Hearing the Earth
One of the best ways to learn about conditions underground is to bang on the Earth. Much as family doctors (remember them?) thunked your back to find phlegm in your chest, oil prospectors have long used loud noises to find oil in what's called "seismic" work.

There are two reasons to use seismic soundings: sound sounds different when bounced off hard, non-porous rocks like granite compared to soft, porous, and possibly oil-bearing rocks like sandstone. "The geophone [earth-hearing microphone] can collect vibrations as they bounce off rocks," says Larry Nation, communications director, American Association of Petroleum Geologists. The data are stuffed into computers that then draw a picture that makes sense to the human eye.

And the second reason? The old-time prospectors made those big noises by setting off TNT -- and getting paid for it!

It is our sad duty to report that truck-mounted "thumpers" and underwater gas guns have largely displaced pyrotechnics in seismic searching.

At first, a seismic peek inside the Earth produced two-dimensional (2-D) maps. Although helpful, these maps had as little depth as a computer screen. In the 1980s, however, properly located microphones that could detect the direction of a sound wave's origin added a third dimension, depth, to the maps (with the help of considerable computerized data massaging).

A decade ago, 3-D seismic started becoming as popular in the oil patch as mammoth mergers are today. In 1989, only 5 percent of new wells in the Gulf of Mexico used 3-D, compared to 80 percent in 1996. One reflection of that advance was a rise in the success rate for new wells in the Gulf from 19 percent in 1985 to 40 percent in 1994.

One more D?
The latest bells and whistle is 4-D modeling, which adds the dimension of time. Don't be duped: this complicated-sounding method simply compares 3-D maps taken months or years apart. Four-dimensional modeling can answer some practical questions: How fast am I sucking this field dry? Where is the rest of the oil hiding, and how can I get my filthy mitts on it?

As with earlier seismic techniques, 4-D depends on microphones and microprocessors. Once the domain of major oil companies, the drop in computer prices has made it available to smaller ones as well.

Four-dimensional modeling is especially helpful for finding isolated pockets of oil, which are increasingly found in many played-out oil fields in the United States and the North Sea. Overall, the technique is part of what Nation calls an effort to "find out what's beneath the surface of the Earth without spending more than the oil is worth."

Measurements while drilling
Because dumbly drilling and hoping to drench yourself in a geyser of oil is, well, dumb, geologists appreciate a constant stream of information from the hole.

The first detailed data on what's underground came from instruments that were lowered into the hole. To use this "well-logging" technology, you would simply pull your entire drill string -- which can weigh dozens of tons -- out of a hole that may be a mile or more deep. Then you would lower a package of instruments while taking measurements. After withdrawing the instrument package, you would simply re-insert the drill string.

Although a considerable improvement over blind luck, well-logging gobbles hours of expensive drilling time. Far slicker would be piggy-backing the detectors on the drill -- if you can figure out how to marry a bunch of sensitive instruments to a giant steel bit that's spinning 100 times a minute in a maelstrom of drilling mud and bucking worse than a rodeo bull.

The first so-called "measurements while drilling" packages lasted only a few hours before the instruments croaked. These days, says David Bergt, a drilling expert from Schlumberger, a giant oil-field services company, an instrument package can last 1,000 hours or more -- plenty long enough to drill a monster hole.

One problem solved. But how do the instruments "phone home" from deep underground? Radio waves can't penetrate the Earth, so you can throw the idea of using a cell phone down the hole. Instead, the instruments use a variation on Morse code. They alter pressure in the drilling fluid, and a pressure detector at the surface reads this as binary code.

Bergt says these signals have many purposes, but first among equals is alerting drillers that they have actually reached oil. That was less problematic in the olden days, when petroleum was often pressurized enough to blow out of the hole in a gusher. These days, gushers are frowned upon by environmentalists and oil drillers alike: the former for wasting the environment, and the latter for wasting oil.

To prevent gushers, the weight of drilling mud -- the lubricant that floats drill cuttings to the surface -- creates pressure that counterbalances the upward pressure of petroleum. Using mud, a hole "can drill right through the pay zone and you wouldn't know it," says Bergt.

It's irresistible
Instruments, however, allow you to continuously drill and only stop when you've found oil.

For an exploration well, you can put around a dozen fancy instruments on a drill. But if you're working in an established field, you are most interested in porosity -- so the oil can flow into the well -- and the electrical resistivity of the rock.

Petroleum, limestone and sandstone have very high resistivity when dry -- they are insulators. But most underground rock is saturated with salty water, giving it great conductivity, but almost zero resistivity. It's the in-between zone that interests drillers, Bergt says. "Since oil is non-conductive, the overall formation resistivity goes up when oil pushes out the water, so now we have a way to detect the presence of oil."

Data from subsurface instruments can also guide a "steerable" drill bit through a narrow plane of oil. Each time the drill leaves the "pay zone," the bit can be redirected to return to the good black crude.