GEOPHYSICS

1. n. [Geophysics] An instrument used to measure the dip of the Earth's magnetic field. See: magnetics

inclinometer

1. n. [Geophysics] An electromagnetic method that uses electrodes with time-varying currents and voltages to map the variation of electrical permittivity (dielectric constant) in the Earth at low frequencies. Induced polarization is observed when a steady current through two electrodes in the Earth is shut off: the voltage does not return to zero instantaneously, but rather decays slowly, indicating that charge has been stored in the rocks. This charge, which accumulates mainly at interfaces between clay minerals, is responsible for the IP effect. This effect can be measured in either the time domain by observing the rate of decay of voltage or in the frequency domain by measuring phase shifts between sinusoidal currents and voltages. It is often used in exploration for minerals and can sometimes distinguish different types of mineralization. The IP method can probe to subsurface depths of thousands of meters. Alternate Form: IP See: dielectric constant,  electrical permittivity,  electromagnetic method,  IIP,  time domain

induced polarization

1. n. [Geophysics] A seismic line within a 3D survey parallel to the direction in which the data were acquired. In marine seismic data, the in-line direction is that in which the recording vessel tows the streamers. See: acquisition,  crossline,  extended spread,  spread,  streamer,  three-dimensional seismic data

in-line

1. n. [Geophysics] In geophysics, analysis of data to generate reasonable models and predictions about the properties and structures of the subsurface. Interpretation of seismic data is the primary concern of geophysicists. See: autotrack,  check-shot survey,  geophysicist,  mis-tie,  model,  phantom,  pick,  processing,  seismic modeling,  tie,  work station

interpretation

1. n. [Geophysics] The elapsed time between two seismic events. See: event

interval time

1. n. [Geophysics] 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: delta t,  slowness,  transit time See: drift,  wave

interval transit time

1. n. [Geophysics] The velocity, typically P-wave velocity, of a specific layer or layers of rock, symbolized by vint and commonly calculated from acoustic logs or from the change in stacking velocity between seismic events on a common midpoint gather. See: acoustic log,  Dix formula,  P-wave,  velocity

interval velocity

1. n. [Geophysics, Formation Evaluation, Enhanced Oil Recovery] A technique for measuring a signal that is broadcast from a transmitter or source located in one well, to a receiver array placed in a neighboring well. This technique is used to create a display of formation properties such as acoustic velocity and attenuation, seismic reflectivity, or electromagnetic resistivity in the area between wells. The reservoir-scale data acquired with this technique can be used to bridge the gap between wellbore measurements and surface measurements. See: tomography

interwell tomography

1. n. [Geophysics] The problem of determining the value or spatial variation of a physical property or feature by comparing measurements to the predictions of a model. For example, seismic traveltimes from a source to a receiver can be used to build a model of seismic velocity in the Earth, or earthquake arrival times can be used to determine the timing and focus (location) of an earthquake. A typical inverse problem in electromagnetics is to determine the variation of electrical conductivity in the Earth from measurements of induced electric and magnetic fields. A forward problem, in contrast, involves taking an assumed model and calculating what the observed values should be, such as the predicting seismic traveltimes between a source and a receiver given a velocity model. See: forward problem

inverse problem

1. n. [Geology] The reversal of features, particularly structural features such as faults, by reactivation. For example, a normal fault might move in a direction opposite to its initial movement. See: fault,  normal fault,  structure

inversion

1. n. [Geophysics] An electromagnetic method that uses electrodes with time-varying currents and voltages to map the variation of electrical permittivity (dielectric constant) in the Earth at low frequencies. Induced polarization is observed when a steady current through two electrodes in the Earth is shut off: the voltage does not return to zero instantaneously, but rather decays slowly, indicating that charge has been stored in the rocks. This charge, which accumulates mainly at interfaces between clay minerals, is responsible for the IP effect. This effect can be measured in either the time domain by observing the rate of decay of voltage or in the frequency domain by measuring phase shifts between sinusoidal currents and voltages. It is often used in exploration for minerals and can sometimes distinguish different types of mineralization. The IP method can probe to subsurface depths of thousands of meters Alternate Form: induced polarization

IP

1. n. [Geophysics] A line joining points of equal time or age, such as a reflection in a seismic profile or contours in an isochron map. See: contour,  isochron map,  isochron map,  reflection

isochron

1. n. [Geophysics] A contour map that displays the variation in time between two seismic events or reflections. See: difference map,  isochron

isochron map

1. n. [Geophysics] A correction for variations in the density or thickness of the Earth's crust. Isostatic corrections are commonly applied to gravity data and are made according to a specific model for isostasy. See: gravity,  isostasy

isostatic correction

1. n. [Geophysics] Archaic slang for a geophone. Synonyms: geophone,  receiver,  seismometer See: geophone

jug

1. n. [Geophysics] Slang term for a member of a seismic acquisition crew or party who lays out cables and plants geophones for seismic acquisition and collects them after surveying. See: acquisition,  cable,  geophone,  party,  plant

jug hustler

1. n. [Geophysics] A mathematical representation of the principle that a wavefield at a given point in space and time can be considered as the superposition of waves propagating from adjacent points and earlier times. It is an integral form of the wave equation in which the wave function at a point is represented as the sum (integral) of contributions from a surface enclosing the given point. The Kirchhoff equation (also called the Kirchhoff integral) is the basis for Kirchhoff migration. See: Kirchhoff migration,  wave equation

Kirchhoff equation

1. n. [Geophysics] 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. Synonyms: diffraction stack See: diffraction,  Kirchhoff equation,  migration,  ray tracing

Kirchhoff migration

1. n. [Geophysics] 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. See: variogram

kriging

1. n. [Geophysics] 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. See: arrival,  event,  weathering

lag