GEOPHYSICS

1. n. [Geophysics] One of two elastic constants named for French mathematician Gabriel Lamé (1795 to 1870). The first Lamé constant is λ, the bulk modulus (K) less two-thirds of the shear modulus (μ): λ = K − (2/3)μ The second Lamé constant is the shear modulus (μ): μ = τ / γ = (ΔF/A) / (ΔL/L), where μ = Shear modulus τ = Shear stress = ΔF/A ΔF = Increment of shear force A = Area acted on by the shear force γ = Shear strain = ΔL/L ΔL = Increment of transverse displacement parallel to A L = Original length. Lamé constants derived from elastic-wave velocities: λ = ρ(VP2 − 2VS2) μ = ρVS2 λ/μ = (VP/VS)2 − 2, where λ = Lamé's first constant μ = Lamé's second constant, the shear modulus VP = Compressional-wave (P-wave) velocity VS = Shear-wave (S-wave) velocity ρ = Density. See: bulk modulus,  elastic constants,  shear modulus,  P-wave,  S-wave

Lamé constant

1. n. [Geophysics] A partial differential equation that governs potential fields (in regions where there are no sources) and is equivalent, in three dimensions, to the inverse square law of gravitational or electrical attraction. In Cartesian coordinates, the Laplace equation equates the sum of the second partial (spatial) derivatives of the field to zero. (When a source is present, this sum is equal to the strength of the source and the resulting equation is called Poisson's equation). The differential equation is named for French mathematician Pierre-Simon de Laplace (1749 to 1827), and applies to electrical, gravity and magnetic fields. ∇2u = ∂2u/∂x2 + ∂2u/∂y2 + ∂2u/∂z2 = 0, where u(x,y,z) is a potential function. See: potential field,  spherical harmonic

Laplace equation

1. n. [Geophysics] A method of seismic inversion whereby the effects of rock layers having different seismic characteristics are removed from layers below. See: inversion

layer stripping

1. n. [Geophysics] The fastest route that a seismic ray can travel between two points, generally dictated by Fermat's principle. See: Fermat's principle,  Snell's law

least-time path

1. n. [Geophysics] A seismic inversion technique that attempts to describe lithology of individual rock layers and evaluate properties and distribution of pore fluids through analysis of variation of reflected seismic amplitude with offset. See: amplitude variation with offset,  inversion,  offset

lithostratigraphic inversion

1. n. [Geophysics] Magnetic interference caused by nearby structures such as metallic rigs and wells. The magnetic permeability of drillstrings and the remanent magnetization in drillstrings contribute to perturbations of the measured magnetic field. Operators may use nonmagnetic drill collars to reduce these effects along with software techniques to compensate for them. See: main magnetic field,  crustal magnetic field,  external disturbance field

local magnetic interference

1. n. [Geophysics] A type of multiply-reflected seismic energy that appears as an event. Long-path multiples generate distinct events because their travel path is much longer than primary reflections giving rise to them. They typically can be removed by seismic processing. See: event,  multiple reflection,  noise,  peg-leg multiple,  primary reflection,  short-path multiple,  simple multiple

long path multiple

1. n. [Geophysics] A type of multiply-reflected seismic energy that appears as an event. Long-path multiples generate distinct events because their travel path is much longer than primary reflections giving rise to them. They typically can be removed by seismic processing. See: event,  multiple reflection,  noise,  peg-leg multiple,  primary reflection,  short-path multiple,  simple multiple

long-path multiple

1. n. [Geophysics] A type of surface wave in which particles oscillate horizontally and perpendicularly to the direction of wave propagation. Synonyms: Q-wave See: wave

Love wave

1. n. [Geophysics, Geology] Also known as weathered layer, a near-surface, possibly unconsolidated layer of low seismic velocity. The base of the weathered layer commonly coincides with the water table and a sharp increase in seismic velocity. The weathered layer typically has air-filled pores. Alternate Form: low-velocity layer See: pore,  static correction,  weathering,  weathering correction

low velocity layer

1. n. [Geology, Geophysics] Also known as weathered layer, a near-surface, possibly unconsolidated layer of low seismic velocity. The base of the weathered layer commonly coincides with the water table and a sharp increase in seismic velocity. The weathered layer typically has air-filled pores. See: low velocity layer,  pore,  static correction,  weathering,  weathering correction

low-velocity layer

1. n. [Geophysics] Another term for magnetic permeability, the ratio of the density of the magnetic flux, B (in units of teslas), to the strength of the magnetic field, H (in units of amperes/meter), typically expressed in units of H/m. See: magnetics,  skin depth

magnetic constant

1. n. [Geophysics] The magnetic field measured near the Earth’s surface is the superposition of magnetic fields arising from various time-varying physical processes that are grouped into four general components: the main magnetic field the crustal field external disturbance field local magnetic interference. The significance of these contributions to direction, strength and stability of the magnetic field varies with geographic region and with magnetic survey direction. See: main magnetic field,  crustal magnetic field,  external disturbance field,  local magnetic interference

magnetic field

1. n. [Geophysics] The ratio of the density of the magnetic flux, B (in units of teslas), to the strength of the magnetic field, H (in units of amperes/meter), typically expressed in units of henries per meter (H/m). See: magnetics,  skin depth

magnetic permeability

1. n. [Geophysics] The modulus of the magnetic field vector. The magnetic total field is the magnitude, or absolute value, of the magnetic field vector. The magnetic total field describes the strength, or intensity, of the magnetic field, which is measured in units of nanoTesla (nT). The symbol for the magnetic total field is often F or Btotal. See: magnetic field,  main magnetic field

magnetic total field

1. n. [Geophysics] The study of the Earth's magnetic field, a branch of geophysics that began with the observation by British scientist William Gilbert (1544 to 1603) that the Earth is a magnet. Variations in the magnetic field can be used to determine the extent of sedimentary basins and the depth to basement rocks, as well as to differentiate between igneous rocks and certain sedimentary rocks such as salt. High-resolution magnetic surveys can also be used to determine the locations of oil pipelines and production equipment. See: deep tow,  downward continuation,  electromagnetic method,  geophysics,  inclinometer,  permeability,  potential field,  remote sensing,  spherical divergence,  survey,  upward continuation

magnetics

1. n. [Geophysics] An instrument used to measure the strength or direction of the Earth's magnetic field. See: aeromagnetic survey,  bird,  detector

magnetometer

1. n. [Geophysics] An electromagnetic method used to map the spatial variation of the Earth's resistivity by measuring naturally occurring electric and magnetic fields at the Earth's surface. These natural EM fields are generated (at all frequencies) in the Earth's atmosphere mainly by lightning strokes and by interactions between the solar wind and the ionosphere. In the most general MT method, the horizontal components of the electric field and all three components of the magnetic field are measured at the surface. The measurements are used to determine specific ratios of electric to magnetic field components called tensor impedances. The technique was introduced the French geophysicist Louis Cagniard in the 1950s and has been popular for mineral exploration and regional geophysical mapping. It is used in oil exploration for low-cost reconnaissance of sedimentary basins and for exploration in areas where seismic surveys are difficult because of severe topography or the presence high-impedance volcanic rocks near the surface. The resolution of MT surveys is limited by the diffusive nature of EM propagation in the earth; it is usually on the order of hundreds of meters to kilometers. But the MT method can probe the Earth to depths of several tens of kilometers. Alternate Form: MT See: electromagnetic method,  Occam's inversion,  probe

magnetotelluric method

1. n. [Geophysics] Earth’s main magnetic field generated in the Earth’s fluid outer core by a self-exciting dynamo process. Approximately 95% of the total magnetic field measured at Earth’s surface comes from this main field, a significant portion of which may be described as the field of a dipole placed at the Earth’s center and tilted approximately 11° from the Earth’s rotational axis. The magnitude of the main magnetic field is nearly 60,000 nT near the Earth’s poles and about 30,000 nT near the equator. However, there are significant nondipole contributions to the main magnetic field that complicate its mathematical and graphical representation, including that the relative strengths of nondipole components change. As additional complications, the main field varies slowly because of changes within the Earth’s core and the magnetic dipole axis pole position itself wanders over time. See: dipole field,  nondipole field,  crustal magnetic field,  external disturbance field,  local magnetic interference

main magnetic field

1. n. [Geophysics] A widespread distinctive rock unit that can be correlated readily over a large area. The most useful marker beds tend to form rapidly, such as during volcanic or geologically instantaneous depositional events, and have unusual seismic, magnetic, electrical or other physical properties that aid geological or geophysical interpretation. Coal beds and volcanic ash falls are examples of marker beds. See: correlate,  igneous,  pick

marker bed