Amplitude: Definition, Seismic Wavelet, and Reflection Strength

In seismic exploration, amplitude is the maximum displacement of a seismic wavelet measured from the zero-crossing baseline to a peak or trough. More precisely, amplitude equals half the peak-to-trough excursion of a single seismic cycle: if a trace swings from +120 to -120 digital counts, the amplitude is 120 counts and the peak-to-trough value is 240 counts. This distinction matters because some software packages report peak-to-trough values, and confusing the two can produce a factor-of-two error in any quantitative attribute calculation. Amplitude is the single most analyzed attribute in the seismic interpreter's toolkit because it encodes the contrast in acoustic properties between rock layers, making it a primary tool for mapping lithology, fluid content, and reservoir quality across an exploration acreage.

Key Takeaways

• Amplitude equals the maximum displacement of a seismic trace from its baseline to a peak or trough; peak-to-trough equals twice the amplitude.
• Measured in digital counts (or normalized units) on a workstation, amplitude reflects the product of the reflection coefficient, the source wavelet, geometric spreading, and absorption losses.
• Automatic Gain Control (AGC) equalizes trace amplitudes along a time gate and must be removed or avoided before any quantitative amplitude analysis, particularly AVO studies.
• Amplitude extraction along a seismic horizon, expressed as peak, trough, RMS, or average absolute amplitude, produces a two-dimensional map that can delineate reservoir extent and fluid contacts.
• Thin-bed tuning at the quarter-wavelength thickness causes constructive interference that artificially boosts amplitude, creating false direct hydrocarbon indicators if not recognized and corrected.

How Amplitude Is Generated: The Physics of Seismic Reflection

A seismic wave generated by an air gun array (marine surveys) or a vibroseis truck (land surveys) travels downward through the earth. At every interface between two rock layers with contrasting acoustic impedance, part of the wave energy is reflected back toward the surface and part continues downward as a transmitted wave. Acoustic impedance (Z) is defined as the product of bulk density (rho, in g/cm³) and P-wave velocity (Vp, in m/s or ft/s): Z = rho x Vp. The fraction of energy reflected at normal incidence is governed by the reflection coefficient (RC):

RC = (Z2 - Z1) / (Z2 + Z1)

where Z1 is the impedance of the upper layer and Z2 is that of the lower layer. Reflection coefficients range from -1 to +1; most geological interfaces produce values between -0.1 and +0.1. The recorded seismic trace is the convolution of this earth reflectivity series with the source wavelet and an additive noise term. The amplitude of a particular reflection event on the recorded trace is therefore a function of the underlying RC, scaled and shaped by the wavelet. In practice, three additional physical processes modify the amplitude before it reaches the recording hydrophone or geophone: (1) geometric spreading, which attenuates amplitude proportionally to the inverse of travel distance; (2) anelastic absorption, which preferentially attenuates high frequencies and reduces amplitude at a rate characterized by the quality factor Q; and (3) transmission losses at each interface the wave passes through above the target reflector.

Processing workflows apply corrections for geometric spreading (a deterministic correction based on velocity and travel time) and sometimes for absorption (Q compensation or inverse-Q filtering). The resulting amplitude, after these corrections, more closely represents the true reflectivity of the subsurface and is said to be a "relative amplitude preserved" or "true relative amplitude" dataset. This is the requisite starting point for any AVO analysis or rock-physics inversion. In contrast, datasets processed with AGC cannot be used for quantitative amplitude work because AGC applies a time-varying scalar that normalizes trace energy within a sliding window, destroying the relative amplitude relationships between different reflectors.

Automatic Gain Control and Relative Amplitude Preservation

AGC was developed in the early analogue era of seismic recording to compensate for the large dynamic range of seismic traces, making deep reflections visible on paper sections alongside shallow high-amplitude events. The AGC operator computes the RMS amplitude within a user-defined time window (typically 500 to 2,000 milliseconds) centered on each sample and divides the sample value by that RMS. The result is that every part of the trace has approximately the same visual energy level, regardless of true geological amplitude. For structural interpretation, AGC-processed sections are effective because the interpreter is primarily mapping the geometry of reflectors rather than measuring their strength. However, for any analysis that depends on knowing whether a particular reflector is anomalously bright or dim relative to background, AGC is destructive. Modern processing flows apply AGC only as a final display step, if at all, and deliver a separate relative-amplitude-preserved volume for attribute analysis. The seismic interpreter must always confirm with the processing contractor which volume has been AGC-corrected and which has not.

Amplitude Measurement Types and Extraction Methods

Several distinct amplitude metrics are applied in seismic interpretation, each suited to different geological questions:

• Peak amplitude: the maximum positive value within a defined time window around a mapped horizon. Used to characterize the amplitude of a positive-polarity reflection such as a hard kick from a carbonate top.
• Trough amplitude: the maximum negative value within a window. Used for negative-polarity reflections, for example the top of a gas sand that creates a soft reflection below a shale.
• RMS amplitude (root-mean-square): the square root of the mean of the squared samples within a window. Sensitive to both peaks and troughs; commonly used for bright-spot detection. Mathematically, RMS amplitude = sqrt[(1/N) x sum(xi^2)] for N samples. Expressed in digital counts in an unnormalized volume, or as a fraction of the normalization scalar in a normalized volume.
• Average absolute amplitude: the mean of the absolute values within the window. Less sensitive to isolated high-amplitude spikes than RMS, and often preferred where noise contamination is a concern.
• Maximum absolute amplitude: the largest absolute value in the window, essentially the louder of peak or trough. Susceptible to noise spikes; typically used only after careful noise attenuation.
• Amplitude envelope (instantaneous amplitude): computed from the analytic signal (Hilbert transform), giving a smoothly varying positive amplitude that is independent of wavelet phase. The envelope is used in reservoir characterization to map gross sand reflectivity without polarity ambiguity.

Amplitude extraction is performed along a seismic horizon, which is a surface of constant two-way travel time (TWT in milliseconds) or depth (in metres or feet) mapped through the 3D volume. The interpreter picks the horizon corresponding to the top or base of a target reservoir interval and instructs the workstation to extract one of the amplitude metrics within a symmetric or asymmetric window around that horizon. The result is a two-dimensional grid of amplitude values that can be posted as a color map over the structural surface, producing an "amplitude map" or "horizon slice." These maps are among the most powerful tools in reservoir delineation because they image variations in fluid content and porosity that are invisible on structural maps alone.