Glossary of Geophysical Terms (D,E,F)

Decay
In nuclear physics, the process of disintegration of an unstable radioisotope by the spontaneous emission of ged particles or photons.
Decentralize
Forcing a logging probe against one side of the drill hole.
Deconvolution
A data processing technique applied to seismic reflection data to improve the detection and resolution of reflected events. The process reverses the effect of linear filtering processes (convolution) that have been applied to the data by recording instruments or other processes.
Dense-non-aqueous-phase liquids (DNAPLs)
Organic liquids that are more dense than water. They often coalesce in an immiscible layer at the bottom of a saturated geologic unit.
Density log
Also called gamma-gamma log; gamma photons from a radioactive source in the sonde are backscattered to a detector; the backscattering is related to the bulk density of the material around the sonde.
Departure curves
Graphs that show the correction that may be made to logs for some extraneous effects, such as hole diameter, bed thickness, temperature, etc.
Depth of investigation
Limit of signal penetration.
Depth reference or datum
Zero reference point for logs of a borehole.
Depth section
A cross section to which a velocity function has been applied, thus converting arrival times of reflections to depths.
Detector
Any kind of a sensor used to detect a form of energy, but usually refers to nuclear detectors, such as scintillation crystals.
Deviation
The departure in degrees between the drill hole or probe axis and vertical.
Dielectric constant
A measure of the ability of a material to store charge when an electric field is applied.
Dielectric permittivity
Describes the ge separation or polarization in a medium.
Differential log
A log that records the rate of change of some logged value as a function of depth; the differential log is sensitive to very small changes in absolute value.
Digital log
A log recorded as a series of discrete numerical values (compare analog recording).
Dipmeter
A multi-electrode, contact-resistivity probe that provides data from which the strike and dip of bedding can be determined.
Dipole
A pair of equal charges or poles of opposite signs. In resistivity, a pair of closely spaced electrodes approximating a dipole.
Directional survey
A log that provides data on the azimuth and deviation of a borehole from the vertical.
Dispersion
A property of seismic surface waves in which their velocity (as well as their penetration into the subsurface) is frequency dependent. The basis of methods such as MASW in which seismic wave velocity is analyzed as a function of wave frequency.
Displacement currents
The movement of charge within a material by polarization, as opposed to the flow of free ions or electrons. Related to the applied electric field by the electric permittivity (dielectric constant).
Dual laterolog
A focused resistivity log with both shallow and deep investigation; usually gamma, SP, and microfocused logs are run simultaneously.
Effective porosity
The amount of interconnected pore space through which fluids can pass. Effective porosity is usually less than total porosity because some dead-end pores may be occupied by static fluid.
Elastic moduli (elastic constants)
Elastic moduli specify the stress- strain properties of isotropic materials in which stress is proportional to strain. They include bulk and shear moduli.
Electric field
A vector field describing the force on a unit electrical charge [newtons/coulomb = volts/meter].
Electrical logs
Provide information on porosity, hydraulic conductivity, and fluid content of formations drilled in fluid-filled boreholes. This record is based on the dielectric properties (e.g., electrical resistivity) of the aquifer materials measured by geophysical devices lowered down boreholes or wells.
Electrode
A piece of metallic material (typically stainless steel) acting as an electric contact with a non-metal substance.
Electromagnetic-casing inspection log
The effects of eddy currents on a magnetic field are used to provide a record of the thickness of the casing wall.
Electromagnetic method
A method which measures magnetic and/or electric fields associated with subsurface currents.
Electron volt
The energy acquired by an electron passing through a potential difference of one volt (eV); used for measuring the energy of nuclear radiation and particles, usually expressed as million electron volts (MeV).
Equipotential map
A plot in which points of equal hydraulic head are connected.
Fan shooting
A seismic refraction technique where the sensors (geophones) are deployed on a segment of a circle centered on the seismic source. Variations in the time of arrival are caused by radial variations in the velocity structure. Could be used, for example, to search for low velocity anomalies caused by buried waste.
Ferrimagnetic
Substances having positive and relatively large magnetic susceptibility as well as generally large hysteresis and remanence. This is due to the interaction of atoms and the coupling of magnetic moments aligned in opposition, which result in non-zero net moments. Ferrimagnetic minerals have this property.
Field
That space in which an effect, such as gravity or magnetism, is measurable.
Field print
A copy of a log obtained at the time of logging that has not been edited or corrected.
Filtering
a) The attenuation of a signal’s components based on a measurable property (usually frequency). Filtering usually involves a numerical operation that enhances only a portion of the signal. b) Fluid passage through a material that retains particles or colloids above a certain size.
First reading
The depth at which logging began at the bottom of the hole.
Flexural Waves
Flexural waves occur in bars and refers to the flexing, or bending, of a bar. Thus they can be created in shafts by impacting the side of a shaft. The velocity of flexural waves depends on their wavelength.
Flowmeter
A logging device designed to measure the rate, and usually the direction, of fluid movement in a well; most are designed to measure vertical flow.
Fluid sampler
An electronically controlled device that can be run on a logging cable to take water samples at selected depths in the well.
Flushed zone
The zone in the borehole wall behind the mudcake that is considered to have had all mobile native fluids flushed from it.
Focused log
A resistivity log that employs electrodes designed to focus the current into a sheet that provides greater penetration and vertical resolution than unfocused logs.
Formation
Used in well-logging literature in a general sense to refer to all material penetrated by a drill hole without regard to its lithology or structure; used in a stratigraphic sense, formation refers to a named body of rock strata with unifying lithologic features.
Formation-resistivity factor
Formation factor (F) is the ratio of the electrical resistivity of a rock 100 percent saturated with water (Ro) to the resistivity of the water with which it is saturated (Rw). F = Ro/Rw.
Frequency domain
In geophysics, refers to measurements analyzed according to their constituent frequencies. The usual alternative is time domain measurements.