GEOPHYSICS 3

Question 1

In gravity surveying, a correction of 0.3086 mGal/m [0.09406 mGal/ft] added to a measurement to compensate for the change in the gravitational field with height above sea level, assuming there is only air between the measurement station and sea level.

Answer 1

Faye correction

Question 2

In seismic surveying or processing, the use of a function of frequency rather than time to express an independent variable or measurement. In contrast, in the time domain, variables are expressed as a function of time instead of frequency.

Answer 2

FD/ Frequency domain

Question 3

The principle that the path taken by a ray of light from one point to another is that which takes the minimum time (or the maximum time in select cases), named for its discoverer, French mathematician Pierre de Fermat (1601 to 1665). Snell’s law and the laws of reflection and refraction follow from Fermat’s principle. Fermat’s principle also applies to seismic waves.

Answer 3

Fermat’s principle

Question 4

An iterative computer algorithm to perform the Fourier transform of digitized waveforms rapidly.

Answer 4

FFT/ fast fourier transform

Question 5

A magnetic tape containing data recorded in the field, abbreviated FT.

Answer 5

field tape

Question 6

To remove undesirable portions of data during seismic processing to increase the signal-to-noise ratio of seismic data. Filtering can eliminate certain frequencies, amplitudes or other information.

Answer 6

filter

Question 7

A process or algorithm using a set of limits used to eliminate unwanted portions of seismic data, commonly on the basis of frequency or amplitude, to enhance the signal-to-noise ratio of the data or to achieve deconvolution.

Answer 7

filter

Question 8

The earliest arrival of energy propagated from the energy source at the surface to the geophone in the wellbore in vertical seismic profiles and check-shot surveys, or the first indication of seismic energy on a trace. On land, first breaks commonly represent the base of weathering and are useful in making static corrections.

Answer 8

first break/ first arrival

Question 9

An acquisition technique commonly used in electromagnetic methods whereby the energy source or transmitter is kept in the same position, and detectors or receivers are moved to different spots to compile a profile or map.

Answer 9

fixed-source method

Question 10

The use of frequency (abbreviated as f) and wavenumber (k, the reciprocal of wavelength) as the reference framework, obtained by using the Fourier transform over time and space.

Answer 10

f-k domain

Question 11

A graphical technique to distinguish subsets of data according to their direction and velocity by plotting and contouring frequency and wavenumber.

Answer 11

f-k plot

Question 12

A seismic section that has been redisplayed such that a reflection of interest not horizontal in the original display appears horizontal and flat. Such displays can shed light on geological conditions at the time a given sedimentary layer accumulated.

Answer 12

flattened section

Question 13

A wave-like geologic structure that forms when rocks deform by bending instead of breaking under compressional stress. Anticlines are arch-shaped folds in which rock layers are upwardly convex. The oldest rock layers form the core of the fold, and outward from the core progressively younger rocks occur. A syncline is the opposite type of fold, having downwardly convex layers with young rocks in the core. Folds typically occur in anticline-syncline pairs. The hinge is the point of maximum curvature in a fold. The limbs occur on either side of the fold hinge. The imaginary surface bisecting the limbs of the fold is called the axial surface. The axial surface is called the axial plane in cases where the fold is symmetrical and the lines containing the points of maximum curvature of the folded layers, or hinge lines, are coplanar. Concentric folding preserves the thickness of each bed as measured perpendicular to original bedding. Similar folds have the same wave shape, but bed thickness changes throughout each layer, with thicker hinges and thinner limbs.

Answer 13

fold

Question 14

A measure of the redundancy of common midpoint seismic data, equal to the number of offset receivers that record a given data point or in a given bin and are added during stacking to produce a single trace. Typical values of fold for modern seismic data range from 60 to 240 for 2D seismic data, and 10 to 120 for 3D seismic data. The fold of 2D seismic data can be calculated by dividing the number of seismometer groups by twice the number of group intervals between shotpoints.

Answer 14

fold

Question 15

The practice of taking a model and calculating what the observed values should be, such as predicting the gravity anomaly around a salt dome using a gravity model or predicting the traveltime of a seismic wave from a source to a receiver using a velocity model.

Answer 15

forward problem

Question 16

Three-dimensional (3D) seismic data acquired at different times over the same area to assess changes in a producing hydrocarbon reservoir with time. Changes may be observed in fluid location and saturation, pressure and temperature. 4D seismic data is one of several forms of time-lapse seismic data. Such data can be acquired on the surface or in a borehole.

Answer 16

four-component seismic data

Question 17

Three-dimensional (3D) seismic data acquired at different times over the same area to assess changes in a producing hydrocarbon reservoir with time. Changes may be observed in fluid location and saturation, pressure and temperature. 4D seismic data is one of several forms of time-lapse seismic data. Such data can be acquired on the surface or in a borehole.

Answer 17

four-dimensional seismic data

Question 18

A mathematical algorithm designed by geometrician and physicist Baron J.B.J. Fourier to determine the frequency distribution within a wave pattern as a series of sine waves. Fourier analysis is also used to study any series of repeated signals or patterns. This analysis is sometimes used to study patterns in images such as thin sections, and in geostatistics and log analysis.

Answer 18

Fourier analysis

Question 19

The process of decomposing a function of time or space into a sum (or integral) of sinusoidal functions (sines or cosines) with specific amplitudes and phases.

Answer 19

Fourier analysis

Question 20

The process of reconstructing a function of time or space from its sinusoidal components determined in Fourier analysis.

Answer 20

Fourier synthesis

Question 21

In gravity surveying, a correction of 0.3086 mGal/m [0.09406 mGal/ft] added to a measurement to compensate for the change in the gravitational field with height above sea level, assuming there is only air between the measurement station and sea level.

Answer 21

free-air correction

Question 22

The rate of repetition of complete wavelengths of electrical signals, light, sound and seismic waves measured in cycles per second, or hertz, and symbolized by f. Typical recorded seismic frequencies are in the range of 5 to 100 hertz.

Answer 22

frequency

Question 23

In seismic surveying or processing, the use of a function of frequency rather than time to express an independent variable or measurement. In contrast, in the time domain, variables are expressed as a function of time instead of frequency.

Answer 23

frequency domain / FD

Question 24

A frequency- and range-dependent area of a reflector from which most of the energy of a reflection is returned and arrival times differ by less than half a period from the first break, named for French physicist Augustin-Jean Fresnel (1788 to 1827). Waves with such arrival times will interfere constructively and so be detected as a single arrival. Subsurface features smaller than the Fresnel zone usually cannot be detected using seismic waves.

Answer 24

Fresnel zone

Question 25

A magnetic tape containing data recorded in the field, abbreviated FT

Answer 25

FT (Field tape)

Question 26

A set of mathematical formulas used to convert a time function, such as a seismic trace, to a function in the frequency domain (Fourier analysis) and back (Fourier synthesis). The function is expressed as a convergent trigonometric series, similar to that first formulated by French mathematician Jean-Baptiste-Joseph, Baron Fourier (1768 to 1830). The Fourier transform is used extensively in signal processing to design filters and remove coherent noise. Many filtering operations are performed in the frequency domain. The Fourier transform has applications in image analysis and in pattern recognition in geological systems.

Answer 26

FT (Fourier transform)

Question 27

A single-vessel technique of acquiring marine seismic data at a complete range of azimuths by towing streamers in a circular path.

Answer 27

Full-azimuth towed-steamer acquisition

Question 28

The change in the amplitude of an electrical signal from the original input to the amplified output.

Answer 28

gain

Question 29

An S-wave parameter for a medium in which the elastic properties exhibit vertical transverse isotropy. Gamma (γ) is the S-wave anisotropy parameter and is equal to half the ratio of the difference between the horizontally and vertically traveling SH-wave velocities squared divided by the vertically traveling SH-wave velocity squared; an SH-wave is a shear wave that is horizontally polarized. γ ≡ ½ [(C66 − C44) / C44] = ½ [(VSH⊥2 − VSHˆ¥2) / VSHˆ¥2]. S-wave parameter (γ) for a medium in which the elastic properties exhibit vertical transverse isotropy, where C66 is the modulus for a horizontally polarized and horizontally traveling S-wave (perpendicular to the symmetry axis), C44 is the modulus for a horizontally polarized and vertically traveling S-wave (parallel to the symmetry axis), VSH⊥ is the velocity for a horizontally polarized and horizontally traveling S-wave and VSHˆ¥ is the velocity for a horizontally polarized and vertically traveling S-wave.

Answer 29

gamma

Question 30

A subsurface leakage of gas from a poorly sealed hydrocarbon accumulation. The gas can cause overlying rocks to have a low velocity. Gas chimneys are visible in seismic data as areas of poor data quality or push-downs.

Answer 30

gas chimney

Question 31

A display of seismic traces that share an acquisition parameter, such as a common midpoint gather, which contains traces having a common midpoint.

Answer 31

gather

Question 32

How Earth’s magnetic field varies with time. These time variations, called secular variations, necessitate periodic updating of magnetic field maps and models. Two types of processes in the Earth’s core produce these variations. One process is related to variations in Earth’s main dipole field, which operate on time scales of hundreds or thousands of years. The other process is related to variations in Earth’s nondipole field, which operate on time scales on the order of tens of years.

Answer 32

geomagnetic secular variation

Question 33

Pertaining to variation of the survey geometry while maintaining the frequency of electromagnetic surveying. In contrast, parametric pertains to keeping frequency the same while varying the geometry.

Answer 33

geometric

Question 34

A geometrical arrangement of seismic receivers (geophones) with signals recorded by one channel. The array can contain numerous closely spaced geophones.

Answer 34

geophone array

Question 35

A bundle of electrical wires that connects geophones and relays data to the seismic recording truck or seismic vessel.

Answer 35

geophone cable

Question 36

The distance between geophones or the centers of groups of geophones.

Answer 36

geophone interval/ group interval

Question 37

A geometrical arrangement of seismic receivers (geophones) with signals recorded by one channel. The array can contain numerous closely spaced geophones.

Answer 37

geophone pattern

Question 38

A device used in surface seismic acquisition, both onshore and on the seabed offshore, that detects ground velocity produced by seismic waves and transforms the motion into electrical impulses. Geophones detect motion in only one direction. Conventional seismic surveys on land use one geophone per receiver location to detect motion in the vertical direction. Three mutually orthogonal geophones are typically used in combination to collect 3C seismic data. Hydrophones, unlike geophones, detect changes in pressure rather than motion.

Answer 38

geophone/ jug/ receiver/ seismometer

Question 39

A scientist trained in the study of the physics of the Earth, particularly its electrical, gravitational and magnetic fields and propagation of elastic (seismic) waves within it. In the petroleum industry, geophysicists perform a variety of functions, chiefly the processing and interpretation of seismic data and generation of subsurface maps on the basis of seismic data. Such interpretations enhance understanding of subsurface geology.

Answer 39

geophysicist

Question 40

The study of the physics of the Earth, especially its electrical, gravitational and magnetic fields and propagation of elastic (seismic) waves within it. Geophysics plays a critical role in the petroleum industry because geophysical data are used by exploration and development personnel to make predictions about the presence, nature and size of subsurface hydrocarbon accumulations.

Answer 40

geophysics

Question 41

A short-path multiple, or a spurious reflection that occurs when seismic energy initially reverberates upward from the shallow subsurface and then is reflected downward, such as at the base of weathering or between sources and receivers and the sea surface.

Answer 41

ghost

Question 42

A device used to measure the acceleration due to gravity, or, more specifically, variations in the gravitational field between two or more points.

Answer 42

gravimeter

Question 43

The measurement of gravity or the study of its variations.

Answer 43

gravimetry

Question 44

The Earth's gravitational field, or the attractive force produced by the mass of the Earth. Variations in the gravitational field can be used to map changes in the density of formations in the Earth. Gravity surveys can be used to map the extent or depth of sedimentary basins or even individual hydrocarbon prospects.

Answer 44

gravity

Question 45

The difference between the actual value of gravity measured at a location and the value predicted by a particular Earth model. Gravity anomalies are usually determined by adjusting the known value of (absolute) gravity at a reference station by Bouguer, free-air or other corrections and subtracting the final predicted value from the measurement. (A different description is that the various corrections are subtracted from the data to reduce it to the reference level. Both interpretations are valid provided it is remembered that the resulting gravity anomaly can be caused by density anomalies-i.e., differences in density between Earth and the theoretical model-that can lie anywhere either above or below the reference level.)

Answer 45

gravity anomaly

Question 46

The measurement of gravitational acceleration over an area, usually presented as a map or profile of Bouguer or free-air anomalies.

Answer 46

gravity survey

Question 47

A regular spatial arrangement of points, such as x-y coordinates.

Answer 47

grid

Question 48

To convert irregularly spaced points to a regular spacing by interpolation

Answer 48

grid

Question 49

A type of coherent noise generated by a surface wave, typically a low-velocity, low-frequency, high-amplitude Rayleigh wave. Ground roll can obscure signal and degrade overall data quality, but can be alleviated through careful selection of source and geophone arrays, filters and stacking parameters.

Answer 49

ground roll

Question 50

A set of seismometers whose output is sent to a common data channel to record a seismic trace. A large group is known as a patch.

Answer 50

group

Question 51

The distance between geophones or groups of geophones.

Answer 51

group interval/ geophone interval

Question 52

The velocity that wave energy€”comprised of a wave group, train or packet of individual wave phases or components€”travels through a medium. The wave energy may be grouped into an envelope that is shrink-wrapped around it. The shape of the envelope around the wave-energy group changes with distance because the individual wave phases move apart from one another. vg = vp ˆ’ λ (ˆ‚vp/ˆ‚λ) = vp + f (ˆ‚vp/ˆ‚f) Relation of group velocity to phase velocity. As a wave travels through a medium, its energy moves at the group velocity (vg) and its individual phases, or components, move at their phase velocity (vp). The wave changes shape with distance as each frequency (f), or wavelength (λ), component moves at its separate phase velocity through the phenomenon of dispersion. Relative to the group velocity, each component moves with faster or slower phase velocity, depending on how phase velocity changes with wavelength or frequency.

Answer 52

group velocity

Question 53

A type of elastic wave propagated and confined in a layer whose velocity is lower than that of the surrounding layers, such as a layer of coal.

Answer 53

guided wave

Question 54

Abbreviation for air gun or water gun. An air gun is a source of seismic energy used in acquisition of marine seismic data. This gun releases highly compressed air into water. Air guns are also used in water-filled pits on land as an energy source during acquisition of vertical seismic profiles. A water gun is a source of energy for acquisition of marine seismic data that shoots water from a chamber in the tool into a larger body of water, creating cavitation. The cavity is a vacuum and implodes without creating secondary bubbles. This provides a short time signature and higher resolution than an air-gun source.

Answer 54

gun

Question 55

 A device used to perforate oil and gas wells in preparation for production. Containing several shaped explosive charges, perforating guns are available in a range of sizes and configurations. The diameter of the gun used is typically determined by the presence of wellbore restrictions or limitations imposed by the surface equipment.

Answer 55

gun

Question 56

An anomaly that occurs as a ring around a feature, such as electrical or geochemical rings around hydrocarbon accumulations.

Answer 56

halo effect

Question 57

A nonlinear change in waveform in which simple multiples of (1,2, ... n times) the input frequencies, or harmonics, are generated.

Answer 57

harmonic distortion

Question 58

A pressure wave in the borehole fluid generated by the passage of either the acoustic compressional wave or the shear wave in the formation. These pressure waves are recorded by logging tools using hydrophones and are the basis for the sonic log. A head wave is generated only when the compressional or the shear speed is faster than the fluid speed. In slow formations, where the shear speed is less than the fluid speed, no shear head wave is created.

Answer 58

head wave

Question 59

A wave entering a relatively high-velocity medium whose incident and refracted angle is the critical angle.

Answer 59

head wave/ refraction

Question 60

In a gathering system, a pipe arrangement that connects flowlines from several wellheads into a single gathering line. A header has production and testing valves to control the flow of each well, thus directing the produced fluids to production or testing vessels. Individual gas/oil ratios and well production rates of oil, gas and water can be assigned by opening and closing selected valves in a header and using individual metering equipment or separators.

Answer 60

header

Question 61

The location, acquisition and processing parameters, and other pertinent information attached to a well log, seismic record and traces.

Answer 61

header

Question 62

The unit of measurement of frequency, equivalent to one cycle per second and symbolized by Hz. The unit is named after German physicist Heinrich Hertz (1857 to 1894), who discovered electromagnetic waves.

Answer 62

Hertz

Question 63

A crossplot of two components of particle motion over a time window. Hodograms are used in borehole seismology to determine arrival directions of waves and to detect shear-wave splitting. Data recorded along two geophone axes are displayed as a function of time.

Answer 63

hodogram

Question 64

A graph or curve that displays time versus distance of motion.

Answer 64

hodogram

Question 65

An interface that might be represented by a seismic reflection, such as the contact between two bodies of rock having different seismic velocity, density, porosity, fluid content or all of those.

Answer 65

horizon

Question 66

An informal term used to denote a surface in or of rock, or a distinctive layer of rock that might be represented by a reflection in seismic data. The term is often used incorrectly to describe a zone from which hydrocarbons are produced.

Answer 66

horizon

Question 67

A map view of a particular reflection in a 3D seismic survey, as opposed to a horizontal (depth) slice or at a given time (a time slice). Slices are convenient displays for visual inspection of seismic attributes, especially amplitude.

Answer 67

horizon slice

Question 68

Transverse isotropy that has a horizontal axis of rotational symmetry. In vertically fractured rocks, properties are uniform in vertical planes parallel to the fractures, but vary in the direction perpendicular to the fractures and across the fractures.

Answer 68

Horizontal Transverse Isotropy

Question 69

Abbreviation for horizontal transverse isotropy. Transverse isotropy that has a horizontal axis of rotational symmetry. In vertically fractured rocks, properties are uniform in vertical planes parallel to the fractures, but vary in the direction perpendicular to the fractures and across the fractures.

Answer 69

HTI

Question 70

"A type of seismic amplitude anomaly, seismic event, or characteristic of seismic data that can occur in a hydrocarbon-bearing reservoir. Although ""bright spots,"" as hydrocarbon indicators are loosely called, can originate in numerous ways, they are not all indicative of the presence of hydrocarbons. Criteria to distinguish true hydrocarbon indicators (sometimes called HCIs) from other types of amplitude anomalies include: • amplitude variation with offset • bright or dim spot(s) in amplitude as a result of variations in lithology and pore fluids, sometimes occurring in groups of stacked reservoirs • change or reversal in polarity because of velocity changes, also called phasing • conformity with local structures • diffractions that emanate from fluid contacts • flat spot that represents a fluid (gas-oil or gas-water) contact, which can also show the downdip limit of the reservoir in some cases • gas chimneys above leaking reservoirs • shadow zones below the accumulation • velocity push-down because of lower velocities of hydrocarbons than rocks • difference in response between reflected pressure and shear energy. Hydrocarbon indicators are most common in relatively young, unconsolidated siliciclastic sediments with large impedance contrasts across lithologic boundaries, such as those in the Gulf of Mexico and offshore western Africa. An ongoing issue in exploration for hydrocarbon indicators is the difficulty in distinguishing between gas accumulations and water with a low degree of gas saturation (""fizz water"")."

Answer 70

hydrocarbon indicator

Question 71

A device designed for use in detecting seismic energy in the form of pressure changes under water during marine seismic acquisition. Hydrophones are combined to form streamers that are towed by seismic vessels or deployed in a borehole. Geophones, unlike hydrophones, detect motion rather than pressure.

Answer 71

hydrophone

Question 72

Inductive-source induced polarization.

Answer 72

IIP

Question 73

In remote sensing, to record and interpret electromagnetic energy from the surfaces of planets or satellites using photographic displays.

Answer 73

image

Question 74

The apparent source of a received wave. The image is the point in the subsurface that the rays would appear to have come from if they were not reflected, but were shot up from below. A ray that travels from a source and is reflected to a geophone has the same appearance as a ray that travels straight from the image and up to the geophone.

Answer 74

image

Question 75

A representation that depicts the subsurface in two or more dimensions.

Answer 75

image

Question 76

In electromagnetics or electrical circuit theory, the ratio of voltage to current when these are represented by phasor quantities in alternating current circuits. (A phasor is a complex number that represents the amplitude and phase of a quantity that varies sinusoidally in time.) Electrical impedance, also symbolized by Z, is a complex number that has the same units (ohms) as resistivity.

Answer 76

impedance

Question 77

In acoustics, the product of velocity times density, also called acoustic impedance and symbolized by Z. The reflection coefficient of an interface depends on the contrast in acoustic impedance of the rock on either side of the interface.

Answer 77

impedance

Question 78

Seismic data whose energy source is impulsive and of short duration, as with an air gun, rather than vibratory, as with a vibrator.

Answer 78

impulsive seismic data

Question 79

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.

Answer 79

in line

Question 80

The acute angle at which a raypath impinges upon a line normal to an interface, such as a seismic wave impinging upon strata. Normal incidence is the case in which the angle of incidence is zero, the wavefront is parallel to the surface and its raypath is perpendicular, or normal, to the interface. Snell's law describes the relationship between the angle of incidence and the angle of refraction of a wave.

Answer 80

incident angle

Question 81

An instrument used to measure the dip of the Earth's magnetic field.

Answer 81

inclinometer

Question 82

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.

Answer 82

in-line

Question 83

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.

Answer 83

interpretation

Question 84

The elapsed time between two seismic events.

Answer 84

interval time

Question 85

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.

Answer 85

interval transit time

Question 86

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.

Answer 86

interval velocity

Question 87

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.

Answer 87

interwell tomography

Question 88

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.

Answer 88

Inverse problem

Question 89

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.

Answer 89

Inversion

Question 90

The atypical appearance of structural and topographic features, such as an anticline being exposed in a valley instead of as a hill; also called inverted relief.

Answer 90

Inversion

Question 91

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

Answer 91

IP/ Induced polarization

Question 92

Abbreviation for the Institute of Petroleum, a standardization body for the petroleum industry in Europe. Several industry-standard drilling-fluid tests are adopted from IP and ASTM procedures.

Answer 92

IP/ Induced polarization

Question 93

A line joining points of equal time or age, such as a reflection in a seismic profile or contours in an isochron map.

Answer 93

Isochron

Question 94

A contour map that displays the variation in time between two seismic events or reflections.

Answer 94

Isochron map

Question 95

A contour map showing the traveltimes to one particular seismic event or reflection.

Answer 95

Isochron map

Question 96

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.

Answer 96

Isostatic correction

Question 97

Archaic slang for a geophone.

Answer 97

Jug

Question 98

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.

Answer 98

Jug hustler

Question 99

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.

Answer 99

Kirchhoff migration

Question 100

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.

Answer 100

Kirchoff equation