Temporal and Spatial Sampling

Vertical or temporal resolution is primarily about well-sampled frequency content. Strong ultra-low frequency amplitudes reduce side lobes on zero-phase wavelets and reduce interference between adjacent thin beds—if the amplitudes are balanced for all frequencies. The general rule of thumb is that provided at least two octaves of frequencies are contained in the signal, the maximum temporal resolution is roughly proportional to the highest recoverable frequency.

Nearly all 3D marine seismic surveys acquired since the 1980s have used arrays of airguns, a small pneumatic device that rapidly releases compressed air through a series of ports into the surrounding water. The frequency content of most air gun designs decays rapidly below about 7 Hz. Although broadband GeoStreamer imaging is routinely able to recover useful low-frequency amplitudes down to about 3 Hz, alternative novel pneumatic source concepts are required to emit significant signal amplitudes in the 3-4 Hz range.

TGS is able to operate both the “Gemini extended frequency” source developed by ION Geophysical and the “Tuned Pulse Source (TPS)” source developed by LISS / Sercel. Both sources can be deployed from TGS vessels. Common motivations to operate low-frequency sources are the improved stability of Full Waveform Inversion (FWI) in surveys that are likely to also acquire long offsets. A common survey design ambition when using low-frequency source concepts is that two or more source types are used together: A ‘conventional’ source used to acquire 0-8 km offsets, and a ‘low-frequency’ source used to acquire offsets in the 8-20 km range (depending upon vessel configuration).

The temporal frequency content and recording parameters used for acquisition with airguns typically restrict the maximum useful frequency to about 250 Hz. There are scenarios, however, where frequencies in excess of 1000 Hz are desired for very shallow seismic imaging and characterization pursuits. Applications include offshore wind farm preparations (i.e., shallow boulder detection) and seafloor mining. In these scenarios, electric source concepts such as Sparker sources deployed within UHR3D towed streamer acquisition can deliver such specifications.

Towed-streamer acquisition historically used hydrophone-only streamers that recorded band-limited temporal frequencies due to ghost reflections from the free surface of the ocean. All ghost effects are now completely avoided when towing multi-sensor GeoStreamers and applying wavefield separation in the processing and imaging workflow. Source-side ghost effects are also robustly removed during GeoStreamer data imaging.

Note that the temporal sampling rate used during recording must decrease as the maximum unaliased (Nyquist) frequency increases, which will translate to large data file sizes for storage.

Theory says that given an accurate velocity model, the Fresnel zone is collapsed by migration to the spatial sampling intervals used during acquisition, which therefore determines the horizontal (or spatial) resolution. Spatial sampling parameters are also customized to record signal and noise wavefields without aliasing of temporal and spatial frequencies. Sampling intervals must reduce as the maximum desired temporal frequency increases (e.g., shallow targets, resolution of stratigraphic features) and as the maximum desired spatial wavenumber increases (e.g., imaging of steep dips, resolution of fault planes and event truncations, resolution of salt body contacts). Multi-dimensional pre-stack noise removal, wavefield regularization, and imaging steps all explicitly benefit from dense spatial sampling. Shot domain imaging solutions such as Full Waveform Inversion (FWI), Reverse Time Migration (RTM), and Separated Wavefield Imaging (SWIM) explicitly benefit from densely sampled streamer spreads with high streamer counts.

Increasing the number of sources being towed may be used to improve the crossline spatial sampling of the emitted source wavefield. This may increase the high-frequency content of migrated seismic images. If ‘wide-tow’ multi-source shooting is used the near offset distribution also becomes more uniform in comparison to traditional multi-source shooting. The resolution and seismic image quality of shallow geology is correspondingly improved.

Similarly, towing streamers with close separation will improve the crossline spatial sampling of the recorded receiver wavefield and may increase the high-frequency content of migrated seismic images.