WASHINGTON (CBS.MW) -- As politicians, oil companies and environmentalists brace for battle once again over the Arctic National Wildlife Refuge, the debate's biggest unknown lies thousands of feet below the tundra.
Is there any oil down there?
And, if so, how much? On this point, like almost everything about this vast Alaskan tract, opinions vary enormously because the main studies on the issue are nearly 20 years old and open to debate. And the only way to get a clearer picture is to drill using modern technology.
"There have been enormous advances in seismic testing since (the last tests) were done," said Ed Porter, research manger for the American Petroleum Institute, the Washington-based industry group that has been pushing for exploration.
Invigorated by even bigger House and Senate majorities, Republican lawmakers are widely expected to revive their push for oil exploration in the 15 million acres of federally-held land, a feat that has eluded pro-development forces for nearly two decades.
But estimating recoverable oil supplies is a constantly changing science, a shocking lesson learned by investors earlier this year when Shell and El Paso dramatically revised the accounting of their reserves.
Petroleum geologists believe the biggest concentration of oil in ANWR lies under a 1.5 million-acre strip of the Alaskan coastal plain called section 1002, the area lawmakers have in their sights.
Data used by the government to pinpoint rock formations likely to contain oil are limited due to congressional restrictions on land studies. Most of the data stem from a single seismic survey performed by a group of oil companies in 1984 and 1985. The effort was overseen by the U.S. Geological Survey.
Estimates of how much oil lies trapped beneath ANWR and how much can actually be brought to the surface depend on these 20-year-old seismic tests, numerous assumptions about geology, and about a dozen test wells drilled on land and offshore outside the area's perimeter.
The seismic survey initially led the USGS in 1987 to estimate Section 1002 held between 4.8 billion and 29.4 billion barrels of oil.
A more recent, three-year study of the same data published by the USGS in 1998 boosted the numbers substantially. In this second report, the USGS calculated that possible oil supplies in ANWR more likely totaled 15.6 billion to 42.3 billion barrels.
However, the agency warned there was a 95 percent chance that only 37 percent of these resources are "technically recoverable." This leaves a "mean estimate" of technically recoverable oil of 10.4 billion barrels, or 1.4 million barrels a day - or about 7 percent of what the U.S. now burns on a daily basis.
The increase over earlier estimates was chalked up to a wider survey area that included state waters up to three miles off the coast and native lands within Section 1002 that were not part of the 1987 study. Excluding native lands and offshore areas, the 1998 study concluded that from 4.3 billion to 11.8 billion barrels, or a mean estimate of 7.7 billion barrels, could be pumped from the area.
The USGS also reprocessed and re-examined the seismic data it collected back in 1985, drawing on new computer programs that filter out seismic "noise" and gave a far clearer picture of the rock strata deep within the earth.
Another key difference was that the later study listed deposits likely to contain only 100 million barrels of oil. Back in the mid-1980s, a field needed at least 400 million barrels to be considered worth pursuing, a threshold dropped by subsequent advances in drilling techniques.
"What that means is there is a whole new class of exploration targets within ANWR," said Dave Houseknecht, research geologist with USGS. The search for these smaller, so-called stratigraphic traps has "revolutionized" oil production, he said.
Better readings of the seismic data also led USGS to shift its conclusion on where most of the oil in ANWR is likely to be found.
The 1987 report suggested 75 percent of the oil was in the southeast part of Section 1002. The 1998 report completely reversed this opinion, claiming nearly 85 percent was likely to reside in the northwestern corner.
But Houseknecht admits the USGS information is not a complete record.
For starters the government doesn't have access to independent seismic tests or proprietary oil industry data gathered from nearby test wells. Exxon Mobil, BP and ChevronTexaco have all been actively surveying or drilling in areas surrounding ANWR.
Furthermore, the USGS seismic survey was conducted using two-dimension technology. Since 1985, the use of three-dimension seismic testing has become widespread, providing far more detailed visual models of potential hydrocarbon-bearing zones.
Energy Secretary Spencer Abraham, when recently asked whether the government would launch a new round of seismic tests if Congress decides to open ANWR, replied: "I don't really know if USGS will do that." He said any decision on conducting a new survey would be left to the Department of Interior.
Abraham said he believes "the estimates we've seen are ones that I think would be sufficient to generate interest" of companies.
Companies that successfully bid for federal exploration leases in the region would likely conduct additional tests of their own, Porter said.
"I think it would be essential for [three dimensional] data to be collected" before drilling, Houseknecht said. He also questioned whether oil companies would be willing to bid on leases made available in the area without further seismic testing.
Houseknecht admits that there is a huge "unknown" within ANWR's Section 1002 -- a large geological structure known as the Niguanak High that has not been test drilled. "It is probably the biggest onshore structure in a U.S. oil province that has not been tested," he said.
The 1998 USGS study concluded that the reservoir rocks that trap oil were missing.
"There is no question that we admit that this is the single greatest uncertainty in ANWR," Houseknecht said. If reservoir rock is present estimates of the oil in ANWR could be much higher.