GEOPHYSICS

1. n. [Geophysics] The state of being attached to another entity: A well-planted geophone has a coupling to the Earth's surface or to a borehole wall that allows it to record ground motion during acquisition of seismic data. See: geophone,  plant,  transition zone,  tube wave

coupling

1. n. [Geophysics] Abbreviation for common receiver gather. A display of seismic traces that share a receiver. Alternate Form: common receiver gather See: seismic trace,  trace

CRG

1. n. [Geophysics] The angle of incidence according to Snell's law at which a refracted wave travels along the interface between two media. It can be quantified mathematically as follows: sin θc = V1 / V2, where θc = the critical angle V1 = velocity of the first medium V2 = velocity of the first medium, which is greater than V1. See: angle of incidence,  critical reflection,  head wave,  refraction,  refractor,  Snell's law

critical angle

1. n. [Geophysics] The minimum damping that will prevent or stop oscillation in the shortest amount of time, typically associated with oscillatory systems like geophones. Critical damping is symbolized by μc. See: damping,  geophone

critical damping

1. n. [Geophysics] A reflection, typically at a large angle, that occurs when the angle of incidence and the angle of reflection of a wave are equal to the critical angle. See: angle of incidence,  critical angle,  reflection

critical reflection

1. vb. [Geophysics] The comparison of different waveforms in digital form to quantify their similarity. A normalized crosscorrelation, or a correlation coefficient, equal to unity indicates a perfect match, whereas a poor match will yield a value close to zero. See: autocorrelation,  correlation,  lag,  waveform

crosscorrelate

1. n. [Geophysics] The comparison of different waveforms in digital form to quantify their similarity. A normalized crosscorrelation, or a correlation coefficient, equal to unity indicates a perfect match, whereas a poor match will yield a value close to zero. See: autocorrelation,  correlation,  lag,  waveform

crosscorrelation

1. n. [Geophysics] A seismic line within a 3D survey perpendicular to the direction in which the data were acquired. See: acquisition,  in-line,  three-dimensional seismic data

crossline

1. n. [Formation Evaluation, Geophysics] A crosswell seismic technique that incorporates reflection traveltimes and direct traveltimes into a tomographic inversion algorithm to produce images of seismic velocity between wells. See: crosswell seismic tomography,  tomography

crosswell reflection tomography

1. n. [Formation Evaluation, Geophysics] A survey technique that measures the seismic signal transmitted from a source, located in one well, to a receiver array in a neighboring well. The resulting data are processed to create a reflection image or to map the acoustic velocity or other properties (velocities of P- and S-waves, for example) of the area between wells. Placement of the source and receiver array in adjacent wells not only enables the formation between wells to be surveyed, it also avoids seismic signal propagation through attenuative near-surface formations. Another advantage is that it places the source and receiver near the reservoir zone of interest, thereby obtaining better resolution than is possible with conventional surface seismic surveys. This technique is often used for high-resolution reservoir characterization when surface seismic or vertical seismic profile (VSP) data lack resolution, or for time-lapse monitoring of fluid movements in the reservoir. See: tomography

crosswell seismic tomography

1. n. [Enhanced Oil Recovery, Formation Evaluation, Geophysics] A technique for measuring a signal that is broadcast from a transmitter or source located in one well, to a receiver array placed in a neighboring well. This technique is used to create a display of formation properties such as acoustic velocity and attenuation, seismic reflectivity, or electromagnetic resistivity in the area between wells. The reservoir-scale data acquired with this technique can be used to bridge the gap between wellbore measurements and surface measurements. See: tomography

crosswell tomography

1. n. [Geophysics] The magnetic field associated with the Earth’s crust arises from induced and remanent magnetism. The crustal field—also referred to as the anomaly field—varies in direction and strength when measured over the Earth’s surface. It is relatively strong in the vicinity of ferrous and magnetic materials, such as in the oceanic crust and near concentrations of metal ores, and is a focus of geophysical mineral exploration. See: main magnetic field,  external disturbance field,  local magnetic interference

crustal magnetic field

1. n. [Geology] Abbreviation for coal seam gas. Natural gas, predominantly methane [CH4], generated during coal formation and adsorbed in coal. Natural gas adsorbs to the surfaces of matrix pores within the coal and natural fractures, or cleats, as reservoir pressure increases. Production of natural gas from coal requires decreasing the pore pressure below the coal’s desorption pressure so that methane will desorb from surfaces, diffuse through the coal matrix and become free gas. Because the diffusivity and permeability of the coal matrix are ultralow, coal must have an extensive cleat system to ensure adequate permeability and flow of methane to wellbores at economic production rates. Coal seams are typically saturated with water. Consequently, the coal must be dewatered for efficient gas production. Dewatering reduces the hydrostatic pressure and promotes gas desorption from coal. As dewatering progresses, gas production often increases at a rate governed by how quickly gas desorbs from coal, the permeability of the cleat and the relative permeability of the gas-water system in the cleat. Eventually, the rate and amount of gas desorption decreases as the coal seam is depleted of its gas, and production declines. Coal seams with no water (dry coal) have been discovered and commercially exploited. In these reservoirs, the adsorbed gas is held in place by free gas in the cleats. Consequently, gas production consists of both free gas from the cleat system and desorbed gas from the matrix. Synonyms: coalbed methane,  coalbed methane,  coalbed methane,  CBM Alternate Form: coal seam gas,  coal seam gas See: unconventional resource

CSG

1. n. [Geophysics] A local geophysical anomaly generated by a man-made feature, such as electrical and communications wires, steel beams and tanks and railroad tracks. See: cultural noise

cultural anomaly

1. n. [Geophysics] Undesirable energy, or noise, generated by human activity, such as automobile traffic that interferes with seismic surveying, or electrical power lines or the steel in pipelines that can adversely affect electromagnetic methods. See: cultural anomaly,  electromagnetic method,  noise,  survey

cultural noise

1. n. [Geophysics] The generation of a theoretical equation to define a given dataset. In contrast, curve matching involves the comparison of well-understood data to a dataset of interest. See: curve matching

curve fitting

1. n. [Geophysics] The graphical comparison of well-understood datasets, called type curves, to another dataset. If a certain type curve closely corresponds to a dataset, then an interpretation of similarity can be made, although, as Sheriff (1991) points out, there might be other type curves that also match the data of interest. Curve matching differs from curve fitting in that curve fitting involves theoretical models rather than actual examples. See: curve fitting,  type curves

curve matching

1. n. [Geophysics] An anomalously high transit time in a log, such as a continuous velocity log, observable as a spike on the log, commonly caused by the presence of fractures, gas, unconsolidated formations, aerated drilling mud and enlarged boreholes. See: borehole,  drilling mud,  fracture,  transit time

cycle skip

1. n. [Geophysics] The opposition, slowing or prevention of oscillation, or decreasing vibration amplitude, as kinetic energy dissipates. Frictional damping can be important in the use of geophones for seismic surveys, since a vibrating instrument is difficult to read. Eddy currents can produce electromagnetic damping. The classic example of damping from physics is the slowing of a swinging pendulum unless it has a steady supply of energy. See: critical damping,  eddy current,  geophone,  seismic survey

damping

1. n. [Geophysics] An agreed and known value, such as the elevation of a benchmark or sea level, to which other measurements are corrected. In seismic data, the term refers to an arbitrary planar surface to which corrections are made and on which sources and receivers are assumed to lie to minimize the effects of topography and near-surface zones of low velocity. See: benchmark,  datum correction,  elevation correction,  receiver,  source

datum