GEOPHYSICS

1. n. [Geophysics] A value added to reflection times of seismic data to compensate for the location of the geophone and source relative to the seismic datum. See: datum,  geophone,  reflection,  source

datum correction

1. n. [Geophysics] The unit of measurement to compare the relative intensity of acoustic or electrical signal, equal to one-tenth of a bel, named for American inventor Alexander Graham Bell (1847 to 1922). The logarithm of the ratio of the sound or signal to a standard provides the decibel measurement. The symbol for the unit is dB. Humans typically hear sounds in the range of 20 to 50 dB in conversation, and upwards of 90 dB when exposed to heavy machinery or aircraft. Alternate Form: decibel

dB

1. n. [Geophysics] The unit of measurement to compare the relative intensity of acoustic or electrical signal, equal to one-tenth of a bel, named for American inventor Alexander Graham Bell (1847 to 1922). The logarithm of the ratio of the sound or signal to a standard provides the decibel measurement. The symbol for the unit is dB. Humans typically hear sounds in the range of 20 to 50 dB in conversation, and upwards of 90 dB when exposed to heavy machinery or aircraft. Alternate Form: dB See: acoustic,  bel,  dynamic range,  signal

decibel

1. n. [Geophysics] A step in seismic signal processing to recover high frequencies, attenuate multiples, equalize amplitudes, produce a zero-phase wavelet or for other purposes that generally affect the waveshape. Deconvolution, or inverse filtering, can improve seismic data that were adversely affected by filtering, or convolution that occurs naturally as seismic energy is filtered by the Earth. Deconvolution can also be performed on other types of data, such as gravity, magnetic or well log data. See: autocorrelation,  backscatter,  deterministic deconvolution,  filter,  inverse filter,  processing,  resolution,  signature deconvolution,  zero-phase

deconvolution

1. n. [Geophysics] A seismic profile recorded specifically to study the lower crust, the Mohorovicic discontinuity and the mantle of the Earth, typically using refraction methods. Most standard seismic reflection profiles record only a small fraction (typically, on the order of 10 km [6 miles]) of the Earth's crust, which is 5 to 75 km [3 to 45 miles] thick. See: DSS,  Mohorovicic discontinuity,  reflection,  refraction

deep seismic sounding

1. n. [Geophysics] A method of marine seismic acquisition in which a boat tows a receiver well below the surface of the water to get closer to features of interest or to reduce noise due to conditions of the sea. Deep tow devices are used for some side-scan sonar, gravity and magnetic surveys. See: acquisition,  gravity,  gravity survey,  magnetics,  noise,  seismic acquisition,  side-scan sonar

deep tow

1. n. [Geology] An area of deposition or the deposit formed by a flowing sediment-laden current as it enters an open or standing body of water, such as a river spilling into a gulf. As a river enters a body of water, its velocity drops and its ability to carry sediment diminishes, leading to deposition. The term has origins in Greek because the shape of deltas in map view can be similar to the Greek letter delta. The shapes of deltas are subsequently modified by rivers, tides and waves. There is a characteristic coarsening upward of sediments in a delta. The three main classes of deltas are river-dominated (Mississippi River), wave-dominated (Nile River), and tide-dominated (Ganges River). Ancient deltas contain some of the largest and most productive petroleum systems. See: depositional environment,  petroleum system,  sediment

delta

1. n. [Geophysics, Formation Evaluation] Also called interval transit time, the amount of time for a wave to travel a certain distance, proportional to the reciprocal of velocity, typically measured in microseconds per foot by an acoustic log and symbolized by t or DT. P-wave interval transit times for common sedimentary rock types range from 43 (dolostone) to 160 (unconsolidated shales) microseconds per foot, and can be distinguished from measurements of steel casing, which has a consistent transit time of 57 microseconds per foot. Synonyms: interval transit time,  slowness See: acoustic,  drift,  P-wave

delta t

1. n. [Geophysics] The variation in the mass per unit volume of rocks, which affects the local gravitational field of the Earth. A density contrast also contributes to an acoustic impedance contrast, which affects the reflection coefficient. See: acoustic impedance,  density,  gravity,  reflection coefficient

density contrast

1. n. [Geophysics] A series of gravity measurements made along a line or over an area of a locally high topographic feature to remove or compensate for the effect of topography on deeper density readings. See: topographic map

density profile

1. n. [Geophysics] A device used in acquisition of marine seismic data that keeps streamers at a certain depth in the water. See: streamer

depth controller

1. n. [Geophysics] 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. See: acoustic log,  check-shot survey,  depth map,  depth section,  pull-up,  push-down,  seismic processing,  seismic section,  velocity,  vertical seismic profile

depth conversion

1. n. [Geophysics] A two-dimensional representation of subsurface structure with contours in depth that have been converted from seismic traveltimes. See: depth conversion,  structure map,  traveltime

depth map

1. n. [Geophysics] 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. See: depth section,  migration,  processing,  shotpoint,  time migration,  traveltime,  velocity

depth migration

1. n. [Geophysics] 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. See: common depth point,  displacement

depth point

1. n. [Geophysics] 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. See: depth conversion,  depth migration

depth section

1. n. [Geophysics] 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. See: vertical resolution,  wavelength

detectable limit

1. n. [Geophysics] A sensor or receiver, such as a geophone or hydrophone, gravimeter or magnetometer.

detector

1. n. [Geophysics] 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. See: inverse filter

deterministic deconvolution

1. vb. [Perforating, Geophysics] 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. See: blasting cap,  cap,  detonating cord

detonate