Listening for deep oil deposits.

Fossil fuels lie buried within Earth. The problem is finding them. To plumb those depths, scientists set off surface explosions and then listen for the returning shock waves. Such seismic rumbles give telltale clues of hidden oil reserves.

Geophysicists dig a hole some 100 meters deep, pack in explosives, seal the hole, and set off a downward-directed kaboom. Some of the shock waves from this focused blast radiate through Earth's stratified layers and reverberate back. Bouncing off rock, hydrocarbon reserves, or other sediments, these reflected waves bear unique signatures from which scientists can decode the properties of underground materials.

Susan E. Minkoff and William W. Symes, both mathematicians at Rice University in Houston, report a new analytical method for estimating the energy of the explosion and the characteristics of the reflected shock wave. Their work represents a novel approach to seismic inversion.

"Inversion techniques are very common," Minkoff notes. "But for some reason, no one has tried to estimate both the energy source and the reflectivity of a seismic event using an inversion method."

Geophysicists correlate the waves' velocities with the densities of underground materials, Minkoff says. Based on this information, they figure out the types of rocks and locations of hydrocarbon deposits. "In order to do that, they need to know the energy source," she says. Since scientists can't determine exactly how much of the explosive energy is focused directly into a shock wave, they estimate it. "This inversion method gives them a more accurate way to estimate that energy source," Minkoff explains.

Minkoff and Symes tested the inversion method with data from a seafloor experiment in the Gulf of Mexico. "When we did an inversion for both the energy source and the reflectivity, we cut the error rate in half," she says. "That's the bottom line."