Subsurface detection

Making inferences about the nature and structure of buried rock bodies, without access to them, is called subsurface detection. Using geophysical techniques, we obtain data at the surface that characterize the feature buried below. Then we construct models of the feature, trying to invent combinations of reasonable rock bodies which are consistent with all of the observations. Finally, using intuition, logic, and guesswork, we may select one or a few of the models as representing the most likely subsurface situation.

Seismic techniques

An earthquake generates seismic waves which can travel through the entire Earth . If you stamp on the ground you make the same kinds of waves, although obviously they are much weaker and do not travel as far. We know a great deal about how these waves travel through rock. By generating waves and then carefully timing how long it takes them to travel different distances we can learn a lot about the structures of rock units at depth.

Seismic reflection

The next time you go to a shopping mall on a bright afternoon, notice your reflection in the two sets of double doors at the entrance. You'll probably see yourself reflected twice, once from the outer set, and once from the inner set. Unlike opaque mirrors , glass doors permit some light to go through them, while at the same time they reflect some light back. Rock layers behave in a similar manner with respect to seismic waves. An explosion detonated at the surface will reflect off of many layers at depth. Detectors set up in an array can time the arrival of each of these reflected waves. Just as bats and dolphins can use the echoes of sound waves to locate food, geophysicists can use the echoes of seismic waves to locate reflecting boundaries at depth. These reflecting boundaries occur where there is an abrupt contrast in some seismic wave transmission properties (usually velocities) of the material. Most often this is a result of the sedimentary layering. A seismic reflection profile, which actually shows reflection horizons, is usually interpreted as revealing the structure of the underlying layers. The oil bearing structures in many productive oil and gas fields were located using reflection seismic surveys, so this technique has been very important to the petroleum industry.

At first seismic reflection profiling was used only by oil companies. Because it is economical to recover oil only if it is relatively near the surface, such surveys did not seek out much information at great depths. Over the last fifteen years or so, however, scientists have adapted the technique to probe the very deepest parts of the crust. One remarkable discovery is a nearly horizontal fault underlying much of Georgia and the adjacent continental shelf .

Explosives are still used to generate seismic waves in some areas, particularly at sea, but in many places they have been supplanted by special trucks which vibrate the ground beneath them with hydraulic jacks . The signals generated by these vibrations reflect just like any other seismic waves, but because the energy is put into the ground over a period of time, instead of instantly, there is less damage to structures in the area.

Seismic refraction

If you live in a good sized city and want to travel a few blocks, you would probably take the direct route and put up with the stoplights and traffic. If you want to go further, though, you might find it takes less time to go out of your way to use an expressway. Although you have to travel a greater distance, you save time because the expressway is so much faster.

Similarly, seismic waves may go further, but reach faster layers at depth and arrive at a sensor before those taking the direct route do. When this occurs the path of the waves bends as it crosses boundaries between layers of different velocities, a phenomenon called " refraction." This technique can be used to determine how thick the soil is above bedrock . (This might be an important consideration in siting a landfill , for example.) Solid rock has faster seismic velocities than soil. If the depths of interest are small, the source of the seismic waves does not need to be very energetic. A sledgehammer, a dropped weight, or a blasting cap might be used. A detector located near this source will pick up the waves traveling through the soil. By plotting how long it takes them to travel different distances, their velocity through the soil can be determined. As the detector is moved further from the source, however, a point will be reached where the waves traveling through the bedrock start arriving first. This is equivalent to the distance you would need to go (in the city example) before it was quicker to use the expressway. Continuing to measure travel times for more distant stations permits the seismic velocity in the bedrock to be found. Knowing the physics of refraction, the two velocities, and the location where the bedrock path became faster yields the thickness of the soil layer.

The same principles can be applied to problems where the depths of interest are much greater, but the source of the seismic waves must be more energetic. When Andrija Mohorovicic observed a similar set of two lines for the arrival times of earthquake generated seismic waves, he realized that the earth must have a layered structure. In this case the upper layer was the crust, and it was about 18.5 mi (30 km) thick. The transition zone between the core and mantle now bears his name, the Mohorovicic discontinuity.

Potential field methods

Some properties of a material can be sensed at a distance because they generate potential fields. Gravitational fields are produced by any object with mass , and magnetic fields can be produced or distorted by objects with appropriate magnetic properties. Techniques to measure and interpret such fields are extremely useful in subsurface detection.

Gravity

Every object with mass produces a gravitational field. In theory it should be possible to detect objects which are denser than average or less dense than average lying beneath the surface. A large body of lead ore , for example, should cause the gravitational field above it to be somewhat greater than normal. A deep trough of loose sediments should result in a weaker gravitational field. The problem is that the gravitational attraction of the planet is huge, so that it is difficult to separate and measure that little modification produced by an anomalous mass.

Halite is a rock made of salt , NaCl. Therefore, rock salt, which is composed of halite, is much lighter than most other rocks , and deforms very easily. Big blobs of it sometimes develop at depth and rise toward the surface in bubble-like structures called diapirs or salt domes. As they move up, they can warp the sedimentary rock units they move through, producing rich, but localized, accumulations of oil. Much of the oil found in the Gulf Coast states occurs in association with these salt domes, and so the petroleum industry had considerable incentive to develop techniques to detect them. As a result, the gravity meter, or gravimeter, was invented.

Essentially a very delicate bathroom scale, the gravimeter measures how much a spring is extended by the gravitational force acting on a mass at the end of the spring. What makes it work, and also makes it expensive, is that this extension can be measured with great precision, generally one part per million or better. That precision is equivalent to measuring a mile to within a sixteenth of an inch.

Such instruments immediately proved their worth, successfully detecting scores of salt domes beneath the nearly flat Gulf Coast states. Extending this technique to regional surveys, involving larger areas extending over greater elevation ranges, required refinements in our models for the gravitational field of the planet. This work has continued in conjunction with ever more refined gravity surveys. In the process, a major rift running down the center of North America has been discovered, subsurface continuations of rock units cropping out at the surface have been delimited, and even precursors for earthquakes have been detected.

Magnetism

Few phenomena seem so magical as the invisible attraction and repulsion we can feel when we play with magnets. When we stick a paper clip on one end of a magnet, it becomes a magnet, too, capable of holding up another paper clip. Some rock types exhibit their own magnetic fields, much like the magnets on a refrigerator door. Others distort Earth's magnetic field , similar to the way a paper clip temporarily becomes a magnet if it is in contact with a kitchen magnet. Sedimentary rocks rarely exhibit either magnetic behavior, and so they are effectively transparent to the magnetic signal.

Using a magnetometer we can measure the strength of the magnetic field anywhere on Earth. Often this is done with airborne surveys, which cover tremendous areas in little time. The results are mapped and the maps are used in several different ways.

The thickness of the sedimentary cover over igneous and metamorphic rocks (often called the "depth to basement") can often be inferred qualitatively from the magnetic maps. Just as a newspaper picture looks like a gray block when seen from a distance, a photograph when seen from arm's length, and a collection of printed dots when seen under a magnifying lens , so too the magnetic signal from the basement looks considerably different when seen from different distances. Little detail and subdued images suggest a thick blanket of sedimentary rocks, often miles. Sweeping patterns or textures, caused by the combination of many outcrops involved in the same tectonic deformation, suggest a sedimentary cover of moderate thickness. If we can see distinct outlines, produced by the basement rock's outcrop patterns, we can safely infer that there is little or no sedimentary cover.

Magnetic maps are also utilized to map the continuation of units from places where they crop out into areas where they are buried. Much of the recent increase in our knowledge of the geology of the Adirondack Mountains in New York stems from this use of magnetic maps.

A third technique uses the strength and form of the magnetic signals to put limits on the geometry of buried units. This is similar to the situation with gravity, where models are developed and tested for consistency with the data. Often magnetic data can be used to constrain gravity models, and vice versa.

Electric techniques

The various responses of Earth materials to electric fields of different types permits additional characterization of the subsurface. Natural and artificial signals at a variety of different frequencies can be observed to travel through different parts of Earth. Measurements are made of how the signals are modified in their paths, and then models are constructed which try to emulate this behavior. Often these techniques are most useful where the extension of a geological body with distinctive electrical properties is sought in the subsurface. Metallic ore deposits or ion-rich pollutant plumes are good examples. Measurements of electrical resistivity, the resistance of a material to passing a current of electricity , have been used for decades in the oil industry to help locate oil- or gas-bearing rock units.

Nuclear survey methods

Nuclear survey methods are of two basic types. The more common of the two involves measurement of natural radioactivity in rocks or soils. This method might be used, for example, to identify potential uranium ores for mining . In this case a hand-held geiger counter could be employed. Another use is to measure the natural gamma ray emissions from rock formations in a drill hole when searching for oil. A gamma ray counter is lowered down the hole on a wire, and the natural gamma ray emissions of the rocks in the borehole wall are measured. Different rocks exhibit various levels of radioactivity, making remote identification of rock type possible. Geophysical surveys of boreholes that are done in this manner are called wireline surveys or wireline bogs.

The second type of nuclear survey method is stimulated radioactivity. In this method, a radioactive source is used to bombard a rock and induce radioactive decay in the rock. The level of induced radioactive decay is measured and the rock type is interpreted from the measurements. Wireline surveys employing this method are often used when exploring for oil.

Satellite altimeter data

Satellites can measure the elevation of the sea level surface with astonishing precision. Because their path crosses the same place on Earth over and over again, the effects of waves and tides can be accounted for, and the actual variations in the elevation of sea level can be determined. These variations are most often the result of relief on the ocean floor. Where there is a mountain on the ocean floor, its additional mass draws extra water toward it, which will elevate the level of the sea above it. Where a trench exists on the ocean floor, the absence of mass attracts less water and a depression in the elevation of sea level will occur above it. Many of the detailed maps of the sea floor were obtained indirectly in this way.

The inverse problem

Subsurface detection relies on solutions to what is often called the "inverse problem." A particular set of data are observed, and then models are developed which attempt to fit all the data. Sometimes, when a very nice match is made, it is tempting to assume that a particular model is the only model which can fit the data, although this is rarely true.

An example may illustrate this: Imagine that I am trying to figure out the value of the change you have in your pocket. Suppose I have a device which will detect how many coins you have, and it says you have seven. Then I would know that you have at least $0.07 and at most $1.75. Suppose I have another device which tells me that the coins are of two different sizes, with four of them larger than the other three. The big ones might be quarters, leaving a range from $1.03 to $1.30, or they could be nickels, leaving a range from $0.23 to $0.50. Finally, if I were to think about this more carefully, I would see that not all of the values in these ranges are possible. So you could have $1.03, $1.12, $1.15, $1.21, $1.30 if the larger coins are quarters, or $0.23, $0.32, $0.41, $0.50, if the larger coins are nickels. We have reduced the number of possibilities to nine, but there is no way we can use these "subsurface detection" techniques to constrain things further. Because each of these nine possibilities fits the data perfectly, we might find one and erroneously conclude that because it fit the data so well it must be true. Assumptions were built into our conclusions, also; we assumed it is United States currency, and no half dollars or dollar coins. Such assumptions make sense for most pockets we are likely to run into in this country, but are obviously not valid in other countries.

This example illustrates the nature of subsurface detection. Results are usually somewhat ambiguous, depend on assumptions, and do not directly give the answers we seek. Yet they can provide important constraints. I may be able to make some additional assumptions from other data, hunches, or guesses. For instance, I may know you well enough to figure that you would not keep pennies, which would reduce the number of options to three.

Usefulness of subsurface detection

The techniques described here are very often used together to improve our understanding of what exists below the surface. By adjusting the sensitivity of the instruments and the spacing of the measurements, the scale and the depth of interest may be varied. The same theory and principles used with magnetic techniques which delineate the rift running through the crust of North America, at a depth of 18 mi (30 km), can be used to locate buried pipes and cables at a depth of less than 10 ft (3 m).

Often, subsurface detection is the only way to study the area of interest, because it lies too deep to be reached directly. Other times it is used because it is less expensive than digging or drilling, or because it disrupts the environment less.

See also Seismograph.

Resources

books

Press, F., and R. Siever. Understanding Earth. 3rd ed. New York: W.H Freeman and Company, 2001.

Telford, William Murray, L.P. Geldart, and R.E. Sheriff. Applied Geophysics. Cambridge; New York: Cambridge University Press, 1990.

Otto H. Muller

KEY TERMS

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Reflection

—When some of the energy of a seismic wave bounces off a boundary between two media, instead of traveling through it.

Refraction

—The bending of light that occurs when traveling from one medium to another, such as air to glass or air to water.

Seismic wave

—A disturbance produced by compression or distortion on or within the earth, which propagates through Earth materials; a seismic wave may be produced by natural (e.g. earthquakes) or artificial (e.g. explosions) means.