Frequency (f): Definition, Seismic Bandwidth, and Reservoir Resolution
What Is Frequency?
Frequency measures the rate of repetition of complete wavelengths in electrical, acoustic, or seismic signals, expressed in cycles per second or hertz (Hz) and symbolized by f. Reflection seismic surveys for petroleum exploration typically record useful energy between 5 and 100 Hz, with higher frequencies controlling vertical resolution and lower frequencies penetrating deeper through the attenuating subsurface.
Key Takeaways
- Frequency equals the inverse of period and is measured in hertz, one cycle per second.
- Higher seismic frequencies improve vertical resolution but attenuate faster with depth.
- The Nyquist frequency equals half the sampling rate and caps recoverable bandwidth.
- Dominant reflection frequencies usually fall between 20 and 50 Hz in clastic basins.
- Wireline and acoustic logging tools operate at far higher frequencies, from kHz to MHz.
How Frequency Works in Geophysics
Frequency relates to wavelength (λ) and propagation velocity (V) through V = fλ. A 30 Hz signal travelling through a 3,000 m/s (9,843 ft/s) sandstone produces a 100 m (328 ft) wavelength, and vertical resolution under the Rayleigh limit equals one quarter of that wavelength, or 25 m (82 ft). Higher source frequencies therefore detect thinner beds but penetrate less rock before attenuation reduces the signal below noise.
Acquisition crews control bandwidth through source selection (Vibroseis sweep, dynamite, marine air gun arrays) and through sampling rate. Sampling at 2 ms gives a Nyquist frequency of 250 Hz; sampling at 4 ms drops Nyquist to 125 Hz. Anti-alias filters in the recording instrument cut energy above Nyquist to avoid aliasing artifacts. Processing flows then apply deconvolution and spectral whitening to compensate for earth attenuation and recover suppressed frequencies.
Frequency Across International Jurisdictions
Canada. AER Directive 056 governs seismic licensing in Alberta, and the Canadian Association of Petroleum Producers (CAPP) Stratton geophysical guidelines define dominant frequency targets for Western Canadian Sedimentary Basin surveys, typically 20 to 80 Hz for Montney and Duvernay plays.
United States. BOEM and BSEE administer marine seismic permitting in the Gulf of Mexico, where broadband ocean-bottom node surveys routinely record 2 to 200 Hz for sub-salt imaging. The Society of Exploration Geophysicists (SEG) publishes resolution and bandwidth conventions referenced across the Permian and Eagle Ford.
Norway and the North Sea. Sodir requires operators to file processed datasets to DISKOS, the Norwegian shelf database. Broadband marine acquisition on the Johan Sverdrup and Troll fields targets 3 to 150 Hz to improve chalk and Jurassic reservoir imaging.
Australia. NOPSEMA regulates offshore seismic environmental plans for the Carnarvon and Browse basins, where ocean-bottom-cable surveys typically aim for 5 to 120 Hz to image deep-water reservoirs.
Fast Facts
Equinor reported recording usable seismic energy down to 2 Hz across the 2018 Johan Sverdrup broadband ocean-bottom-node survey, extending low-frequency content needed for full-waveform inversion under chalk overburden.
Frequency Components, Sampling, and Resolution
Three frequency descriptors matter most to interpreters. Dominant frequency is the peak of the amplitude spectrum and controls the apparent wavelet shape. Bandwidth is the range between low-cut and high-cut frequencies inside which signal exceeds noise. Nyquist is the upper recoverable limit set by sampling. Vertical resolution under Widess (1973) becomes ambiguous below one-eighth of the dominant wavelength, where tuning effects dominate amplitude.
Logging measurements use very different frequencies. Sonic tools operate around 10 to 30 kHz, dipole shear tools near 1 to 5 kHz, and dielectric tools above 1 GHz. Vertical seismic profile surveys bridge the surface seismic and log frequency gap, recording 10 to 250 Hz at depth.
Tip: When evaluating whether a target reflector can be resolved, calculate the dominant wavelength using a representative interval velocity and compare a quarter of that wavelength against the expected bed thickness. If the bed is thinner, expect tuning, not a clean two-event response.
Frequency Synonyms and Related Terminology
Frequency is also known as:
- Hz (hertz), the SI unit equal to one cycle per second
- f, the standard symbol used in wave equations
- Spectral content, when describing the full distribution across bandwidth
Related terms: Seismic, Resolution, Attenuation, Deconvolution, Vertical Seismic Profile
Frequently Asked Questions
What is the typical frequency range of reflection seismic data?
Most onshore reflection surveys carry useful information between 7 and 80 Hz, with dominant frequencies usually falling between 20 and 50 Hz. Marine broadband acquisition extends the low end to 2 to 3 Hz and the high end above 100 Hz. Frequencies above 100 Hz are usually too attenuated to image targets below 1,500 m (4,921 ft) of overburden.
Why does seismic frequency decrease with depth?
Anelastic absorption in rock removes high frequencies preferentially. The quality factor Q, typically 50 to 200 in clastic sections, governs how rapidly amplitude decays with travel time. By 3 km (9,843 ft) of overburden, a 100 Hz signal may lose more than 30 dB while a 20 Hz signal loses under 6 dB, shifting the dominant frequency downward.
How does Nyquist frequency limit interpretation?
Energy above the Nyquist frequency aliases back into the recorded spectrum as false low frequencies and cannot be recovered. Crews therefore choose a sampling rate that places Nyquist comfortably above the highest expected reflection frequency, typically 2 ms sampling for modern broadband acquisition giving a 250 Hz Nyquist ceiling.
Why Frequency Matters in Oil and Gas
Frequency controls what operators can see and where the well will drill. Resolving stratigraphic traps, mapping thin pay zones in the Montney or Eagle Ford, and inverting for reservoir properties all depend on bandwidth. Broadband acquisition adds millions of dollars to a survey budget but routinely pays back through improved trap definition, fewer dry holes, and better drainage planning in unconventional and deepwater plays alike.