GEOPHYSICS 2 Flashcards
The process of transforming seismic data from a scale of time (the domain in which they are acquired) to a scale of depth to provide a picture of the structure of the subsurface independent of velocity. Depth conversion, ideally, is an iterative process that begins with proper seismic processing, seismic velocity analysis and study of well data to refine the conversion. Acoustic logs, check-shot surveys and vertical seismic profiles can aid depth conversion efforts and improve correlation of well logs and drilling data with surface seismic data.
depth conversion
A two-dimensional representation of subsurface structure with contours in depth that have been converted from seismic traveltimes.
depth map
A step in seismic processing in which reflections in seismic data are moved to their correct locations in space, including position relative to shotpoints, in areas where there are significant and rapid lateral or vertical changes in velocity that distort the time image. This requires an accurate knowledge of vertical and horizontal seismic velocity variations.
depth migration
A point on the surface for which the depth to a horizon has been calculated in a refraction seismic survey. The term is commonly misused as a synonym for common depth point.
depth point
A display of seismic data with a scale of units of depth rather than time along the vertical axis. Careful migration and depth conversion are essential for creating depth sections.
depth section
The minimum thickness necessary for a layer of rock to be visible or distinct in reflection seismic data. Generally, the detectable limit is at least 1/30 of the wavelength. Acquisition of higher frequency seismic data generally results in better detection or vertical resolution of thinner layers.
detectable limit
A sensor or receiver, such as a geophone or hydrophone, gravimeter or magnetometer.
detector
A type of inverse filtering, or deconvolution, in which the effects of the filter are known by observation or assumed, as opposed to statistical deconvolution.
deterministic deconvolution
To set off an explosive material. Explosive sources are used in seismic acquisition and explosive charges are used to perforate wells in preparation for production or injection.
detonate
A device containing primary high-explosive material that is used to initiate an explosive sequence. The two common types of detonators are electrical detonators (also known as blasting caps) and percussion detonators. Electrical detonators have a fuse material that burns when high voltage is applied to initiate the primary high explosive. Percussion detonators contain abrasive grit and primary high explosive in a sealed container that is activated by a firing pin. The impact force of the firing pin is sufficient to initiate the ballistic sequence that is then transmitted to the detonating cord. Several safety systems are used in conjunction with detonators to avoid accidental firing during rig-up or rig-down. Safety systems also are used to disarm the gun or ballistic assembly if downhole conditions are unsafe for firing.
detonator/ blasting cap
A small, electrically activated explosive charge that explodes a larger charge. Detonators, also called caps, seismic caps or blasting caps, are used for seismic acquisition with an explosive source to achieve consistent timing of detonation.
detonator/ blasting cap
A material used in a capacitor to store a charge from an applied electrical field. A pure dielectric does not conduct electricity.
dielectric
The degree to which a medium resists the flow of electric charge, defined as the ratio of the electric displacement to the electric field strength. It is more common to use the relative dielectric permittivity.
dielectric constant/ dielectric permittivity
A map that represents the change from one map to another, such as a reservoir map of an area made from two different seismic surveys separated in production history (one possible product of 4D seismic data), or an isochron map that displays the variation in time between two seismic events or reflections.
difference map
A type of static correction that compensates for delays in seismic reflection or refraction times from one point to another, such as among geophone groups in a survey. These delays can be induced by low-velocity layers such as the weathered layer near the Earth’s surface.
differential weathering correction
A type of event produced by the radial scattering of a wave into new wavefronts after the wave meets a discontinuity such as a fault surface, an unconformity or an abrupt change in rock type. Diffractions appear as hyperbolic or umbrella-shaped events on a seismic profile. Proper migration of seismic data makes use of diffracted energy to properly position reflections.
diffraction
Also known as Kirchhoff migration, a method of seismic migration that uses the integral form (Kirchhoff equation) of the wave equation. All methods of seismic migration involve the backpropagation (or continuation) of the seismic wavefield from the region where it was measured (Earth’s surface or along a borehole) into the region to be imaged. In Kirchhoff migration, this is done by using the Kirchhoff integral representation of a field at a given point as a (weighted) superposition of waves propagating from adjacent points and times. Continuation of the wavefield requires a background model of seismic velocity, which is usually a model of constant or smoothly varying velocity. Because of the integral form of Kirchhoff migration, its implementation reduces to stacking the data along curves that trace the arrival time of energy scattered by image points in the earth.
diffraction stack/ Kirchhoff migration
The process by which particles move over time within a material due to their kinetic motion. The term is most commonly used in pulsed neutron capture logging and in nuclear magnetic resonance (NMR) logging. In a pulsed neutron capture log, the term refers to the spread of neutrons away from the neutron generator. In NMR logging, diffusion refers to the movement of gas, oil or water molecules within the pore space.
diffusion
The movement of ions or molecules from regions of high concentration to low concentration within a solution.
diffusion
The conduction of heat by the movement of molecules.
diffusion
A partial differential equation describing the variation in space and time of a physical quantity that is governed by diffusion. The diffusion equation provides a good mathematical model for the variation of temperature through conduction of heat and the propagation of electromagnetic waves in a highly conducting medium. The diffusion equation is a parabolic partial differential equation whose characteristic form relates the first partial derivative of a field with respect to time to its second partial derivatives with respect to spatial coordinates. It is closely related to the wave equation. ∇2E = j ω μ σ E. whereE = electrical fieldω = angular frequencyμ = magnetic permeabilityσ = electrical conductivity∇ = vector differential operator.
diffusion equation
A fundamental differential equation obtained by combining the continuity equation, flow law and equation of state. Most of the mathematics of well testing were derived from solutions of this equation, which was originally developed for the study of heat transfer. Fluid flow through porous media is directly analogous to flow of heat through solids. Solutions used in well testing usually assume radial flow and homogenous, isotropic formations.
diffusion equation
The increase in the volume of rocks as a result of deformation, such as when fractures develop.
dilatancy
A possible explanation for volume changes in rocks due to strain, such as microfracturing or cracking, and the accompanying change in the ratio of P- to S-wave velocity. Support for dilatancy theory comes in the form of porosity increases from 20 to 40% that have been measured in laboratory experiments using rock samples.
dilatancy theory