GEOPHYSICS

1. n. [Geophysics] The inability of a system to exactly match input and output, a general example being an electronic amplifier and the classic example being a home stereophonic amplifier. Synonyms: amplitude distortion See: bias,  dispersion,  dynamic range,  harmonic distortion,  zero-phase

distortion

1. n. [Geophysics] The daily variation in properties of the Earth, such as the temperature or the local geomagnetic field, or the daily change in sunlight. Such variations depend in part on latitude, proximity to the ocean, the effects of solar radiation and tides and other factors.

diurnal variation

1. n. [Geophysics] The loss of energy from a wavefront as a consequence of geometrical spreading, observable as a decrease in wave amplitude. Spherical divergence decreases energy with the square of the distance. Cylindrical divergence decreases energy with the distance. See: spherical divergence

divergence

1. n. [Geophysics] An equation used to calculate the interval velocity within a series of flat, parallel layers, named for American geophysicist C. Hewitt Dix (1905 to 1984). Sheriff (1991) cautions that the equation is misused in situations that do not match Dix's assumptions. The equation is as follows: Vint = [(t2 VRMS22 − t1 VRMS12) / (t2 − t1)]1/2, where Vint = interval velocity t1 = traveltime to the first reflector t2 = traveltime to the second reflector VRMS1 = root-mean-square velocity to the first reflector VRMS2 = root-mean-square velocity to the second reflector. Reference: Dix CH: "Seismic Velocities from Surface Measurements," Geophysics 20, no. 1 (January 1955): 68–86. See: interval velocity,  reflector,  root-mean-square velocity,  traveltime

Dix formula

1. n. [Geophysics] The difference in the arrival times or traveltimes of a reflected wave, measured by receivers at two different offset locations, that is produced when reflectors dip. Seismic processing compensates for DMO. Alternate Form: dip moveout

DMO

1. n. [Drilling] A particularly crooked place in a wellbore where the trajectory of the wellbore in three-dimensional space changes rapidly. While a dogleg is sometimes created intentionally by directional drillers, the term more commonly refers to a section of the hole that changes direction faster than anticipated or desired, usually with harmful side effects. In surveying wellbore trajectories, a standard calculation of dogleg severity is made, usually expressed in two-dimensional degrees per 100 feet [degrees per 30 m] of wellbore length. There are several difficulties associated with doglegs. First, the wellbore is not located in the planned path. Second is the possibility that a planned casing string may no longer easily fit through the curved section. Third, repeated abrasion by the drillstring in a particular location of the dogleg results in a worn spot called a keyseat, in which the bottomhole assembly components may become stuck as they are pulled through the section. Fourth, casing successfully cemented through the dogleg may wear unusually quickly due to higher contact forces between the drillstring and the inner diameter (ID) of the casing through the dogleg. Fifth, a relatively stiff bottomhole assembly may not easily fit through the dogleg section drilled with a relatively limber BHA. Sixth, excessive doglegs increase the overall friction to the drillstring, increasing the likelihood of getting stuck or not reaching the planned total depth. Usually these problems are manageable. If the dogleg impairs the well, remedial action can be taken, such as reaming or underreaming through the dogleg, or even sidetracking in extreme situations. See: bottomhole assembly,  casing string,  directional driller,  inside diameter,  ream,  sidetrack,  total depth,  underream

dogleg

1. n. [Geophysics] The set of values an independent variable can take. For example, the independent variable of the time domain is time; and for the frequency domain, it is frequency. See: absorbing boundary conditions,  f-k domain,  frequency domain,  time domain

domain

1. n. [Geophysics] Slang term to describe a seismologist performing seismic field work.

doodlebugger

1. n. [Geophysics] The splitting of an incident wave into two waves of different velocities and orthogonal polarizations. Double refraction, or birefringence, occurs in optical mineralogy (see petrography) when plane-polarized light passes through an anisotropic mineral and emerges as two rays traveling at different speeds, the difference between which is characteristic of a mineral. In seismology, incident S-waves can exhibit birefringence as they split into a quasi-shear and a pure-shear wave. Although birefringence was first described by Danish physician Erasmus Bartholin (1625 to 1698) in crystals in 1669, the phenomenon was not fully understood until French physicist Etienne-Louis Malus (1775 to 1812) described polarized light in 1808. Synonyms: birefringence See: anisotropy,  S-wave,  seismology,  velocity,  petrography

double refraction

1. n. [Geophysics] A receiver located in a wellbore, as opposed to a location on the Earth's surface. See: geophone,  wellbore

downhole receiver

1. n. [Geophysics] A seismic source located in a wellbore rather than at the Earth's surface. See: source

downhole source

1. n. [Geophysics] A technique used to estimate the value of a potential field or seismic data at a surface beneath a measured surface. The method is risky because it assumes continuity of the field, so anomalies affect predictions, especially if they occur beneath the measured surface. Noise can be exaggerated and affect calculations adversely. Antonyms: upward continuation See: anomaly,  gravity,  magnetics,  seismic

downward continuation

1. n. [Geology, Geophysics, Formation Evaluation] Test in which the pore fluid in the sample is able to flow and equilibrate to imposed pore pressure conditions; the fluid mass and volume will vary but its pressure will be constant. A drained test could be on a dry sample. See: undrained test

drained test

1. n. [Drilling] A term to describe the inclination from vertical of a wellbore. See: deviation survey,  inclination

drift

1. n. [Geophysics] A technique for acquiring a vertical seismic profile that uses the noise of the drill bit as a source and receivers laid out along the ground or seabed. In deep water, the receiver arrays can be deployed vertically. Acquisition and processing are typically more challenging than in the more conventional types of VSPs, but the technique can yield time-depth information and, less frequently, reflection information, while the well is being drilled. The information from a drill-noise VSP can be used to improve time-depth conversions while drilling, decide where to set casing in a well and evaluate drilling hazards, such as anomalous pore pressure. Alternate Form: drill-noise vertical seismic profile See: acquisition,  noise,  receiver,  reflection,  source,  vertical seismic profile

drill noise vertical seismic profile

1. n. [Geophysics] A technique for acquiring a vertical seismic profile that uses the noise of the drill bit as a source and receivers laid out along the ground or seabed. In deep water, the receiver arrays can be deployed vertically. Acquisition and processing are typically more challenging than in the more conventional types of VSPs, but the technique can yield time-depth information and, less frequently, reflection information, while the well is being drilled. The information from a drill-noise VSP can be used to improve time-depth conversions while drilling decide where to set casing in a well evaluate drilling hazards, such as anomalous pore pressure. Alternate Form: drill noise vertical seismic profile See: acquisition,  noise,  receiver,  reflection,  source,  vertical seismic profile

drill-noise vertical seismic profile

1. n. [Geophysics] The loss of information from a magnetic tape that occurs if the tape is damaged or exposed to dirt. See: field tape

dropout

1. n. [Geophysics] A seismic profile recorded specifically to study the lower crust, the Mohorovicic discontinuity and the mantle of the Earth, typically using refraction methods. Most standard seismic reflection profiles record only a small fraction (typically, on the order of 10 km [6 miles]) of the Earth's crust, which is 5 to 75 km [3 to 45 miles] thick. Alternate Form: deep seismic sounding

DSS

1. n. [Geophysics] A time-variant operation performed on seismic data. Normal moveout (NMO) is a dynamic correction. See: normal moveout,  processing,  static correction

dynamic correction

1. n. [Geophysics] The ratio of or difference between the highest and the lowest reading, or strongest and weakest signal, that can be recorded or reproduced by an instrument without distortion. See: decibel,  distortion,  signal

dynamic range