GEOPHYSICS 4 Flashcards
A statistical technique used with variograms, or two-point statistical functions that describe the increasing difference or decreasing correlation between sample values as separation between them increases, to determine the value of a point in a heterogeneous grid from known values nearby.
Kriging
The delay or difference in the arrival time of seismic events that can result from weathering of the rocks or variations in geologic structures in the subsurface.
lag
A term used in seismic processing to describe the interval between the zero-time of a crosscorrelation between two traces and the point of maximum correlation.
lag
The time delay of the onset of one sinusoidal oscillation, or frequency component of a trace, relative to another. Also known as a “phase-lag.”
lag
The distance between the static measure point and the dynamic measure point of a logging measurement. For nuclear logs and any others that must be recorded over a significant time period, there is a difference between the measure point with the tool stationary and moving. If the tool is moving during this period, the effective center of measurement will be a certain distance from the point at which the measurement started. This distance is the lag. The lag depends on the logging speed and the sampling interval.
lag
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 modulusτ = Shear stress = ΔF/AΔF = Increment of shear forceA = Area acted on by the shear forceγ = Shear strain = ΔL/LΔL = Increment of transverse displacement parallel to AL = 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 constantμ = Lamé’s second constant, the shear modulusVP = Compressional-wave (P-wave) velocityVS = Shear-wave (S-wave) velocityρ = Density.
Lame constant
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.
Laplace equation
A method of seismic inversion whereby the effects of rock layers having different seismic characteristics are removed from layers below.
layer stripping
The fastest route that a seismic ray can travel between two points, generally dictated by Fermat’s principle.
least-time path
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.
litostratigraphic inversion
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.
local magnetic interference
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.
long-path multiple
A type of surface wave in which particles oscillate horizontally and perpendicularly to the direction of wave propagation.
love wave
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.
low-velocity layer
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.
magnetic constant
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, the external disturbance field and 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.
magnetic field
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).
magnetic permeability
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.
magnetic total field
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.
magnetics
An instrument used to measure the strength or direction of the Earth’s magnetic field.
magnetometer
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.
Magnetotelluric method
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.
Main magnetic field
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.
Marker bed
“A group of four partial differential equations that describe all classical phenomena, involving electric and magnetic fields. James Clerk Maxwell (1831 to 1879), a British physicist, first wrote out this complete set of equations: (1.) ∇·D = ρ, (2.) ∇×H = J + (∂D/∂t), (3.) ∇·B = 0, (4.) ∇×E = −(∂B/∂t). whereD = electric displacementρ = electric charge densityH = magnetic field strengthJ = electric current densityB = magnetic flux densityE = electric field strength.
Equation (1) is equivalent to Coulomb’s law, the inverse square attraction of static electric charges. Equation (2) is Ampere’s law relating magnetic fields and currents, which was extended by Maxwell to include induction of a magnetic field by a time-varying electric displacement. Equation (3) is Coulomb’s law for magnetic flux, expressing the absence of isolated magnetic charges. Equation (4) is Faraday’s law of induction, relating an electric field to a time-varying magnetic flux. Maxwell’s equations are the starting point for all calculations involving surface or borehole EM methods.
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Maxwell’s equations
The halfway point between a seismic source and a receiver at the Earth’s surface.
Midpoint
To execute a step inseismic processingin which reflections in seismic data are moved to their correct locations in x-y-time space of seismic data. Migration improves seismic interpretation and mapping because the locations of geological structures, especially faults, are more accurate in migrated seismic data. Proper migration collapses diffractions from secondary sources such as reflector terminations against faults and corrects bow ties to form synclines. There are numerous methods of migration, such as dip moveout (DMO), frequency domain, ray-trace and wave-equation migration.
Migrate
For hydrocarbons to move from theirsourceinto reservoir rocks. The movement of newly generated hydrocarbons out of their source rock is primary migration, also called expulsion. The further movement of the hydrocarbons into reservoir rock in a hydrocarbon trap or other area of accumulation is secondary migration. Migration typically occurs from a structurally low area to a higher area because of the relative buoyancy of hydrocarbons in comparison to the surrounding rock. Migration can be local or can occur along distances of hundreds of kilometers in large sedimentary basins, and is critical to the formation of a viable petroleum system.
Migrate
A step in seismic processing in which reflections in seismic data are moved to their correct locations in the x-y-time space of seismic data, including two-way traveltime and position relative to shotpoints. Migration improves seismic interpretation and mapping because the locations of geological structures, especially faults, are more accurate in migrated seismic data. Proper migration collapses diffractions from secondary sources such as reflector terminations against faults and corrects bow ties to form synclines. There are numerous methods of migration, such as dip moveout (DMO), frequency domain, ray-trace and wave-equation migration.
Migration
The movement of hydrocarbons from their source into reservoir rocks. The movement of newly generated hydrocarbons out of their source rock is primary migration, also called expulsion. The further movement of the hydrocarbons into reservoir rock in a hydrocarbon trap or other area of accumulation is secondary migration. Migration typically occurs from a structurally low area to a higher area because of the relative buoyancy of hydrocarbons in comparison to the surrounding rock. Migration can be local or can occur along distances of hundreds of kilometers in large sedimentary basins, and is critical to the formation of a viable petroleum system.
Migration
The fastest route that a seismic ray can travel between two points, generally dictated by Fermat’s principle.
Minimum time path
A situation in interpretation of seismic data in which predicted and actual values differ, or when an interpreted reflection does not close, or tie, when interpreting intersecting lines; or when interpreted seismic data do not match results of drilling a well. Mis-ties commonly occur when data of different phases, rather than uniformly zero-phase data, are interpreted together, or data that have different datum corrections are tied. Mis-ties are described as static if they involve a bulk shift of data (as in the case of tying seismic sections with different datum corrections) or dynamic if the magnitude of the mis-tie varies with time (as in the case of data that have been migrated differently).
mis tie
A situation in interpretation of seismic data in which predicted and actual values differ, or when an interpreted reflection does not close, or tie, when interpreting intersecting lines; or when interpreted seismic data do not match results of drilling a well. Mis-ties commonly occur when data of different phases, rather than uniformly zero-phase data, are interpreted together, or data that have different datum corrections are tied. Mis-ties are described as static if they involve a bulk shift of data (as in the case of tying seismic sections with different datum corrections) or dynamic if the magnitude of the mis-tie varies with time (as in the case of data that have been migrated differently).
Mis-tie
To produce a representation or a simulation of a problem.
Model
A conceptual, three-dimensional construction of a reservoir or oil and gas field. The model is constructed from incomplete data and much of the interwell space must be estimated from nearby wells or from low vertical resolution data, such as seismic data. The construction of models can be performed by deterministic methods or geostatistical methods, or a combination of both.
Model
A representation of a physical property or entity that can be used to make predictions or compare observations with assumptions. Mathematical velocity models are commonly used to predict the depth to a formation of interest. Physical models, such as layers of clay or putty, can be used to simulate rock layers. As Sheriff (1991) points out, agreement between data and a model does not prove that the model is correct, since there can be numerous models that agree with a given data set.
Model/ Modeling
The act of constructing a model.
modeling/ model
Another term for bulk modulus, the ratio of stress to strain, abbreviated as k. The bulk modulus is an elastic constant equal to the applied stress divided by the ratio of the change in volume to the original volume of a body.
modulus of compression
Any one of 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.
Modulus of elasticity/ elastic constants/ elastic moduli
Another term for shear modulus, an elastic constant for the ratio of shear stress to shear strain. The shear modulus is one of the Lamé constants. It can be expressed mathematically as follows: μ = τ / γ = (ΔF/A) / (ΔL/L), where μ = Shear modulusτ = Shear stress = ΔF/AΔF = Increment of shear forceA = Area acted on by the shear forceγ = Shear strain = ΔL/LΔL = Increment of transverse displacement parallel to AL = Original length.
Modulus of rigidity
A relatively permanent, fixed marker used in surveying, such as a concrete block or steel plate, with an inscription of location and elevation.
Monument/ benchmark
