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Volume 23, Number 6November/December 1972

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Positively Earthshaking

Good vibrations and turned-on color ...

Written by Brainerd S. Bates

Out in the broad expanse of the Saudi Arabian desert where everything is normally quite static, there has recently been a strange kind of movement. Five enormous vehicles, with their huge tires and oddly shaped but functional bodies looking from afar much like giant beetles, seem almost to be participating in some kind of outlandish, stylized dance. Lined up in oblique formation, they move forward a short way, pause as if straining at an imaginary leash, advance just so far, pause again, letting out another frustrated whine, then lurch ahead another time the same short distance. Maintaining near-perfect positions one on another, the five grotesque creatures inch their way across the sands, repeating their weird rite over and over for hours at a time, day after day.

Fantasy aside, these vehicles are engaged in doing some serious business, and they look the way they do because they're built to do it. A close-up look at the rear portion of one of the beetle-vehicles in action, for example just as it begins the "pause" cycle of its maneuver, helps explain the complicated mechanical structure riding on the chassis. A large flat pad is pushed down hydraulically by two thick, gleaming shafts until it touches the earth and this new surface contact slightly raises the rear wheels off the ground. During the brief period the pad is in this lowered position a bystander might sometimes feel slight tremors under his feet. The five enormous vehicles moving rhythmically across the desert in a line are real earth-shakers, and what they are doing out there is making good vibrations to help the Arabian American Oil Company (Aramco) in its search for oil.

It is axiomatic in the oil industry that the only certain way to find out whether the earth under any given spot contains oil is to drill a hole and look for it. But it's an equally well-known fact that even under the most favorable circumstances drilling is a chancey—and expensive—proposition. For these reasons, over the years exploration experts have devised a variety of techniques which can tell drillers which location under the ground is more likely to yield petroleum than another (Aramco World , January-February 1966). Every one of these methods came into being to help discover the possible existence of cracks (faults) and dome-like porous structures (anticlines) in the subsurface having impervious coverings under which oil might be trapped under pressure. Figuratively, all the various techniques used to get this information simply try to take out a huge, deep slice of the earth and examine it in cross section.

At various times Aramco has used several different tools to help do this. One is the gravity meter, which measures minute variations in gravity pull caused by undulations in the earth's crust. A second is the magnetometer, used to gauge variations of magnetic forces. Astute interpretations of subsurface differences revealed by gravity meters and magnetometers can tell something about underlying structures. But while these two instruments are relatively inexpensive exploration tools, the information they impart about the earth's substructures is much less precise and reliable than that put out by the newer—and costlier—seismographic techniques which have now replaced them.

Seismo is derived from a Greek word meaning "shaking"; seismographic can be loosely translated as "writing shakes." Oil explorers send shock waves or impulses into the ground and measure with sensitive instruments the length of time it takes for them to be reflected back to the surface. By studying the visual records made by these instruments a geophysicist can derive a great deal of structural information about the area being explored. Even in seismic exploration, however, techniques are constantly being refined and improved.

The earliest seismic exploration employed small charges of explosives to produce shock waves, creating what amounted to miniature earthquakes. Generating impulses by mechanical means is a more recent development. One method in this category uses a three-ton slab of steel mounted on the back of a specially designed truck and dropped in a vertical free fall at very brief, regular intervals as the truck moves along at a set slow speed. Another kind of sand-tired vehicle, advancing abreast of several others just like it, carries a gas-driven chamber which also delivers a sharp thump onto the ground. The vibrating earthshakers are the latest word in seismic exploration, and using them effectively is a complex operation involving a skilled team of men and a wide array of sophisticated equipment.

Early last summer an exploration party under contract to Aramco was working in an area far south of al-Hasa Oasis assigned to it by the oil company's Exploration Department. Aramco supplied the party—designated Seismo 7—with maps indicating the precise lines and directions it was to follow. A surveying team, keeping four or five miles ahead at all times, staked out the path in the trackless desert which the main body of explorers would be taking. Back down the line were those five vibrating units working their way fitfully forward in oblique formation along the route laid out. Buried just under the surface of the sand for some 13,000 feet ahead of the five trucks and 4,000 feet behind them were 96 "arrays" of geophones—altogether about 3,400 extremely sensitive receivers for picking up the impulses sent into the ground by the vibrators, all interconnected by a ¾-inch-thick cable containing 150 pairs of insulated wires. And somewhere out in front of the muscular earthshaking vibrating units was the nerve center, a recording truck whose windowless, air-conditioned body was jammed with computers and other assorted electronic equipment.

The vibrating units themselves are highly specialized vehicles designed to do one specific job and one job only—and look it. They come equipped with crab axles, and all four wheels are steer-able for greater maneuverability. Both front and rear axles are driving axles. Power steering and full power shift transmissions are standard equipment. Their cabs are air conditioned, not so much for the comfort of the drivers as to pamper the small computers each carries inside. When the full 15-ton weight of these vehicles bears down on their vibrating pads, the section of ground on which they rest really feels their presence—"like grabbing a piece of earth and shaking it," as one member of the Seismo 7 party describes it.

The computer in each cab receives a signal from the recording truck which activates its hydraulic mechanism to lower the vehicle's 12-foot-square pad to the ground. Instantaneously the recording truck sends out a radioed command for the pads to vibrate in unison, beginning at a low selected frequency and increasing uniformly to a high frequency for precisely seven seconds to make one "sweep." After a six-second pause the pads make another simultaneous seven-second sweep in the same position before the vehicles advance exactly 15 feet during the next six-second interval. On their second station the vibrating units again make two sweeps of seven seconds each, with a six-second pause in between. Always obeying the radioed orders from the recording truck, and taking time out only for some such necessary detour as skirting the slipface of a sand dune and repositioning, the vibrating units stick to their 13-second advance-and-pause cycles from dawn to mid-afternoon, seven days a week. On their best days they can vibrate up a line as long as seven miles.

The action of the vibrating units is divided into 13-second cycles because this is precisely the amount of time the magnetic-drum memory core in the recording truck takes to make one revolution. The memory core continues to revolve until it has made from 12 to 16 records while the distant vehicles vibrate in six to eight positions. The records made during these revolutions result in one stacked, or composited, record which goes on a digital magnetic tape.

Just as a shortwave radio listener tries to tune out interference on his receiver to pick up a broadcast as static-free as possible, the geophysicist attempts to remove unwanted noises from his seismic recordings. Advanced seismic techniques call for an ever more sophisticated array of computers whose principal function is to filter out electronically as many undesirable noises as possible, noises such as those caused by wind blowing across the ground surface which are picked up by the geophones. Electronic equipment in the recording truck does some of the preliminary filtering. Later, the two or more tapes which represent a day's work in the field are sent to London, where highly specialized computers undertake more filtering and further enhance the seismic data by advanced digital processing involving the use of a selection from a library of 300 programs.

Early in the proceedings the auditory seismic waves reflected by the substrata are turned into visual representations—broad strips of photographic paper covered by squiggly horizontal lines which tell a significant story about the underground structures being examined to anyone trained to interpret them. Not too long ago, experts developed a sophisticated refinement of this basic technique. Vivid and distinctive colors can now be added to the visual cross sections, colors which are keyed to the varying compositions and alignments of such rocks as limestone, shale and sandstone which the waves pass through. To experts, the result gives the most accurate look yet at the "deep slice of earth" below the earthshaking vehicles, and the formations which might hold oil. To the layman its simply a turned-on, tuned-in electronic prism.

Actually moving a derrick to the site and drilling down deep into the earth is still the only way to "discover" oil. But these new forms of earthshakers, plus these increasingly accurate and easier-to-read visual "pictures" of the shock waves they generate, are making interpretations of these records ever more credible. Today, when a geophysicist looks down at a concession map, points and says, "This is where I think we should drill," the industry's decision makers, thanks to these new applications of physics, electronics and communications to petroleum geology, have better reasons than ever to take him at his word.

Brainerd S. Bates, a regular contributor to Aramco World, is Aramco's chief writer on petroleum

This article appeared on pages 8-12 of the November/December 1972 print edition of Saudi Aramco World.


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