Gulf focus shifts, but where is all the oil?

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With BP's leaking well in the Gulf of Mexico finally capped, the focus shifts to the surface clean-up and the question on everyone's lips is: where is all the oil?

For three long months a massive slick threatened the shorelines of Louisiana and other southern US Gulf Coast states as BP tried everything from top hats to junk shots and giant domes to stanch the toxic sludge.

A cap stopped the flow on July 15 after between 2.8 and 4.5 million barrels (117.6 million and 189 million gallons) had gushed out. Only one quarter of that was collected by BP's various collection and containment systems.

But thanks to frantic efforts to skim and burn the crude on the surface - some 34.7 million gallons of oil-water mix have been recovered and 411 burns have been conducted - the real difficulty now is finding any oil to clean up.

Dozens of reconnaissance planes fly constant sorties from Florida to Texas noting any oil sightings, while flat-bottomed boats trawl the marshes for lumps of tar too large to biodegrade.

"What we have is an aggregation of hundreds of thousands of patches of oil and the challenge is to find out where they are at right now because they are widely dispersed," said US spill response chief Thad Allen.

Pressed further on the patches, Allen relented: "Maybe patches is a misnomer on my part. What we're seeing are mats, patties, small concentrations, very hard to detect, but they're out there.

"What we're trying to figure out is where is all the oil at and what can we do about it."

The figures speak for themselves. Before the cap went on, some 25,000 barrels of oil a day were being skimmed from the thickest part of the slick near the well site.

By the time Tropical Storm Bonnie arrived last week, the take was down to a pitiful 56 barrels, begging the question of what to do with the fleet of 800 skimmers, many of them run by disgruntled fishermen.

Allen said he was already looking at trying to redeploy the so-called vessels of opportunity in surveillance and testing programs and was meeting local leaders this week to discuss options.

As to where all the oil that hasn't been skimmed or burned off has gone, opinions vary: some experts say it has been broken down naturally by the elements and by microbes in the ocean, while others fear it could be lingering undetected in underwater plumes.

Only weeks ago, the slick was an unstoppable force that couldn't be prevented from swamping shorelines and slowly choking helpless pelicans; now the oil is an elusive enemy, one that has to be tracked down.

The latest "Nearshore Surface Oil Forecast" from the US government's National Oceanic and Atmospheric Administration (NOAA) indicated only seven sizeable patches of surface oil, all light sheen.

An overflight Sunday identified one thicker patch of emulsified crude. A flotilla of skimmers was immediately dispatched and made short work of mopping it up.

With hopes high that the well will be sealed for good next week, Allen was conscious of the need to plan for the next phase, admitting, "we are going to have to figure out how we make a transition in our resources."

Sophisticated underwater operations involving fleets of robotic submarines at brain-crunching depths will make way for the less glamorous but equally complex work of Shoreline Clean-up Assessment Teams, SCATs for short.

"They will sign off literally mile by mile where we've had oil impact," said Rear Admiral Paul Zukunft, a government on-scene coordinator. "And that is the very long phase of this operation where you ultimately determine how clean is clean."

Approximately 638 miles (1,027 kilometers) of Gulf Coast is officially listed as "oiled," 362 miles in Louisiana, 109 miles in Mississippi, 70 miles in Alabama, and 97 miles in Florida.

The beaches should be relatively painless to mop up, but cleaning up the maze of marshes, where there's nothing to stand on and shallow-bottomed boats are needed to navigate the narrow channels, is a logistical nightmare.

Geologist Ed Owens, a world authority on protecting shorelines from oil spills contracted by BP to lend his expertise to the response effort, gave an upbeat assessment on Monday, saying the marshes should recover in a matter of months as only a "tiny fraction" of oil had infiltrated them.

Other leading scientists though have warned of a decades-long effect on marine life that could lead to a shift in the overall biological network in the Gulf of Mexico.