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 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.

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.