Li-Fi for marine surveys

Li-Fi stands for light fidelity which is a play on Wi-Fi (wireless fidelity or wireless network). This interesting technology could change how data is collected in geophysics.

Geophysics in most applications uses tools to image the interior of the earth. These tools measure some property of the earth like gravity or magnetic field strength. The success of any geophysics survey is limited by the technology used. There are entire companies that just focus on developing better technology for certain types of surveys. It is amazing how complex the instruments have become. One good example of this is seismic surveys conducted in the ocean.

Seismic surveys are used to characterize the subsurface geology. A source is used to make vibrations that travel through the subsurface and bound off of the geologic structures. The receivers record the bounced around waves.

A towed seismic source and receiver array.

The traditional method to conduct surveys is to tow the source and receivers behind a boat. The streamers (lines of receivers) can be kilometers long! This makes it difficult to keep straight lines. Another method used is ocean bottom cables.

Ocean bottom cables waiting to be laid on the ocean floor. 

These cables are laid down by a boat around the area of interest. They are usually put down in parallel lines. These are better than the towed array but the lines cannot be placed underneath an oil platform which is usually where the survey needs to be conducted. Enter ocean bottom nodes (OBN). OBN are geophones that are completely detached from any cable. Inside the cylinderical shape is every instrument needed, batteries, and a way to record the data.

An ocean bottom node from CGGVeritas.

These are placed on the ocean floor by a remotely operation underwater vechicle (ROV). They stay on the ocean floor until the survey is complete which is sometimes months. Then the ROV comes by and picks them up. Once they get back to the surface the data is downloaded from their memory.

A cartoon of an ROV placing nodes on the ocean floor.

The bad part of OBN is that you have to wait until the nodes are back on the surface to see the quality of the data. Problems could occur and you wouldn't know until the whole survey is complete. Better data is always the goal and with the high cost of these surveys it would help if the data could be streamed up to the boat or to the ROV. How might this be accomplished?

There is a idea out there to use light to transmit data exactly like how your wireless internet streams data to your computer or a radio tower communicates with your phone. Harald Haas, a professor from University of Edinburgh, explains and demonstrates the principle in the TED video below.

The basic idea is that an LED light can be turned off and on extremely fast so fast that humans can't see the variation. The light on equals a one and the light off equals a zero. So binary code is being transmitted by the light. The receiver takes that information and turns it into a video or a text or an email. The method still has a ways to go for everyday use with phones or internet but it is looking promising for communicate between nodes and ROVs. The ROV's light could be used to download data from the nodes. This is still just a futuristic idea but it could be a way to make a geophysical tool even more efficient and valuable for surveys.

How do you think data could be retrieved from ocean bottom nodes? Have you heard of any crazy technology that might change the way information is gathered?

Burning ice

Did you know there is a type of ice that can burn while someone holds it? Don't believe me? Well, look at the picture below. It's not a trick!

Methane hydrate burning in someone's hands.

Alright so it's not really ice. But it looks and feels like ice. It's really methane hydrate, a specific type of gas hydrate. The word hydrate is a term used in organic chemistry to indicate that a substance contains  water. So a gas hydrate is a gas trapped in a ice-like cage of water molecules. It's really the methane burning while the water melts - not exactly burning ice but that's what it looks like!

A diagram of how gas hydrates form.

In addition to looking super cool when it burns, methane hydrates have some important impacts on energy and climate change. Methane hydrate forms in sediments beneath the ocean floor requiring low temperatures and high pressures to form but its usually located shallower than oil reservoirs. The methane hydrates are a problem for the guys drilling wells because it could blow-up. But methane is also a source of energy (natural gas) and ideally the methane hydrate could increase the amount of domestic energy products but we would need to figure out how to extract it. Methane is very bad for our atmosphere (actually worse than CO2) and if the methane hydrates were to melt and release the methane it could be the tipping point to start an even faster global warming.

In a paper published in a 1996 issue of  Science, one of the most highly regarded scientific journals, Steve Holbrook, Hartley Hoskins, Warren Wood, Ralph Stevens, and Daniel Lizarrale report their findings of the amount of methane hydrates in three wells located in the Blake Ridge area, offshore South Carolina. (I lived in SC for 10 years without knowing we had gas hydrates or any energy near!) This paper came out during the early investigations into methane hydrates and everyone was still unsure how much of this stuff was out there. These guys concluded that the volume of hydrates located in the area offshore of South Carolina had been over estimated and that global estimates could be 3 times too high. 

The location of Blake Ridge offshore of SC.

Methane hydrates are found by acquiring seismic data over the location. A seismic survey is where a boat tows a source (air gun - makes vibrations) and receivers (hydrophones - they listen for the reflected vibrations) over the ocean floor. The methane hydrates are found by the presence of a bottom simulating reflector (BSR) in the data. Basically the difference from the presence of gas hydrates creates a reflection that shows up as a line in the seismic profile. The image below depicts the data from the 1996 paper.

Seismic data of the bottom simulating reflector (BSR) highlighted in blue. 

So in 1996 it looked like estimates for methane hydrates were not going to be as large as originally thought but because they could actually provide us with energy and could cause more climate change the research continued. It was actually written into the Energy Act of 2005 that the research would keep getting funded. And now after years of more research the estimate of  global methane hydrate volume is around 700,000 Tcf (trillion cubic feet) which is enormous when compared to the 200 Tcf of worldwide natural gas reserves! Since 1996 lots of methane hydrate was found in the Arctic permafrost and Gulf of Mexico. The estimates from the seismic data were too simplified and the complexity of gas hydrates is now better understood. The first methane was produced from a well going through the permafrost in northern Canada in 2008. Experiments continue but it looks promising that this form of energy can be produced safely. But the future is never certain and with the worry of releasing too much into the atmosphere it is worth taking the risks to get more energy? What do you guys think? Are you just fascinated with the burning ice?