Ocean bottom cable acquisition
Ocean bottom cable acquisition is a seismic acquisition technique used in marine conditions to acquire seismic data. The cables are laid at the ocean floor where they record seismic data. Ocean bottom cables can be used in many different industries interested in ocean exploration, but in the past they have mainly been used by the oil industry. The use of ocean bottom cables has been in decline over the past few decades though as ocean bottom nodes are now preferred by most oil companies and ocean bottom cables are more expensive than streamers typically used in research by other industries.
How it works
Ocean bottom cables are typically an assembly of geophones and hydrophones attached together by electrical wires and placed at the bottom of the ocean floor to record seismic data and relay that data to a recording vessel. The cables can be attached to a vessel that pulls the cables into place or they be placed individually in a desired location or series by a bouye to accommodate the survey. A seismic source, typically an airgun, is created by one vessel that moves across the survey grid sending out seismic waves. The seismic waves will reflect and refract off the seafloor and the subsequent layers below it, which the geophones and hydrophones on the seafloor will record. Then they either send the data to a second recording vessel to analyse the data or it is stored directly by the geophone and hydrophone, requiring retrieval later for analysis.
Ocean bottom cables can be dated back to the 1950s, where they were basically just an extension of land seismic cables in very shallow water. They did not come become widely used until the 1990s when Joe Sanders and Fred Barr combined the use of geophones and hydrophones. Less than a decade later though, most companies lost interest in the ocean bottom cable industry due to increased cost and time over towed streamers and an oil industry downturn in 2008. The superiority of ocean bottom seismic was recognized during this time, just not in the form of cables. Most of the funding and projects are now being granted to ocean bottom nodes instead of cables.
Advantages and Disadvantages
Advantages over towed streamers
- A quieter environment- By placing the geophones and hydrophones on the seafloor, we can limit our exposure to surface factors like weather, waves and swells which will improve our signal-to-noise ratio allowing for better imaging of deep reflectors.
- Use of geophones- Hydrophones detect changes in pressure under water while geophones detect motion in one direction. That being said, the streamer method only uses hydrophones which will only give one component of seismic data. Ocean bottom cables offers the ability to combine these technologies and give us more components of seismic data. Three geophones can be used to measure motion in the X, Y and Z direction and the hydrophone can measure pressure, giving us four components, or 4C seismic data.
- Converted waves- Shear waves, or S-waves, cannot be transmitted through liquids therefore cannot be captured by the streamer method. With the receivers on the ocean floor, the S-wave data can now be gathered from the layers below. S-waves can be very important to image gas in the subsurface. Gas reservoirs can absorb and distort P-waves which can obscure the reservoir. S-waves on the other hand are not affected by the pore fluids since they can propagate through the rock matrix without being affected by the pore fluids.
- Flexibility- Towed streamers can be limited by surrounding infrastructure and geography since it needs to be able to fit a boat with several kilometers of streamers towed behind without tangling the streamers or running the boat aground. Ocean bottom cables can work around these impedances. Another advantage of ocean bottom cables over towed streamers is the flexibility it allows for in designing a survey. Having the sources and receivers detached from each other can allow for a full azimuth data. This ability offers improved imaging for many subsurface features like fault definition and sub-salt imaging. Ocean bottom cables also grants the ability to use longer offsets, resulting in improved imaging of deeper seismic horizons.
Disadvantages over towed streamers
- Coverage and Speed- Towed streamers have an inherent advantage over ocean bottom cables since they are just towed behind a single vessel rather than being placed or dragged into place on the seafloor. This allows for quicker coverage over large areas, especially in deep water environments.
- Cost- Streamers only need one vessel to tow them and do not require any retrieval of equipment allowing for a lower cost over a greater area of coverage. Geophones are not used on streamers as well, so the cost of materials is also lower.
- Speed and Cost- Ocean bottom nodes are much bulkier and heavier than cables. Due to this they need to be placed on the seafloor and retrieved by remote operating vehicles, or ROVs. ROVs deploy the nodes at a slow rate which increases the cost of the operation. The nodes must also be retrieved by ROVs to analyse the data.
- Depth- Where cables are limited in depth by the length of the cables present, usually no more than one kilometer, nodes are untethered. They can be placed as deep as needed and can be left for as long as needed to acquire seismic data. This is a very large advantage for oil companies looking into deep water drilling.
The importance of multi-component sensors for hydrocarbons
One of the biggest advantages of ocean bottom cables over towed streamers is the ability to use 4C sensors which can measure S-waves along with P-waves. This ability creates many different applications not available to towed streamers. When anisotropy is present, S-waves will split into two waves. These waves provide information about fracture density and orientation which is very helpful in finding carbonate reservoirs. Amplitude anomalies will also help determine the presence of hydrocarbons and other subsurface lithologies.
The Future of Ocean Bottom Cables
Even though the majority of funding is now in ocean bottom nodes, there are a few companies trying to combine the cost effectiveness of streamers with the data quality and freedom of nodes in the form of ocean bottom cables. One of those companies is Magseis, which is using technology developed by the US military and the telecommunications industry to create lightweight, autonomous nodes. These nodes are held together by cables containing no electronics resulting in reduced costs. With this technology, a single vessel can hold thousands of nodes over several hundred kilometers of cables and can be deployed much faster than single nodes with ROVs.
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