directivity, Oil & Gas Glossary

What Is Directivity in Oil and Gas Geophysics?

Directivity in oil and gas geophysics refers to the angular dependence of seismic source energy radiation — the variation in amplitude, frequency content, and wavelet shape as a function of the angle from the source to the receiver — arising from the physical dimensions, geometry, and activation pattern of the seismic source, with directivity effects being most significant when the source array is large relative to the seismic wavelength and requiring correction in seismic processing to avoid amplitude distortions that could be misinterpreted as formation properties.

Key Takeaways

A point source radiates energy equally in all directions (isotropic); a distributed source array has directivity because constructive and destructive interference between the individual source elements creates angle-dependent radiation patterns.
Marine airgun arrays have strong vertical directivity (good downward energy for reflection seismic) and are designed to minimise upward bubble energy and improve the signal-to-noise ratio in the water column.
Directivity creates ghost effects in both marine sources (air-water interface reflection) and receivers (streamer ghost and OBN ghost) that notch the amplitude spectrum at ghost frequencies.
In borehole acoustics, directivity refers to the ability of azimuthally sensitive acoustic tools (acoustic imagers, dipole shear tools) to resolve directional properties of the formation from different azimuths around the wellbore.
Directivity correction in seismic processing compensates for the offset- and azimuth-dependent amplitude effects of source and receiver arrays before amplitude variation with offset (AVO) analysis.

Seismic Source Directivity: Physical Basis

The radiation pattern of a seismic source is controlled by the spatial distribution of its energy-emitting elements. For a marine airgun array consisting of 20-30 individual airguns deployed over a 20-30 metre lateral extent, the array acts as a spatially distributed source. At any specific azimuth and angle from the array, the seismic signals from each individual gun arrive with slightly different travel times that depend on the path length from each gun to the receiver. At near-vertical incidence (receiver directly below the source), all guns are nearly equidistant from the receiver and their signals arrive nearly simultaneously, adding constructively to produce a strong, coherent pulse. At oblique angles (receiver at large offset), the path length differences between the guns become significant relative to the signal wavelength, causing partial destructive interference for the higher frequencies. The result is that at large offsets, the amplitude of the airgun array signal is reduced (particularly at high frequencies) compared to the near-vertical direction.

This offset-dependent amplitude reduction from source directivity is one of several effects that contribute to offset-dependent amplitude patterns in seismic data. If not corrected, source directivity mimics an AVO effect — reducing amplitude at far offsets relative to near offsets — which could be misinterpreted as a formation fluid effect. Spherical divergence corrections, source directivity corrections, and receiver array directivity corrections are all applied during seismic processing to remove these geometric amplitude effects and recover the true amplitude variation that reflects formation properties such as Poisson's ratio and fluid saturation.

Directivity Applications Across International Jurisdictions

In Canada, seismic source directivity considerations are relevant for both marine acquisition offshore Newfoundland and Nova Scotia and for dynamite and Vibroseis land seismic in the WCSB. Vibroseis arrays (multiple vibrators operated in a swept frequency pattern over a 30-60 metre spatial extent) have directivity characteristics that affect the high-frequency content at large source-receiver offsets, reducing resolution for deep targets in wide-azimuth land seismic surveys. AER exploration well seismic data submissions for exploration licence applications must meet minimum data quality standards; seismic processing documentation typically describes the directivity corrections applied. For the Jeanne d'Arc Basin offshore Newfoundland, marine seismic acquired with large airgun arrays requires directivity-corrected processing to ensure accurate AVO analysis of Hibernia and Terra Nova Formation reservoir targets.

In the United States, marine seismic directivity is a critical processing issue for Gulf of Mexico deepwater exploration where broad-azimuth (BAZ) and wide-azimuth (WAZ) acquisition uses very large airgun arrays to improve illumination of sub-salt targets. At extreme offsets (6-10 km in deep-water WAZ surveys), airgun array directivity significantly attenuates high-frequency content, reducing resolution at the deepest pre-salt targets. BSEE marine protected species regulations on seismic source arrays constrain the maximum airgun array volume and operational parameters, which in turn affect the achievable directivity pattern. In Norway, Sodir's seismic data standards require documentation of source and receiver array specifications used in NCS exploration surveys, with directivity information needed to properly process the data in future reprocessing projects. In the Middle East, high-resolution Vibroseis land seismic for near-surface mapping of the Arabian Shield and for shallow gas hazard identification uses small arrays with controlled directivity to achieve high vertical resolution in the shallow (<500 metre) section.

Fast Facts

The ghost reflection — the seismic energy reflected from the sea surface downward immediately after the source fires or immediately before the receiver detects the signal — is fundamentally a directivity effect. For a marine airgun source at 6-metre tow depth, the ghost arrives approximately 8 milliseconds after the direct downgoing pulse (2 × 6 metres / 1,500 m/s water velocity ≈ 8 ms). This ghost superposition creates a notch in the amplitude spectrum at frequency f_ghost = 1 / (2 × tow depth / water velocity) = 125 Hz for a 6-metre source. At this frequency, the direct source pulse and the ghost (which has opposite polarity) cancel completely, creating a zero in the spectrum. Deeper tow depths push the ghost notch to lower frequencies (degrading low-frequency content) while shallower tow pushes it to higher frequencies (degrading resolution-relevant high frequencies) — the source tow depth is therefore an important acquisition design parameter that affects achievable bandwidth.

Directivity in Borehole Acoustic Logging

In borehole acoustic tools, directivity has a different meaning: it refers to the sensitivity of the tool's acoustic transmitters and receivers to specific azimuths around the wellbore. Omnidirectional monopole acoustic tools (standard sonic logs) transmit and receive acoustic energy equally in all azimuthal directions, measuring the compressional and shear wave velocities averaged over the full borehole circumference. Dipole shear wave tools (such as the Baker Hughes XMAC or Schlumberger DSI) generate a flexural wave by firing an asymmetric dipole transmitter that preferentially excites shear wave particle motion in one horizontal direction. By firing two perpendicular dipole directions, the tool measures the fast and slow shear wave velocities corresponding to the two principal directions of shear wave splitting induced by formation stress anisotropy or aligned fractures. Acoustic borehole imagers (Schlumberger's CBIL, Baker Hughes' CAST-V) use a rotating focused beam to achieve azimuthal directivity that produces a 360-degree image of the borehole wall acoustic reflectivity, resolving fractures, bedding, and borehole breakouts at centimetre resolution.

Tip: When reviewing AVO analysis results from a seismic dataset acquired with a large airgun array, always request the processing report documentation of the source and receiver directivity corrections applied. If the data shows a systematic amplitude decrease at far offsets across all lithologies (not just in specific AVO anomalous zones), the directivity correction may have been inadequate — particularly for data acquired with large arrays at deep tow depths where the ghost notch falls within the usable frequency band. An uncorrected directivity effect that attenuates far-offset amplitudes will appear as a negative AVO gradient on the seismic data, potentially misidentifying gas-free sands as having gas-like AVO character. Applying a directivity-corrected amplitude surface calibrated to a well tie is the recommended quality control check before committing to a fluid identification based on far-offset amplitude anomalies.

Directivity is also referenced as:

Radiation pattern — the physics term for the angular dependence of source amplitude, used in the seismic source design literature; "radiation pattern modelling" is used when describing the calculation of expected amplitude and phase as a function of angle for a given source array configuration
Array response — used when describing the frequency-dependent amplitude and directivity characteristics of a source or receiver array; "array response function" is the mathematical expression of how the array modifies the amplitude spectrum as a function of azimuth and offset
Ghost — the specific manifestation of source or receiver directivity arising from the reflection at the sea surface or near-surface reflectors; "source ghost" and "receiver ghost" are the two components that create the combined ghost notch in marine seismic data

Related terms: AVO, airgun, seismic acquisition, dipole shear, ghost

Frequently Asked Questions

How does source directivity affect AVO analysis in deep water?

In deepwater seismic acquisition, the airgun source is towed at 6-12 metres depth and the hydrophone streamer is towed at 7-15 metres depth. The ghost reflections from the sea surface for both source and receiver create notches in the combined amplitude spectrum that are offset-dependent because the ghost delay time is fixed (determined by tow depth) while the recording time window increases with offset. The ghost effect therefore changes the wavelet shape with offset in a way that mimics AVO — at some frequencies, the ghost constructively interferes at near offsets and destructively at far offsets, creating an apparent amplitude decrease with offset that has nothing to do with the formation's AVO response. In quantitative AVO studies aimed at fluid typing, a directivity correction that removes the ghost effect at all offsets is essential before measuring amplitude gradients. The industry approach of deghosting the data (estimating and subtracting the ghost from the primary signal) using adaptive methods is now standard in pre-AVO processing workflows, but its quality depends critically on accurately knowing the source and receiver tow depths during acquisition.

What is azimuthal directivity in borehole formation imaging?

Azimuthal directivity in borehole imaging tools refers to the angular resolution of the acoustic or electrical measurement system — its ability to distinguish features at different azimuths around the wellbore. A focused acoustic beam that is 10 degrees wide in azimuth has an azimuthal directivity that resolves features separated by more than 10 degrees (approximately 9 cm of borehole circumference in an 8.5-inch borehole). Ultrasonic borehole imagers achieve azimuthal resolution of 1-3 degrees through a combination of narrow beam focus and tool rotation speed relative to the data acquisition rate. Microresistivity imagers (FMI, STAR) achieve comparable azimuthal resolution through an array of closely-spaced button electrodes on pad contacts around the borehole circumference. The azimuthal directivity of these tools is what enables the identification of individual fractures, bedding surfaces, and formation heterogeneities at the borehole scale, providing the detailed formation characterisation needed for fracture-stimulated completion design and geological structural dip analysis.