt, annihilated or modified by the folding and thrusting.


Rotating electrode near the drillbit provides a spiral electrical conductivity image while drilling with WBM.

RAB, MicroScope

RAB, MicroScope
Viewing the BHI as a matrix, the elements of that matrix. May be originally measured values or a transform of them (e.g. by normalization 2).

relative bearing

Relative bearing
Angle from the intersection of a vertical plane with an equatorial (x-y) plane of a logging instrument in the equatorial plane to a reference point such as the extension of the reference sensor parallel to the z-axis to the plane.

resistivity imager

Resistivity imager
A borehole imager which measures the potential drop over a short interval within a larger scale potential gradient. OBMI.

shale mechanics

Shale mechanics
Although shales are often referred to as weak and soft, this is usually only in the direction normal to lamination plane L. The stiffness along L can be twice as high as across L. Compressive strength along and across L can be similar but is usually much smaller oblique to L. Poisson’s ratios measured by expansion along L due to stress normal or parallel to L are similar but smaller than that measured from expansion normal to L from stress along L. The resistance to shear along L is much smaller than across it. Evaluation of stress in shales, particularly if no principal stress axis is normal to L, requires numerical methods.

shale reservoir

Shale reservoir
Hydrocarbons generated in a shale during/after burial have not yet been able to escape via fractures or by diffusion, and are stored in nano-size porespace. They can be liberated through fractures created by hydraulic pressure and held open by injection of particles of sufficient compressive strength.


Equivalent to tool, logging tool, but referring more to the sensor equipment than the tool body.


Dipmeter tool with 4 arms, may be considered a prototype for FMS and FMI, also equipped with solid-state navigation permitting speed-correction.


Power displayed as a function of frequency. Used f.x. on dipmeter curves to analyze the fineness of lamination or periodicity of bedding. Can be calculated quickly for 2n samples by the Cooley-Tukey “fast Fourier transform” after tapering both ends.


Six-arm multi-electrode electrical conductivity imager for WBM. Detail of rock texture is similar to FMI or Formation micro-imager.


Rotating electrode near the drillbit provides a spiral electrical conductivity image while drilling with WBM.


A projection of normal vectors to dips (2) onto the equatorial plane of the directional sphere. Preserves angles. Also called Wulff diagram.


Paper with a printed net, used for counting angles between poles and for rotation of poles in stereograms. now superseded by computer graphics programs such as Interpret-2 in Recall.

sticking (tool sticking)

Sticking (tool sticking)
Tool decelerates or stops completely its alonghole motion due to excessive friction against the wellbore, either forever, or until cable tension builds up to overcome it.

speed correction

Speed correction
Calculation of the exact location of each measurement, permitting an image to be constructed. involves integration of the z-axis accelerometer.


The relative incremental deformation of a solid due to stress, described by a second rank tensor with 6 independent components.


Azimuth of the intersection between the horizontal and the dip-2.


Oriented pressure in a solid, described by a second rank tensor with 6 independent components.


The appearance of the internal structure of a rock on a borehole image.


Tool measure point offset. for a ‘train’ of logging tools with sensors at multiple z-distances, the end point of the tool combination or any other depth may be chosen as the reference point. The reference point is used to synchronize simultaneous measurements, or inversely, to correlate depths observed at different times by sensors placed at different levels (z-distances).

tool (logging tool)

Tool (logging tool)
A physical measurement device, usually built into a tubular structure equipped with sensor arms, designed to acquire data while passing through a borehole, propelled either by gravity, a cable, a tractor or a drillstring.

tool coordinate system

Tool coordinate system
The z-axis is the centerline of a cylindrical tool, pointing in the direction of decreasing measured depth, the x-axis is perpendicular to the z-axis and penetrates a reference point such as a reference sensor, the y-axis is perpendicular to both. x and y have origo in their intersection with z-axis. y increases to the right if looking towards origo from the positive x direction.

true vertical depth (TVD)

True vertical depth (TVD)
The vertical distance between a subsurface event and a surface reference, f.x. drill floor or mean sea level.


Ultrasound transducer spinning on the tool axis directly in the mud provides spiral image of acoustic reflectance, suitable for imaging high-contrast non-planar features such as vuggy carbonate.


A computer system, developed and owned by Eriksfiord, for analyzing stress in wellbores, based on analytic solutions and linear elastic mechanics.

water base mud

Water base mud (WBM)
Drilling fluid which is a solution of salts and suspension of solids in water. High salinity is an advantage for galvanic imagers. Low density and viscosity is an advantage for acoustic imagers.


A cable which propels a logging tool along the wellbore, provides electric power to the tool, and conveys data from the tool to the surface computer during logging.


A 6 arm electrical imager for WBM, similar to STAR.