Seismic Interpretation glossary

Clear, one-line definitions of the Seismic Interpretation terms used across the OgbonLab textbooks. Each entry links to the interactive sections where the idea is taught.

4d amplitude difference

The amplitude difference between baseline and monitor surveys, mapping reservoir changes such as gas exsolution, water sweep, or compaction.

4d time shift

Differences in reflection traveltime between baseline and monitor surveys, caused by velocity changes from pressure, temperature, or saturation changes.

acoustic impedance

The product of density and P-wave velocity, ρVp; contrasts in acoustic impedance generate normal-incidence reflections.

See: From acoustic impedance to the elastic rock, Acoustic impedance and reflection coefficients

anisotropy

Direction-dependence of elastic wave speeds; common in shales and fractured rocks, requiring anisotropic velocity models for accurate imaging.

See: Anisotropy, What Anisotropy Is

attribute

Any quantity derived from the seismic trace, amplitude, phase, frequency, coherence, used to highlight features beyond the raw reflection image.

See: Frequency attributes: spectral decomposition, Geometric attributes: dip, azimuth, and curvature

avo anomaly

An offset- or angle-dependent amplitude behavior diagnostic of fluid or lithology contrasts, classified into AVO classes I-IV.

baseline

The first (reference) seismic survey in a time-lapse (4D) program against which later monitor surveys are differenced.

bright spot

A localized high-amplitude reflection, often indicating a gas-charged sand against shale; a classic direct hydrocarbon indicator.

brittleness index

An elastic-attribute combination (often a function of Young's modulus and Poisson's ratio) used to predict fracability in unconventional reservoirs.

coherence

An attribute measuring the local trace-to-trace similarity; faults, channels, and other discontinuities appear as coherence lows.

See: Coherence and discontinuity attributes: seeing the unseen faults

crossline

In a 3D seismic survey, the direction perpendicular to the inline (cable) direction; crossline sections cut across the receiver lines.

See: Inline / crossline templates

density

Mass per unit volume of a rock, ρ; combines with velocity to give acoustic impedance and is one of three pre-stack inversion outputs.

See: The Density Log, Porosity from Density

dhi

Direct hydrocarbon indicator: any seismic anomaly, bright spot, flat spot, dim spot, polarity reversal, low-frequency shadow, that suggests in-situ hydrocarbons.

facies classification

The assignment of seismic samples to discrete rock-type categories using attributes, well-log calibration, and supervised or unsupervised learning.

See: Machine-learning QI: neural networks for facies classification, Probabilistic facies classification: uncertainty at every voxel

fault

A fracture surface across which rocks have moved relative to each other; on seismic data, recognized by reflector offset, drag, or termination.

faults

Fracture surfaces with relative displacement, classified as normal, reverse, or strike-slip depending on the sense of motion.

See: Faults in the Model, Horizons, Faults, and Zones

flat spot

A horizontal reflector that cuts across structural dip, interpreted as a fluid contact (gas-oil, gas-water, or oil-water).

fluid

In reservoir interpretation, the pore fluid (water, oil, or gas) occupying rock pore space and influencing seismic amplitudes via density and bulk modulus.

four-way structural closure

A trap configuration in which a reflector dips downward in every direction from a crest, forming a structurally bounded hydrocarbon column.

horizon

A continuous seismic reflection interpreted as the time or depth surface of a geological boundary, such as a formation top or unconformity.

See: Horizons, Faults, and Zones, Picking horizons: methodology and practice

hti

Horizontal transverse isotropy: anisotropy with a horizontal symmetry axis, used to model vertically fractured media.

inline

In a 3D seismic survey, the acquisition direction parallel to the receiver cables; inline sections show data along that orientation.

See: Inline / crossline templates

instantaneous frequency

The time derivative of the instantaneous phase; highlights tuning, attenuation, and frequency anomalies such as low-frequency shadows.

instantaneous phase

The phase angle of the complex trace at each sample; emphasizes lateral continuity of reflectors regardless of amplitude.

low-frequency shadow

A localized loss of high-frequency energy beneath a reservoir, caused by attenuation in gas- or fluid-charged rock; used as a DHI.

mean curvature

A seismic geometric attribute equal to the average of the two principal curvatures of a horizon; highlights gentle bowls and domes on a picked surface.

See: Gaussian and Mean Curvature of Surfaces

monitor

A repeated seismic survey acquired after the baseline to detect time-lapse production-induced changes in the reservoir.

normal faults

Dip-slip faults where the hanging wall moves down relative to the footwall, indicating extensional tectonics.

orthorhombic

An anisotropy class with three mutually perpendicular symmetry planes; models a vertically fractured horizontally layered medium.

polarity

The sign of the reflection coefficient, positive (hard kick) or negative (soft kick), determined by the impedance change across an interface.

See: Polarity, phase, and wavelet conventions

pre-stack inversion

Inversion that uses angle gathers to simultaneously estimate P-impedance, S-impedance, and density via the linearized Zoeppritz equations.

reflectivity series

A depth or time sequence of normal-incidence reflection coefficients computed from impedance contrasts; the earth's response convolved by the wavelet.

rms amplitude

The root-mean-square amplitude computed over a time window; a robust amplitude attribute correlated with bright spots and reservoir quality.

seismic inversion

The process of converting reflection amplitudes into a layer-property volume such as acoustic impedance, used for quantitative reservoir characterization.

semblance attribute

Variance-based coherence measure used as a 3D attribute; sensitive to faults, channels, and stratigraphic edges.

shear modulus

The elastic constant μ relating shear stress to shear strain; together with density it sets the S-wave velocity, Vs = √(μ/ρ).

spectral decomposition

An attribute that decomposes the trace into narrow-band frequency components, revealing thin-bed tuning and channel geometries.

See: Frequency attributes: spectral decomposition

strike-slip

A fault on which the dominant displacement is horizontal and parallel to the fault strike, often seen as flower structures on seismic.

See: Structural styles: extension, compression, strike-slip, salt, gravity

structural framework

The interpreted network of faults and key horizons that defines a survey's structural compartments and serves as the skeleton for geologic models.

See: Build the Structural Framework, Building the Structural Framework

sweetness

A composite attribute defined as envelope amplitude divided by the square root of instantaneous frequency; highlights clean, low-frequency sand-prone intervals.

synthetic seismogram

A modeled seismic trace generated by convolving a wavelet with a reflectivity series computed from well-log impedance, used for well-to-seismic ties.

See: Closing the loop: synthetic seismograms and inversion

thomsen parameters

Three dimensionless coefficients (ε, δ, γ) introduced by Thomsen to describe weak elastic anisotropy in transversely isotropic media.

See: The Thomsen Parameters

time map

A 2D map of a horizon's two-way travel time across a survey; the standard structural map produced before depth conversion.

time slice

A horizontal section through a 3D seismic volume at a constant two-way time, used to map plan-view geological features.

tti

Tilted transverse isotropy: VTI with a tilted symmetry axis, common in dipping shale beds adjacent to salt or in thrust belts.

tuning effects

Constructive or destructive interference of reflections from a thin bed, peaking when bed thickness equals one-quarter of the dominant wavelength.

unconformity

A surface of erosion or non-deposition separating older rocks below from younger rocks above; expressed on seismic as truncation, onlap, or downlap patterns.

vp/vs

The ratio of P-wave to S-wave velocity; a sensitive indicator of fluid type and lithology, with low values typical of gas sands.

vti

Vertical transverse isotropy: anisotropy with a vertical symmetry axis, typical of horizontally layered shales.

well-to-seismic tie

The process of relating well-log data to seismic data via a synthetic seismogram, providing depth-time conversion and wavelet calibration.

zero-crossing

The time at which a wavelet changes sign; commonly used as the picking reference for high-resolution horizon tracking.

zoeppritz equations

Four equations expressing the reflection and transmission coefficients at an elastic interface as functions of incidence angle and elastic properties on both sides.