Seismic Unwired: Going Wireless in Difficult Terrain
Authors: Rassenfoss Stephen 
Published: February 2015

Seismic Unwired

The Cook Inlet has a long history of oil production, with more than 1.3 billion bbl produced and the potential for more. But ever since production peaked at 230,000 b/d in the 1970s, there has been little exploration as it dwindled to around 12,000 b/d.

Stories about the decline point out that the Cook Inlet was eclipsed by far larger finds on Alaska’s North Slope, such as Prudhoe Bay.

Still it does offer the potential for significant conventional oil finds onshore and in shallow waters offshore.  A 2011 assessment by the US Geological Survey (USGS) covering the entire inlet estimated there could be up to 1.3 billion bbl of oil to discover—its mean estimate was nearly 600 mm bbl of oil plus 13.7 Tcf of natural gas from conventional formations. Plus significantly more gas from unconventional ones.

Unlike the North Slope, there is a ready market for natural gas. The nearby city of Anchorage depends on the inlet for natural gas and is concerned about dwindling local supplies, and the location offers transport links not available in the far north.

Past exploration suggested the underground structures likely to hold hydrocarbons could readily be identified using 3D seismic.

But there have been few surveys using advanced seismic technology and they generally covered small areas. That is not surprising considering the inlet’s treacherous tides, punishing winter weather, endangered Beluga whales, and the brown bears thriving in the wildlife preserves on its shores. Not to mention the permitting process.

That all changed when Apache Corp. began an aggressive leasing program a few years ago, gaining rights to explore more than 1 million acres. The big independent exploration company saw the potential for discoveries, particularly in unexplored deeper horizons, and based on its experience elsewhere it believed it could deal with the problems associated with doing a large-scale seismic survey.

Obtaining permits to shoot seismic in the inlet required creating a monitoring program, with specially trained and equipped observers making sure the operation followed regulations that can shut down seismic work if certain endangered mammals are within a 6-mile radius, and where sounds could affect animals.

Putting out the sensors needed to record those echoes presented another set of natural challenges in waters where tides can change the water level by as much as 35 ft. Currents up to 7 mph (6 knots) rule out towing streamers that move with the current, which also has presented problems for strings of seafloor receivers wired together using cables.

“There were horror stories in the past of cables drifting in the currents. Currents are a big problem,” said Mike Yates, senior staff geophysicist for Apache, which also experienced electrical problems with a cabled system, caused by the salt water.

On land, laying out long lines of receivers connected by cables was a problem because the permits banned cutting through the brush to create the relatively straight paths needed for cables, and forced detours around protected areas, such as bear dens.