Nodes are finally making their mark

摘要

With their ability to better image difficult geology, seismic nodal systems have gone from idea to reality. Ever since researchers first embraced the concept of multi-component seismic, acquiring shear- as well as compressional-wave data, the industry has been at odds over how best to gather this information, particularly offshore. Since shear waves don't travel through water, the choice was to place sensors on the ocean bottom or measure "converted" shear waves with towed streamers. Purists preferred the first method. But the cost of deploying and recovering these sensors was cost-prohibitive. An interim solution was found with ocean-bottom cables (OBC), which could be lain on the seafloor and recovered after the survey was complete. The deployment was expensive, and while the data were generally of better quality than stremer data, complete coupling to the seafloor could not be guaranteed. OBC equipment could also be subject to damage from fishing vessels and other ship traffic, and it wasn't much easier than streamers to deploy around infrastructure. An increase in commodity prices coupled with the industry's desperate need to learn more about its assets has propelled a new technology forward - the ocean-bottom node. Nodes are placed in a grid using remotely operated vehicles (ROVs), left in place for several days while the survey is being shot and recovered afterwards for data download. While these surveys are quite expensive, they are sometimes the only method available in situations such as heavy infrastructure, environmentally delicate areas or ultradeep water, where working with a cable is difficult.