ocean bottom seismic

 

Ocean Bottom Seismic has repeatedly demonstrated to be an effective technology for risk reduction in numerous challenging acquisition environments. By placing receivers on the seafloor and decoupling the source from these receivers, the resulting data offers improved imaging through improved multiple suppression and better subsurface illumination, while also accessing areas not open to conventional streamer operations.

 

There is an increasing realisation that OBS data provides a significant advantage in providing both improved image quality and also coverage in inaccessible areas.

 

Each sensor within a multicomponent recording cable comprises three orthogonally oriented geophones/accelerometer, plus a hydrophone, giving us a 4C recording system. Compression P-waves are recorded on both the vertical component (Z) and the hydrophone, while the converted wave data is recorded by the X & Y component geophones/accelerometers.

 

Ocean Bottom recording is necessary to capture converted wave data, as shear wave data cannot travel through water, and therefore will not be recorded by streamer sensors. Additional benefits include improved signal-noise from stationary sensors in a quieter environment, and better target illumination via a wider diversity of ray paths from full azimuth acquisition.

 

typical OBC setup

 

Advantages of Ocean Bottom Seismic data acquisition

 

  1. Imaging where towed streamer fails: in fields with multiple surface obstructions, streamer acquisition can leave holes in the seismic coverage. This can be addressed successfully with the controlled positioning of cables and nodes. In shallow waters near to the coast, streamer vessel operations are compromised due to the limitations on the vessel turning in shallow waters. OBS operations can acquire data up to, and on to, the beach.

  2. Increased resolution through PZ summation: Dual sensor summation (P+Z) removes energy reverberations from the water column, this is important in hard water bottom areas since the seabed multiple is often the highest amplitude data in the seismic record.

  3. Deeper targets requiring longer offsets and lower frequencies: Source-receiver decoupling means any offset - azimuth combination is achievable when imaging deep targets. Additionally, many OBS systems have improved low frequency response for improved imaging of deeper targets.

  4. 4D Accuracy and repeatability: Receiver geometry can be re-created accurately, without the problems caused by currents in towed streamer acquisition, improving the 4D response on monitors surveys.

  5. Inversion: low frequencies and complete far offset data obtained with an OBS survey will give improved confidence in identifying sands and fluids via inversion studies.

  6. Source and Receiver positioning: Decoupling the source and receiver can be of considerable advantage in high dip environments such as those associated with salt intrusions. Additionally, by placing the sensors on the seafloor, two sources of noise which impact both data quality and operational performance of towed streamer surveys are avoided:- • The noise arising from towing the sensors through the water • The noise induced by the movement of the sea surface, so called weather noise.

  7. Cost: Improved operational efficiency and experience with Ocean Bottom operations combined with increasing competition in the OBC marketplace means an OBC survey may not be as commonly expensive as perceived, with the added advantages such as less weather down time, shooting when streamer cannot.

 

typical patch layout

 

 

Applications of Multicomponent seismic technology

 

  • Fracture density and orientation
    In the presence of anisotropy, shear waves will typically split into two waves, fast and slow. These split waves are very sensitive to fractures and can provide information about fracture density (fracture porosity) and orientation (directions of preferred permeability). A delay time will be observed between the 2 shear waves. This delay will be proportional to (1) The degree of anisotropy, (2) The distance travelled in the anisotropic medium. This technique is especially applicable in carbonate reservoirs.

  • Gas clouds & seepages
    P-wave energy may be attenuated by gas trapped in the subsurface. Shear waves can be used to help clarify the subsurface image because they are unaffected by pore fluids, an important attribute that can improve seismic imaging and highlight information valuable for reservoir characterization, reservoir monitoring, and well planning.

  • Direct hydrocarbon and lithology indicators
    Differentiation of P-P and P-S amplitude anomalies will give credibility that the observed anomaly is very likely a fluid effect, and that the amplitudes are consistent with presence of hydrocarbons. Converted shear waves can provide valuable insights into the nature of subsurface lithologies and pore-saturating fluids.

  • Stealth reservoirs
    Low compression wave reflectivity (transparent) reservoirs where there is little or no acoustic (P-wave) impedance contrast between the reservoir sands and the surrounding overburden, can be successfully imaged using converted S-wave reflectivity.

 

typical shooting configuration

 

Companies offering OBS acquisition services:-

 

Company Technology Crew Count
WesternGeco Q-Seabed 1
Geokinetics Sercel SeaRay 1
CGGVeritas Sercel SeaRay (OBC) / Trilobit (OBN) 2
Global Sercel 408 3
RXT VectorSeis Ocean 2
BGP OYO GeoRes 1
Geo-COSL Sercel SeaRay 1
Seabed CASE Nodes 1
FairfieldNodal Z3000 & Z700 Nodes 2

 

For more information about Ocean Bottom Seismic and how this technology can help your company optimise its potential please contact us.

If you would like to talk to us about your Ocean Bottom Seismic requirements please call us on...

 

  • +44 1872 870 766 (UK)
  • +44 7703 74 11 24 (UK)

 

Alternatively, click here to send us an email.