Velocity Definitions: T T XV XV T +
Velocity Definitions: T T XV XV T +
Velocity Definitions: T T XV XV T +
The majority of definitions given here are derived either from Sheriff 1991 or Hubral and Krey
1980.
Abnormal Moveout Velocity (VAMO),
Abnormal moveout velocity is defined as the VAMO term in Taner and Koehlers (1969)
moveout equation
T x = T0 + x VNMO x V AMO T0
2
Tx is the reflection time of an event at an offset x, T0 is the vertical reflection time and VNMO is
the normal moveout (q.v.). In the case of horizontal homogeneous layers
[(
V AMO = VRMS 4
4
RM4
)]
14
4
RMS
VRM4 = VIk Tk T0
4
k=0
VIk being the interval velocity (q.v.) in the kth layer and Tk the two-way vertical travel time in
the kth layer, there being n horizontal layers.
Air Wave Velocity (Vair),
The velocity with which sound travels through air, approximately 1050 + 1.1F ft/sec where F
is the temperature in degrees Fahrenheit. Such energy can give rise to unwanted signal on
seismic field records.
Apparent Velocity (Vapp),
1. The phase velocity which a wavefront appears to have along a line of geophones. If the
wavefront makes an angle with the spread and the true velocity of the wavefront is V,
then the apparent velocity is V/Sin.
2. The inverse of the slope of a refraction time-distance curve.
3. An estimate of average velocity (q.v.) calculated from seismic reflection times and
measured well depths (usually in the absence of a borehole velocity survey). Also known
as effective velocity (q.v.) and pseudo velocity (q.v.).
Average Interval Velocity (VAI),
An interval velocity calculated from two average velocities (q.v.) and commonly referred to as
the interval velocity (q.v.).
Average Stacking Velocity (VAS),
An estimate of average velocity computed from a series of interval stacking velocities (q.v.). It
is a closer approximation to the average velocity (q.v.) than is the stacking velocity (q.v.).
Average Velocity (VA),
1. The distance traversed by a seismic wavelet divided by the time required, both with
respect to some particular travel path to a reference datum. For reflections, often refers to
a ray reflected at normal incidence, sometimes to a vertical travel path.
The ratio of a given distance along a certain path to the time required for a seismic wave to
traverse the path. While it has meaning only with respect to a particular path, a vertical path is
often implied, that is, it is given by a depth divided by seismic travel time to that depth, usually
assuming straight raypath travel. If the section is made up of parallel horizontal layers of
velocity VIj and thickness zj so that the travel time across each layer is tj = zj / VIj, then the
average velocity is
VDIX =
[( V
2
RMS,n
) (T T )]
2
Tn VRMS
,n 1 Tn 1
12
n 1
where VRMS,n-1 and VRMS,n are the root mean square velocities (q.v.) from the datum to the
reflectors above and below the layer and Tn-1 and Tn are the vertical incidence reflection
times. This formula is typically used substituting stacking velocity (q.v.) values for the root
mean square velocity values, see interval stacking velocity.
Effective Velocity,
An estimate of average velocity (q.v.) calculated from seismic reflection times and measured
well depths (usually in the absence of a borehole velocity survey). Also known as apparent
velocity (q.v.) and pseudo velocity (q.v.).
Elevation Velocity,
Subweathering velocity (q.v.).
Equivalent Velocity,
1. Stacking velocity (q.v.), the constant velocity which gives nearly the same normal moveout
as observed in a velocity analysis.
2. Apparent velocity (q.v.).
Formation Velocity ,
The interval velocity (q.v.) associated with a geological formation.
Ground Roll Velocity,
The velocity of surface waves which travel along or near the surface of the ground.
Characteristically a low velocity. Rayleigh waves are usually the main source of ground roll
and the term pseudo-Rayleigh wave is sometimes used.
Group Velocity,
1. The velocity with which the energy in a wavetrain travels. In dispersive media where
velocity varies with frequency, the wavetrain changes shape as it progresses so that
individual wave crests travel at a different velocity (see phase velocity) than does the
envelope of the wavetrain.
2. In anisotropic media it is the velocity of energy transport in the direction radially outward
from a point source and it is associated with angular dispersion.
3. When frequency and angular dispersion occur together the velocity of the wave envelope
and energy velocity are not the same.
Head Wave Velocity,
The velocity of a head wave or refracted wave, in particular the velocity of the first break.
Horizontal Velocity,
The velocity of a wave measured in the horizontal direction i.e., in the x-y plane.
H-wave Velocity,
Another term for hydrodynamic wave velocity (q.v.).
Hydrodynamic Wave Velocity (VH),
1. The velocity of a Rayleigh wave characterised by prograde elliptic particle motion.
2. The velocity of waves on the surface of a fluid.
T x = T0 + x VNMO
2
where T x is the reflection time at a small offset x and To is the vertical reflection time at zero
offset. In the case of flat, parallel, isotropic layers, it is almost equal to the root mean square
velocity (q.v.).
Particle Velocity,
Particle velocity refers to the actual motion of a local portion of the medium, it is a function of
the disturbance of the medium rather than a property of the medium. Whereas seismic
velocities have magnitudes of thousands of metres per second, particle velocities typically
have magnitudes of millionths of a metre per second.
Phase Velocity,
1. The velocity with which any given phase (such as a trough or a wave of single frequency)
travels; it may differ from group velocity (q.v.) because of dispersion. It is sometimes
known as trough velocity or peak velocity.
2. The velocity perpendicular to a surface of constant phase in anisotropic media.
3. Sometimes means apparent velocity (q.v.).
Primary Velocity,
A term sometimes used to describe the stacking velocity (q.v.) of a primary reflection event.
Primary Wave Velocity (Vp),
Another term for compressional wave velocity (q.v.).
Pseudo Velocity,
An estimate of average velocity (q.v.) calculated from seismic reflection time and measured
well depth (usually in the absence of a borehole velocity survey). Also known as effective
velocity (q.v.) and apparent velocity (q.v.).
Pseudo Rayleigh Wave Velocity,
Another term for ground roll velocity (q.v.).
p-Wave Velocity (Vp),
Another term for compressional wave velocity (q.v.).
Rayleigh Wave Velocity,
The velocity of a Rayleigh wave. Rayleigh waves propagate along a free surface in the
presence of layers showing dispersion. There are two branches of the dispersion relation.
The M1 branch, whose velocity is denoted VR and was the original Rayleigh wave, is
characterised by retrograde elliptic particle motion at the surface. The M2 branch, whose
velocity is denoted by VH and is also known as a hydrodynamic wave (q.v.), is characterised
by a prograde elliptic particle motion. Rayleigh waves are a major constituent of ground roll. A
Rayleigh wave along a free surface can be thought of as a special case of a Stoneley wave
which travels along an interface.
Refraction Velocity,
The velocity of a layer determined from refracted or head waves. Energy travels obliquely
downwards from a source until it encounters an interface with a higher velocity medium, the
energy meeting the interface with an angle of incidence greater than or equal to the critical
angle will be refracted, it will travel parallel to the (flat) interface within the higher velocity
medium, energy will emerge at the critical angle and travel obliquely upwards.
Replacement Velocity,
The velocity used in static corrections to compensate for low-velocity near surface materials.
Root Mean Square Velocity (VRMS),
The root mean square velocity VRMS is given by the following formula:
VRMS =
[
k =1
12
2
Ik
V Tk T 0
where VIk is the interval velocity (q.v.) in the kth layer, Tk the two way vertical travel time in the
kth layer and T0 the vertical incidence reflection time, there being n horizontal layers.
Root Mean Square Interval Velocity,
The interval velocity (q.v.) calculated from the root mean square velocities (q.v.) to the top
and bottom of the interval (Dix interval velocity (q.v.)) is the root mean square velocity of the
interval. If the interval consists of n segments (or layers) each one having a uniform velocity
Vik, the root mean square velocity of the interval is
VDIX = [
VIk
k =1
(t
k +1
tk ) ( tn +1 t1)
1/ 2
where tk is the normal incidence travel time to the top of the kth segment.
Sample Velocity,
The velocity measured on a rock sample in the laboratory. The sample may be wet, dry, at
some confining pressure etc.
Seam wave velocity,
Another term for channel wave velocity (q.v.).
Secondary wave velocity (Vs),
Another term for shear wave velocity (q.v.).
Seismic wave velocity,
The velocity of an elastic disturbance propagating from point to point through a medium.
Many field, laboratory and theoretical studies have been devoted to the problem of
determining seismic wave velocities and establishing empirical and analytical laws between
these velocities and the various parameters that influence them.
Sezawa M2 Wave Velocity (VH),
Another term for hydrodynamic wave velocity (q.v.).
Shear Wave Velocity (Vs),
The velocity of a body wave in which the particle motion is perpendicular to the direction of
propagation. When shear waves are transmitted through media having azimuthal anisotropy
then birefringence, or polarisation of the wave, occurs, leading S1 and S2 waves - fast swaves and slow s-waves. Fluids will not transmit shear waves. In seismic surveys shear
waves are frequently generated at long offsets by mode conversion at layer boundaries with
high acoustic impedance contrasts.
sh-Wave Velocity (Vsh),
1. The velocity of a horizontally polarised shear wave.
2. The velocity of the horizontal component of any shear wave.
3. The velocity of a surface Love wave which is sometimes mistaken for and called an shbodywave.
Small Aperture Migration Velocity,
A specific form of migration velocity (q.v.). It is the migration velocity derived by fitting an
approximating second order time surface, which is a hyperboloid but not necessarily a
rotational one, to a small number of short offset traces (the aperture), in areas of complex
structure when the 3D diffracting surfaces do not closely approximate rotational hyperboloids
and thus wide aperture migration fails.
Sonic Velocity,
1. The velocity at a point in the subsurface as determined from a sonic log.
2. The velocity of acoustic or p-waves in fluid, sometimes includes other wave modes and
hence becomes synonymous with seismic and elastic.
Stacking Velocity (VS),
Stacking velocity is a number or function having the dimension of length/time used to stack
seismic traces in an optimum way and is usually determined by maximising a statistical
function (semblance, coherency, etc.,). Most schemes fit a hyperbolic curve to the moveout;
see, for example, Taner and Koehler (1969). In the limit as offset approaches zero the
stacking velocity, sometimes erroneously referred to as root mean square velocity (q.v.),
approaches the normal moveout velocity (q.v.). Fitting a normal moveout equation to common
midpoint data is equivalent to assuming an ellipsoidal wavefront, yielding the stacking velocity
of the horizontal component, Nonelliptical wavefronts yield offset-variable stacking velocities.
Stoneley Wave Velocity,
The velocity of a Stoneley wave which is a type of seismic wave propagated along an
interface. These waves are always possible at solid/fluid interfaces and under very restricted
conditions at solid/solid interfaces. Stoneley waves occur as surface waves in boreholes.
Subweathering Velocity,
The velocity immediately below the base of weathering. It is sometimes taken as the velocity
of a refraction at the base of weathering.
Surface sh-wave Velocity (VL),
Another term for Love wave velocity (q.v.).
Surface Wave Velocity,
The velocity with which energy travels along, or near to, the surface. In seismic exploration
the term usually refers to the ground roll velocity (q.v.) but there are other forms of surface
waves such as Rayleigh waves, Love Waves, Stoneley waves and hydrodynamic waves.
sv-Wave Velocity (Vsv),
The velocity of shear wave energy polarised so that the particle motion is in the vertical plane
which also contains the direction of propagation.
s-Wave Velocity (Vs),
Another term for shear wave velocity (q.v.).
Transverse Wave Velocity,
Another term for shear wave velocity (q.v.).
Trapped Wave Velocity,
Another term for channel wave velocity (q.v.).
Tube Wave Velocity,
The velocity of a surface wave in a borehole, see Stoneley wave velocity. The term is often
applied to the surface wave which travels along the casing in a borehole.
Uphole Velocity,
The velocity of the near surface determined by firing a shot at one or more depths in a
borehole and recording the uphole times; the average velocity (q.v.) of the near surface as
measured in a shallow borehole.
Vertical Velocity,
Velocity determined in a vertical direction.
Water Velocity (VW),
The speed of a compressional wave in water is a function of temperature T in degrees
Celsius, the salinity S in parts per thousand, and the depth Z below the surface in metres and
is given by:
Wave Velocity,
The speed with which a wave advances in a given direction.
Weathering Velocity,
The velocity, usually of compressional waves, in the near surface. Seismically it is usually the
section above the watertable where the pore spaces contain air.