GEOPHYSICS 7 Flashcards
The elapsed time for a seismic wave to travel from its source to a given reflector and return to a receiver at the Earth’s surface. Minimum two-way traveltime is that of a normal-incidence wave with zero offset.
TWT/ Two-way traveltime
Abbreviation for unconfined compressive strength, or uniaxial compressive strength.
UCS
A measure of a material’s strength. The unconfined compressive strength (UCS) is the maximum axial compressive stress that a right-cylindrical sample of material can withstand under unconfined conditions—the confining stress is zero. It is also known as the uniaxial compressive strength of a material because the application of compressive stress is only along one axis—the longitudinal axis—of the sample.
unconfined compressive strength / UCS/ Uniaxial
A technique for acquisition of seismic data beneath areas that are difficult to access at the surface of the Earth, such as near rivers, drilling rigs, production platforms, environmentally sensitive areas or around seismically problematic features such as salt domes, which introduce uncertainty because of their high velocity. The sources and receivers are located on opposite sides of the feature.
undershooting
An undrained test is one in which the fluid in the sample is not able to flow and equilibrate to imposed pore pressure conditions; the fluid mass remains the same while the fluid volume and pressure will vary.
undrained test
A measure of a material’s strength. The uniaxial compressive strength (UCS) is the maximum axial compressive stress that a right-cylindrical sample of material can withstand before failing. It is also known as the unconfined compressive strength of a material because confining stress is set to zero.
uniaxial compressive strength
The use of measurements of a field at one elevation, level or surface to determine the values of the field at a higher level. The technique is most often used on potential fields, such as gravity or magnetic fields, to reduce scattered measurements to a common level for a simpler interpretation.
upward continuation
A two-point statistical function that describes the increasing difference or decreasing correlation, or continuity, between sample values as separation between them increases.
variogram
A two-point statistical function that describes the increasing differences or decreasing correlation, or continuity, between sample values as separation between them increases. The term variogram is sometimes used incorrectly in place of semivariogram. The two differ only in that the semivariogram uses each pair of data elements only once, whereas the variogram uses all possible data pairs. Semivariograms are usually used instead of variograms, but opposite vector directions (for example, north and south) are recognized as representing the same thing and having identical ranges, sills, nugget points and the like.
variogram
The process of calculating seismic velocity, typically by using common midpoint data, in order to better process seismic data. Successful stacking, time migration and depth migration all require proper velocity inputs. Velocity or stacking velocity can be calculated from normal moveout, or the change in arrival time produced by source-receiver offset.
velocity analysis
A feature in seismic data that results from changes in velocity, both laterally and vertically. Pull-up and push-down are examples of velocity anomalies.
Velocity anomaly
A change made in seismic data to present reflectors realistically. Velocity corrections typically require that assumptions be made about the seismic velocities of the rocks or sediments through which seismic waves pass.
Velocity correction
Those thicknesses of rock or sediment that have a common velocity, as opposed to the sedimentary layering or bedding of the rock or sediments.
Velocity layering
Measurements used to determine average velocity versus depth, such as from an acoustic log or check-shot survey. Acquiring a velocity survey is also known as “shooting a well.”
Velocity survey
The rate at which a wave travels through a medium (a scalar) or the rate at which a body is displaced in a given direction (a vector), commonly symbolized by v. Unlike the physicist’s definition of velocity as a vector, its usage in geophysics is as a property of a medium-distance divided by traveltime. Velocity can be determined from laboratory measurements, acoustic logs, vertical seismic profiles or from velocity analysis of seismic data. Velocity can vary vertically, laterally and azimuthally in anisotropic media such as rocks, and tends to increase with depth in the Earth because compaction reduces porosity. Velocity also varies as a function of how it is derived from the data. For example, the stacking velocity derived from normal moveout measurements of common depth point gathers differs from the average velocity measured vertically from a check-shot or vertical seismic profile (VSP). Velocity would be the same only in a constant velocity (homogeneous) medium.
Velocity/ acoustic velocity/ seismic velocity
An adjustable mechanical source that delivers vibratory seismic energy to the Earth for acquisition of seismic data. Mounted on large trucks, vibrators are commonly used for acquisition of onshore seismic data.
Vibrator
Seismic data whose energy source is a truck-mounted device called a vibrator that uses a vibrating plate to generate waves of seismic energy; also known as Vibroseis data (Vibroseis is a mark of Conoco). The frequency and duration of the energy can be controlled and varied according to the terrain and type of seismic data desired. The vibrator typically emits a linear “sweep” of at least seven seconds, beginning with high frequencies and decreasing with time (“downsweeping”) or going from low to high frequency (“upsweeping”). The frequency can also be changed in a nonlinear manner, such that certain frequencies are emitted longer than others. The resulting source wavelet is not impulsive. Vibrators are employed in land acquisition in areas where explosive sources cannot be used, and more than one vibrator can be used simultaneously to improve data quality.
Vibratory seismic data
A class of borehole seismic measurements used for correlation with surface seismic data, for obtaining images of higher resolution than surface seismic images and for looking ahead of the drill bit; also called a VSP. Purely defined, VSP refers to measurements made in a vertical wellbore using geophones inside the wellbore and a source at the surface near the well. In the more general context, VSPs vary in the well configuration, the number and location of sources and geophones, and how they are deployed. Most VSPs use a surface seismic source, which is commonly a vibrator on land and an air gun in offshore or marine environments. VSPs include the zero-offset VSP, offset VSP, walkaway VSP, walk-above VSP, salt-proximity VSP, shear-wave VSP, and drill-noise or seismic-while-drilling VSP. A VSP is a much more detailed survey than a check-shot survey because the geophones are more closely spaced, typically on the order of 25 m [82 ft], whereas a check-shot survey might include measurements of intervals hundreds of meters apart. Also, a VSP uses the reflected energy contained in the recorded trace at each receiver position as well as the first direct path from source to receiver. The check-shot survey uses only the direct path traveltime. In addition to tying well data to seismic data, the vertical seismic profile also enables converting seismic data to zero-phase data and distinguishing primary reflections from multiples.
VSP (Vertical Seismic Profile)
