GEOPHYSICS

1. n. [Geophysics] A type of explosive used as a source for seismic energy during data acquisition. Originally, dynamite referred specifically to a nitroglycerin-based explosive formulated in 1866 by Alfred Bernhard Nobel (1833 to 1896), the Swedish inventor who endowed the Nobel prizes. The term is incorrectly used to mean any explosive rather than the original formulation. See: air shooting,  shot depth

dynamite

1. n. [Geophysics] A form of azimuthal anisotropy that occurs when fractures or microcracks are not horizontal. Waves that travel parallel to the fractures have a higher velocity than waves traveling perpendicular to fractures. Alternate Form: extensive dilatancy anisotropy

EDA

1. n. [Geophysics] An alternating or transient electrical current in a conductive medium in the presence of a time-varying magnetic field. The eddy current generates its own electromagnetic field. See: damping,  electromagnetic method

eddy current

1. n. [Geophysics] A hydrophone array in a cable that can be attached to a streamer for acquisition of marine seismic data. The eel can be suspended from the streamer so that the eel is close to the seafloor but the streamer remains high enough to avoid obstacles on the seafloor such as reefs or debris from human activity.

eel

1. n. [Geophysics] The variation of elastic properties with direction. For example, elastic anisotropy occurs when seismic, or elastic, waves travel through rock at differing velocities in various directions. Elastic anisotropy occurs if there is a preferred alignment of a material’s fabric elements—crystals, grains, cracks, bedding planes, joints or fractures—on a scale smaller than the length of the wave. This alignment causes waves to propagate fastest in its direction. Elastic anisotropy is sometimes called seismic anisotropy, velocity anisotropy, traveltime anisotropy, acoustic anisotropy or slowness anisotropy.

elastic anisotropy

1. n. [Geophysics] A set of constants, also known as elastic moduli, that defines the properties of material that undergoes stress, deforms, and then recovers and returns to its original shape after the stress ceases. The elastic constants include the bulk modulus, Lame constant, Poisson's ratio, shear modulus, and Young's modulus. Elastic constants are important in seismology because the velocity of waves depends on the elastic constants and density of the rock. Synonyms: elastic moduli,  modulus of elasticity See: bulk modulus,  elastic,  Lamé constant,  Poisson's ratio,  shear modulus,  Young's modulus

elastic constants

1. n. [Geology] The deformation that can be recovered when an applied stress has been removed. When the elastic limit of a material has been exceeded, nonrecoverable, permanent deformation occurs. See: elastic limit,  plastic deformation,  rheology

elastic deformation

1. n. [Geophysics] The product of the density of a medium and its shear wave velocity. See: acoustic impedance

elastic impedance

1. n. [Geophysics] A set of constants that defines the properties of material that undergoes stress, deforms, and then recovers and returns to its original shape after the stress ceases. The elastic constants include the bulk modulus, Lame constant, Poisson's ratio, shear modulus, and Young's modulus. Elastic constants are important in seismology because the velocity of waves depends on the elastic constants and density of the rock. Synonyms: elastic constants,  modulus of elasticity See: bulk modulus,  elastic,  Lamé constant,  Poisson's ratio,  shear modulus,  Young's modulus

elastic moduli

1. n. [Geophysics] A seismic or acoustic wave, such as a P-wave. See: acoustic emission,  acoustic wave,  channel wave,  P-wave,  seismic wave

elastic wave

1. n. [Geophysics] The ability of a material to support the flow of an electrical current. In linear isotropic materials, the electric current density at any point in space is proportional to the electric field; the constant of proportionality is the electrical conductivity. Conductivity is the inverse of resistivity in isotropic materials, and is measured in siemens per meter or the archaic units of mhos per meter. The electrical conductivity of the Earth can be measured by electromagnetic methods. The electrical conductivity symbol is σ. See: electromagnetic method

electrical conductivity

1. n. [Geophysics] The ability of a material to store a charge from an applied electrical field without conducting electricity. Synonyms: permittivity See: dielectric,  electromagnetic method

electrical permittivity

1. n. [Geophysics] The ability of a material to resist or inhibit the flow of an electrical current, measured in ohm-meters. Resistivity is the reciprocal of conductivity. The electrical resistivity symbol is ρ.

electrical resistivity

1. n. [Geophysics] A group of techniques in which natural or artificially generated electric or magnetic fields are measured at the Earth's surface or in boreholes in order to map variations in the Earth's electrical properties (resistivity, permeability or permittivity). Most applications of surface electromagnetic methods today are for mineral and groundwater exploration or for shallow environmental mapping. Electromagnetic or electrical logging is, however, the main technique used in oil exploration to measure the amount of hydrocarbons in the pores of underground reservoirs. Inductive electromagnetic (EM) methods include a variety of low frequency (a few Hz to several kHz) techniques deploying large or small wire coils at or near the surface. In older usage, "electromagnetic method" tended to refer only to inductive methods. This term is now commonly used for any method employing electromagnetic fields, including methods that use direct current (electrical or resistivity methods) and induced polarization (IP), methods that use microwave frequencies (ground-penetrating radar), and methods that use natural electromagnetic fields (magnetotelluric methods). See: array,  cultural noise,  diffusion equation,  dipole,  eddy current,  electrical conductivity,  electrical permittivity,  fixed-source method,  geometric,  induced polarization,  magnetics,  magnetotelluric method,  Maxwell's equations,  moving-source method,  Occam's inversion,  parametric,  perpendicular offset,  probe,  reflection,  skin depth,  survey,  transient electromagnetic method,  transverse electric mode,  transverse magnetic mode

electromagnetic method

1. n. [Geophysics] Any compensating factor used to bring measurements to a common datum or reference plane. In gravity surveying, elevation corrections include the Bouguer and free-air corrections. Seismic data undergo a static correction to reduce the effects of topography and low-velocity zones near the Earth's surface. Well log headers include the elevation of the drilling rig's kelly bushing and, for onshore locations, the height of the location above sea level, so that well log depths can be corrected to sea level. See: Bouguer correction,  free-air correction,  gravity survey,  kelly bushing,  static correction

elevation correction

1. n. [Geophysics] The shape of a wavelet produced by reflection of an actual wave train at one interface with a positive reflection coefficient. The embedded wavelet is useful for generating a convolutional model, or the convolution of an embedded wavelet with a reflectivity function and random noise, during seismic processing or interpretation. Alternate Form: basic wavelet See: reflection coefficient,  seismic processing

embedded wavelet

1. n. [Geophysics] A P-wave parameter for a medium in which the elastic properties exhibit vertical transverse isotropy. Epsilon (ε) is the P-wave anisotropy parameter and equal to half the ratio of the difference between the horizontal and vertical P-wave velocities squared divided by the vertical P-wave velocity squared. ε ≡ ½ [(C11 − C33) / C33] = ½ [(VP⊥2 − VP∥2) / VP∥2] P-wave parameter (ε) for a medium in which the elastic properties exhibit vertical transverse isotropy, where C11 is the horizontal P-wave modulus (perpendicular to the symmetry axis), C33 is the vertical P-wave modulus (parallel to the symmetry axis), VP⊥ is the horizontal P-wave velocity and VP∥ is the vertical P-wave velocity. Reference: Thomsen L: “Weak Elastic Anisotropy,” Geophysics 51, no. 10 (October 1986): 1954–1966. See: delta,  gamma (γ),  eta (η)

epsilon (ε)

1. n. [Geophysics] A technique to map a potential field generated by stationary electrodes by moving an electrode around the survey area. Synonyms: applied-potential method

equipotential method

1. n. [Geophysics] A measure of the anellipticity of the P-wave phase slowness—the inverse of P-wave phase velocity—in rock exhibiting vertical transverse isotropy. η = (ε − δ) / (1 + 2δ) Anellipticity of P-wave phase slowness for a medium in which the elastic properties exhibit vertical transverse isotropy. Eta (η) is the anellipticity and ε and δ are the P-wave anisotropy parameters. When ε and δ are equal, η = 0 and the P-wave phase slowness is an ellipse. When ε = δ = 0, the P-wave phase slowness is isotropic. Reference: Alkhalifah T and Tsvankin I: “Velocity Analysis for Transversely Isotropic Media,” Geophysics 60, no. 5 (September–October 1995): 1550–1566. See: epsilon (ε),  delta

eta (η)

1. n. [Geophysics] An appearance of seismic data as a diffraction, reflection, refraction or other similar feature produced by an arrival of seismic energy. An event can be a single wiggle within a trace, or a consistent lining up of several wiggles over several traces. An event in a seismic section can represent a geologic interface, such as a fault, unconformity or change in lithology. See: abnormal events,  attenuate,  bow tie,  character,  coherence filtering,  dim spot,  interval time,  isochron map,  lag,  long-path multiple,  multiple reflection,  peg-leg multiple,  phantom,  pick,  primary reflection,  processing,  seismic section,  short-path multiple,  sideswipe,  signature,  smile,  space frequency domain,  tuning effect

event